Ethiopian airliner down in Africa

[infrequentflyer789]

Since this video may not be coming back I'll post a sort of transcript from memory since I was lucky enough to get to watch it. I'll try and just describe what I saw and keep my commentary/speculation to a minimum - in <italic>. Bear in mind this is a non-pilot observer's take on it from flaky memory:


There is a fair bit of intro talk then the sim.

They are in NG sim, not MAX, no surprise there. Mentour in left seat.

They start at low altitude, I think it was commented on, not at takeoff, I can't remember if they showed putting flaps up or started with flaps up and said it was just after flaps up.

No stick shaker, that I saw.
<presume means no elevator feel shift either, and that actual AOA error is not being simulated>

IAS disagree then simulated - or they just did the memory items, I didn't notice warning light (not that I would) and he didn't note it, there was an aural alarm but I think that was for AP disconnect
<I don't think this is exactly what the accident flights faced, they had shaker and IAS disagree from takeoff>

IAS disagree memory items, AP/AT OFF, FDs OFF, 75% N1, 4degrees

Throttles are pushed forward to get 75%, he comments that at low alt with denser air this is a lot of thrust and hence speed will increase

Trim runs, obviously sim doesn't have MCAS so not sure if they are simulating runaway or if he is doing it with the column switches

Comment is made that he was _expecting_ trim to run, because speed is changing, he says it will take a couple of cycles of fighting it with the switches before being definite that it is wrong

They discuss (he raises) that he is having flight control issue, they conclude it is trim that is the problem, they now run the stab trim runaway memory items finishing by hitting the switches. At this point he is clearly holding quite a bit of pressure on the column, but not obviously losing control of pitch

Now he points out that trim has only got down to (I think) 3 degrees on the indicator (zero being full nose down, I think) and that they are now going to "try something" - video captions say "don't try this at home", like we've all got a FFS in our shed or a 737 on the drive :-)

At this point the guy in the right seat uses the handle to wind the trim further _forward_. He stops when he clearly cannot wind it further - but they point out that it's still not at full nose down

Mentour is obviously struggling to keep the nose up at this point. I noticed he didn't seem to have the shoulder straps on (right seat did), possibly because he was frequently turning round to talk to the camera. That probably didn't help, but on the other hand he didn't have stick shaker or feel shift to cope with either.

Now, with Mentour pulling back, the right seat guy tries to wind the trim back with the handle - and he struggles to move it at all

They stop the sim, I think it was after stopping that he noted that at that point they were at 310 (maybe he said 340) knots.

I think there was then some further commentary, at one point I think he mentioned the rollercoaster to unload the stab and allow trimming, but commented that close to the ground the instinct is to pull back - I may be imagining that or remembering it from another video though.

<
And that was about it. I can try to answer questions on the video if anyone has any. The eye-opener for me wasn't the force on the control column (which was held anyway), it was that the trim couldn't be moved, in either direction. From somewhere I had got the impression that those wheels had a lot of leverage, due to the gearing, which was why a lot of turns would be required, but it seems that a lot of turns would be required and a gorilla to make them...
>

[gums]

Salute!

Good thots, FDR and Murphy is correct, I like the generic FBW laws and 'bus implementaion for the most part.

Without control surface feedback, you can satisfy the FAR requirements for column force versus AoA because the control stick/wheel can use a simple spring as does the 'bus. There is no relationship to compare. The FBW pitch law commands a modified gee, and not AoA. I can tell you that trim technique for the Viper was just like any other plane I flew. You trimmed to have a neutral stick pressure/force. If you were trimming for an attitude, no problem. But you could not trim for an AoA. So we were speed neutral, kinda like the 737 seems to be or they would not need STS, huh?

Late breaking news - - maybe the CVR mentioned a birdstrike, huh? Still comes down to a single point failure nd confusing procedures for stall and whacko trim.

Gums sends...

[ecto1]

Gums,

Yeah, simple FBW gives you that. But each philosofy has pros and cons, as you know.

You cited a big pro of simple (closed loop control of pitch rate, for instance) FBW. It will follow whichever control force feedback law you want. That's true, regardless of aerodinamic characteristic of the plane.

I'll give you some cons. In such FBW, a lot of feedback is lost. For instance, it's much harder to tell how authority you have left. 50% deflection at the stick will give you whatever deg/s roll regardless of the plane needing 5% aileron or 95% aileron. Controls do not get more sensible or harder at higher airspeed, so your subconscious mind do not have that clue to compute the actual airspeed. And you cannot feel trouble: sticky control surfaces, vibrations, hard spots or friction, all gets lost. Plane is trying to keep wings level if you don't touch the controls, which is nice, but it is also nice to know how hard it's trying. Otherwise it may surprise you when/if authority is depleted.

Of course you can make FBW with force feedback proportional to hydraulic pressure and stick deflection proportional to surface position. Only use FBW to "cheat" a little bit when you want to place protections (eg: if you are going fast and go for 90% stick deflection, force feedback will artificially ramp up a lot to keep you from easily ripping your wings out) or with dutch roll or overall damping (opposing a bit of control surface to a given pitch or roll rate). That would be my ideal system. I think it is not given sufficient thought most of the time because at first it seems a lot less fancy to directly correlate stick to control surfaces position instead of something with a much better sound like "target AOA that progressively changes to a target g when you accelerate". But it will have many advantages, you would be able to see and feel what autopilot does, feel the most subtle abnormality in the plane (cofg, drag, you name it)...

[ProPax]

(Not allowed to post links, hence the weird format)
Just read this:
flightglobal.com/news/articles/report-that-crashed-737-crew-shut-off-mcas-raises-ne-457224/

Source:
wsj.com/articles/ethiopian-airlines-pilots-initially-followed-boeings-required-emergency-steps-to-disable-737-max-system-11554263276

It looks like the pilots did turn MCAS off following Boeing's procedure, which was Boeing's trump card in defending the system's propriety. It didn't do them any good, though, it seems. But then again, this thread is not called "Rumours" for nothing. "Unnamed sources" is also not very encouraging in terms of credibility. However, if true, it casts an even darker shadow on the whole MCAS calamity.

MCAS saga continues...

[Icarus2001]

Controls do not get more sensible or harder at higher airspeed, so your subconscious mind do not have that clue to compute the actual airspeed.

You must mean the Airbus FBW system. The Boeing and Embraer FBW has artificial feel. Works very well.

[Seat4A]

Reuters - APRIL 3, 2019 / 7:07 PM ET / UPDATED 29 MINUTES AGO

FAA launches new review of Boeing 737 MAX to ensure safety

https://www.reuters.com/article/us-ethiopia-airplane-faa-regulation/faa-launches-new-review-of-boeing-737-max-to-ensure-safety-idUSKCN1RF2VY?il=0

QUOTES:

WASHINGTON (Reuters) - The Federal Aviation Administration said late on Wednesday it is launching a new review of the safety of the now-grounded Boeing 737 MAX that will be headed by a formal top U.S. safety official.

The FAA said it is establishing a Joint Authorities Technical Review “to ensure the safety of the Boeing 737 MAX” and scrutinize an anti-stall software that’s been questioned in two fatal crashes since October.

The review will be chaired by former National Transportation Safety Board chairman Christopher Hart “and comprised of a team of experts from the FAA, NASA, and international aviation authorities.”

The FAA did not immediately disclose who was taking part but a Canadian government spokeswoman said Canada would join."

[Loose rivets]

When we gave the Americans the flying tailplane, was it hinged at the front?

Pivoted at the rear, or nearer to, it is inherently unstable. The nut failure, or some such would mean it thwacking over full deflection, but it hasn't happened, has it? Okay, so we've not had a wing detaching g force that I can remember, but now we've got a new reason to pivot at the front. The loads would be smooth and progressive, and even if it had been cranked all the way, hand-winding it back would now be aided rather than opposed.

The first scenario we can discount because of history. The second issue a major change in design philosophy, but not a huge change in pilot handling and seemingly needed. I'd never dreamed of having to unload it in a series of switchback rides. It would be so easy to crank it with a front fulcrum and I'd guess a lot cheaper than gum's "rehanging the engines".

Where to put the Jack? I'd have cut a slot out of the rudder before I'd have put the fulcrum at the rear. But seriously, a horizontal jack would only need a redesigned lever and the loads would be less and far less consistent.

[568]

Salute!

Good thots, FDR and Murphy is correct, I like the generic FBW laws and 'bus implementaion for the most part.

Without control surface feedback, you can satisfy the FAR requirements for column force versus AoA because the control stick/wheel can use a simple spring as does the 'bus. There is no relationship to compare. The FBW pitch law commands a modified gee, and not AoA. I can tell you that trim technique for the Viper was just like any other plane I flew. You trimmed to have a neutral stick pressure/force. If you were trimming for an attitude, no problem. But you could not trim for an AoA. So we were speed neutral, kinda like the 737 seems to be or they would not need STS, huh?

Late breaking news - - maybe the CVR mentioned a birdstrike, huh? Still comes down to a single point failure nd confusing procedures for stall and whacko trim.

Gums sends...

Very good explanation

[568]

In total agreement.
As far as I am aware no simulator training for the 737 type mentions this even at FAA/ICAO training establishments.

[dingy737]

1. Sorry if this was answered before, but assuming the flaps to be still extended during the Ethiopian 302 MCAS event, how would any other failure or failed sensor allow allow MCAS to operate? ( MCAS is electrically engineered NOT to function with flaps extended)
2. If at 400 feet descending fast and PM manual operation of the trim wheel too slow, would reengaging the trim cutout switches to use electrical column trim switches to override MCAS and provide a faster rate of corrective trim be a “ Hailmary” consideration?
THANKS.

[RickNRoll]

Since this video may not be coming back I'll post a sort of transcript from memory since I was lucky enough to get to watch it. I'll try and just describe what I saw and keep my commentary/speculation to a minimum - in <italic>. Bear in mind this is a non-pilot observer's take on it from flaky memory:


There is a fair bit of intro talk then the sim.

They are in NG sim, not MAX, no surprise there. Mentour in left seat.

They start at low altitude, I think it was commented on, not at takeoff, I can't remember if they showed putting flaps up or started with flaps up and said it was just after flaps up.

No stick shaker, that I saw.
<presume means no elevator feel shift either, and that actual AOA error is not being simulated>

IAS disagree then simulated - or they just did the memory items, I didn't notice warning light (not that I would) and he didn't note it, there was an aural alarm but I think that was for AP disconnect
<I don't think this is exactly what the accident flights faced, they had shaker and IAS disagree from takeoff>

IAS disagree memory items, AP/AT OFF, FDs OFF, 75% N1, 4degrees

Throttles are pushed forward to get 75%, he comments that at low alt with denser air this is a lot of thrust and hence speed will increase

Trim runs, obviously sim doesn't have MCAS so not sure if they are simulating runaway or if he is doing it with the column switches

Comment is made that he was _expecting_ trim to run, because speed is changing, he says it will take a couple of cycles of fighting it with the switches before being definite that it is wrong

They discuss (he raises) that he is having flight control issue, they conclude it is trim that is the problem, they now run the stab trim runaway memory items finishing by hitting the switches. At this point he is clearly holding quite a bit of pressure on the column, but not obviously losing control of pitch

Now he points out that trim has only got down to (I think) 3 degrees on the indicator (zero being full nose down, I think) and that they are now going to "try something" - video captions say "don't try this at home", like we've all got a FFS in our shed or a 737 on the drive :-)

At this point the guy in the right seat uses the handle to wind the trim further _forward_. He stops when he clearly cannot wind it further - but they point out that it's still not at full nose down

Mentour is obviously struggling to keep the nose up at this point. I noticed he didn't seem to have the shoulder straps on (right seat did), possibly because he was frequently turning round to talk to the camera. That probably didn't help, but on the other hand he didn't have stick shaker or feel shift to cope with either.

Now, with Mentour pulling back, the right seat guy tries to wind the trim back with the handle - and he struggles to move it at all

They stop the sim, I think it was after stopping that he noted that at that point they were at 310 (maybe he said 340) knots.

I think there was then some further commentary, at one point I think he mentioned the rollercoaster to unload the stab and allow trimming, but commented that close to the ground the instinct is to pull back - I may be imagining that or remembering it from another video though.

<
And that was about it. I can try to answer questions on the video if anyone has any. The eye-opener for me wasn't the force on the control column (which was held anyway), it was that the trim couldn't be moved, in either direction. From somewhere I had got the impression that those wheels had a lot of leverage, due to the gearing, which was why a lot of turns would be required, but it seems that a lot of turns would be required and a gorilla to make them...
>

From another forum.


I agree it's a flawed design. And I used to work there. I'm glad I don't now.

Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper.
Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
As I said, I'm glad I'm no longer there.


From an ex Boeing employee.

[GarageYears]

Ok, I have to confess I’m confused.

All this talk of manual trim forces, etc.

MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.

If MCAS runs again, again any pilot trim action defeats MCAS.

There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.

Am I wrong?

- GY

[dingy737]

Garage years I agree. However if you initially allow some trim rotation assuming the STS system is operating. Then when you feel the aircraft coming out of trim you identify the problem as a runaway stab. The memory items that exist do not state as you recommend. They state Disconnect AP/AT if engaged and if it continues then stab switches off, if not engaged switches OFF. Seeing that no MCAS memory items exist then they use the most relevant which is runaway stab, where no attempt to electrically trim aircraft first exist.

[TriStar_drvr]

Ok, I have to confess I’m confused.

All this talk of manual trim forces, etc.

MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.

If MCAS runs again, again any pilot trim action defeats MCAS.

There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.

Am I wrong?

- GY

I agree. When did we stop becoming pilots?

[GordonR_Cape]

Ok, I have to confess I’m confused.

All this talk of manual trim forces, etc.

MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.

If MCAS runs again, again any pilot trim action defeats MCAS.

There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.

Am I wrong?

- GY

If the runaway MCAS is countered early enough, then nose up trim, and disabing the cutoff switches is sufficient. That was the official version from Boeing and the FAA, until shortly after the second MAX crash. If left too long, the situation enters uncharted territory, and nobody has come out alive (except those in the simulator).

Several sources indicate that electric trim was intentionally limited in scope, to avoid unintentional runaway nose up trim (whether by the pilot or by a wiring fault). Runaway nose up trim may be just as deadly as nose down trim, so there was logical justification for this restriction.

It has been suggested that pitted against runaway MCAS, the electric trim never wins enough authority to recover from severe nose down trim, where there is aerodynamic loading of the horizontal stabiliser. Whether electric trim would be sufficient against the upgraded MCAS is not clear, and that risk needs to be scrutinised.

Several tests, leaks, and EASA documentation have knocked a huge hole in the initial assertions. I don't have detailed references handy for all of those points, but they have been interspersed throughout the last few days posts. I expect to see more media articles and blogs on these topics.

Edit: The link posted by ProPax gives the latest overview: https://www.flightglobal.com/news/articles/report-that-crashed-737-crew-shut-off-mcas-raises-ne-457224/

The FAA is presumably not happy with all of this contradictory information, and being made to look sheepish by ongoing media revelations. The certification review should require detailed evidence, rather than the bland reassurances we had last November after the first MAX crash.

[Dee Vee]

: https://www.flightglobal.com/news/articles/report-that-crashed-737-crew-shut-off-mcas-raises-ne-457224/

Don't you just love the spin Boeing are putting out, right now its:

Boeing warns against drawing conclusions before investigators release more details

You can bet your bottom dollar after the findings are released it'll all be:

"We don't want to comment on the past, we want to look to the future, as we make a safer aircraft even safer, and we've ALREADY released software to do that, nothing to see here, move along"........

[gmx]

Edit: The link posted by ProPax gives the latest overview: https://www.flightglobal.com/news/articles/report-that-crashed-737-crew-shut-off-mcas-raises-ne-457224/

The FAA is presumably not happy with all of this contradictory information, and being made to look sheepish by ongoing media revelations. The certification review should require detailed evidence, rather than the bland reassurances we had last November after the first MAX crash.

The article doesn't seem to know what it's talking about. MCAS is (temporarily) disabled by pilot use of the electric trim. There's no two steps down, one step up scenario at play here, where MCAS has greater authority over the stabiliser than the pilot. The only way MCAS puts the aircraft in an unrecoverable dive is if the pilot fails to trim out MCAS nose down inputs and then activates the stab trim cutout switches, leaving the pilot with manual trim only.

[Water pilot]

This is my understanding also, based on the schematic posted before:

https://www.pprune.org/tech-log/615709-737max-stab-trim-architecture-9.html#post10433110

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1278x850/b737_max_columnswitchingmodule_2cf8055695fc24da4521cb26ab661 afac41f93a3.jpg
When MCAS is engaged, control column "extreme deflection" towards pull will NOT disable MCAS trim down, but it will NOT disable your thumb switch trim up either, so you still win.

In other words, this battle will not be affected by too much pull.
So that is a selonoid on the column cutout override switch, right? What happens if it sticks ON, perhaps because someone installed the wrong one? (I have seen this.). Since MCAS is never engaged normally such a fault could lie in wait a long time. What woukd be the consequence, there is not much detail in that schematic.

[GordonR_Cape]

The article doesn't seem to know what it's talking about. MCAS is (temporarily) disabled by pilot use of the electric trim. There's no two steps down, one step up scenario at play here, where MCAS has greater authority over the stabiliser than the pilot. The only way MCAS puts the aircraft in an unrecoverable dive is if the pilot fails to trim out MCAS nose down inputs and then activates the stab trim cutout switches, leaving the pilot with manual trim only.

I agree the article is badly worded. The point is, do you believe anything that Boeing and the FAA tell you at this stage? More proof is needed to regain trust, rather than assertions and paper circuit diagrams. Since MCAS cannot be independently turned on or off, there is no way for a maintenance technician to test any of this, nor a pre-flight check on a specific aircraft.

So that is a selonoid on the column cutout override switch, right? What happens if it sticks ON, perhaps because someone installed the wrong one? (I have seen this.). Since MCAS is never engaged normally such a fault could lie in wait a long time. What woukd be the consequence, there is not much detail in that schematic.


More questions for each answer in this thread! Having a non-redundant column cutout switch solenoid would be another single point of failure. Presumably the fact that there are two control columns has something to do with it. Since MCAS is wired into both columns, then either one can be used to control the stabiliser trim in the event that the other fails.

[gmx]

I agree the article is badly worded. The point is, do you believe anything that Boeing and the FAA tell you at this stage? More proof is needed to regain trust, rather than assertions and paper circuit diagrams. Since MCAS cannot be independently turned on or off, there is no way for a maintenance technician to test any of this, nor a pre-flight check on a specific aircraft.


I will wait for the official report. MCAS is clearly an unintuitive and potentially lethal system when triggered by faulty AoA data. I'm not going to blame Boeing, the FAA, the pilots or airlines until we know the exact failure modes triggered in these two accidents.

[GordonR_Cape]

I will wait for the official report. MCAS is clearly an unintuitive and potentially lethal system when triggered by faulty AoA data. I'm not going to blame Boeing, the FAA, the pilots or airlines until we know the exact failure modes triggered in these two accidents.

I am not a pilot, and probably being adversarial, but the question that might be asked is: Will waiting for the grounding to be lifted be enough, or the FAA oversight review, or until either or both of the accident reports, which ever comes later?

The FDR readouts provide a lot of information, but the state of the AOA sensor, cutoff switches, and flight computers is impossible to determine after the crashes. We may never know the the full answers, given the complex fault tree. Could this saga take years, like the Comet metal fatigue investigation?

[HundredPercentPlease]

It could take years, but it may be more like the rudder hardover problem. Two fatal accidents, keep the thing flying, quite a few hairy moments, then find and fix the problem years later.

[EDLB]

Could this saga take years, like the Comet metal fatigue investigation?
Very unlikely. The modern digital FDR used store lots of parameters and with the CVR synced there will be enough data to come up with a very good picture of what happened why. There are lots of grounded MAX too where you can look into and test fly them to check your assumptions about the events.

[sky9]

Boeing's mistake was to use the stabiliser to actuate MCAS. If they had followed the example of the BAC 1-11 and the deep stall problem in the 1960's they would have had a stick shaker followed by a stick pusher on the elevators. At least the pilot was in the loop throughout the period of designed operation, even if as I suspect like MCAS the pusher was there to satisfy the certification regime and unlike the 737 probably never actuated in flight.

The only issue then would be the small size of the elevators on the 737 but that could have been resolved.

[IFixPlanes]

So that is a selonoid on the column cutout override switch, right? What happens if it sticks ON, perhaps because someone installed the wrong one? (I have seen this.). Since MCAS is never engaged normally such a fault could lie in wait a long time. What woukd be the consequence, there is not much detail in that schematic.
The system (stabilizer trim control column switching module) must be checked every 6000 FH according MPD.

[EDLB]

I have a simple question:
What happens if the trim cutoff switches are in on position (electrical trim possible), MCAS wan’t to trim the stabiliser nose down while the pilot holds his dual trim switches on the colom for nose up?

Who wins?

According to water_pilot the PF win’s but is it verified?

With the outcome of ET302 I have my doubts.

[ferry pilot]

It appears that MCAS can in ten seconds move the stab whenever it's in the mood, but the pilots need considerably longer to bring back the stab with dozens of cranks of manual trim - provided that airload allows them to move the trim.

Very much an unequal contest.

​​​​​​MCAS really shouldn't be putting in more trim than can be corrected by the crew in the interval before it reactivates.

But then, limiting MCAS authority might fail to achieve required stick force increase approaching stall. The software will have to get even fancier to satisfy 10E-9 reliability.

