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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 3rd Apr 2019, 20:02
  #3001 (permalink)  
 
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CT:
It is a shame that the Mentour Pilot video has been disabled, I managed to watch it and it showed what happens when the IAS disagree and runaway trim checklist are followed in the (presumed) ET MCAS case. Overall fairly similar to your scenario.

The pilots in the sim calmly and methodically followed the checklists in what I would call a demonstrating or teaching mode, certainly not stressed or surprised.
Don't believer they even had a stick shaker going.

Even so they ended up in a state where manual trim inputs were physically close to impossible after electrical trim cutout due to the air speed.
My takeaway was that unless the pilot trimmed close to neutral before electrical tim cutout there was no way to manually trim.
Note that this should be possible (as shown in both lion air flights) since pilot trim cancels (for 5 seconds) MCAS trim actions.

One other observation from the video is that from the jumpseat camera point of view the trim wheel action is very obvious. I suspect it might be much less so for stressed pilots trying to make sense of the situation. This may be why the jumpseat pilot was able to save the penultimate Lion air flight.

Gums: If you are referring my post re making a full FBW 737, that was meant as a thought experiment on automation more than a practical suggestion, if for no other reason that it 737 FBW would certainly require more than a slideware conversion course.
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Old 3rd Apr 2019, 20:24
  #3002 (permalink)  
 
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Given the known the MCAS problem, and Boeing air directive, I would use flaps 1 for as long as possible to prevent MCAS from engaging if pilot sees IAS disagree. Once atleast 2500 agl retract flaps to see if MCAS engages. If yes, extend flaps immediately and return to airport. WSJ reported MCAS engaged in this flight at 450 agl which suggests plane already in clean configuration. Interesting to see what fdr shows for flap settings and retraction.
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Old 3rd Apr 2019, 20:47
  #3003 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
CT:
It is a shame that the Mentour Pilot video has been disabled, I managed to watch it and it showed what happens when the IAS disagree and runaway trim checklist are followed in the (presumed) ET MCAS case. Overall fairly similar to your scenario.

The pilots in the sim calmly and methodically followed the checklists in what I would call a demonstrating or teaching mode, certainly not stressed or surprised. Don't believer they even had a stick shaker going.

Even so they ended up in a state where manual trim inputs were physically close to impossible after electrical trim cutout due to the air speed.
My takeaway was that unless the pilot trimmed close to neutral before electrical tim cutout there was no way to manually trim.
Note that this should be possible (as shown in both lion air flights) since pilot trim cancels (for 5 seconds) MCAS trim actions.

One other observation from the video is that from the jumpseat camera point of view the trim wheel action is very obvious. I suspect it might be much less so for stressed pilots trying to make sense of the situation. This may be why the jumpseat pilot was able to save the penultimate Lion air flight.

Gums: If you are referring my post re making a full FBW 737, that was meant as a thought experiment on automation more than a practical suggestion, if for no other reason that it 737 FBW would certainly require more than a slideware conversion course.
I never got to see Mentour Pilot's latest video, but a previous one from 2 weeks ago on runaway stabiliser trim is still available. It shows a much 'tamer' scenario, with speed < 250kts, and positive pitch, but indicates that manual trim can be difficult. Worth watching a minute of footage (start at 18m35s), for those who can't visualise what actually happens in such a scenario:
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Old 3rd Apr 2019, 21:03
  #3004 (permalink)  
 
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Originally Posted by RatherBeFlying View Post
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.


...... The software will have to get even fancier to satisfy 10E-9 reliability.

.
There is no such requirement applicable, besides you probably wouldn't believe it anyhow.

It's really a balance between the pilot and the design. If the pilot can't be relied on, then the design is not good enough
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Old 3rd Apr 2019, 21:20
  #3005 (permalink)  
 
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Originally Posted by jagema View Post
Control column cutout switches do not inhibit MCAS trimming (which makes sense if you think why the system was designed in the first place, but also no sense to a 737 pilot that doesn't expect the trimwheel to move AND when pulling up, pre-MCAS discovery).
The control column trim switches are not the "cutout" switches. I'm inclined to believe [Late edit: it turns out, wrongly] that central cutout switches have authority over MCAS, but control column trim switches do not, contrary to
Originally Posted by ecto1 View Post
Column switch vs mcas, column wins.
But again, do we have an authoritative answer, based on some circuit diagram, manual, first-hand knowledge?