I'm with gums. Dump MCAS and fix the nacelle aerodynamics.
Me too, and I would be willing to bet quite a few Boeing engineers long before anyone else ever heard of it.

[ecto1]

I have a simple question:
What happens if the trim cutoff switches are in on position (electrical trim possible), MCAS wan’t to trim the stabiliser nose down while the pilot holds his dual trim switches on the colom for nose up?

Who wins?

According to water_pilot the PF win’s but is it verified?

With the outcome of ET302 I have my doubts.
PF wins, see FDR traces.

On a different topic, failure of control column deflection cutoff override to always closed would mean that if the trim thumb button is pressed nose down, or fails to that position, it will not be cut if the pilot pulls.

Very rare coincidence, but previous posters are right in that periodical check would be needed.

It is in my eyes as dangerous and as rare as the control column defection switches failing to always open, and they installed an override for that (before mcas)

[Mike Flynn]

Apart from Boeing the FAA will be left holding the legal responsibility here.

Trying to get the Max back in the air and fully certified will not be easy.

This is not a quick fix and rubber stamp job.

Added to that the passenger viewpoint of an old design past its sell by date and clearly not safe and you have a whole bundle of problems for the manufacturer.

[ATC Watcher]

I was talking this week end with a senior from one of the European airline affected, they had last week decided stop training on the Max, transfer all the pilots to their other types for upgrade and strike out the type from their summer schedule. Whether the airline will survive this financially is in the balance now as the Summer season is where the money is made... this will have rippling effect on the industry well beyond Boeing..:(

[KelvinD]

So the prelim report, issued by the Ethiopian government says the crew "repeatedly followed procedures recommended by Boeing.."
https://www.bbc.co.uk/news/business-47812225

[STBYRUD]

Repeatedly?! How is that possible?

[compressor stall]

Interesting. When it appears in print and is verified, I await all those that said that "sub standard training was the cause", and that "it would never have happened in the US" to say they were wrong.

[commsbloke]

From the Ethiopian Airlines press release,
"The preliminary report clearly showed that the Ethiopian Airlines Pilots who were commanding FlightET 302/10 March have followed the Boeing recommended and FAA approved emergency procedures to handle the most difficult emergency situation created on the airplane. Despite their hard work and full compliance with the emergency procedures, it was very unfortunate that they could not recover the airplane from the persistence of nose diving"

[warbirdfinder]

The Wall Street Journal, relying on sources who have seen the flight data recorder readout, reported that the pilots, upon experiencing uncommanded nose-down trim, used the 737’s stabilizer trim cutout switches. And while the 737 MAX 8 retains the manual trim wheels it has had from day one, it’s not known if they used these to re-trim the aircraft. The sources told the Journal that the pilots appeared to have reengaged the stabilizer trim cutout switches, which would have re-enabled the MCAS stall protection system.

The underline is mine. Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?

[oversteer]

No “foreign object damage” or “structural design problem” identified

That’s likely to be a significant finding

[Albino]

Interesting. When it appears in print and is verified, I await all those that said that "sub standard training was the cause", and that "it would never have happened in the US" to say they were wrong.

I think it would be wise to wait for the report. If the aircraft wasn't put in trim using the switches as per Boeing recommendation before placing them to cutout and it turns out that the switches were then turned back on, again contrary to guidance, the press release from Ethiopian is misleading to say the least.

​​​​​​

[FullWings]

Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?
Maybe because the control loadings at the speed they were doing made it difficult/impossible to manually trim, so they tried the electric trim again? If you’ve got both (or even four) hands on the control column trying to stop the aircraft pitching down, there’s not many hands left for the manual trim...

[jagema]

Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?

Speculatively with both pilots hauling back on the control column and no electric trim, the moment any of them lets go to give the wheel a try the nose would dip down again. Additionally, at nose low and high speed with stab overloaded the wheel might have been much too stiff to both move and do so enough times to make an impact.
I hate to speculate on this but it points to proper action by crew finding themselves unable to bring nose up with manual means and reactivating the cutouts to regain electric trim capability. (Which should come back and if used should stop MCAS either way, unless...)

[ManaAdaSystem]

I think it would be wise to wait for the report. If the aircraft wasn't put in trim using the switches as per Boeing recommendation before placing them to cutout and it turns out that the switches were then turned back on, again contrary to guidance, the press release from Ethiopian is misleading to say the least.

​​​​​​

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.

[kwh]

Speculatively with both pilots hauling back on the control column and no electric trim, the moment any of them lets go to give the wheel a try the nose would dip down again. Additionally, at nose low and high speed with stab overloaded the wheel might have been much too stiff to both move and do so enough times to make an impact.
I hate to speculate on this but it points to proper action by crew finding themselves unable to bring nose up with manual means and reactivating the cutouts to regain electric trim capability. (Which should come back and if used should stop MCAS either way, unless...)

If this turns out to be correct, could it be because the Boeing recommended response to the unwelcome MCAS activation was developed in a simulator incapable of generating the forces that the real world can generate, so the pilots developing & “testing” the protocol could pull back the sim yoke with one hand, while rotating the easy to spin manual trim wheel with the other? “Don’t worry, this is easy to get out of if we just add a page to the manual to tell people what to do, watch...”. Also “Flight test the fix on a real aircraft? Sounds like a lot of work, dude... let’s not, OK?”

No, well spotted, I’m not a pilot, but I assume that not every control in a simulator for an aircraft where the force felt through the controls relates to the forces the control surfaces they connect to are experiencing [i.e. a non-fully-FBW plane] will be 100% accurate?

[bill fly]

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.

And that highlights a big difference between runaway stab. and MCAS unwanted operation:

With runaway stab you can’t catch it by trimming - it is running away...

With MCAS unwanted (for want of a better description) operation you can theoretically catch (reverse) it by trimming.

Another difference is that in an AoA fault caused unwanted MCAS operation other factors also come into play - even before MCAS operation on flap retraction:

Autothrottle, Instrument anomaly, Stall warning etc. which can cause:

Unwanted rise in speed, initial pilot input nose down response, cockpit confusion.

Therefore the standard drill for stab runaway does not necessarily apply. Trimming to neutral and ATS disconnect might well have priority over stab trim cutoff operation.

Of course we didn’t know this, the poor crews didn’t know it and it is a scenario which Boeing will be looking at hard before submitting a suitable procedure for recertification, I am sure.

[ecto1]

[QUOTE=ManaAdaSystem;10438363]

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.[/QOTE]
And that highlights a big difference between runaway stab. and MCAS unwanted operation:

With runaway stab you can’t catch it by trimming - it is running away...

With MCAS unwanted (for want of a better description) operation you can theoretically catch (reverse) it by trimming.

Another difference is that in an AoA fault caused unwanted MCAS operation other factors also come into play - even before MCAS operation on flap retraction:

Autothrottle, Instrument anomaly, Stall warning etc. which can cause:

Unwanted rise in speed, initial pilot input nose down response, cockpit confusion.

Therefore the standard drill for stab runaway does not necessarily apply. Trimming to neutral and ATS disconnect might well have priority over stab trim cutoff operation.

Of course we didn’t know this, the poor crews didn’t know it and it is a scenario which Boeing will be looking at hard before submitting a suitable procedure for recertification, I am sure.



You are both right. But Boing introduced a very "clever" sentence in the AD:

"Electric estabilizer trim can be used to neutralize control column pitch forces before moving the stab trim cutout switches to cutout"

Can: means that if you do it and you crash, they would say they didn't tell you to do it, but if you don't do it and you crash they will say that they mentioned it for something.

Not that any amount of words is going to change the fact that they are responsible. If only they were as clever in the design as they are with legal-related documents.

[compressor stall]

They should've kept airframe and hydraulics (99% of them, just a touch here and there)

DO you have any idea of how much of the 737 does not meet the modern day safety standards and is grandfathered? That's probably worthy of a thread of its own.

Nearly 25 years ago Airbus complained to JAA about Boeing's grandfather rights that were letting the 737 get away with nearly half a dozen more seats than a comparable newly certified airliner could, all being equal.

[Albino]

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.

Have you read the Boeing bulletin?

​​​​​​Under operating instructions
'Electric stabilizer trim can be used to neutralise control column pitch forces before moving the stab trim switches to cutout'

Why wouldn't you?

[quentinc]

Maybe because the control loadings at the speed they were doing made it difficult/impossible to manually trim, so they tried the electric trim again? If you’ve got both (or even four) hands on the control column trying to stop the aircraft pitching down, there’s not many hands left for the manual trim...

Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?

[RickNRoll]

Ok, I have to confess I’m confused.

All this talk of manual trim forces, etc.

MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.

If MCAS runs again, again any pilot trim action defeats MCAS.

There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.

Am I wrong?

- GY

They were already trying to deal with a stickshaker event. Then they followed Boeings list and disabled electrical trim. They then found that they could not use manual trim to recover. As can be seen in videos, manual trim can be a lot harder for some people than others.

[SLF3]

Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....

[Switchbait]

Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?


actually it does.......


“Control airplane pitch attitude manually with control column and main electric trim as needed”

its the point in the memory items right after disengagjng the autopilot......

[bill fly]

Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?

Yes that’s the Point, There should be a runaway stab checklist and a different one for unwanted MCAS operation.

This should specify the possible symptoms.
It should then cover the AD items and go further to disconnecting ATS and flying pitch and thrust. It could well suggest reselecting flap and landing as soon as possible.

[jagema]

Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....
The report should have been issued four hours ago if you go by what the Ethiopian authorities had said. Its release seems to have been delayed, but we are getting bits of information from different sources (ie. Ethiopian Twitter).

Fairly poor handling of a serious investigation if you ask me.

[gmx]

Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?

It's a reasonable point. However, after the LionAir investigation, everyone knew the electric trim could be used to neutralize MCAS, even if temporarily (as the accident crew had done 20-odd times on the disaster flight). Having MCAS run the trim to its maximum nose down position, and being fully aware of the LionAir situation, wouldn't it make sense to re-trim the aircraft to neutral attitude *before* hitting the cutout switches? Having said that, the pilots have clearly encountered a terrifying circumstance. The only thing you might hope is that after the LionAir incident they were better equipped to deal with it than what seems to have been the case.

[fotoguzzi]

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off.

(Not a pilot) This random internet grab from 2009 (authenticity unknown to me) does say to use electric trim and does not say to apply it again if runaway continues.

As these are terse instructions, if one might soon be grabbing a rotating wheel, and not long after that, hand cranking it, [would he] not want to do the best possible work with the electric trim before cutout?

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1080x1920/screenshot_20190404_025252_838cc488abeb1203e969edc9e2d2b8cf5 e7ab074.jpg

[safetypee]

What is the direction of the pitching moment when extending the air brake in the 737 NG / Max, and separately (or as operationally used), when selecting flap from a clean configuration ?
How would slat extension affect these changes ?
Would the logical reverse be a reasonable assumption ?


GordonR, re PM, :ok:
and https://www.pprune.org/showpost.php?p=10437830&postcount=3014 (https://www.pprune.org/showpost.php?p=10437830&amp;postcount=3014)

[Captain Biggles 101]

Clearly these incidents have raised many questions, especially for the trained professionals that operate the 737. The consequences of these accidents could maybe also demonstrate issues relating to trim on other 737 variants including the 737 NG. We must remain open to learning the lessons and exploring the wider implications.

If the Ethiopian crew did follow the Stabiliser Runaway memory actions as per QRH NNC, then clearly the very next action that would have followed would have been an attempt to use the manual trim handles. Should the excessive control loading have either blocked or restricted movement of that control due to control loading, the crew would find themselves in a perilous situation and dive. At that point out of desperation perhaps they tried to use the electric trim again and reactivated the electric trim cutout switches. I don't think that is too extraordinary to have attempted out of desperation if the trained procedures were failing were failing.




Manual control of trim on the 737 NG is usually straightforward, but it not impossible to find yourself possibly in a nasty situation. I have once experienced in a level D 738 sim following double engine flameout, high power nose high prior to failure, to a quick ensuing nose low dive. It shocked me the difficulty to raise the nose again and high loading on the manual trim handles. We recovered, but recall unloading the control column back pressure for a short time to help having read about such possible scenarios.




It is too early to say, but there could be lessons here not only just applicable to the Max but perhaps even the NG with regards to control loading whilst using manual trim. If we consider accidents including the 738 Fly Dubai nose dive, control loading implications to manual trim operation are important. After any stab trim runaway, it is essential that we have confidence in the effectiveness of the manual trim throughout the full range of movement and control loadings.




More attention is needed imo to why repeated AOA failures are happening on both 737 NG & Max variants.




For the crew operating these aircraft, there is little doubt that they were suddenly faced with multiple simultaneous failures and warnings. They had seconds to react to complex failures that could easily overload competent crews. If faced with Airspeed Unreliable, possibly altitude disagree, stick shaker, possible other master caution warnings, Stabiliser Runaway, increasing pitch down descent rate, followed by possible manual trim control being restricted by control loading (not proven yet) then they really did have a lot to deal with and the odds were stacked against them.




I think we should all be respectful to the families of the professionals that died trying in vain to regain control. It is too easy for armchair pilots to be judgmental without understanding the human factors and technical issues involved on the flight deck in these incidents. There is no evidence that crew training or that the airlines themselves had much if any involvement in outcome pf these accidents.

[Just This Once...]

So in sum, we have a non-DAL A system integrated with a secondary control system with no redundancy, monitoring, crew warnings or a dedicated means to disengage. Strap it to an aircraft where the trim system is more powerful than the primary controls and give it authority to move the stab at a far faster rate than normal trim and give it no limits of travel other than mechanical end-stops. If it goes wrong, force the crews to disable all powered stab control and equip them with manual trim wheels that are difficult and slow to operate at the best of times. Modify said wheels to make them slightly smaller and insert a more powerful damper to counteract the spring and backlash effect of the stab being moved at a faster rate than ever before - requiring an undocumented increase in physical effort. Finally inform the crews of this system's existence and get the company test pilots to retest all of this after a quick fatal crash, but do so only at a relatively slow speed. Second accident crew become the unexpected test pilots collecting a datapoint that suggests the trim wheel is impossible to move when at a higher airspeed. Cumulogranite awaits.

Apart from the flight envelop defining the configuration, CofG, AuW and airspeed/mach beyond which the manual wheels cannot be moved at an effective rate, what are we missing?

[quentinc]

[would he] not want to do the best possible work with the electric trim before cutout?


Yes... but as we know now, the MCAS system will keep cutting in... So the pilot reaches (4) in the check list. That step is clear... operate the the CUTOUT... If already trimmed down and speed a little high, the pilot is now in a very dangerous position. All following the FAA AD and Boeing check list.

[kristofera]

Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....

...and many of those newspaper articles are referring to ET's press release as being the preliminary report...
https://www.documentcloud.org/documents/5793704-Press-Release-Ethiopian-Statement-on-ET-302.html

[A0283]

A few minutes ago the Ethiopian CAA website was still silent on this. There now only is minute information on the fact that there is an investigation and a ban.

In the case of recent Asian crashes the official websites were also hardly used or completely bypassed for either facebook and twitter reporting.

Lots of rules already, but another thing ICAO might take a look at. What is the minimum standard on reporting.

In many cases in any news situation the press gets an early view of things with an embargo till a certain time on publication. Might be the case here.

Another thing is the exceptional role the Ethiopian Airlines CEO has carved out for himself. He was one of the first on scene, shown handling evidence, reporting on the investigation, and continuing to do that. With all due respect, that is amazing, but in this case the only information available,...

[quentinc]

how would the ET302 pilots know

They might notice that the stick shaker was operating on a single side only. Reality is though the point of these devices is to encourage immediate action by the pilot (to avoid an impending stall), not to enter into some slow careful fault diagnosis.

[GordonR_Cape]

They might notice that the stick shaker was operating on a single side only. Reality is though the point of these devices is to encourage immediate action by the pilot (to avoid an impending stall), not to enter into some slow careful fault diagnosis.

There were actually two failures on the MAX: The faulty AOA sensor data which triggered a whole range of spurious warnings, put the pilots in a high workload situation, which on its own was hazardous. Then MCAS comes along, and administers the coup-de-grace while the pilots are busy trying to make sense of the aircraft and their checklists.

[wheelsright]

It's a reasonable point. However, after the LionAir investigation, everyone knew the electric trim could be used to neutralize MCAS, even if temporarily (as the accident crew had done 20-odd times on the disaster flight). Having MCAS run the trim to its maximum nose down position, and being fully aware of the LionAir situation, wouldn't it make sense to re-trim the aircraft to neutral attitude *before* hitting the cutout switches? Having said that, the pilots have clearly encountered a terrifying circumstance. The only thing you might hope is that after the LionAir incident they were better equipped to deal with it than what seems to have been the case.

It is not known that the pickle switches are the solution. It is just a theory. Of course, that theory could be correct but would not address runaway trim unrelated to MCAS. It is not clear that the testing and simulation has been sufficiently close to the real world situation, particularly if there is very little altitude to play with. Does electric trim beat MCAS within the time frame required? It will require detailed investigation and simulation. It will also have to established whether an average pilot could be expected to recover the aircraft.

I have many questions about the control systems in the MAX series that will only be answered by detailed investigation. It is still possible that the best crew could not have recovered. I think that some of the criticism of the pilots has been unfortunate and premature. It may be likely that crew could have done more, but that does not excuse Boeing for having put them in the situation in the first place. No pilot should be placed in the position that an unreliable safety system is trying to crash the aircraft and it is unclear how to recover or even promptly diagnose precisely what is happening. Boeing's response to the LionAir accident was inadequate in my opinion and the LionAir accident report, thus far, was also insufficient to address all of the issues that were apparent. I suspect that politics has interfered and dumbed down safety concerns.

[ecto1]

DO you have any idea of how much of the 737 does not meet the modern day safety standards and is grandfathered? That's probably worthy of a thread of its own.

Nearly 25 years ago Airbus complained to JAA about Boeing's grandfather rights that were letting the 737 get away with nearly half a dozen more seats than a comparable newly certified airliner could, all being equal.

You are of course right. My suggestion would be: Every freshly build plane must comply with the same rules, regardless being a new design or not. Improving a plane shouldn't imply regulatory disadvantages, that's having a rule that discourages improvement. If we all agreeded to let 737 fly all these years is because we think it is safe: then maybe some non-compliances of the 737 are too strict.

Or maybe that decision was a bad one all along. I'm not familiar with the safety record of the 737 vs other "compliant" airframes, the data must be there.

We must focus in safety, not focus in complying with safety related rules. Some of them are not ok, like grandfather rights. If some rules makes building planes objectively too expensive for what users are willing to pay for a ticket, corners will be cut and safety will diminish.

[Capn Bloggs]

They might notice that the stick shaker was operating on a single side only.
No, the columns are interconnected so a single shaker will be felt through both or so my old FCOM says.

[quentinc]

No, the columns are interconnected so a single shaker will be felt through both or so my old FCOM says.
You might be right. The faa ad though says the stick shaker on one side only, is an indication of angle of attack disagreement.

[bill fly]

(Not a pilot) This random internet grab from 2009 (authenticity unknown to me) does say to use electric trim and does not say to apply it again if runaway continues.

As these are terse instructions, if one might soon be grabbing a rotating wheel, and not long after that, hand cranking it, [would he] not want to do the best possible work with the electric trim before cutout?

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1080x1920/screenshot_20190404_025252_838cc488abeb1203e969edc9e2d2b8cf5 e7ab074.jpg

Hello Foto,

Thanks for posting the checklist page.
It is good to follow a non normal or emergency checklist to see what the writer had in mind, before taking to the air sometimes.
In this case the items in point 2. about operating main electric trim are provisional to the autopilot having been on before. Here the writer covers
the case where the autopilot could have been causing the runaway.
Having disengaged the AP there is now the choice of the runaway having stopped, which allows manual flight with normal trim inputs OR if it doesn't stop, no longer is electric trim desired - but stab trim cutout switches off.
Now in these accidents the AP was not engaged, so point 2. did not apply - and as the stab movement continues, pass point 3. and you are left with point 4. - no trim, cutoff switches off.
This is where the "recommended checklist" does not properly cover the case of rogue MCAS operation, despite claims to the contrary. A dedicated checklist is required. A checklist which, if studied on the ground would provide insight into dealing with this complex failure.
And to answer the point further up about, how do the pilots recognise rogue MCAS operation? - at the first accident, they couldn't, since nobody knew of the system - at the second accident after the publicity they may well have realised what was wrong but the runaway checklist is insufficient alone to deal with it (reasons above) and that checklist is what they had.

[mryan75]

I'm sorry, but this is a complete hose job by the Ethiopian CAA authority (which has an annual budget of $89,000) and Ethiopian Airlines. "We did everything perfectly". Well RELEASE THE REPORT. Turning MCAS back on 4 times is not following the AD or the standard trim procedure. They're laying all the blame on Boeing, and then once 18,000 news articles have repeated that, they'll release the report tomorrow. Absolute crap.

[gmx]

It is not known that the pickle switches are the solution. It is just a theory.

You might have to explain that further. The LionAir crash FDR clearly shows short bursts of uncommanded nose-down trim (from MCAS), each one counteracted by PF-commanded nose-up electric trim, maintaining level flight at 5000 feet for roughly six minutes. The uncommanded nose-down trim is never coincident with the PF's nose-up electric trim. They are clearly interleaved, which is exactly the MCAS behaviour described by the updated advice.

The erroneous AoA / AP disconnect / MCAS response chain may have manifested differently on the EA flight, but we'll have to wait for the FDR / CVR data in the report to know one way or the other.

[SLF3]

I'm sorry, but this is a complete hose job by the Ethiopian CAA authority (which has an annual budget of $89,000) and Ethiopian Airlines. "We did everything perfectly". Well RELEASE THE REPORT. Turning MCAS back on 4 times is not following the AD or the standard trim procedure. They're laying all the blame on Boeing, and then once 18,000 news articles have repeated that, they'll release the report tomorrow. Absolute crap.

It maybe complete crap, or it may not. But if it is a hose job, where did they learn it, and who said they were unsophisticated?

[fotoguzzi]

Yes... but as we know now, the MCAS system will keep cutting in... So the pilot reaches (4) in the check list. That step is clear... operate the the CUTOUT... If already trimmed down and speed a little high, the pilot is now in a very dangerous position. All following the FAA AD and Boeing check list.

(Still not a pilot) Thanks. It is understandable that one would not want to remove his hands from the yoke. MCAS seems to give five seconds after the last electric trim, and I presume it does not operate while the trim switch operates. My contention is that if they are reading ahead, they know some cranking is coming, and would naturally trim before cutout. That is, one argument is that MCAS trims faster than the trim switch can, but (unless I am wrong) MCAS will give you as much time as needed to trim before it tries to undo your work.

At what point the combination of nose down and overspeed makes hand cranking unreasonable (or makes even the idea of letting go of the yoke unreasonable), seems yet unclear. The penultimate Indonesian flight continued to destination while the Ethiopian flight lasted minutes.

[infrequentflyer789]

So in sum, we have a non-DAL A system integrated with a secondary control system with no redundancy, monitoring, crew warnings or a dedicated means to disengage. Strap it to an aircraft where the trim system is more powerful than the primary controls and give it authority to move the stab at a far faster rate than normal trim and give it no limits of travel other than mechanical end-stops. If it goes wrong, force the crews to disable all powered stab control and equip them with manual trim wheels that are difficult and slow to operate at the best of times. Modify said wheels to make them slightly smaller and insert a more powerful damper to counteract the spring and backlash effect of the stab being moved at a faster rate than ever before - requiring an undocumented increase in physical effort. Finally inform the crews of this system's existence and get the company test pilots to retest all of this after a quick fatal crash, but do so only at a relatively slow speed. Second accident crew become the unexpected test pilots collecting a datapoint that suggests the trim wheel is impossible to move when at a higher airspeed. Cumulogranite awaits.

Apart from the flight envelop defining the configuration, CofG, AuW and airspeed/mach beyond which the manual wheels cannot be moved at an effective rate, what are we missing?

OTOH:

* build it all in a rush after being caught out by the competition
* while maxxed out on the rush job, expend some precious resources on changing stuff that didn't need to be changed (bigger displays must be better, must-have new feature, just like on phones )
* take a marketing claim - differences training will not need sim time - and turn it into a design requirement
* sign a sales contract that turns it into a financial requirement so we can beat the engineers with that one as well
* screw up the aero modelling of the effect that lead to the system in the first place, find it is much worse in flight tests, make the system several times more powerful to fix it
* don't tell the regulator what you just did
* don't tell aircrews anything because they don't need to know because of the no-training requirement above
* make all failure cases for the system the crew doesn't know about be handled by existing procedures, however mismatched or badly, because can't train any new procedures
* hang it all off a single sensor because if it used two it would need a warning if a disagree caused it to disable itself, and new warning means new training so it can't be done that way
* rewire the cutout switches, which have been the same for decades, so that now a really clever crew cannot turn off just the automatic trim and leave the manual electric on to regain control

And between both of us we've probably still missed something.

[Fortissimo]

I'm sorry, but this is a complete hose job by the Ethiopian CAA authority (which has an annual budget of $89,000) and Ethiopian Airlines. "We did everything perfectly". Well RELEASE THE REPORT. Turning MCAS back on 4 times is not following the AD or the standard trim procedure. They're laying all the blame on Boeing, and then once 18,000 news articles have repeated that, they'll release the report tomorrow. Absolute crap.

I don't think you can point the finger at the Ethiopian CAA here, as they appear to have had control seized by the Govt so the choice of timing is probably not down to the investigators . A UK colleague who flew with the ET302 captain told me he was a very competent operator, but I think many of us could be caught out by the effort required to manual trim at higher speeds and with a significant stick forces involved as well. He may well have opted to try further electric trim inputs to try to manage a situation that only 2 other crews had faced, but that is speculation. We won't know exactly what he was dealing with until the final report is released, when perhaps we will have proper FDR and CVR information to speculate with.

And as for the Ethiopians laying all the blame on Boeing, could someone remind me which manufacturer produced an aircraft that was vulnerable to a catastrophic outcome from a single point of failure and which has been grounded across the globe because, pending modification, the design is currently viewed by regulators as being unsafe?

[737mgm]

I'm sorry, but this is a complete hose job by the Ethiopian CAA authority (which has an annual budget of $89,000) and Ethiopian Airlines. "We did everything perfectly". Well RELEASE THE REPORT. Turning MCAS back on 4 times is not following the AD or the standard trim procedure. They're laying all the blame on Boeing, and then once 18,000 news articles have repeated that, they'll release the report tomorrow. Absolute crap.


I agree 100%. Can we actually read the report and judge for ourselves if the pilots achieved "full compliance with the emergency procedures." Also I find it incredible to read that some people here are suggesting it is too much of pilots to ask if they should trim out the aircraft with the electric trim switch first before setting the stab trim cut out switches to cut out. Boeing pointed out the need for this and it is also obvious that it isn't a good idea to set the stab trim cut out switches to cut out when the airplane is already in a nose dive at more than 250 knots (I am not saying that is what happened with ET 302). Of course in hindsight Boeing is largely to blame for these two accidents because apparently crews are too easily overwhelmed if MCAS malfunctions. Nonetheless, it doesn't make the aviation world safer to always claim the pilots couldn't have done anything. There are plenty of accidents in which a lack of flying skills is the main cause. Systems will continue to fail and then it will be up to the pilots to regain control of the airplane. If pilots are lacking in their ability to do so there needs to be just as much of a focus on dealing with that as there needs to be a focus on making systems less prone to fail.

[gmx]

(Still not a pilot) Thanks. It is understandable that one would not want to remove his hands from the yoke. MCAS seems to give five seconds after the last electric trim, and I presume it does not operate while the trim switch operates. My contention is that if they are reading ahead, they know some cranking is coming, and would naturally trim before cutout. That is, one argument is that MCAS trims faster than the trim switch can, but (unless I am wrong) MCAS will give you as much time as needed to trim before it tries to undo your work.

At what point the combination of nose down and overspeed makes hand cranking unreasonable (or makes even the idea of letting go of the yoke unreasonable), seems yet unclear. The penultimate Indonesian flight continued to destination while the Ethiopian flight lasted minutes.

I think it is generally accepted that full nose-down stabilizer position at high airspeed, low altitude may be unrecoverable via the manual trim wheel.

We think we know that the ET302 crew activated the STAB TRIM CUTOUT, leaving them in a potentially vulnerable position if the stabilizer was already at or near its maximum nose down position. We also think we know that the crew re-enabled the electric trim at some stage -- the FDR should therefore show whether the crew subsequently attempted to level the aircraft via the electric trim after re-enabling it (and by extension) MCAS. I think that will be a key indicator as to the crew's understanding of what the cause of their flight control problem was, and what they understood they needed to do to recover from it.

By that I mean -- when the crew re-enabled electric trim, did they attempt to recover the dive using the electric trim switches ?

[FGD135]

There were actually two failures on the MAX: The faulty AOA sensor data which triggered a whole range of spurious warnings, put the pilots in a high workload situation, which on its own was hazardous. Then MCAS comes along, and administers the coup-de-grace while the pilots are busy trying to make sense of the aircraft and their checklists.
Excellent summation, Gordon.

[fdr]

It could take years, but it may be more like the rudder hardover problem. Two fatal accidents, keep the thing flying, quite a few hairy moments, then find and fix the problem years later.

The outcome of the UAL585 and USAir 427 jammed servo/rudder reversal problem owns a great deal to one certain manager in USAir who doggedly refused to accept the conclusions being drawn on the Aliquippa event. Eventually that led to the in-depth investigation and the surprise revelation of thermal shock from hot hydraulic fluid interacting with a cold actuator. I still have an issue on the rate of roll recorded in 427's case, which is consistent with autorotation, which needs a high AOA to exist in such cases; the rudder alone did not have the secondary roll authority to achieve the rates recorded.

[ecto1]

I Of course in hindsight Boeing is largely to blame for these two accidents because apparently crews are too easily overwhelmed if MCAS malfunctions. Nonetheless, it doesn't make the aviation world safer to always claim the pilots couldn't have done anything.

Disregarding your choice of words, which I believe is unfortunate, we all agree that you need both things to fail (engineering and training) for most accidents to happen.

I also believe that the adequate level of engineering in this particular problem would have been a lot cheaper to achieve than adequate level of training for pilots. I mean, which is more understandable to you:

1) Assume that the new plane is not going to try to kill you (pilot) in a new way nobody explained you beforehand, (that was pilot error)
2) Assume that no crew is going to be "too easily overwhelmed" if the new system you designed, which is susceptible to failure as every other thing in the world, tries to kill them in a brand new way that you decide not to explain or mitigate beforehand. (that was boeing error)

[A0283]

In a news conference in Addis Ababa, Ms Dagmawit (Ethiop.Transport Minister) said: "The crew performed all the procedures repeatedly [that were] provided by the manufacturer but were not able to control the aircraft." Her comments were based on a preliminary report into the crash, which has not been published yet, but could be released by the end of the week.

+

The preliminary report said:

The aircraft had a valid certificate of air worthiness.*
The crew had a licence and qualifications to conduct the flight.*
The takeoff appeared very normal.*
Pilots performed the necessary procedures, as provided by the manufacturer, "repeatedly" to bring the flight under control.But was not able to control the aircraft.*
Ethiopian Airlines Flight 302 experienced “repetitive uncommanded aircraft nose-down conditions” which "continued for the remainder of the flight” before the crash.
Boeing urged to review aircraft flight control system relating to control of the plane by accident investigators.
[Note A0283 - the lines with an asterix * were the literal words used by her during the press conference.]

The Ethiopian authorities did not attribute blame in their preliminary report and did not give detailed analysis of the flight.

Following the Ethiopian disaster, Max jets have been grounded worldwide pending a software fix that Boeing is rolling out, which must still receive approval from the US Federal Aviation Administration (FAA) and other regulators.

Boeing is now being investigated by the US Justice Department, the Transportation Department's inspector general, and congressional committees.

Investigations are also looking into the role of the Federal Aviation Administration in the US, which certified the Max in 2017 and refused to ground the jets after the crash back in October.

The FAA said in a statement it was continuing to work towards understanding what happened.

[gmx]

I don't see the details in the preliminary report about who was flying and instructions to continue trimming, that you are reporting. Perhaps you have seen some other information. I will say the preliminary carries the same information in the appendices about operating CUTOUT. It does NOT say its fine to leave the electrical trim running, provided you are careful to trim up.

That's fair. I'll chase up a reference. Regardless, your characterisation of the FDR (that pilot-trim-up was involved in a losing tug-of-war with MCAS-trim-down) is incorrect. It is only the final 4 MCAS-commanded-nose-down trim commands that result in increasing nose-down attitude. The previous 21 such MCAS-commanded-nose-down trim commands are fully counteracted by the flight crew resulting in essentially level flight at 5000 feet for 6 continuous minutes.

[GordonR_Cape]

Also fair point. Maybe instead of "novel concept" I should have said "undesirable". Similar things are certainly not happening week to week. I'm not out to sink the boot into anyone at this stage. I'm keen to see the report and draw my own conclusions. My point at this stage is that the crew were obviously under stress, and the aircraft was working against them.

I think it is also important to note that both MAX crashes happened during day/VMC flight. I cannot imagine what it might be like to experience the same symptoms at night in IMC conditions.

That's fair. I'll chase up a reference. Regardless, your characterisation of the FDR (that pilot-trim-up was involved in a losing tug-of-war with MCAS-trim-down) is incorrect. It is only the final 4 MCAS-commanded-nose-down trim commands that result in increasing nose-down attitude. The previous 21 such MCAS-commanded-nose-down trim commands are fully counteracted by the flight crew resulting in essentially level flight at 5000 feet for 6 continuous minutes.

Your overall description of JT610 is correct, but the airspeed was relatively low, so it may be premature to judge if electric trim would have had the same effects at speeds > 250kts, when both speed trim and horizontal stabiliser loading come into play.

FDR trace:

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/640x669/jt610_fdr_trace_crash__5013afa901aa3f4a74075ff30e40e0fd8beea 7fb.jpg

gmx The transition from captain to co-pilot is when the last four nose-up trim commands become short blips, instead of the long activation earlier in the FDR.

[gmx]

Your overall description of JT610 is correct, but the airspeed was relatively low, so it may be premature to judge if electric trim would have had the same effects at speeds > 250kts, when both speed trim and horizontal stabiliser loading come into play.

FDR trace:

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/640x669/jt610_fdr_trace_crash__5013afa901aa3f4a74075ff30e40e0fd8beea 7fb.jpg
Thanks Gordon. Any help on the transfer of control from PF to FO at the time the aircraft attitude starts to go south ?

[michaelbinary]

Last year my first ever real in-flight incident was in my CAP-10, flying downwind 1000ft AGL in the circuit. The electric trim ran away to the end stop and suddenly I was pointing at the ground, After pooing myself, I worked out what it was quite quickly and returned the trim to neutral, where it promptly ran away to the end stop again. So I recentralised it, and disabled it. Problem solved, although had to land without flaps.
Gave me a real heart thumping moment until it was sorted.
The stick forces even in my small plane were quite high.
At the next opportunity I went up to 5000 ft and had a practice while slowly moving the trim to its end stops to see if the plane was controllable, which it was. Uncomfortable, but controllable.

I don't think anybody should second guess what the pilots should and should not have done, or did and didn't do, until the full report is made public and we can see the facts.

I think Boeing will have a lot to answer to with explanations and money before these planes are certified for flight again. And then you have the public relations problem of getting bums on seats again.

[mryan75]

Last year my first ever real in-flight incident was in my CAP-10, flying downwind 1000ft AGL in the circuit. The electric trim ran away to the end stop and suddenly I was pointing at the ground, After pooing myself, I worked out what it was quite quickly and returned the trim to neutral, where it promptly ran away to the end stop again. So I recentralised it, and disabled it. Problem solved, although had to land without flaps.
Gave me a real heart thumping moment until it was sorted.
The stick forces even in my small plane were quite high.
At the next opportunity I went up to 5000 ft and had a practice while slowly moving the trim to its end stops to see if the plane was controllable, which it was. Uncomfortable, but controllable.

I don't think anybody should second guess what the pilots should and should not have done, or did and didn't do, until the full report is made public and we can see the facts.

I think Boeing will have a lot to answer to with explanations and money before these planes are certified for flight again. And then you have the public relations problem of getting bums on seats again.
Nobody is questioning that what they faced was a challenging situation. But these aren't recreational pilots out on a Sunday afternoon. These are professional revenue pilots, in theory trained to a very high standard. I agree, let's wait for the report, but you can't say "let's wait for the report, and Boeing is going to have a lot to answer for." I will bet anything that if we get a fair, complete report, it will cite design problems, single point of failure issues, yes - BUT I will eat my shoes if pilot training, adherence to standard procedures, maintenance practices, and even possibly parts sourcing and environmental factors (sand, high humidity, etc.) aren't all factors. I fully expect the end result to be a smorgasbord of issues. It usually is.

[737mgm]

Disregarding your choice of words, which I believe is unfortunate, we all agree that you need both things to fail (engineering and training) for most accidents to happen.

I also believe that the adequate level of engineering in this particular problem would have been a lot cheaper to achieve than adequate level of training for pilots. I mean, which is more understandable to you:

1) Assume that the new plane is not going to try to kill you (pilot) in a new way nobody explained you beforehand, (that was pilot error)
2) Assume that no crew is going to be "too easily overwhelmed" if the new system you designed, which is susceptible to failure as every other thing in the world, tries to kill them in a brand new way that you decide not to explain or mitigate beforehand. (that was boeing error)

Fair point, I should have written crews were overwhelmed instead of too easily overwhelmed. Nonetheless, despite all the stress and confusion that was surely occuring, the "only thing" that was happening is that the airplane was trimming nose down. According to Boeing this can be overriden with the electric trim switch and I have not seen any proof yet that this isn't the case. As I already wrote in an earlier post it is just very puzzling to me why a pilot would let his/her aircraft get to an extreme nose down state if it is possible to counter this with electric trimming. We obviously need more information to be able to understand what truly happened.

[Pure Pursuit]

If you're not a pilot, why are you posting on a professional pilot's website?

I would suggest that his input has been far more valuable that yours...

Ultimately, this crash will come down to design and engineering so, let’s not be too precious about pilot only input into this forum.

[FakePilot]

Anyone consider the old "pitch for airspeed" and how counter intuitive slowing down might have seemed?

[jimjim1]

There were actually two failures on the MAX: The faulty AOA sensor data which triggered a whole range of spurious warnings, put the pilots in a high workload situation, which on its own was hazardous. Then MCAS comes along, and administers the coup-de-grace while the pilots are busy trying to make sense of the aircraft and their checklists.

This^^^.

Not one word more is necessary. Thanks.

[infrequentflyer789]

Nonetheless, despite all the stress and confusion that was surely occuring, the "only thing" that was happening is that the airplane was trimming nose down.

Not quite the only thing - also Stick Shaker, elevator feel shift, IAS disagree and ALT disagree to deal with.

Regardless, there is a simple way of proving whether an average crew will be overwhelmed - test it in the sim. During the likely extent of the MAX grounding there must be enough NG/MAX crew getting a sim check anyway to give you a large dataset to be sure.

If the average crew is overwhelmed, either the system isn't safe or the training isn't good enough, or both. If the average crews cope just fine, then these crashes are just pilot error, nothing to fix, move along...

Of course, the industry can't do that because sims that can replicate mcas are rare as rocking horse sh*t outside of Boeing itself, because they aren't needed, because you can learn all you need to know about the MAX from an NG sim. All except how not to crash.

[KelvinD]

Referring to the same post And perhaps you could remind us how US crews were somehow able to fly this dangerously designed airplane for two full years without a single MCAS activation.
Isn't there a report by an AA pilot concerning MCAS causing problems in the safety reporting database (can't remember the name of it just now)?

[GordonR_Cape]

Anyone consider the old "pitch for airspeed" and how counter intuitive slowing down might have seemed?

I have repeatedly suggested that excessive airspeed may have played a role, particularly based on the ADS-B data for the ET302 crash. Given the simultaneous stick shaker and other warnings, reducing speed may not have been the first choice, and was certainly not on any checklists.

The downside of reducing thrust with underslung engines, is that this reduction produces a slight nose down pitch. It is unlikely that a reduction in speed would have been sufficient in the time available, given the low altitude and 'point of no return' reached by ET302

[ecto1]

Sim pilots also put Sully plane back on a runway.

In this and many other scenarios, figuring out what to do is a lot more challenge than doing it. That sim test is worth nothing if you know which specific failure is going to happen at your specific flight.

[GarageYears]

http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

- GY

[SRMman]

CNN say they have a copy of the preliminary Report - see their website - and have quoted some interesting facts.

[bill fly]

Hi Gordon,
Your post surprises me a bit. I can't see that I suggested a checklist for AoA disagree here...I did say that the much touted so called "correct" checklist for Stab trim runaway was unsuitable for the rogue MCAS cases which have occurred and that a dedicated checklist should be made for that.But there can't be checklist, study or training until Boeing has the final (final...) design.

[GarageYears]

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1203x661/et302_fdr_9f3a192af7b7f6efbc48538385146e0f8dd5ed20.jpg

[wheelsright]

You might have to explain that further. The LionAir crash FDR clearly shows short bursts of uncommanded nose-down trim (from MCAS), each one counteracted by PF-commanded nose-up electric trim, maintaining level flight at 5000 feet for roughly six minutes. The uncommanded nose-down trim is never coincident with the PF's nose-up electric trim. They are clearly interleaved, which is exactly the MCAS behaviour described by the updated advice.

The erroneous AoA / AP disconnect / MCAS response chain may have manifested differently on the EA flight, but we'll have to wait for the FDR / CVR data in the report to know one way or the other.

Quite simply the LionAir data does not establish that it is possible to win the battle between Electric Trim and MCAS (particularly if time is of the essence). With combined opposing authority it may only have have a marginal effect on trim. While that may be sufficient to maintain control if the trim is close to optimum it may not be enough if the trim is already way off. Like you say, the data on Ethiopian has not been released yet. However, the published information on the LionAir incidents is not sufficient to draw firm conclusions. I will concede that in the LionAir incident it would appear to be the case that the situation could have been resolved by acting sooner; as in the previous flight. It is not clear that the same is true in the Ethiopian accident. It also may be of some importance that the situation developed at a later stage in the previous LionAir flight. Despite reading most of what is available, I still have more questions than there are answers. In the LionAir incident (crash) stick-shaker started at the beginning of the take-off roll... it is a stage that puts particular stress on the crew to properly interpret and react.

[jagema]

Preliminary report and FDR data are out.

It appears MCAS did in fact make several Stab Nose-down inputs.

Pilots trimmed back up and selected switches to CUTOUT. But the stab remained in a nose down angle. Crew seen pulling back on elevator continuously. They decide to re-energize the stab cutouts so as to trim nose up again (only slightly). It is then when MCAS goes "I've had enough" and adds another 2,5degs nose down. Airplane dives.