Last edited by fgrieu; 4th Apr 2019 at 06:04.
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Old 3rd Apr 2019, 21:32
  #3006 (permalink)  
 
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An update from ABC News:

Sensor damaged by a foreign object on Ethiopian Airlines 737 MAX triggered fatal crash: Sources

The preliminary crash findings are expected to be released on Thursday.

By David Kerley and Jeffrey Cook Apr 3, 2019 5:04 PM

The Ethiopian Airlines Boeing 737 MAX, which crashed in March and killed 157 people, suffered a damaged angle-of-attack sensor upon takeoff from a bird or foreign object, triggering erroneous data and the activation an anti-stall system -- called MCAS -- sending the pitch of the plane downward and ultimately crashing into the ground, two aviation sources familiar with the investigation told ABC News.

As the jet was nose diving, the Boeing 737 MAX pilots did not try to electronically pull the nose of the plane up before following Boeing's emergency procedures of disengaging power to the horizontal stabilizer on the rear of the aircraft, according to the sources.

One source told ABC News that they manually attempted to bring the nose of the plane back up by using the trim wheel. Soon after, the pilots restored power to the horizontal stabilizer.

With power restored, the MCAS was re-engaged, the sources said, and the pilots were unable to regain control and the plane crashed.

The preliminary findings in the crash investigation are expected to be released by transportation officials in Ethiopia on Thursday morning.
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Old 3rd Apr 2019, 21:42
  #3007 (permalink)  
 
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I believe the basic problem is lift from the engine cowlings at high AoA in a turn with flaps up this I think pulls the CL in front of the C of G so the nose keeps pitching up.

I seem to remember Learjet had a problem with a T tail model that at high angel of attack could blank the tail so entering a deep stall, so required a stick shaker.

So to over come the problem they fitted under the rear fuselage what they called Delta Fins these at high AoA produced lift and pushed the nose down so no stick shaker or any other aid required. May be the fix the Max needs. Just a thought.
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Old 3rd Apr 2019, 22:09
  #3008 (permalink)  
 
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Originally Posted by fgrieu View Post
The control column trim switches are not the "cutout" switches. I'm inclined to believe that central cutout switches have authority over MCAS, but control column trim switches do not, contrary to

But again, do we have an authoritative answer, based on some circuit diagram, manual, first-hand knowledge?
Yes, we do. We are talking 3 sets of switches in series here: 1) thumb (normally open), 2) control column "extreme deflection" (normally closed) and 3)pedestal cutouts (normally closed).

What I meant is that if you electrically trim nose up with your thumb, MCAS stops trimming nose down right then, so you win. Plus, there is a 5 second delay between you releasing your thumb and MCAS trying to nosedive again. That's your timeframe to kill it with pedestal cutout switches.

I don't know where I learnt it first, but that fact is "authoritatively" shown on FDR from both lionair flights, to begin with. Also, reductio ad adsurdum: Otherwise no way the scenario is survivable for any amount of time with cutout switches to "normal". 60 seconds after first MCAS activation you are diving vertical, regardless of elevator position.

In fact, if you could hear it trim, (you cant because stick shaker) it would be quite trivial to discover that, as soon as you trim with your thumb switch, it stops. I'd bet the accidents would have not happened.

The only ongoing debate now* is (and as far as I know, there is no consensus yet but there will be soon) if you were slow with your thumb and got to a point in which you need a lot of pull to remain level, your control column "extreme deflection" trim cutout switches may inhibit your trim inputs and then (possibly) MCAS wins by lack of opposition. (because at that moment you will not recall yet another system working silently against you and release control column to regain the ability to trim).

Which, if comfirmed, is a frightening situation, because it only takes one panicking pilot pulling too hard to prevent both from trimming up and wining the fight against MCAS. Thats a good theory to explain what happened in the last seconds of the first crash.

*Edit: I'm quite convinced after examining again the schematics, that MCAS vs thumb trim switch battle is not affected by control column deflection switches.