[Semreh]

Fair point, I should have written crews were overwhelmed instead of too easily overwhelmed. Nonetheless, despite all the stress and confusion that was surely occurring, the "only thing" that was happening is that the airplane was trimming nose down. According to Boeing this can be overriden with the electric trim switch and I have not seen any proof yet that this isn't the case. As I already wrote in an earlier post it is just very puzzling to me why a pilot would let his/her aircraft get to an extreme nose down state if it is possible to counter this with electric trimming. We obviously need more information to be able to understand what truly happened."Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope.
In addition, the autopilot has the authority to trim the airplane in these conditions.
The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane
throughout the flight envelope."Reference: Explanatory Note to TCDS IM.A.120 - Boeing 737 Issue 10 page 15 [Ref 1]. My reading of this, and the preceding paragraph in the referenced document, plus an equivalent safety issue on the 747-8 / -8F [Ref 2], is that the aisle stand trim switches can be use throughout the flight envelope, but the yoke trim switches cannot.

Ref 1 - www.easa.europa.eu/sites/default/files/dfu/IM.A.120%20Boeing737%20TCDS%20APPENDIX%20ISS%2010.pdf
Ref 2 - www.easa.europa.eu/sites/default/files/dfu/ESF%20B-13.pdf

[LaissezPasser]

"The crew performed all the procedures repeatedly provided by the manufacturer, but was not able to control the aircraft," said Ethiopian Transport Minister Dagmawit Moges, unveiling results of the preliminary probe into the crash.

Presumably Moges refers to "STAB TRIM CUT OUT switches (both) ... CUT OUT" on the Runaway Stabilizer Checklist.
Does the ET302 Preliminary FDR data support the assertion that the crew did this, and if so, when?

[Airbubba]

Narrative of the flight from the preliminary report:

1 FACTUAL INFORMATION

1.1 HISTORY OF FLIGHT

On March 10, 2019, at about 05:44 UTC (All times listed is Universal Coordinated Time (UTC), as recorded on the FDR.), Ethiopian Airlines flight 302, a Boeing 737-8 (MAX), Ethiopian registration ET-AVJ, crashed near Ejere, Ethiopia, shortly after takeoff from Addis Ababa Bole International Airport (HAAB), Ethiopia. The flight was a regularly scheduled international passenger flight from Addis Ababa to Jomo Kenyatta International Airport (HKJK), Nairobi, Kenya. There were 157 passengers and crew on board. All were fatally injured, and the Aircraft was destroyed.

The following is based on the preliminary analysis of the DFDR, CVR and ATC communications. As the investigation continues, revisions and changes may occur before the final report is published.

At 05:37:34, ATC issued take off clearance to ET-302 and to contact radar on 119.7 MHz.

Takeoff roll began from runway 07R at a field elevation of 2333.5 m at approximately 05:38, with a flap setting of 5 degrees and a stabilizer setting of 5.6 units. The takeoff roll appeared normal, including normal values of left and right angle-of-attack (AOA). During takeoff roll, the engines stabilized at about 94% N1, which matched the N1 Reference recorded on the DFDR. From this point for most of the flight, the N1 Reference remained about 94% and the throttles did not move. The N1 target indicated non data pattern 220 seconds before the end of recording. According to the CVR data and the control column forces recorded in DFDR, captain was the pilot flying.

At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1° then increased to 35.7° while value of right AOA indicated 14.94°. Then after, the left AOA value reached 74.5° in ¾ seconds while the right AOA reached a maximum value of 15.3°. At this time, the left stick shaker activated and remained active until near the end of the recording. Also, the airspeed, altitude and flight director pitch bar values from the left side noted deviating from the corresponding right side values. The left side values were lower than the right side values until near the end of the recording.

At 05:38:43 and about 50 ft radio altitude, the flight director roll mode changed to LNAV.

At 05:38:46 and about 200 ft radio altitude, the Master Caution parameter changed state. The First Officer called out Master Caution Anti-Ice on CVR. Four seconds later, the recorded Left AOA Heat parameter changed state.

At 05:38:58 and about 400 ft radio altitude, the flight director pitch mode changed to VNAV SPEED and Captain called out “Command” (standard call out for autopilot engagement) and an autopilot warning is recorded.

At 05:39:00, Captain called out “Command”.

At 05:39:01 and about 630 ft radio altitude, a second autopilot warning is recorded.

At 05:39:06, the Captain advised the First-Officer to contact radar and First Officer reported SHALA 2A departure crossing 8400 ft and climbing FL 320.

Between liftoff and 1000 ft above ground level (AGL), the pitch trim position moved between 4.9 and 5.9 units in response to manual electric trim inputs. At 1000 ft AGL, the pitch trim position was at 5.6 units.

At 05:39:22 and about 1,000 feet the left autopilot (AP) was engaged (it disengaged about 33 seconds later), the flaps were retracted and the pitch trim position decreased to 4.6 units.

Six seconds after the autopilot engagement, there were small amplitude roll oscillations accompanied by lateral acceleration, rudder oscillations and slight heading changes. These oscillations continued also after the autopilot was disengaged.

At 05:39:29, radar controller identified ET-302 and instructed to climb FL 340 and when able right turns direct to RUDOL and the First-Officer acknowledged.

At 05:39:42, Level Change mode was engaged. The selected altitude was 32000 ft. Shortly after the mode change, the selected airspeed was set to 238 kt.

At 05:39:45, Captain requested flaps up and First-Officer acknowledged. One second later, flap handle moved from 5 to 0 degrees and flaps retraction began.

At 05:39:50, the selected heading started to change from 072 to 197 degrees and at the same time the Captain asked the First-Officer to request to maintain runway heading.

At 05:39:55, Autopilot disengaged,

At 05:39:57, the Captain advised again the First-Officer to request to maintain runway heading and that they are having flight control problems.

At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.

At 05:40:03 Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.

At 05:40:05, the First-Officer reported to ATC that they were unable to maintain SHALA 1A and requested runway heading which was approved by ATC.

At 05:40:06, left and right flap position reached a recorded value of 0.019 degrees which remained until the end of the recording.

The column moved aft and a positive climb was re-established during the automatic AND motion.

At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the DFDR and the stabilizer moved in the ANU direction to 2.4 units. The Aircraft pitch attitude remained about the same as the back pressure on the column increased.

At 05:40:20, approximately five seconds after the end of the ANU stabilizer motion, a second instance of automatic AND stabilizer trim occurred and the stabilizer moved down and reached 0.4 units.

From 05:40:23 to 05:40:31, three Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.

At 05:40:27, the Captain advised the First-Officer to trim up with him.

At 05:40:28 Manual electric trim in the ANU direction was recorded and the stabilizer reversed moving in the ANU direction and then the trim reached 2.3 units.

At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.

At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position

At 05:40:44, the Captain called out three times “Pull-up” and the First-Officer acknowledged.

At 05:40:50, the Captain instructed the First Officer to advise ATC that they would like to maintain 14,000 ft and they have flight control problem.

At 05:40:56, the First-Officer requested ATC to maintain 14,000 ft and reported that they are having flight control problem. ATC approved.

From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position. The left indicated airspeed increased from approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was approximately 20-25 kt higher than the left.

The data indicates that aft force was applied to both columns simultaneously several times throughout the remainder of the recording.

At 05:41:20, the right overspeed clacker was recorded on CVR. It remained active until the end of the recording.

At 05:41:21, the selected altitude was changed from 32000 ft to 14000 ft.

At 05:41:30, the Captain requested the First-Officer to pitch up with him and the First-Officer acknowledged.

At 05:41:32, the left overspeed warning activated and was active intermittently until the end of the recording.

At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.

At 05:42:10, the Captain asked and the First-Officer requested radar control a vector to return and ATC approved.

At 05:42:30, ATC instructed ET-302 to turn right heading 260 degrees and the First-Officer acknowledged.

At 05:42:43, the selected heading was changed to 262 degrees.

At 05:42:51, the First-Officer mentioned Master Caution Anti-Ice. The Master Caution is recorded on DFDR.

At 05:42:54, both pilots called out “left alpha vane”.

At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough.

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right Indicated Airspeed reached 500 kts at the end of the recording. The last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right.

[GarageYears]

However, you might add, at no point was thrust reduced from 94% N1, which allowed the airspeed to reach around 375kts.... which at the final ND trim was unrecoverable.

None of the crew trim inputs at any point were sufficient to counter the MCAS ND trim events, which is really where things went wrong. Had they trimmed sufficiently and hit the cutouts, we'd not be have this exact discussion.

Also, the AOA was not offset for the entire flight, it appears to an event that occured some8 seconds after takeoff. Bird strike maybe?

- GY

[GordonR_Cape]

http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

- GY

Some very strange error messages, not just AOA offset:
At 05:38:46 and about 200 ft radio altitude, the Master Caution parameter changed state. The
First Officer called out Master Caution Anti-Ice on CVR. Four seconds later, the recorded
Left AOA Heat parameter changed state.

Edit: I thought it was not possible to engage the autopilot while there was AOA disagree, but it seems that it did happen for a short interval.

Edit: The sequence in this crash was slightly different, with autopilot engaged before flap retraction. The first MCAS activation was not immediately after flaps up, but only after the autopilot disengaged a few seconds later. A subtle difference perhaps?

[DaveReidUK]

With apologies to the mods for the size:
https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/2000x1250/general_overview_of_flight_8b8dd9189cd9fcafbf7029c8cda62b59d 4ab09b9.jpg

[sooty655]

However, you might add, at no point was thrust reduced from 94% N1, which allowed the airspeed to reach around 375kts.... which at the final ND trim was unrecoverable.

None of the crew trim inputs at any point were sufficient to counter the MCAS ND trim events, which is really where things went wrong. Had they trimmed sufficiently and hit the cutouts, we'd not be have this exact discussion.

Also, the AOA was not offset for the entire flight, it appears to an event that occured some8 seconds after takeoff. Bird strike maybe?

- GY
The left AOA also appears to correct itself (very briefly) at 05:43:25, and immediately go haywire again. Very strange.

[Avionista]

Flaps UP at 05:39:45 but there appears to be two Automatic Trim Down commands prior to this which produced some Down Pitch Trim.
Is this STS, or MCAS operating when the flaps are down?

[Organfreak]

I'm interested in this "bird (or other) strike." Where did that info come from? Is this a real event? How do we know that, CVR?

[GarageYears]

Flaps UP at 05:39:45 but there appears to be two Automatic Trim Down commands prior to this which produced some Down Pitch Trim.
Is this STS, or MCAS operating when the flaps are down?

Those both occur while the AP was engaged, which is a 'disable' input for MCAS, hence I think we can assume those are STS.

-GY

[moosepig]

Link to the preliminary report please...
Post #3187.

[T28B]

I'll put it here again since the thread is moving at a nice clip:

Preliminary Report (http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e)

Dear readers, please have a look at the Preliminary Report (http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e); it may answer some of your questions "up front." You'll probably have some questions that it does not answer.

[bill fly]

Narrative of the flight from the preliminary report:

Thanks for the tabulated data Bubba,
That puts it in pilot's perspective and makes terrible reading.
I hate to think what those poor fellows felt like.

Also opens up new questions which as usual we will guess at, until a reliable report emerges.

[mryan75]

I owe the first officer, Ahmed Nur Mohammod, a true and heartfelt apology. He called the stab trim cutout and performed admirably. Two-hundred hours or not, he did his job well.

[infrequentflyer789]

Some very strange error messages, not just AOA offset:

And in the maint log too - and like Lion all were tested using BITE and found "ok".

I can't see that it is confirmed anywhere that they re-engaged electric trim
- FO called for stab trim cutout, and in the narrative it is after they trimmed up (anyone going to change their appraisal of him? edit: seems someone did, good)
- one period of MCAS AND has no effect (so cutouts worked)
- later two manual elec trim inputs seem to be ineffective, maybe they did turn back on but too short to see the effect?
- Another MCAS input and this time the stab goes down
Given the info we have, it seems that both "switches were re-activated" and "switches failed to stop MCAS" are plausible.

And then there is the AP - they got the AP to engage (left-side, despite dud AOA, WTF?) but it tripped out 33 secs later. The AP appears to have been trimming nose down too, and failing to climb, and then we have:
Six seconds after the autopilot engagement, there were small amplitude roll oscillations accompanied by lateral acceleration, rudder oscillations and slight heading changes. These oscillations continued also after the autopilot was disengaged.

First impressions are that this ain't just a software fix, yes MCAS dumped them in the ground but even without that this was a brand new plane that was seriously sick.

All from one dud AOA sensor? - or something in the vicinity of the (left) ADIRU?

[Water pilot]

The left AOA also appears to correct itself (very briefly) at 05:43:25, and immediately go haywire again. Very strange.
Wire chafe? Something got pinched or partially shorted (changing resistance) and briefly released? I have no idea how one would prove that given how little of the planes are left. A misrouted wiring harness on a new plane kind of fits the bill but there are a lot of other possibilities. Do they take detailed production photos?

[LaissezPasser]

Had they trimmed sufficiently and hit the cutouts, we'd not be have this exact discussion.

But they trimmed repeatedly and did hit the cutouts. From the preliminary report:

At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.

At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position

[GarageYears]

But they trimmed repeatedly and did hit the cutouts. From the preliminary report:

I suggest you look at the FDR traces and then tell me what you think. I did...

- GY

[GoodTimes]

Why no mention of the fact that they reengaged the stab trim cutout switches at 5:43:11? Seems to me as though they reengaged the system, applied nose up trim via the electric trim switches and left the stab trim cutout switches engaged thus allowing MCAS to activate again once they stopped trimming.

[patplan]

Taken from ET-AVJ Preliminary Accident Report PP 26-27:
https://i.ibb.co/5TjHfNC/pp26-ed2.jpg (https://ibb.co/qBgQPth)
..

https://i.ibb.co/tBknbYf/pp27-ed.jpg (https://ibb.co/p2GN34D)

[dlen]

From the outside, after looking carefully at the Ethiopian crash flight recorder plot:

Systems should nowadays be able to detect faulty AOA sensor (or every other sensor) output automatically by checking whether it is congruent with the set of information available. Flight parameters are not independent of each other.
To be more specific, a correctly detected sudden increase in the AOA can have a limited number of causes:
a) very strong upward winds - very improbable, and if, only possible for a couple of seconds. Anyway, even then there has to be a corresponding spike in upward acceleration to be detected.
b) corresponding increase in longitudinal pitch
c) corresponding hefty decrease in airspeed
As none of those were present, a sensor failure should have been detected, the pilots informed and the sensor input to other systems blocked.
A similar reasoning can be set up for airspeed sensors.

If the pilots had consistently nullified the automatic downwards trim by manual upwards trim, they would have made it. So clearly they did not identify the chain of problems (wrong AOA, MCAS reacting) correctly. The stick shaker certainly didn't help here for a clear analysis.

[Lonewolf_50]

At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. *
At 05:41:54, the First-Officer replied that it is not working This is a puzzling excerpt from the CVR. It appears that the FO had the right idea, in terms of getting control of the trim back via manual. By "not working" - I wonder what he meant.
(As I look at the Airspeed value, I keep thinking, will ask another question in a bit ...
The left indicated airspeed increased from approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was approximately 20-25 kt higher than the left.
At 05:41:20, the right overspeed clacker was recorded on CVR. It remained active until the end of the recording.)

*As I read this, they had already put stab trim cut out.
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First- Officer confirmed stab trim cut-out.
At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position

[LaissezPasser]

I suggest you look at the FDR traces and then tell me what you think. I did...

- GY
The callout box with the arrow pointing to the aqua blue line where it says "Automatic Trim Command With No Change In The Pitch Trim" indicates to me (and apparently to the investigators) that the cutout switches were in the "cut out" position.

I'm thinking wiring issue.

[infrequentflyer789]

This is a puzzling excerpt from the CVR. It appears that the FO had the right idea, in terms of getting control of the trim back via manual. By "not working" - I wonder what he meant.
​​​​​​​(As I look at the Airspeed value, I keep thinking, will ask another question in a bit ...)

It's a shame that Mentour Pilot took his video down, I think it showed exactly what he meant (physically unable to move trim wheel), I also think their sim session was lower airspeed (310?).

[futurama]

I'm a real pilot, too. The one thing that really stands out to me in the prelim is that they didn't reduce power. I just took a nice big bite of humble pie regarding the first officer, so I'm not going to sling any more mud, but one could reasonably have assumed they would have done so, wouldn't you say? While trying to fight a nose-down situation? Unusual attitude recovery 101.

They probably thought they did: "At 05:39:42, Level Change mode was engaged. The selected altitude was 32000 ft. Shortly after the mode change, the selected airspeed was set to 238 kt."

While attempting to sort out the trim situation, they didn't see that the airspeed kept increasing.

[MurphyWasRight]

I've read the entire report and still see no mention of a bird (or other foreign object) strike to the left AoA sensor. Where did that tantalizing hint come from??? CVR?
Also, the media coverage (and even posts here) keep harping on the idea that all Boeing recovery procedures were followed, and yet they turned the trim cutout switches back ON in their desperation, which was certainly NOT part of the prescribed procedures. Seems as if that is some serious "dumbing-down" of the info.

The press conference statement was 'no foreign object' as a I recall. This is not the same as conclusively denying a bird strike.
The FDR data supports a sudden event on the AoA system with at least 2 hints that it was at the sensor:

1: Sudden jump to extreme non changing value until late in flight with partial freeing near end. Bird guts blew off?
2: Master caution anti ice caution and 'Primary AOA heat L' to off coincident with above. This is consistent with significant physical damage to AoA sensor causing an open in heater circuit.

I suspect there may be internal disagreement on the possible cause of this so they left it out but a bird strike is a strong contender in my view.
Could be the initial bird strike reports followed the above reasoning but may not be a way to dis/prove the theory.

[patplan]

ET-AVJ Preliminary Accident Report
Maintenance Log pp. 20-21
https://i.ibb.co/R3q8xFf/top-maint-min.jpg (https://ibb.co/5rPZN0m)
https://i.ibb.co/2jVCrDH/bot-pre-min.jpg (https://ibb.co/7GMZDwL)

[GordonR_Cape]

I'm a real pilot, too. The one thing that really stands out to me in the prelim is that they didn't reduce power. I just took a nice big bite of humble pie regarding the first officer, so I'm not going to sling any more mud, but one could reasonably have assumed they would have done so, wouldn't you say? While trying to fight a nose-down situation? Unusual attitude recovery 101.

I've been suggesting that speed was a factor in not being able to trim up, which is probably true. However a careful look at the FDR shows that everything was stable for several minutes, provided no speed/thrust or trim changes were made. The aircraft certainly could not land safely in that configuration.

The high speed alone was not the direct cause of the crash, though it may have severely limited their options. In theory speed could have been reduced gradually, though it is not known what effect the reduction of engine thrust would have on pitch.

IMO the combination of speed, and the final MCAS activation of nose down trim was fatal. Conversely, MCAS activation at low speed earlier in the flight was recoverable, because they could trim up manually. The fault-tree is complex...

Edit: pilotmike If I remove the word slight will that make you happy?

Edit: In retrospect I stand by my statement for muiltiple reasons:
1. Pilots are not the only people who know about aerodynamics.
2. The discussion is around flight close to VMO, at which speed the aerodynamic forces far outweigh the pitch moment of the engine thrust.
3. Most discussion of engine pitch up moment is close to VMC, in which case the aerodynamic forces are proportionately small.
4. Most discussion of underslung engines centers around TOGA thrust, which could not be further from this flight regime.
Nobody has come up with any hard numbers, so these factors may remain an unknown.

Edit: Similar mention of speed/thrust from justthisonce

[wheelsright]

The biggest issue that I see from the data so far is the massive authority that MCAS has and how small the authority of the STS/ pickle switch is. Two massive MCAS AND trim adjustments in the space of 29 secs sealed the fate of the flight. If is not clear if it was possible to manually trim given the airspeed but unlikely due to control pressure. The ground proximity probably precluded thrust reduction or runaway trim recovery procedure.

The crew would have had to correctly diagnose the problem and flicked the trim disconnect switches in just over 60s from stick-shaker to unwanted MCAS AND. It seems the crew did not make all the right choices but hardly reckless driving either. I suspect that the overall picture is that that Boeing are asking way too much of their customers to fly this aircraft. It is an accident waiting to happen. Broadly speaking the data shows a horrible control system that does not work properly and could have been much more safely and better designed. It also shows that pilots may not always make all the best decisions when under high levels of stress.

Being a little flippant... a logical solution is to automatically disconnect electric trim if there is a left side stick-shaker condition... or instead of alpha vane disagree indication it could automatically disconnect MCAS (slightly more serious solution). Behind the systemic problems is the question of why the AOA sensors failed... is this a freak, or is there something more fundamental at work?

[4listair]

COTCHETT, PITRE & McCARTHY, LL
https://www.cpmlegal.com/media/news/302_Stumo_Complaint.pdf

IN THE UNITED STATES DISTRICT COURT FOR THE NORTHERN DISTRICT OF ILLINOIS EASTERN DIVISION

MICHAEL STUMO and NADIA MILLERON, as Personal Representatives of the Estate of SAMYA STUMO, deceased,
Plaintiffs,
v.
THE BOEING COMPANY, a Delaware corporation;
ETHIOPIAN AIRLINES, a foreign corporation;
ETHIOPIAN AIRLINES ENTERPRISE;
ETHIOPIAN AIRLINES GROUP, INC.;
and
ROSEMOUNT AEROSPACE, INC., a Delaware corporation;
Defendants.

A Claim against the FAA has also been filed by the same partners, ahead of a possible suit.

[Just This Once...]

The autopilot behaviour differed considerably from the Boeing documents:

By 05:38:45 the left AoA sensor was reporting (but not displaying) over 74deg AoA, yet the LH AP was engaged successfully some 37 seconds later and remained engaged until time 05:39:55. By that time MCAS had completed 3 discrete pitch-down trim inputs before the AP actually disengaged.

George the autopilot wants to go up whilst HAL the MCAS wants to go down.

Eeek.

[EDLB]

One common pattern between the Lion air and Ethopian air crash is, that the later/last manual trim up commanded from the coloum switches are in both cases only short blips, followed by 5 second trim down from the MCAS system.

I find it hard to believe that both pilots clinging on the control coloums for their dare life to get nose up do not try harder/longer on the electric trim as long as engaged and not cut out.

I assume that there is more to the story.

[GarageYears]

The autopilot behaviour differed considerably from the Boeing documents:

By 05:38:45 the left AoA sensor was reporting (but not displaying) over 74deg AoA, yet the LH AP was engaged successfully some 37 seconds later and remained engaged until time 05:39:55. By that time MCAS had completed 3 discrete pitch-down trim inputs before the AP actually disengaged.

George the autopilot wants to go up whilst HAL the MCAS wants to go down.

Eeek.

Really?

Aside from why the AP was successfully engaged, the three trim ND FCC commands (and one NU for good measure), DURING autopilot engagement are brief and likely STS driven - not MCAS. That comes after the AP is disconnected.

The first MCAS ND command occurs on deselection of the AP, and runs for 10 seconds as the system is expected to do. You can clearly see that in the FDR traces.

The crew then input a shorter NU pickle switch trim input (which did not equal the MCAS ND demaind).

MCAS reset and 5 seconds after the crew ceased their input, it runs again for less than 10 seconds this time, since the crew again provided NU pickle switch trim, which shut off MCAS for another 5 seconds, at which point the crew had selected the cutoff switches.

MCAS ran again (since it was reset) but had no effect, since it was disabled by the cutoffs.

At this point the aircraft was roughly pitch trim of 2.5 units (so ND). The crew then tried to use manual trim (which failed) and then hauled back on the column and tried to control the aircraft via the yoke.

Later two very short pickle switch inputs NU were input indicating the crew re-engaged the electric trim, but they were very short and had a very small effect on trim.

5 seconds later MCAS, having been reset by the crew electric trim input, and now being active again, ran for less than 10 seconds and seems to have been interrupted by the Left AOA value changing.

Unfortunately at this point the aircraft was very fast >375kts and the ND input was too much to overcome.

- GY

[ecto1]

One common pattern between the Lion air and Ethopian air crash is, that the later/last manual trim up commanded from the coloum switches are in both cases only short blips, followed by 5 second trim down from the MCAS system.

I find it hard to believe that both pilots clinging on the control coloums for their dare life to get nose up do not try harder/longer on the electric trim as long as engaged and not cut out.

I assume that there is more to the story.

That the trim wheels didn't move because electric trim is not powerful enough at such high speed?

[hawk76]

5 seconds after T/O with the stick-shaker going and a brace of captions is not the time to pull the power back.

It is time to put the flaps up? Would, perhaps, Airspeed Unreliable apply?

[slacktide]

How much would you have reduced power by, with unreliable airpeed, stick shaker going off, low altitude, while you've seemingly lost pitch control? Bearing in mind no reference value is provided in the handbook, and reducing power pushes the nose down?

Well, if I were the pilot, I would have set power to 75% N1. This is the reference value which is provided in the handbook, AKA the unreliable airspeed NNC, and it is a memory item that pilots are expected to execute without needing to reference the QRH. As a note, the QRH specifically states that stick shaker, overspeed warning, and airspeed low alerts may sound erroneously or simultaneously, and this situation is regularly practiced in the simulator.

[MurphyWasRight]

That the trim wheels didn't move because electric trim is not powerful enough at such high speed?
Except that trim did move slightly (as expected given short blips) and the FDR shows the switch activation directly, 2 short blips about 5s apart.

[threemiles]

I've read the entire report and still see no mention of a bird (or other foreign object) strike to the left AoA sensor. Where did that tantalizing hint come from??? CVR?
Also, the media coverage (and even posts here) keep harping on the idea that all Boeing recovery procedures were followed, and yet they turned the trim cutout switches back ON in their desperation, which was certainly NOT part of the prescribed procedures. Seems as if that is some serious "dumbing-down" of the info.

The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.

[GordonR_Cape]

Why would pilots in a severe AND trim crisis merely blip the switches?

Is there any possibility that the extreme column forces could have interfered with the switches, either mechanically (including wiring etc) or biomechanically.?

By biomechanically I mean could the hand position and forces make the pilot believe he was pressing the switches when in fact he was not?

The FDR shows a 'bunt' with significant g-force due to the final MCAS stabiliser nose down trim. Until that point the trim was stable nose low but unvarying, so they may not have seen the need for (or possibility of) nose up trim. They may have severely underestimated the force of the nose down trim at high airspeed, compared to the earlier MCAS activation. Within 10 seconds they were experiencing negative 2g and 40 degrees nose down pitch on the FDR, so moving arms may have been impossible. During that time the aft forces on the yoke reached extreme values, and it is probably impossible to pull and trim at the same time. Hope that makes sense.

The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.

Not true at all. There is a long period where the trim is stable, and the cutoff switches have clearly been disabled, inhibiting MCAS. This is shown on the FDR. Later there is a sudden uncommanded nose down trim, consistent with MCAS. The only logical way this could have happened is for the switches to have been been enabled. To suggest that this is pure speculation, flies in the facts shown on the FDR.

[slacktide]

The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.

I would state that it is pure inference, rather than speculation.

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

That could not have happened if the cutout switches had not been turned back on.

[Organfreak]

The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.

Oh! :ouch: I do apologize if I have posted wrong information. I had thought it was established fact.

[Albino]

Not only is turning the stab trim cutout switches back on NOT a part of the documented and approved recovery procedure, the FCOM specifically cautions against turning electric stab trim back on after it has been selected off.

"Ensure that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight."

It's disingenuous to state that the recovery procedure was followed, when it was only halfway followed, and other basic elements of pilotage were neglected. If they had monitored and controlled the airspeed within limits, and followed the published recovery procedure, they would have been able to manually trim the airplane using the trim wheel.

Regarding my language "monitored and controlled their airspeed." I know someone will pipe up about stick shakers and clackers and cockpit confusion. Yes, the left side airspeed was unreliable, and the left side stick shaker was going off. But the right side airspeed was not unreliable, and the BU airspeed was not unreliable, and the right side stick shaker was not going off. Isn't unreliable airspeed a well documented scenario which is regularly practiced in the simulator, with simultaneous shaker and clacker? Isn't it the second item on the very first page of the QRH? Are the appropriate initial pitch and power settings to be used during unreliable airspeed not memory items?

Couldn't agree more.

Memory Items for Airspeed Unreliable?
Memory Items for Unscheduled Stab trim?

It appears the aircraft was never put back in trim after the initial MCAS input and although an attempt was made with the cutouts the aircraft was out of trim. Turned back on again contrary to advice (accept this is speculation at present) and the system was allowed to trim forward again.

Unfortunately these points will be lost to the majority on here who simply want to flame Boeing.

[AfricanSkies]

Thoughts.

This aircraft had multiple pitot static malfunctions in the days prior to the accident as per the tech log quoted in the report. Altimeter and VSI erratic.

If they had had a bird strike on the vane, they would have noticed the thump and said something. I think we can rule a bird strike out. And frozen things. It was 17 degrees C on the ground.

Just after takeoff the crew got an almost immediate
Stick shaker
Airspeed disagree
Altitude disagree
F/D disagree
Which constitutes an unreliable airspeed non normal
(Together with an anti ice caution)
And did not apply the memory items for that , and seemed to be prepared to continue to FL320.
The trim problems started once the flaps were retracted , however the flaps seem to have been retracted despite the left stick shaker being continually active and the aircraft in an unreliable airpseed state which calls for a pitch and power setting with the flaps down (if I recall, 10deg NU, 85%N1) but the power wasn't reduced from 94% either.
The stabtrim cutout switches seem to have stopped the MCAS from moving the stab, but then the switches were re-engaged (2 ANU trims were recorded 2,5 min after the stab trim switches were set to cutout) and this then allowed the MCAS to pitch it down again.
As regards thrust and pitching moments I personally would prefer to take my chances with speedbrake, idle thrust and a bit of a dip than two engines at 94% when I'm pointing downhill.
And I don't think I'd be getting ATC involved until I had the beast back under some sort of control.
This seems to be more than an MCAS thing, it may be a pitotstatic adiru thing.
If one sensor gets extreme readings or if there's a sensor disagree, then no system should rely upon that sensor data.
Systems shouldn't rely on one sensor only.

[MurphyWasRight]

Originally Posted by MurphyWasRight https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-post10438980.html#post10438980)
Why would pilots in a severe AND trim crisis merely blip the switches?

Is there any possibility that the extreme column forces could have interfered with the switches, either mechanically (including wiring etc) or biomechanically.?

By biomechanically I mean could the hand position and forces make the pilot believe he was pressing the switches when in fact he was not?

The FDR shows a 'bunt' with significant g-force due to the final MCAS stabiliser nose down trim. Until that point the trim was stable, so they may not have seen the need for (or possibility of) nose up trim. They may have severely underestimated the force of the nose down trim at high airspeed, compared to the earlier MCAS activation. Within 10 seconds they were experiencing negative 2g and 40 degrees nose down pitch on the FDR, so moving arms may have been impossible. During that time the aft forces on the yoke reached extreme values, and it is probably impossible to pull and trim at the same time. Hope that makes sense.
https://www.pprune.org/images/statusicon/user_online.gif https://www.pprune.org/images/buttons/report.gif (https://www.pprune.org/report.php?p=10439007)
The 2 blips occurred before the final MCAS action, in fact likely triggered it so the extreme final G load is not a factor. They were flying but with (great?) difficulty before then.

Not sure 3 units AND (from trace) while 'stable' is not correct. From the report it appears they tried and failed to use manual trim wheel after cutout and then (last ditch option) re-enabled it to attempt electrical trim.

Gets back to the question of why 2 short blips instead of continuous until collum force approaches neutral?

[Artus KG]

Any ideas on why the faulty left AoA goes to values even lower than the assumed correctly working right AoA at the same time the terminal descent starts? MCAS works for a while, left AOA goes down, stick shaker is happy, MCAS stops.

[Just This Once...]

Really?

Aside from why the AP was successfully engaged, the three trim ND FCC commands (and one NU for good measure), DURING autopilot engagement are brief and likely STS driven - not MCAS. That comes after the AP is disconnected.

The first MCAS ND command occurs on deselection of the AP, and runs for 10 seconds as the system is expected to do. You can clearly see that in the FDR traces.


I'm not seeing it that way. With the AP engaged the 3x AND auto trim events kill the pitch attitude - from 8 deg positive decreasing all the way to zero by the time AP tripped-out. At that stage with the AP engaged, FD commanding a climb and with throttles parked forward with excess speed in hand the aircraft should have been going up like the proverbial. Indeed, the pitch attitude only becomes positive again when PF takes control and applies a generous amount of aft stick and ANU blips. Even then he does not achieve anything like the 8 deg positive achieved earlier.

Clearly I know what the aircraft 'should' be doing, but I'm just not finding it in the data provided.

If you see it differently please shout out as even after years of flight testing this has left me scratching my head.

[ecto1]

Except that trim did move slightly (as expected given short blips) and the FDR shows the switch activation directly, 2 short blips about 5s apart.

Yeah, from 2.1 to 2.3 units, I know.

It could very well be that the initial blip moved the trim slightly and then stopped, they released and try again but no avail. Each square in the FDR is 3 seconds, so the second "short" blip is 2s or so. Between the two, looks to me in the 3s area. (one full square) Let's say 2.5 to be safe.

The final AND input by MCAS was 5s and moved 1.3 units. The last pilot input ANU was 2.5s (combined) and moved 0.2 units. Something odd is there.

It is a theory, I could be mistaken. But it's plausible to my eyes.

[infrequentflyer789]

Later two very short pickle switch inputs NU were input indicating the crew re-engaged the electric trim, but they were very short and had a very small effect on trim.


I am struggling to see any effect, and I didn't see any other indication in the report that the crew re-engaged.

I suspect that cutout switch position is not directly recorded and has to be inferred, possibly this is where the rumours of disagreements and CVR reading problems originate from - maybe there are sounds or speech on the CVR that could indicate re-engagement but it isn't definite, or agreed. Any reference to the disputed sounds may then have been excised from the report.


5 seconds later MCAS, having been reset by the crew electric trim input, and now being active again, ran for less than 10 seconds and seems to have been interrupted by the Left AOA value changing.


This MCAS signal got through, which some will take as proving the switch position was changed, there is so much else going wrong not sure I would agree (that it is proven). Also, this still doesn't look right per the descriptions we've had:
- MCAS is supposed to have less authority at higher speed (maybe that's it, it is shorter input?)
- If AOA reduces then MCAS is apparently supposed to put the trim back up, if no pilot trim input - yet there isn't and it doesn't

[YYZjim]

Re: The FDR plot

Note the call-out box which states "Automatic Trim Command With No Change In Pitch Trim". However, there is another downwards blip of automatic trim command shortly before the crash, which clearly causes more nose-down trim. This must be the result of the MCAS being re-enabled. For some reason, the CVR transcript does not say so, but the pilots must have re-enabled the Cutout Switch.

[Just This Once...]

Gets back to the question of why 2 short blips instead of continuous until collum force approaches neutral?

Could be a DFDR artefact. On another aircraft type I am more familiar with a continuous blip when the trim motor does not run just poles the DFDR data flag on and off again. This single pole will appear as a spike when graphed.

In the accident graph the 2 final electric trim blips do not appear to move the pitch trim, either due to cutout or aerodynamics overcoming the drive.

[MurphyWasRight]

Yeah, from 2.1 to 2.3 units, I know.

It could very well be that the initial blip moved the trim slightly and then stopped, they released and try again but no avail. Each square in the FDR is 3 seconds, so the second "short" blip is 2s or so. Between the two, looks to me in the 3s area. (one full square) Let's say 2.5 to be safe.

The final AND input by MCAS was 5s and moved 1.3 units. The last pilot input ANU was 2.5s (combined) and moved 0.2 units. Something odd is there.

It is a theory, I could be mistaken. But it's plausible to my eyes.

Unfortunately the plot has artificial slopes on binary events so without the raw data is hard to tell the actual length of short inputs so the total 'blip' could be significantly less than 2.5s.

Interesting idea that when they tried the electrical trim the system stalled so they tried again, hard to tell without raw data and CVR though.

[GordonR_Cape]

This MCAS signal got through, which some will take as proving the switch position was changed, there is so much else going wrong not sure I would agree (that it is proven). Also, this still doesn't look right per the descriptions we've had:
- MCAS is supposed to have less authority at higher speed (maybe that's it, it is shorter input?)
- If AOA reduces then MCAS is apparently supposed to put the trim back up, if no pilot trim input - yet there isn't and it doesn't

AFAIK MCAS uses the same rate of nose down trim of 0.27 units per second, it is the duration that varies, with a maximum of 9 seconds.
MCAS will 'unwind' the trim only when the AOA reduces below the 10 degree threshold, not when AOA reduces relative to its previous value.

[OldnGrounded]

[ [Stab trim cutout switches.] Turned back on again contrary to advice (accept this is speculation at present) and the system was allowed to trim forward again.


Unless I misread the preliminary report, it appears that the "advice" that was followed was that of the FO, who advised that manual operation of the trim wheels was not possible. It's fairly easy to understand why the crew decided to flip those switches back on (if they did).

[737mgm]

The preliminary report leaves me with several questions:

Shortly after Takeoff the Captain who was pilot flying had stick shaker on his side only. Why not transfer control to the first officer at this point or at least a little while later? That is actually what the Captain of the Lion Air flight previous to Lion Air 610 flight did. If you see that there is stick shaker on your side but not on the other wouldn't it make sense to rely on the FCC B side in this case? Why engage the autopilot on the A side that has the stick shaker? The captain tried to engage Autopilot A two times unsuccessfully at 400 Ft and 630 Ft. That would be another indication to either continue manually or at least try the other side. Finally he succeeded in turning the autopilot A on at 1000 Ft (for 33 seconds). Possibly Autopilot B would have worked since it seems like there was no false sensor indications on that side an MCAS would not have activated (speculation).

Based on the report the situation for ET302 was basically exactly the same as for Lion Air 610. Stick shaker right after Takeoff and then as the flaps were retracted MCAS started trimming the nose down.

At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.

Knowing about the Lion Air Accident as the ET302 crew must have, were they expecting the nose down trim as they retracted the flaps? Based on their actions it seems like they did not even though the circumstances were the same. Would this not have been the point to carry out the runaway stab trim memory items?

At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the DFDR and the stabilizer moved in the ANU direction to 2.4 units. The Aircraft pitch attitude remained about the same as the back pressure on the column increased.

Instead the crew trimmed the aircraft nose up from 2.1 units back to 2.4 units. Why not trim back to the original 4.6 units before MCAS engaged so that it isn't necessary to increase back pressure on the column? Some people on this forum suggest the electric trim switch will not work sufficiently against MCAS. The preliminary report on Lion Air 610 shows otherwise as they countered MCAS up to 30 times.

By now we know that MCAS activates again 5 seconds after the electronic trim switches have been used. They were used by the crew but apparently only to offset the MCAS trim input to a small degree and now MCAS starts trimming the nose down again:

At 05:40:20, approximately five seconds after the end of the ANU stabilizer motion, a second instance of automatic AND stabilizer trim occurred and the stabilizer moved down and reached 0.4 units.

This time the crew did trim against MCAS from 0.4 units back to 2.3 units therefore basically reversing the input from MCAS and showing that this is possible.

Now the First Officer suggested to set the stab trim cut out switches to cut out and MCAS was stopped.

Would it have been possible at this point to reduce the power and then trim the aircraft manually?
At the time the crew did try to trim the aircraft manually the aircraft was flying at the edge of it's envelope at 340 Knots according to the left side or outside of it already at 365 according to the right side. I am not surprised that at this point it was not possible to trim manually anymore.

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units. At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds.

Since the crew said at 05:41:46 that electric trim did not work and now it was possible for them to trim electrically again it is likely that the stab trim switches were set back to their original position before being set to cut out. Aside from not reducing the power even when there was the overspeed warning, why did the crew not trim continuously as they did before in order really raise the nose? Also why did they then not set the stab trim switches back to cut out since it was to be expected that MCAS would engage again 5 seconds after release as it did before?

I am not judging the pilots or saying the same thing could not have happened to me. I am just asking myself these questions based on what I have read.

[Albino]

Here are the memory items for "Runaway Stabilizer":

I. Runaway Stabilizer

CONTROL COLUMN - HOLD FIRMLY
AUTOPILOT (if engaged) - DISENGAGE Do not re-engage the autopilot. Control airplane pitch attitude manually with control column and main electric trim as needed
If the Runaway Continues
STAB TRIM CUTOUT SWITCHES (both) - CUTOUT
If the Runaway Continues
STABILIZER TRIM WHEEL - GRASP and HOLD

Since the condition before step three says, "If the Runaway Continues", I would infer that at that point the aircraft is no longer in trim. Could you identify for me which subsequent step would put it back into trim?
It is looking very likely that moving the trim wheel with the trim set too far nose down is not physically possible, especially at high airspeed. As a last desperate measure, they tried something that wasn't in the checklist.

Step 2 - Control airplane pitch attitude manually with control column and main electric trim as needed.

Was this step done? 2 units is nowhere near in trim and significant control column input would've been needed. Full stabilizer authority was available to the pilots using the electric trim.

If runaway continues... I'd argue people are getting hooked up on semantics here. You can't put a long winded paragraph in covering every eventuality. The system is still not performing as expected so you cut out the stabs, ensuring step 2 is complete and the aircraft is in trim.

The bulletin Boeing sent to all operators and should have been read by all pilots was very clear on this point in 'Operating Instructions'.

[GordonR_Cape]

737mgm

That would be another indication to either continue manually or at least try the other side. Finally he succeeded in turning the autopilot A on at 1000 Ft (for 33 seconds). Possibly Autopilot B would have worked since it seems like there was no false sensor indications on that side an MCAS would not have activated (speculation).

Based on the report the situation for ET302 was basically exactly the same as for Lion Air 610. Stick shaker right after Takeoff and then as the flaps were retracted MCAS started trimming the nose down.

There is a subtle but perhaps crucial difference between the flights. With Lion Air 610 the autopilot was never engaged, and MCAS activated as soon as the flaps retracted. With ET302 the autopilot was engaged before flaps were retracted, and remained engaged for a few seconds longer, during which time MCAS was inhibited. The trigger for MCAS activation was autopilot disengage, not flaps retraction.

Cognitively this is a very different situation, and focusing on why the autopilot disengaged may have been a distraction from the MCAS activation. Conversely flaps retraction is a non-event cognitively, and more attention could have been given to MCAS activation. I do not know whether any of this is relevant, but it is perhaps important to see the whole sequence of events.

[Joejosh999]

Are pilots not used to using those thumb/pickle switches too much? Or perhaps trained to just blip/use sparingly?
It seems we’ve seen two crews (FO on Lion and now on ET) appear to just “blip” when more extended engagement might have been better?

Is there any reason they might be reluctant to go hard and extended on that thumb switch? I mean, if I saw it appear to counter that bad trim, I’d be jumping on it with both feet!

Just wondering why the apparently sparing use of the switch which had potential to correct trim....?

[Albino]

Unless I misread the preliminary report, it appears that the "advice" that was followed was that of the FO, who advised that manual operation of the trim wheels was not possible. It's fairly easy to understand why the crew decided to flip those switches back on (if they did).

I don't disagree and I'd have probably done the same. Surely the moment they were back on though you'd be trimming hard?

From the narrative it sounds as if the Capt hasn't grasped what is happening and it's a suggestion from the FO to cut the switches out in the first place. So when they turn them back on is he 'in the loop'?

[DaveReidUK]

That's the first time I've read about the nationality of the deadhead who'd hitchhiked Flight JT043... Care to show us your source?

There are multiple reports suggesting that the deadheading pilot was a Batik Air captain. I haven't seen any that mentioned their nationality, and I note that the poster who suggested it was a Brit hasn't come back with any confirmation of that.

[GordonR_Cape]

How can the airplane be in trim AFTER a continued runaway? How do you know it is running away if it is not moving away from a trimmed condition?

In Mentour Pilot's video he demonstrates the process in the simulator. If (big IF) runaway trim is detected and stopped quickly enough, the aircraft is still flyable. AFAIK there is no specific term for that situation, in which it is not perfectly trimmed, and significant yoke forces are required to keep the nose level. It is also not so out of trim, that yoke forces cannot keep the nose level, and the aircraft goes into a dive. In/out are not absolutes,

[jan99]

I surmise: killed by airspeed. By the time the cut-out switches were used it was already too late for nose up by any means.

[Intruder]

A couple things I drew from the report:

At no time did they reduce power. The airplane would be MUCH more sensitive to pitch commands as it neared VMO, hence much less controllable in the out-of-trim condition. IF the autothrottle was still engaged, they did NOT follow the non-normal procedure (Step 3) for Runaway trim.

At no time did they get the pitch trim back near an in-trim position. Granted, less nose-up trim would be needed at the high speed, but they continued to use aft column force in lieu of more trim.

It is unclear when/if they returned the Stab Trim Cutout switches to the Normal position at the 5:43:11 point. We may see that later IF there is a FDR trace for those switch positions. However, IF one or both of the Cutout switches were returned to normal, why was there NO yoke trim input after that? If I were to try that last-ditch move of re-engaging the stab trim, I would FIRST hold the yoke trim switches to the nose-up position...

[Zeffy]

Ethiopian pilots fought the 737 MAX flight controls almost from take-off, preliminary report shows

Video at the link.

April 4, 2019 at 11:59 am Updated April 4, 2019 at 1:26 pm

https://www.seattletimes.com/business/boeing-aerospace/preliminary-crash-report-reveals-detail-of-ethiopian-pilots-fight-against-the-737-max-flight-controls/

By Dominic Gates
Seattle Times aerospace reporter

The preliminary investigation into the crash of Ethiopian Airlines Flight 302 last month reveals that pilots began fighting against the Boeing 737 MAX’s new automatic flight control system barely a minute after leaving the ground, after a sensor failed immediately on take-off.

Boeing Chief Executive Dennis Muilenburg issued a statement Thursday from the Renton 737 factory expressing “the immense gravity of these events across our company,” and acknowledging the role the new Maneuvering Characteristics Augmentation System, or MCAS, played in the crash.

He pointed to the software fix and associated pilot training Boeing is working on.

“As pilots have told us, erroneous activation of the MCAS function can add to what is already a high workload environment. It’s our responsibility to eliminate this risk,” Muilenburg said. “We own it and we know how to do it.”

The “black box” flight recorder data shows that after MCAS swiveled the plane’s horizontal tail to push the nose sharply down three times in succession, the pilots hit the cut-off switches stopping the automatic action and tried to adjust the tail manually, according to the report by the Accident Investigation Bureau of Ethiopia’s Transport Ministry.

In doing so, they were following instructions provided by Boeing last November, following the crash of Lion Air Flight 610, on how to deal with such an inadvertent triggering of the new flight control system.

Ahead of the release of the full report, Ethiopian Transport Minister Dagmawit Moges held a news conference in the capital, Addis Ababa, that was almost entirely focused on vindicating the actions of the pilots. “The crew performed all the procedures repeatedly provided by the manufacturer but was not able to control the aircraft,” she said.

The report says that while trying to follow Boeing’s directions, about three minutes into the flight, the two pilots found that the manual system for moving the horizontal tail — also known as the stabilizer — “was not working.” This meant they couldn’t move the large stabilizer wheel in the cockpit that is connected via cables to the tail.

Flight-control experts told The Seattle Times earlier this week that was probably because the forces on the tail of the plane moving at high speed made it next to physically impossible to move the stabilizer wheel as Boeing had recommended.

All the while, the control column “stick shaker” was vibrating the control column, and various messages were telling the pilots and that their airspeed, altitude and pitch readings were unreliable. Two minutes into the flight, losing altitude, an audible warning sounded that the plane was too close to the ground: “Don’t Sink!”

About four minutes into the flight, the pilots gave up on the manual stabilizer wheel and switched the electric power to the tail back on, then used the thumb switches on the control column to pitch the nose back up.

But just five seconds later, MCAS kicked in again and once more pushed the nose sharply down.

Just 35 seconds later, six minutes after take-off, the plane rolled over before plowing into the earth in a “high energy impact” at a speed of approximately 575 miles per hour.

This sequence of events was triggered by the failure of the left angle-of-attack sensor on the outside of the fuselage, just 44 seconds after take-off, the data shows.

There are two such sensors one, either side of the aircraft, that measure the angle between the wing and the airflow. Only one is used to trigger MCAS. The data shows that both sensors showed normal readings on the ground during the take-off roll but deviated immediately after lifting off and in less than a minute were divergent by 60 degrees.

Ethiopian Airlines issued a statement Thursday backing the Flight 302 pilots, saying that they “followed the Boeing recommended and FAA approved emergency procedures to handle the most difficult emergency situation created on the airplane.”

“Despite their hard work and full compliance with the emergency procedures … they could not recover the airplane from the persistence of nose diving,” the statement added.

Boeing CEO Muilenburg said while the Ethiopian and Lion Air “tragedies continue to weigh heavily on our hearts and minds,” Boeing remains “confident in the fundamental safety of the 737 MAX.”

“When the MAX returns to the skies with the software changes to the MCAS function, it will be among the safest airplanes ever to fly,” he said.

[BluSdUp]

Just a few observations.
I have read a few hundred Accident reports and this one is one of the scariest!
Why did this Cpt select AP on, with stickshaker going?
Why did he retract flaps when he knew there was a 50/50 chance the MCAS would go off.
Why did he not set a reasonable pwr setting so as to not accelerate out of control?

Must have been confusing for him?

As for Boeing Max
It will never fly again without serious modifications!
Cpt B

[Water pilot]

What caused the bogus airspeed readings in that scenario?

[Water pilot]

Shortly after Takeoff the Captain who was pilot flying had stick shaker on his side only. Why not transfer control to the first officer at this point or at least a little while later?
As as been pointed out many times, the first officer was fairly inexperienced. So imagine that you are an experienced captain in a plane on takeoff that has a fault which you have never seen before. How quickly would you hand your life over to the greenhorn?

[slacktide]

What caused the bogus airspeed readings in that scenario?

As has been previously discussed - the measured AOA and the measured static pressure is used to compute an AOA-corrected static pressure. Airspeed is then calculated based on the difference between the dynamic pressure measured at the Pitot tube and the AOA-corrected static pressure. If you loose AOA, you loose AOA-corrected static pressure, and anything that uses it in it's calculation becomes unreliable.

[737mgm]

As as been pointed out many times, the first officer was fairly inexperienced. So imagine that you are an experienced captain in a plane on takeoff that has a fault which you have never seen before. How quickly would you hand your life over to the greenhorn?

Being a Captain on the 737 myself, I can tell you, I would hand over control to the green horn immediately in that situation. First of all, because it doesn't make sense for me to fly the airplane if it is very likely that -based on the stick shaker on my side- that my instruments are not reliable. Secondly there is no reason for me to assume that the FO is not capable of flying the airplane on a normal climb out as long as his instruments work fine (let's not start the discussion about low hour pilots, this is normal in most parts of the world and works fine).
Another very important reason is that I am probably going to gain much greater situational awareness if I become the pilot monitoring as I can concentrate on getting a grip on the situation instead of concentrating on flying the airplane.

[MrsDoubtfire]

There are multiple reports suggesting that the deadheading pilot was a Batik Air captain. I haven't seen any that mentioned their nationality, and I note that the poster who suggested it was a Brit hasn't come back with any confirmation of that.
If it helps:
Investigators on Thursday confirmed there was a third, off-duty pilot in the cockpit that evening. That was not mentioned in the preliminary report because they had not interviewed the pilot at that stage as they worked to get the report out fast, Utomo said.
Reuters on Wednesday reported it was a captain at Lion Air’s full-service sister carrier Batik Air who solved the flight control problems, according to two sources.
KNKT said the pilot was qualified on the 737 MAX 8 but did not say what airline he worked for or what role he played in assisting the crew.

[mini]

MRYAN75

https://w ww.avsim.com/forums/topic/546259-44-clean-install-question/ (https://www.avsim.com/forums/topic/546259-44-clean-install-question/)

[Airbubba]

Boeing CEO Dennis Muilenburg addresses the Ethiopian Airlines Flight 302 preliminary report:

Boeing: 737 MAX Update (http://www.boeing.com/commercial/737max/737-max-update.page#/message)

[PaxBritannica]

Additional software problem detected in Boeing 737 Max flight control system, officials say (https://www.washingtonpost.com/world/africa/ethiopia-says-pilots-performed-boeings-recommendations-to-stop-doomed-aircraft-from-diving-urges-review-of-737-max-flight-control-system/2019/04/04/3a125942-4fec-11e9-bdb7-44f948cc0605_story.html?utm_term=.6af86daa985e)


BREAKING: A Boeing-led review of a stall-prevention system suspected in the deadly crashes of two of the company’s new 737 Max jetliners has detected an additional software problem that the FAA has ordered fixed before the planes are cleared to fly again, the company acknowledged Thursday. Boeing called the additional problem, which is unrelated to the stall-prevention system,“relatively minor.” Two officials familiar with the FAA investigation said the issue is nonetheless classified as critical to flight safety. Boeing said it expects to have a solution ready “in the coming weeks.”

[BluSdUp]

Let one thing be clear, we now have facts.
The crew was extremely inexperienced :
The FO had grand total 361hrs of wich 207 the last 3 months.
The captain was 29 years. Had an impressive career!
Had 8122hrs total

July 23 2010 he graduated
FO 737-800 31 jan 2011
Then FO 757/767 777 and 787.
BUT!
And here comes the problem: In 26 Okt 2017 he made Cpt 737-800 , SO less then 1.5 years Command.
There is a total of 1477hrs 738 and 103hrs Max.
Of which a lot is FO time!!
So, a low timer indeed!

This is a warning on so many levels!

[Brosa]

Just a few observations.
I have read a few hundred Accident reports and this one is one of the scariest!
Why did this Cpt select AP on, with stickshaker going?
Why did he retract flaps when he knew there was a 50/50 chance the MCAS would go off.
Why did he not set a reasonable pwr setting so as to not accelerate out of control?

Must have been confusing for him?

As for Boeing Max
It will never fly again without serious modifications!
Cpt BIt appears he tried to engage the left autopilot a fourth time, near the end at 05:43:30.

That's the only explanation to the AP Warning they got there.

It appears they

restored the stab trim cutout switches
made two quick manual ANU inputs
hit the autopilot engange button, hoping the autopilot would solve the situation for them

Instead MCAS kicked in again...

[skwdenyer]

It appears he tried to engage the left autopilot a fourth time, near the end at 05:43:30.

That's the only explanation to the AP Warning they got there.

It appears they

restored the stab trim cutout switches
made two quick manual ANU imputs
hit the autopilot engange button, hoping the autopilot would solve the situation for them

Instead MCAS kicked in again...
The pilot may have recalled that MCAS is disabled when AP is enabled. Perhaps engaging AP seemed like a good idea at that point?

[ironbutt57]

so now the spin begins, "they followed all the rules and it still crashed"...well, not quite, how did the elec/auto stab trim find itself functioning again, or did it turn itself back on?

[AVAT]

Why would he "know" that?

Because Boeing stated MCAS inoperative with flaps extended. If crew extended flaps before reengaging electric stabilizer they could have prevented MCAS from operating. That would have allowed them to easily trim plane electrically. With known MCAS issue retracting flaps at 1000 agl seems too soon to me. At that altitude MCAS engagement more difficult to control.

[atakacs]

I'm still baffled by the speed they reached. Having control issues I'd think one would instinctively try to maintain some reasonable power level. It is almost if they flew TOGA till the ground...

[xyze]

''About four minutes into the flight, the pilots gave up on the manual stabilizer wheel and switched the electric power to the tail back on, then used the thumb switches on the control column to pitch the nose back up.

But just five seconds later, MCAS kicked in again and once more pushed the nose sharply down.

Just 35 seconds later, six minutes after take-off, the plane rolled over before plowing into the earth in a “high energy impact” at a speed of approximately 575 miles per hour."


Checkmate - they were damned if they did (use trim cutout switches) and damned if they didn't. Seems that at the point they shut of the system the AND trim was more than the elevators could overcome and with ever increasing speed in the dive manual trim was not an option. what would you do? try flying inverted (as with the air Alaska accident )? May explain the last minute roll .

What a mess.


​​​​

[Thomas coupling]

So why has the CEO/President of Boeing, tonight, said that he now own responsibility for what happened in that cockpit as a result of the MCAS debacle thrown up by the preliminary report?
He went on to state that Boeing had a 'fix' which will/is being implemented as he speaks.
Do you honestly believe a man of this stature is going to make a carefully crafted statement such as this - without advice.

Boeing is in for some serious pain for years to come, over this. Law suits have already been lined up.

The pilots 'appear' to have been cleared of any malpractice, it seems to me.......

[Snyggapa]

to add a non-pilot (burn him, burn him, but sometimes the ignorant can ask relevant questions..) question to the mix.

Could the reason that on both fatal flights that we saw unexpectedly brief presses to the "trim up" pickle switch be :

because that the pilot would have expected that his pressing of the switch to cause the immediate cacophony of noise of the trim wheels spinning , and maybe they didn't due to aerodynamic loads currently on them due to the lack of authority of the electric motor due to increasing speed so they quickly released the switch ("It's not working") . This would have been then followed up by MCAS throwing in it's "i'm OK to go again jack" logic of another bucketload of nose down. And the same side effect of increasing speed could cause the inability to wind back manually leading to the desperate measure of turning the electronic back on.

I guess the question is - is there a combination of speed and nose down trim that can mean that neither the electronic trim nor the hand wind trim can counteract it. And of course if you re-enable the electronic trim to try, having manual trim try and failed, then MCAS gives you an extra dose of "nose hard down"...

[BluSdUp]

Fair summary.
But it is not criminal to be one level above Your Competency Level.
It IS criminal to make such a **** aircraft and certify and sell it like Boeing does!

[MurphyWasRight]

Prelim report is perplexing.At face value it appears that the crew did cutout
the required switches(only after 2 bursts of MCAS ) with stabilizer at 2.1
units but FO couldnt get the manual trim working.FO has 200 hours
so this is not surprising.
...
...

Agree with many of your points, esp on failure to react to ever increasing speed. The report mentions setting a speed target at some point, perhaps they thought auto throttle was working?

That said do disagree with statements on manual trim, pretty strong evidence that at the speed and trim they were in it would be physically very hard or impossible to trim manually without unloading maneuvers that the did not have the altitude for and/or knowledge/training of.

[Organfreak]

so now the spin begins, "they followed all the rules and it still crashed"...well, not quite, how did the elec/auto stab trim find itself functioning again, or did it turn itself back on?

I don't believe it is spin on the part of The Media. It's just too complicated to process all this information into something understandable and digestible for the public.

[ktcanuck]

When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?

[maxxer]

When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?

Maybe it is just simple plain old grounding also why the other sensor messed up , combined with a very bad qa at boeing trying to push more and more of the planes out of the factory more mistakes made
Both incident planes had issues within months of delivery i would love to see the issues on the other 380 they had delivered

[svhar]

There is something seriously wrong with this design.

[OldnGrounded]

Because Boeing stated MCAS inoperative with flaps extended. If crew extended flaps before reengaging electric stabilizer they could have prevented MCAS from operating. That would have allowed them to easily trim plane electrically. With known MCAS issue retracting flaps at 1000 agl seems too soon to me. At that altitude MCAS engagement more difficult to control.

Totally agree that, if the crew actually understood MCAS, and understood that they had a sensor or sensor signal failure that might cause it to activate, they would not have cleaned up when they did. I honestly don't think that the ET crew, or the vast majority of crews at that time, had anything like the understanding of MCAS that we have now, notwithstanding the Lion crash and the subsequent Boeing and FAA notices. At this point, I can't remember airspeed, etc. at the point they reengaged electric trim. I think it may have been high enough that extending flaps would not have come naturally to mind, but I'll have to look at the data again.

[PaxBritannica]

More about the newly-disclosed 'minor' issue with safety-critical software, from the Washington Post:

Additional software problem detected in Boeing 737 Max flight control system, officials say (https://www.washingtonpost.com/world/africa/ethiopia-says-pilots-performed-boeings-recommendations-to-stop-doomed-aircraft-from-diving-urges-review-of-737-max-flight-control-system/2019/04/04/3a125942-4fec-11e9-bdb7-44f948cc0605_story.html?utm_term=.de0d3f624ec5)

But later Thursday, Boeing confirmed to The Washington Post that it had found a second software problem that the Federal Aviation Administration has ordered fixed — separate from the anti-stall system that is under investigation in the two crashes and is involved in the worldwide grounding of the aircraft.

That additional problem pertains to software affecting flaps and other flight-control hardware and is therefore classified as critical to flight safety, said two officials who spoke on the condition of anonymity to discuss the probe.

The realization of a second software problem explains why the timeline that Boeing projected publicly last week for getting hundreds of the aircraft airborne again has slipped, the officials said.
Boeing initially said it planned to submit fixes for its stall-prevention system to the FAA for review last week. On Monday, an FAA spokesman said the agency instead expected to receive the final package of software “over the coming weeks.”

“Obviously, we ended up at a situation that in hindsight was not supposed to happen,” one of the officials familiar with the investigation told The Washington Post. “Now, you don’t want to be in a situation where there was one contributing factor to an accident, and then three weeks later you find another one.

In a statement, Boeing called the additional problem “relatively minor” but did not offer details of how it affects the plane’s flight-control system. “We are taking steps to thoroughly address this relatively minor issue and already have the solution in work to do that.”

[Loose rivets]

There is something seriously wrong with this design.

In all seriousness, I fear you're right.


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/660x371/max_short_legs_2e52839eed24c7697e18903347393fbc500b11b8.jpg

We always used to say, if it looks right, it'll fly right, etc., but I can't recall ever having such an uneasy feeling about an aircraft's ground clearance. Remember, this is the core of the problem we're discussing.

Fuel, payload and a bit of the thumper will make it even more critical, and that's before a hefty cross wind.

[Intrance]

While it seems fair to say the crew made decisions that we, based on FDR plots and from the comfort of our homes, do not understand... I think it is also important to keep in mind the timespan and chaotic situation in the cockpit at the time. Pretty much in the time that it takes you to read the events as they happened, they had to analyze them and troubleshoot them with alarms going off. We can sit down with a cup of tea or coffee and take 10-15 minutes to think about it and decide what they should have done instead.

Pointing at the crew as the cause of this accident is not completely fair in my opinion. They may have made decisions that turn out to be questionable to the skygods here who have figured out exactly what they should have done. But... if they'd had a well designed plane without vague, ill designed and explained "safety" features one could also argue this accident would not have happened. Follow the problem down to the roots and you will IMHO always end up at Boeing and the design decisions made. The crew on this flight could be considered the last slice of the cliché swiss cheese with the hole lined up.

[jimjim1]

I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.

If the vane had been lost the AoA sensor would become unbalanced about its usual axis of rotation. The internal balance weight** would then cause the axle to be subject to movement when the aircraft transitioned from +g to -g. +g would cause the indication of +AoA. (If I have got this the right way round:)

Looking at the FDR traces it can be seen that this appears to be the case. I have drawn four green vertical lines to indicate the transitions from +g to -g and vice versa. In each case they appear to align with a change in the direction of movement of the sensor in the correct sense. Remember that the data consists of discrete samples and we do not know the sample rate and I am assuming that any small discrepancies are due to errors introduced by the sampling.

I have (rather crudely) chopped out a period in the middle of the chart so that it is a bit narrower so that the scale markings can be easily seen. The horizontal blue line in the "g" section of the chart is coincidentally exactly on 0g.

It therefore seems quite likely that the vane was lost or perhaps damaged soon after take off, perhaps by a bird strike or otherwise. Note however that if the vane had been bent back its balance would be moved in the other direction and its aerodynamic influences would still have been felt so I think that the best conclusion consistent with the data is that the vane was lost.

** A post on pprune regarding this incident showed an exploded view of the AoA sensor and it featured an internal balance weight. Unfortunately I cannot recall where it was now.

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/917x767/zero_g_1b_2c3df0ed32b07bbbd97772aa5eb7f0ff480aa824.png

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/253x291/zero_g_2_b90aa19c3ecac1e2d744499796415bc2b96f13ef.png

[MurphyWasRight]

I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.

If the vane had been lost the AoA sensor would become unbalanced about its usual axis or rotation. The internal balance weight** would then cause the axle to be subject to movement when the aircraft transitioned from +g to -g. +g would cause the indication of +AoA. (If I have got this the right way round:)



Excellent post, the tracking is almost too perfect to be believed at first.

This fits with the AoA heater fault as well, whatever happened caused 2 symptoms that are hard to explain except as physical damage to the sensor since the heater supply is independent of the resolver.

Someone commented that the pilots would have heard a bird strike, given the lack of a full CVR transcript we don't know if anything was audible.
I would suspect that the press conference statement of 'no foreign object' would suggest but not prove none was audible.

Anyone know if a departing vane by itself would cause a heater open?

One other observation is that the flight deck actions seem to be mostly normal until the AP disconnect, almost as though the stick shaker was activated but not working or noticed.
If missing from CVR that probably would have been noted.

[AlexGG]

The flight before LionAir crash, had the same symptoms, but did it have the AoA sensor vane missing or shaft failure?

[paperHanger]

Checkmate - they were damned if they did (use trim cutout switches) and damned if they didn't. Seems that at the point they shut of the system the AND trim was more than the elevators could overcome and with ever increasing speed in the dive manual trim was not an option. what would you do? try flying inverted (as with the air Alaska accident )? May explain the last minute roll .
​​​​

If the column forces were too high, one option (if you hadn't worked out how to engage the manual trim, which these guys hadn't) is to turn the trim back on, correct it with the normal electric trim switches, and then turn it off again before MCAS kicks back in. You asked what I would do, that's what I would do.

[MurphyWasRight]

The flight before LionAir crash, had the same symptoms, but did it have the AoA sensor vane missing or shaft failure?
No but it was active with a constant ~20 degree offset to good sensor and no reported AoA heater fail, which would have led to it's replacement.

Clearly a different issue, simplest explanation would be it was stressed on installation causing the vane to slip on shaft but not known if that is possible.

[Loose rivets]

I just can't get to grips with the concept of a weight. Counter-balance or not. My recollection of checking the vanes was that they stayed put to wherever you shoved them. Having any weight would make them subject to accelerative forces - the weight of the vane itself would be almost totally air-supported and while still affected by g, it would be minuscule compared to the pressure of the air.

[GroundGeek]

I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.
...
Looking at the FDR traces it can be seen that this appears to be the case. I have drawn four green vertical lines to indicate the transitions from +g to -g and vice versa. In each case they appear to align with a change in the direction of movement of the sensor in the correct sense. Remember that the data consists of discrete samples and we do not know the sample rate and I am assuming that any small discrepancies are due to errors introduced by the sampling.


You may be on to something. There are some other data points to consider:

The Left AOA divergence begins at 05:38:45 right as the Vertical Acceleration hits a new positive maximum in the climb.

At 05:41:22, that divergence is reduced just a little bit as the Vertical Acceleration hits a new low.

(I don't have enough posts to upload the images).

[MurphyWasRight]

I just can't get to grips with the concept of a weight. Counter-balance or not. My recollection of checking the vanes was that they stayed put to wherever you shoved them. Having any weight would make them subject to accelerative forces - the weight of the vane itself would be almost totally air-supported and while still affected by g, it would be minuscule compared to the pressure of the air.
If they were not balanced they would not stay put wherever you put them, so either the vane itself was balanced or there would need to be a separate internal counter balance.

[OldnGrounded]

Ethiopian Crash Report Indicates Pilots Followed Boeing’s Emergency Procedures (https://www.nytimes.com/2019/04/04/business/boeing-ethiopian-airlines-crash-report.html)

[. . .]
“The captain was not able to recover the aircraft with the procedures he was trained on and told by Boeing,” said Dennis Tajer, a spokesman for the American Airlines pilots union and a 737 pilot, who read the report.

A bit of additional stuff, but mostly I thought Tajer's comment might be of interest.

[jagema]

What I still find baffling is why both pilots kept pulling back on the controls and barely ever tried to trim the forces off.

I'd imagine if I felt the nose heavy (whether it was MCAS at it or not) I'd trim up, which would alleviate the forces AND stop MCAS. Do the pilots just give up trimming up at some point?

FO reported manual trim didn't work. Later on, we see electric trim up as though they'd re-energized the cutouts as a last resort to get the nose up, but the amount of trim used was negligible? Then MCAS with regained access went ahead and sent the plane into its final dive.

Something that I haven't seen pointed out in this thread is that with the MAX, Boeing changed stab trim cutout philosophy from the NG. Previously you'd have 2 switches, MAIN ELEC and AUTOPILOT channels, self explanatory. Now they are PRI and BU for Primary and Backup whereby the cutout of any of them leaves you with absolutely no electric trim, precisely what you don't need when they implement an automatic system capable of limitless authority over the horizontal stabilizer. One would think previous design would work better with MCAS (disable autopilot trim channel via cutout and keep the ability to use the electric trim).

Just my 2c.

[dingy737]

Question:
Why did Boeing give a FULL description of the NG’s , STS, in their FCOM but ONLY mention MCAS in the abbreviation section of the MAX FCOM.
A. Because a full description would highlight an unwanted flight characteristic forced onto a 1967 airframe, pushing it beyond the MAX. ( criminal)
B. It was considered better to cover up & hide this aerodynamic instability, which could open Pandora’s box and affect market $hare against the NEO. ( criminal)
C. Because their partner in crime the FAA, allowed them. ( criminal)
D. Because they thought their magic software would magically make their shoe horned design failure disappear.
E. All of the above.

The answer is always “ all of the above”.

[ACLS65]

Just curious, what position are the vanes in at rest or when say taxiing?

Are they self zeroing or just in a random position until X speed?

When is their position considered live/legit is it based on airspeed?

This might help with what would happen with a damaged or missing vane. It definitely seems like something that would be accounted for in the design or software, but maybe the addition of MCAS meant unexpected side effects.

[KRUSTY 34]

When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?

WTF indeed. So when a critical data input sensor fails, take the system out of the picture. A system that was vital to the certification of the aircraft.

I think you’ll have to do better than that guys!

[FrequentSLF]

I have not seen any comment regarding the force required to crank manually the trim wheels, those are mechanically connected to the stabilizer therefore to change stabilizer position rotating manually the wheels will require to overcame the load on the stabilizer. It is stupid to think that the forces on stabilizer at those speeds made hard if not impossible to manually change stabilizer position?

[George Glass]

05:38:45 Lefthand stick shaker sounds
05:39:00. Captain attempts to engage LH Autopilot.....................

[KRUSTY 34]

I have not seen any comment regarding the force required to crank manually the trim wheels, those are mechanically connected to the stabilizer therefore to change stabilizer position rotating manually the wheels will require to overcame the load on the stabilizer. It is stupid to think that the forces on stabilizer at those speeds made hard if not impossible to manually change stabilizer position?

Are you sure about that?

[RickNRoll]

WTF indeed. So when a critical data input sensor fails, take the system out of the picture. A system that was vital to the certification of the aircraft.

I think you’ll have to do better than that guys!

When AF447 had the pitot tubes freeze up it dropped the plane back into manual flight mode and alternate law. That is OK if you are trained for it and respond accordingly. There was no simulator training for the MAX if the MCAS failed, even if you had the MAX simulator and Boeing said no one needed any simulator time for the MAX.

[OldnGrounded]

Originally Posted by FrequentSLF https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-163.html#post10439287)I have not seen any comment regarding the force required to crank manually the trim wheels, those are mechanically connected to the stabilizer therefore to change stabilizer position rotating manually the wheels will require to overcame the load on the stabilizer. It is stupid to think that the forces on stabilizer at those speeds made hard if not impossible to manually change stabilizer position?
Are you sure about that?

I'm pretty sure we have some confusion here based upon misunderstanding of a post by a non-native writer of English.

FrequentSLF: There have been a number of posts here, and coverage in the news media, addressing the issue of difficulty operating the manual trim wheels under high loads. It's pretty much agreed that it would be very hard. Your post seems to say that you believe that isn't true, but I don't think that's what you meant. Can you help us?

[FrequentSLF]

Originally Posted by FrequentSLF https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-163.html#post10439287)I have not seen any comment regarding the force required to crank manually the trim wheels, those are mechanically connected to the stabilizer therefore to change stabilizer position rotating manually the wheels will require to overcame the load on the stabilizer. It is stupid to think that the forces on stabilizer at those speeds made hard if not impossible to manually change stabilizer position?


I'm pretty sure we have some confusion here based upon misunderstanding of a post by a non-native writer of English.

FrequentSLF: There have been a number of posts here, and coverage in the news media, addressing the issue of difficulty operating the manual trim wheels under high loads. It's pretty much agreed that it would be very hard. Your post seems to say that you believe that isn't true, but I don't think that's what you meant. Can you help us?
thanks, yes i meant that would be pretty hard to operate the manual,trim under high loads, sorry for the confusion i might have created

[jimjim1]

Gulp. Murphy, you're erm . . . right. Not like me to fuzzle mechanical things. Must stop posting at 02 sparrows. :-(

Here is something more to fuzzle with:)

Miraculously I have found the diagram of an AoA sensor that I remembered seeing. It is from a 747 I think it says and has dual encoders but the ideas are all the same.

https://www.satcom.guru/2019/03/aoa-vane-must-have-failed-boeing-fix.html#more

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/760x601/737_aoa_1_0444d7117846428a876d0afbda57b5d767edc152.png
https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/828x751/737_aoa2_4747d42a5dee56d6f2b5d12df7b046b18d84001f.png

[NSEU]

I have not seen any comment regarding the force required to crank manually the trim wheels,....

The Maintenance Manual has upper and lower limits when there are no airloads:

The torque required to turn the wheel is somewhere between 22 and 62pound-inches (2.5 and 7 newton-meters).

What's the radius of the trim wheel at the handle position?

[YRP]

- After autopilot engagement, there were small amplitude roll oscillations accompanied by lateral acceleration, rudder oscillations and slight heading changes; these oscillations also continued after the autopilot disengaged.

So what is the significance or cause of this item in the report? Is this the result of the AoA sensor issue leading to the autopilot trying to track spurious inputs?

Or is it an unrelated problem, something else wrong?

[NSEU]

I just can't get to grips with the concept of a weight. Counter-balance or not. My recollection of checking the vanes was that they stayed put to wherever you shoved them. Having any weight would make them subject to accelerative forces - the weight of the vane itself would be almost totally air-supported and while still affected by g, it would be minuscule compared to the pressure of the air.

Wouldn't the accelerative forces apply to the vane and the counter balance about the pivot point? So possibly no rotation due to equal forces up/down around the pivot point. I also recall the vanes stay roughly where you put them. They feel fluid-damped.

[FGD135]

Not difficult to rotate the trim wheels under certain circumstances, but impossible.

IMPOSSIBLE.

Picture the stab, angled leading-edge up (trimmed for nose down) - then picture the elevator, attached to the rear of the stab, deflected up (to achieve nose up pitch).

The down force at the rear of the stab (due to the elevator deflection) makes stab movement IMPOSSIBLE above a certain speed.

IMPOSSIBLE.

[fotoguzzi]

[Not a pilot] I note that they had time to communicate and navigate. They set altitudes and headings and communicated their intentions.

Back pressure increases, but that is not graphed, so we will have to wait eighteen months to learn how much they were exerting while they were setting altitudes, headings, etc.

Within five seconds the Captain went from climbing to 32,000 ft to requesting to maintain runway heading. Are the voice and data timestamps compatible?

Summary with points of interest:
takeoff trim 5.6
man/elec trim 4.9 to 5.9 end 5.6
trim via ?? Down from 5.6 to 4.6
auto trim Down from 4.60 to 2.1
man/elec trim Up from 2.1 to 2.4
auto trim Down from 2.4 to 0.4

"At 05:40:28 Manual electric trim in the ANU direction was recorded and the stabilizer reversed moving in the ANU direction and then the trim reached 2.3 units." [One of these ANUs is a typo, no?]

man/elec trim Up 0.4 to 2.3
Cutout

"From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units." [Approximately, 0.1 units / min.]

"At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working. At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough."

A minute into the two and a half minutes of nose down trim, the Captain asks a reasonable question. Within a span of eight seconds the FO answers that trim is not working, oddly suggests trying manual, gets permission, and says, Nope. What exactly was the Captain asking? What did the FO mean by manual? The nose down was gradual, but the FO only worked on whatever "manual" was for eight seconds.

They worked together on something for seven seconds. Did each have a yoke in one hand and a hand wheel in the other? So, fifteen seconds of trying did not defeat the gradual nose down of 0.2 units in two and a half minutes. Or, another interpretation: eight seconds of cranking in the wrong direction didn't seem to be helping. This interpretation is at odds with the stated "gradual" nose down.

trim Down via ?? from 2.3 to 2.1
Normal [Inferred]
manu/elec trim Up from 2.1 to 2.3
auto trim Down 2.3 to 1.0
trim via ?? 1.1 to 0.8

[GlobalNav]

Question:
Why did Boeing give a FULL description of the NG’s , STS, in their FCOM but ONLY mention MCAS in the abbreviation section of the MAX FCOM.
A. Because a full description would highlight an unwanted flight characteristic forced onto a 1967 airframe, pushing it beyond the MAX. ( criminal)
B. It was considered better to cover up & hide this aerodynamic instability, which could open Pandora’s box and affect market $hare against the NEO. ( criminal)
C. Because their partner in crime the FAA, allowed them. ( criminal)
D. Because they thought their magic software would magically make their shoe horned design failure disappear.
E. All of the above.

The answer is always “ all of the above”.


I won't pretend to know the answer, but suggest one other possibility, supported by previous posts. Boeing needed to minimize the training load, as a selling point and commitment to its airline customers. The more that goes into the FCOM that is unique to the Max, the more pilot training must address it. It would seem that when assessing the need for FCOM documentation and training, Boeing (and FAA?) focused on how MCAS works when fully operational (no malfunctions), requiring no pilot response. Its function is to make the handling of the Max in pitch at high AoA so similar to the 737NG (not prevent stall as the media seem to be so hooked on).

I can't imagine that Boeing anticipated the combination of conditions the pilots encountered, or even the repeated MCAS trim actions that accumulated so much. If they had, I am confident they would have done something different with the either the Max design, the FCOM and/or the pilot training. Furthermore, I doubt Boeing anticipated notion that the MCAS would create such an adverse trim condition at higher speeds than normally encountered during the takeoff climb. The higher speeds the accident aircraft reached would have made manually correcting the trim practically impossible for the pilots to achieve (ref the Mentor video).

[runner1021]

Not difficult to rotate the trim wheels under certain circumstances, but impossible.
IMPOSSIBLE.
Picture the stab, angled leading-edge up (trimmed for nose down) - then picture the elevator, attached to the rear of the stab, deflected up (to achieve nose up pitch).that
The down force at the rear of the stab (due to the elevator deflection) makes stab movement IMPOSSIBLE above a certain speed.
IMPOSSIBLE.
Hard to imagine that Boeing didn't include a WARNING in the flight manual advising that above certain speeds manual trimming of the stab would be extremely difficult, if not impossible. Surely they must have encountered this situation during flight testing. In their efforts to hide the MCAS system, they failed to make operators aware of a potentially deadly scenario.

[KRUSTY 34]

thanks, yes i meant that would be pretty hard to operate the manual,trim under high loads, sorry for the confusion i might have created

No worries mate.

Glad we cleared that up.

[jimjim1]

Wouldn't the accelerative forces apply to the vane and the counter balance about the pivot point? So possibly no rotation due to equal forces up/down around the pivot point. I also recall the vanes stay roughly where you put them. They feel fluid-damped.

Yes.

That is what happens when everything is working correctly.

The suggestion is that the observed changes to the AoA signal recorded by the FDR can be explained if it is assumed that the vane has fallen off thereby unbalancing the system and making it susceptible to AoA signal changes with changing Vertical Acceleration (g).

[FGD135]

Hard to imagine that Boeing didn't include a WARNING in the flight manual advising that above certain speeds manual trimming of the stab would be extremely difficult, if not impossible. Surely they must have encountered this situation during flight testing. In their efforts to hide the MCAS system, they failed to make operators aware of a potentially deadly scenario.
Boeing did include exactly that warning in the flight manuals, but deleted it - no doubt, due product liability concerns.

[EDLB]

Seems to me that the final hole in the cheese which dictated the outcome of both flights is, that at higher speeds the trim is jammed (in the direction of nose up) if trimmed nose down. On the Ethopian flight a single nose down command by the MCAS system was enough to seal their fate.

I do not believe that in the time where they clinged on the column for nose up, that they did not try to use the trim switches to unload the back pressure. Elevator trimming you learn from day one in your SEP trainer.

On the Lion air you can see that the PF did constant fight the MCAS AND trim with nose up. As soon as he transferred command there are only few blips of nose up trim to see on the FDR. Same here on the Ethopian flight.

I can’t believe that you only try with two short blips if the landscape becomes larger fast.

[rmac2]

Not difficult to rotate the trim wheels under certain circumstances, but impossible.

IMPOSSIBLE.

Picture the stab, angled leading-edge up (trimmed for nose down) - then picture the elevator, attached to the rear of the stab, deflected up (to achieve nose up pitch).

The down force at the rear of the stab (due to the elevator deflection) makes stab movement IMPOSSIBLE above a certain speed.

IMPOSSIBLE.

The more this subject is examined the more convinced I am that it will be IMPOSSIBLE for the MAX to retain its certification in current form. A quick fix is not going to cut it. No regulator will put their head on a chopping block over it.

[EDLB]

Lots more discussion of manual trim, here: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html

Thanks for the link. So Boeing added at least two more lined up holes in the cheese on the MAX. MCAS much more powerful with 2.5 degree steps. Removal of the Autopilot+MCAS trim cut out switch. You now go direct to manual wheel crank trim only, if you have to cut out the influence from any automation to the trim.
And as a third hole that manual trim wheel is jammed if MCAS did his job on false data from a single sensor.

How did they get this changes through certification?

We know now with 20/20 hindsight that you are in deep trouble if any of your flight automation systems goes rough and you have to cut out trim.

Unfortunately for Boeing I do not see the Max getting back in the air soon.

[Derfred]

At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough.

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

This bit baffles me. Apart from the question of why didn't they get the aircraft back in trim earlier, at this point in time the situation looked just salvageable.

Presumably somewhere in-between these two time-stamps, the stab trim cutout switches were turned back on.

There is no mention of any cockpit discussion taking place from the CVR about turning them back on, so perhaps the Captain just reached down and reactivated the stab trim in desperation.

But why only two short blips of trim? They worked! It is stated the trim moved from 2.1 to 2.3 units. Re-engaging the stab trim was working! So why stop there? If only they'd trimmed it back to around 5 units and then hit the cutout switches again it would have become a manageable problem again. Surely every Max pilot in the world would have briefed themselves on this after Lion Air?

This makes the event seem all the more tragic - so close to solving the problem.

[AfricanSkies]

With respect to Why didn't they use the yoke trim switches effectively?
How much or how little, rather, hand flying experience did they have?
Captain was cadet from the start at ET, operated from the get go in an "AP on at 200' and off at 1000'/500' " SOP environment, and the FO was brand new in just such an environment. Unfamiliar with using trim switches whilst flying .. unfamiliar with hand flying, and having to do it with the stick shaker on max continuous with unreliable airspeeds. Ouch

[Alpine Flyer]

And here comes the problem: In 26 Okt 2017 he made Cpt 737-800 , SO less then 1.5 years Command.
There is a total of 1477hrs 738 and 103hrs Max.
Of which a lot is FO time!!
So, a low timer indeed!

This is a warning on so many levels!

Someone who's flown 8000 hours from 2011 to 2019 can hardly be called a "low-timer", that's more than most people flying in Europe (or even the US would rack up in that time).
Most Commanders flew as FOs before (and the saying goes that those who didn't aren't the best to fly with). Every time there's a new type out there, someone's gonna have to start flying it with zero hours on this type.
Where would you procure "ready-made" commanders with experience and command time on type?

If I got you wrong, please elaborate.

[NiclasB]

EASA document from Feb 2016 allegedly states that electric trim would not work above 230 knots. That might explain why they re-engaged trim (if the indication that they did re-engage trim is correct)...

The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said.

It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke.

https://www.reuters.com/article/us-ethiopia-airplane-regulator-insight/regulators-knew-before-crashes-that-737-max-trim-control-was-confusing-in-some-conditions-document-idUSKCN1RA0DP

[SteinarN]

The Maintenance Manual has upper and lower limits when there are no airloads:

The torque required to turn the wheel is somewhere between 22 and 62pound-inches (2.5 and 7 newton-meters).

What's the radius of the trim wheel at the handle position?

I would say the radius at the handle is between 10 and 15cm. So, that would give a force required to manually crank - WITHOUT any air loads - of 2,5 to 7 kg for a 10 cm radius and 1,67 to 4,67 kg for a 15 cm radius wheel.
I am fairly sure that the radius at the handle is closer to 10 cm than to 15 cm.

[TOGA Tap]

C/L is actually trying to do the troubleshooting - first step is to eliminate autopilot false inputs and then if that does not help - trim motor switches go to cut-out.
Should it be the other way around - trim motor switches to cut out first and then troubleshoot if you want.

Especially after the Lion crash when it was widely known that disengaging the autopilot is a pre-condition for erroneous MCAS trim input.

[SLFstu]

(From an ex hardware/software tech writer) - a comment about software rather than aeronautics. If the timeline story (CVR derived) in comparison to the FDR traces is accurate, here is something weird from the preliminary report's data nobody has commented on.

The third MCAS activation lasting 9 or so seconds starting at 05:40:41 did not alter stab pitch trim because the FO had just cut power to the stabilizer jackscrew motor. The report glibly notes that fact. But why did MCAS even try to activate, as if sitting there fat, dumb and stupid? Exactly what type of coding allows the FCC to attempt to command AND trim when both STAB TRIM switches are already set to CUTOUT? Seems the designed use case never anticipated this sequence.

So it appears that this bolted on chunk of MAX-only software was not monitoring real-time trim motor electrics. Hell no, you cannot do that - a loom change might be required and that would disturb the production line and possibly delay certification! Or better yet, a few lines of more rigorous code were required.

Way up in the thread at least 2 incredulous posters suggested, sarcastically I think, that an intern might have programmed MCAS_1? Makes you wonder.

[Cows getting bigger]

Is it not a bit quaint that a 21st century airline manufacturer still believes that a hand-cranked pulley is the best final solution to a complex systems problem?

Surely the biggest lesson to learn from this event is that the Max should be the last of the 737 family? (No disrespect to the 737 which has been an outstanding aircraft......... but is rather long in the tooth).

Interesting fact. The 737 first flew about 50 years ago and, to date, there have been about 11000 manufactured and 4000 on order. The A320 range first flew 30 years ago and there have been about 8500 built and 6000 on order. Sort of explains why Boeing need to stay in the game.

[Chesty Morgan]

units but FO couldnt get the manual trim working.FO has 200 hours
so this is not surprising
I'm sorry but how many hours does one require before one can turn a wheel?

[HundredPercentPlease]

Turning the wheel was on my type rating course, but featured only one other time in my run of recurrent sims. I would guess that the low houred FO probably had the best recency on a bit of wheel turning.

[QuarterInchSocket]

I'm sorry but how many hours does one require before one can turn a wheel?
more than 200. There’s a diff between competence and proficiency.

theres a video of a pair of idiots on YouTube who run through a runaway trim procedure. The ‘fo’ cranks the wheel like he’s rolling a drum - meanwhile, the extendable handle sat comfortably in its retracted position.

[quentinc]

Is it not a bit quaint that a 21st century airline manufacturer still believes that a hand-cranked pulley is the best final solution to a complex systems problem?



It is unfortunate, that following the lion-air crash, Boeing and the FAA detailed an AD, that to address a faulty MCAS system, required pilots to disable a perfectly working electric trim system. For a pilot that happened to be appropriately trimmed already, high and not too fast, trimming by the trim wheels is going to be OK. For pilots in more challenging situations, then this was going to be a problem. Both Boeing and the FAA would have been aware of this.

Once the faulty MCAS design had been recognized, it should have been fixed or at the very least, a mechanism introduced and documented, so that a pilot could disable the errant MCAS system without taking out other, important, working systems.

[ecto1]

I think there is a reasonable consensus (lest's say 50% chances) that vane shaft broke, keeping AOA vane attached or not, and subsequent alarms and checklists (both useful and useless) threw the crew out of balance. They didn't manage airspeed and lost ability to trim. They reengaged electric trim, was not strong enough either, forgot to CUTOUT, mcas trimmed down, EOF.

I think one of the biggest problems here is the compounding of stupidities. Each stupidity on its own is very much survivable, but all of them is a huge mess.

0. Of course MCAS MUST NOT BE operative with AOA disagree. Minimal software mod.

1. one AOA clearly fails, why not use a switch to transfer everything to the other (manually or automatically). It's a 3 way switch (AOA input L/NORM/R). In the event of stick shaker on, AOA disagree, check if any AOA is stupid (75 is quite stupid), switch to the other side, no more alarms in the cabin, crisis over in 10s tops. minimal wiring loom mod.

Even if you don't do it:

2. We have now perfect data about the influence of AOA over airspeed. 30 knots tops over the full AOA range and airspeed. Probably 15 knots 0 to 15 degrees 0 to 300 knts, probably less than 5 knots in the really tricky areas (slow). Upon AOA disagree, both airspeeds should use a default AOA value (4 deg maybe) instead of throwing UAS. and offer a reading with a possible +-7 knot deviation. But keep autothrottle and autopilot, maybe a caution message (airspeed calculation inacurate, stay 20 knots away from limits). Not a really disturbing unreliable airspeed, just because of a few knots. Minimal software mod.

3. Same with altitude. (altitude calculation inaccurate, stay 1000 feet clear from limits). Minimal software mod.

So that the only remaining alarm would be a stick shaker plus AOA disagree, and you still have autopilots. Much, much easier to handle. But if this is still enough for you to have the aircraft out of trim and miss speed management,

4. If speed goes over 280, message: reduce speed to regain trim ability). Minimal software mod.

My point is: most probably ANY of those mods would have saved the day, and all of them are pretty evident.

To me the problem is simply a huge lack of effort at design level to 1) Imagine 2) prepare for failures.

Chances are that the very same secuence of events, without MCAS final strike, has happened more than once before in other 737 variants (AOA fails, unreliable air data, stick shaker, big confusion, lack of proper aviation and or navigation and possibly all the way to overspeed and uneffective trim). Only without MCAS the aircraft would have been more or less in trim and therefore not nosediving and making the news.

[SteinarN]

Bjørn Fehrm at Leeham:
Bjorn’s Corner: ET302 crash report, the first analysis (https://leehamnews.com/2019/04/05/bjorns-corner-et302-crash-report-the-first-analysis/)

At 7 the aircraft nose is dipping (see Pitch Attitude Disp trace) because PF can no longer hold against the Yoke forces we discussed Wednesday (Ctrl Column Pos L/R). PF decides he needs Electric Trim to stop the aircraft from diving. Cut Off switches are put to Electric Trim active. PF successfully trimmed against the last MCAS attack, he can do it again.

The insufficient trim mystery after re-activation of Electric Trim

After 7 PF commands Electric Trim Nose Up in two short cycles. I asked my selves (as did others) why these short trims? They are fighting to get the nose up to the extent they risk switching in the Electric Trim again. Then why not trim nose up continuously or for at least long cycles once Electric Trim is there? It took me several hours to find an explanation. Here my take:

To understand the blip trims one must have flown fast jets at low altitude. At the speed ET302 is flying, 360kts, it’s hypersensitive to trim. The least trim action and the aircraft reacts violently. Any trimming is in short blips.

As PF holds the nose up with a very high stick force, now for a long time, his sensitivity to release stick with trim is not there (this is what Pilots do when they trim nose up, otherwise the aircraft pitches up fast). He trims therefore in short blips and has difficulty to judge the trim effect he has achieved. His is not flying on feel. He can’t, he is severely out of trim, holding on to the Yoke with a strong pull force.

Anyone who has flown a grossly out of trim aircraft at high speeds knows your feel is compromised. The sensors you have to rely on are your eyes, not your hands.

PF has the horizon glued to read the aircraft. The result is the short nose-up trims we see. The nose goes up and the stick force needed is reduced. His judgment is; this is enough for now. Any MCAS attack I now trim against, then I correct my trim if I need to.

But the aggressive MCAS, trimming with a speed 50% higher than the pilot and for a full nine seconds, kicks in at 8 with a force they didn’t expect. Speed is now at 375kts and MCAS was never designed to trim at these Speed/Altitude combinations. Dynamic pressures, which governs how the aircraft reacts to control surface movements, is now almost double it was when last MCAS trimmed (Dynamic pressure increases with Speed squared).

The Pilots are thrown off their seats, hitting the cockpit roof. Look at the Pitch Attitude Disp trace and the Accel Vert trace. These are on the way to Zero G and we can see how PF loses stick pull in the process (Ctrl Column Pos L). He can barely hold on to the Yoke, let alone pull or trim against.

His reduced pull increases the pitch down further, which increases the speed even more. At 05.45.30 the Pilots have hit the seats again (Accel Vert trace and Ctrl Columns force trace) and can start pulling in a desperate last move. But it’s too late. Despite them creating the largest Control Column movement ever, pitch down attitude is only marginally affected.

We have Control Column displacement this time, JT610 was Force. If the elevator reacts to these displacements, at the Dynamic Pressure we have, we should have seen the diving stop. The lack of reaction to the large Control Column displacement of two Pilots pulling makes me think we now have blowback. This is not a design fault, we are well beyond Vmo. But it explains the rapid dive, unhindered by the Pilots’ actions.

It’s easy to say “Why didn’t they trim then?”. Because they are going down at 20 degrees nose down (which is a lot, a normal landing approach is 3°) and at 400kts. Then you just pull for all you have. And the aircraft is not reacting to the largest Control Column displacement since takeoff. This makes them pull even harder, the aircraft is unresponsive and they are fighting for theirs and all the passenger lives.

A final reflection: Once again we have been given no elevator trace. Why? It’s there, why can’t we see it. It would have given us a better understanding of what’s happening in the last part of the flight.

[HundredPercentPlease]

Reading the report fills me with horror. Two of them, fighting to find the solution, with decreasing success as they are overloaded.

There is too much conflicting information to guarantee that a crew resolves the problem.

The aircraft is stalling, yet the pitch/thrust/speed appear correct.
As thrust is applied, the stall condition gets "worse", as you have to apply more back pressure to maintain the pitch.
The two primary airspeeds are different.
The stick shaker is activated, yet the airspeed is high.
The airspeed is high, yet the aircraft pitches down as it speeds up, not up.
As you speed up, you normally trim down. But it feels like I should trim up.


Stuff that I have never did in the 737 sim:

Found myself with a stab trim that overpowered elevator at max deflection.
The yo-yo or rollercoaster manual trim exercise.


Finally, it needs to be remembered that the deafening stick shaker reduces cognitive ability, and is shaking both control columns, and may therefore be prominent as the "primary" problem.

[ecto1]

I've read and imagined the "hipersensitive trim" theory for short blips before the final dive.

I think it is plausible. I think not enough electric trim force is plausible also.

All we need to know the truth is a zoomed in view of the trim units around that time.

I guess it doesn't make any difference, though.

[threemiles]

I think there is a reasonable consensus (lest's say 50% chances) that vane shaft broke, keeping AOA vane attached or not, and subsequent alarms and checklists (both useful and useless) threw the crew out of balance. They didn't manage airspeed and lost ability to trim. They reengaged electric trim, was not strong enough either, forgot to CUTOUT, mcas trimmed down, EOF.

I think one of the biggest problems here is the compounding of stupidities. Each stupidity on its own is very much survivable, but all of them is a huge mess.

0. Of course MCAS MUST NOT BE operative with AOA disagree. Minimal software mod.

1. one AOA clearly fails, why not use a switch to transfer everything to the other (manually or automatically). It's a 3 way switch (AOA input L/NORM/R). In the event of stick shaker on, AOA disagree, check if any AOA is stupid (75 is quite stupid), switch to the other side, no more alarms in the cabin, crisis over in 10s tops. minimal wiring loom mod.

Even if you don't do it:

2. We have now perfect data about the influence of AOA over airspeed. 30 knots tops over the full AOA range and airspeed. Probably 15 knots 0 to 15 degrees 0 to 300 knts, probably less than 5 knots in the really tricky areas (slow). Upon AOA disagree, both airspeeds should use a default AOA value (4 deg maybe) instead of throwing UAS. and offer a reading with a possible +-7 knot deviation. But keep autothrottle and autopilot, maybe a caution message (airspeed calculation inacurate, stay 20 knots away from limits). Not a really disturbing unreliable airspeed, just because of a few knots. Minimal software mod.

3. Same with altitude. (altitude calculation inaccurate, stay 1000 feet clear from limits). Minimal software mod.

So that the only remaining alarm would be a stick shaker plus AOA disagree, and you still have autopilots. Much, much easier to handle. But if this is still enough for you to have the aircraft out of trim and miss speed management,

4. If speed goes over 280, message: reduce speed to regain trim ability). Minimal software mod.

My point is: most probably ANY of those mods would have saved the day, and all of them are pretty evident.

To me the problem is simply a huge lack of effort at design level to 1) Imagine 2) prepare for failures.

Chances are that the very same secuence of events, without MCAS final strike, has happened more than once before in other 737 variants (AOA fails, unreliable air data, stick shaker, big confusion, lack of proper aviation and or navigation and possibly all the way to overspeed and uneffective trim). Only without MCAS the aircraft would have been more or less in trim and therefore not nosediving and making the news.

Fully agreed. There could be much more on the list (e.g. same sound for cabin pressure altitude warning and t/o config, which killed 121 people)
Sensors will continue to break in future. Bits will continue to flip. Wires will continue to fail. Connectors will continue to corrode. People will continue to be humans.
It is the damned duty of every OEM lead design and certification engineer and every FAA, EASA and have you representative to think of possible failure modes and ask for solid and sane design.
It is not the duty to think about saving money by keeping a 40 years old certification in place. Which lets the boss, the boss of the boss and the boss of the boss of the boss smile.
It seems basic airmen and engineering knowledge is lost in the industry and substituted by plain software commodity engineering skills, that are typical for games, desktop software and iOS apps. These do not run airplanes.
Not the only thing that is lost in this world.

[His dudeness]

What moment does one get from deploying speed brakes ? NU or ND ?

(One of the aircraft I have flown had a partial speed brake deployment with elevator stuck in a ND setting in the C/L....)

[ManaAdaSystem]

I'm sorry but how many hours does one require before one can turn a wheel?

Chesty, we discussed this earlier, and you said you have flown the 737 with manual trim and it was no problem.
I’ve only tried this a few times, but found the manual trim to be pretty hard. A friend of mine got «Stab out of trim» enroute and diverted because he thought he had a jammed stab. This after he tried manual trim and found it so hard he concluded he had a jammed stab.
It now looks like manual trim is nearly impossible at higher speeds.
It puzzels me a bit that the stab can be trimmed so far it will bring the aircraft out of the pilot’s control. And then you can’t manually bring it back under control because the trim forces are too high.

How Boeing will solve this is beyond me. MCAS is a killer and if they restrict it it will affect the certification. In order to get rid of MCAS they have to redesign the aircraft.
IMHO the MAX will stay on ground for a long time.

[DaveReidUK]

EASA document from Feb 2016 allegedly states that electric trim would not work above 230 knots. That might explain why they re-engaged trim (if the indication that they did re-engage trim is correct)...

Might be a good time to reiterate the relevant part from the document:

"The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches."

[AfricanSkies]

Is it not a bit quaint that a 21st century airline manufacturer still believes that a hand-cranked pulley is the best final solution to a complex systems problem?.

Are the complex systems not the problem? Do you really need an electric trimmer to move a stabilizer in a 15 degree range? Do you really need a electric motor to close your car boot?

[ivor toolbox]

Fully agreed. There could be much more on the list (e.g. same sound for cabin pressure altitude warning and t/o config, which killed 121 people)
Sensors will continue to break in future. Bits will continue to flip. Wires will continue to fail. Connectors will continue to corrode. People will continue to be humans.
It is the damned duty of every OEM lead design and certification engineer and every FAA, EASA and have you representative to think of possible failure modes and ask for solid and sane design.
It is not the duty to think about saving money by keeping a 40 years old certification in place. Which lets the boss, the boss of the boss and the boss of the boss of the boss smile.
It seems basic airmen and engineering knowledge is lost in the industry and substituted by plain software commodity engineering skills, that are typical for games, desktop software and iOS apps. These do not run airplanes.
Not the only thing that is lost in this world.
Spot on post. Sums it all up succinctly

Ttfn

[SRMman]

I have read all the comments made on this forum concerning the Lion Air, and now the Ethiopian 737 Max crashes. It seems to me that the majority of comments focus on what could, or should, have been done by the crews involved. The Ethiopian pilots especially, whilst doubtless fully knowledgeable of the previous incident, were faced with the extraordinary and bizarre coincidence of a 2nd failure on the Max of the AOA vane, supplying erroneous data to the MCAS. This happened at the worst possible time where they were at their busiest, and with no height or time to play with. Whilst we all now have the benefit of knowledge, hindsight and time to analyse the situation . . . they didn’t. Of course they did their best, attempting to follow the memory items and new procedures as this crisis developed, but who, hand on heart, can lay any blame on them for not getting it exactly right? They may have been an average crew, with maybe less than average experience, but they were a trained and qualified crew and presumably signed off to fly the Max.

Even Boeing chief executive Dennis Muilenburg has publically stated: “As pilots have told us, erroneous activation of the MCAS function can add to what is already a high workload environment”. The Max, and in fact all commercial aircraft, simply have to be designed, built and certified to be within the flying ability of all pilots deemed qualified by their authorities to fly the Max. Two crews have shown sadly that the Max was outside their ability, particularly the 302 crew even with prior knowledge of the potential failure, and the industry therefore has to assume that this is the norm, not the exception.

[fizz57]

...

My point is: most probably ANY of those mods would have saved the day, and all of them are pretty evident.

...


Disagree. May address the issues that caused the latest accident, but contradicts the entire concept of the 737's instrumentation system, and will invalidate the safety calculations on which certification is based, as well as possibly opening the doors to other failure modes.

The 737 instrumentation is a dual-redundant system, comprising two completely separate systems such that in the event of a single failure one completely operational system is still available. With the aid of appropriate disagreement monitors, standby instruments and checklists, pilots are trained to detect and diagnose such failures and take appropriate action.

This system was state-of-the-art at the time the 737 was launched and is still in use in smaller, non-FBW airliners. It is totally adequate for its stated task, that is to provide information to a well-trained human crew. It should never have been allowed to provide inputs to a system that will automatically drive flight control surfaces.

[DaveReidUK]

Do you really need an electric trimmer to move a stabilizer in a 15 degree range?

Of course not, you simply turn the manual trim wheel.

About 250 revolutions from full ANU to full AND ...

[onsoutherntip]

Seems to me that the final hole in the cheese which dictated the outcome of both flights is, that at higher speeds the trim is jammed (in the direction of nose up) if trimmed nose down. On the Ethopian flight a single nose down command by the MCAS system was enough to seal their fate.

I do not believe that in the time where they clung on the column for nose up, that they did not try to use the trim switches to unload the back pressure. Elevator trimming you learn from day one in your SEP trainer.

On the Lion air you can see that the PF did constant fight the MCAS AND trim with nose up. As soon as he transferred command there are only few blips of nose up trim to see on the FDR. Same here on the Ethopian flight.

I can’t believe that you only try with two short blips if the landscape becomes larger fast.
Not a pilot, my field is electronic engineering. The FDR traces on both Lion Air & Ethiopia show short electrical trim ANU blips when longer trim activation would be expected.

I am curious as to where the FDR data for the stabilizer trim is read from - yoke trim switches or the motor drive electronics? Most high power motor drives have feedback that detects if the power demand is exceeded. If this happens, power to the motor is disabled, in order to protect it. Therefore a situation could arise where even though the pilot is activating ANU trim, it results in just a short ANU motor movement, before the motor power limit is exceeded. So if the FDR records from the drive electronics, the trim ANU 'request' by the pilot, will be seen as just a 'blip' although the trim switch is still activated,

This would account for AND trim running the full period as motor power demand is not exceeded (low aerodynamic load), but ANU trim is fighting against high aerodynamic load, which activates the motor protection. Thoughts?

[ecto1]

Disagree. May address the issues that caused the latest accident, but contradicts the entire concept of the 737's instrumentation system, and will invalidate the safety calculations on which certification is based, as well as possibly opening the doors to other failure modes.

.

I read your post and reflected about it. I see your point. We musn't have knee jerk reactions. But open your mind for a second:

Disregarding AOA for speed calculation (for example) will contradict which concept? Which safety calculation will it invalidate? Which other failure mode can you imagine if we do that? (note that the difference is about 5knt at takeoff speed between correct and bat**** crazy AOA, so the error between fixed assumed AOA value and real AOA may be as low as 2knts).

Advicing the crew of inability to trim at high speed will harm how? We think it is worthy to throw them an alarm about the AOA heater not heating, and keep the stick shaker all along, but we believe it would be too distracting to remind them that he will loose trim ability due to high speeds?

Those are the official excuses. I don't buy them. It was honestly a very reasonable system when safety was important but not as important as today, data buses did not exist and pilots came from the military.

Not any more.We have better tools now and even if we break concepts and need to recertify things, it is simply too easy today to fool a 737 computer.

[Freehills]

https://leehamnews.com/2019/04/05/bjorns-corner-et302-crash-report-the-first-analysis/#more-29839

Thinks that the force when electric tirm/ MCAS was switched back on probably bounced the crew out of their seats, causing them to lose/ weaken grip on the controls

[PJ2]

Of course not, you simply turn the manual trim wheel.

About 250 revolutions from full ANU to full AND ...

Is that what it is?...250 turns of the trim wheel from stop to stop? That info wasn't available anywhere in the AMM, the FCOM or even the FCTM. Like some, I wondered how many turns it was, so thanks.

The full range (from the AMM) of the stab is 17°. This range is not available to all controls, (electric, manual, wheel), as noted in this thread, but let us assume the available range is -1 AND to 16 ANU using the big wheel.

Now we can calculate turns and degrees of trim, (remembering the comments on the difficulty of turning the wheel under some circumstances).

250 turns / 17 degrees = 14.7 turns per degree, or,
17 degrees / 250 turns = 0.07° per turn.

If the thinking and the math is correct, we can see that there is a lot of work and time to manually modify the stab trim using the wheel. In fact, those who fly/flew the B727/B737 will recall just how fast the wheel goes around when electrically-trimming in normal flight, flaps-up; the white mark on the wheel is almost a blur...

PJ2