Last edited by ecto1; 3rd Apr 2019 at 22:57.
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Old 3rd Apr 2019, 22:10
  #3009 (permalink)  
 
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Originally Posted by horizon flyer View Post
I believe the basic problem is lift from the engine cowlings at high AoA in a turn with flaps up this I think pulls the CL in front of the C of G so the nose keeps pitching up.

I seem to remember Learjet had a problem with a T tail model that at high angel of attack could blank the tail so entering a deep stall, so required a stick shaker.

So to over come the problem they fitted under the rear fuselage what they called Delta Fins these at high AoA produced lift and pushed the nose down so no stick shaker or any other aid required. May be the fix the Max needs. Just a thought.
Yes, that is also my impression.
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Old 3rd Apr 2019, 22:18
  #3010 (permalink)  
 
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Originally Posted by fgrieu View Post
I'm inclined to believe that central cutout switches have authority over MCAS, but control column trim switches do not
This is correct except that it's the control column cutout switches that do not have authority.

Originally Posted by ecto1
The only ongoing debate now is (and as far as I know, there is no consensus yet but there will be soon) if you were slow with your thumb and got to a point in which you need a lot of pull to remain level, your control column "extreme deflection" trim cutout switches may inhibit your trim inputs and then (possibly) MCAS wins by lack of opposition.
There is a forward and an aft control column cutout switch. Enough movement of the control column in either direction opens the circuit, creating a discontinuity and disallowing trim in the direction of column deflection (ie. You cannot trim up while pushing down) unless STAB TRIM OVRD switch is in OVRD. I guess you refer to the cutout switches installed in the column when speaking of "extreme deflection trim cutout switches". Schematic for clarity.

B737 Trim

edit: this is the schematic for the NG. The MAX introduced changed to the pedestal cutout switches, renamed PRI and B/U respectively.

Last edited by jagema; 3rd Apr 2019 at 22:48.
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Old 3rd Apr 2019, 22:20
  #3011 (permalink)  
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THS Airloads Mitigation Procedure

I am aware of the procedure but was never taught it on type training on the 737 conducted under FAA.

Simply, there is no manner by which such a heroic technique complied with the Type Certification requirements under Part 25. The rules do not permit the application of exceptional force, technique or skill, and any need to have 2 pilots apply force at the same time is grossly unacceptable, breaching the FAA’s own prohibition on control inputs by more than one pilot.

As the aircraft often flies with a seat unoccupied when nature calls, it would be incompatible to have a flight condition where both pilots must be able to have inputs onto controls at all times in flight.

The 737 is grandfathered to revisions of the rules in the 60’s but the basic rationales have not changed that much.

If the the plane has any condition that may necessitate rock n’ roll as a recovery, using both pilots, then I would imagine that it needs a type review and rectification of the underlying design deficiency.

How on on earth would it be acceptable to have a passenger transport with a known need for exceptional training outside of the training for CPL or ATPL/ATP, requiring both pilots simultaneous inputs on the flight controls, necessitating a ballistic flight path, where the pax are not required to be restrained at ALL times, I.e., whenever the flaps are retracted....

it it is not acceptable under the rules, and the plane has been known to have this characteristic for how long?

I’ve flown flown the 737 out way beyond VMO MMO Under controlled conditions, and while careful about tuck considerations, was not aware of a “jack stall” potential on the aircraft. That is troubling.

25.143 provides the general handling qualities requirements, including statements on skill, limit of force on elevator to 70 lbs short term, 2?hands, and 10 lbs long term.

25.255 covers mid trimmed cases, and potentially lets the OEM off the hook, but places the regulations in question. Recovery with a force of 120 lbs should be shown for a maximum runaway trim of 3 seconds of trim motion for the conditions of flight. MCAS action exceeds the 3 seconds, but the OEM is only required to show acceptable characteristics with 3 seconds of trim motion from in trim conditions, which is clearly inadequate to protect the aircraft operation.

Last edited by fdr; 3rd Apr 2019 at 23:12.
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Old 3rd Apr 2019, 22:28
  #3012 (permalink)  
 
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Originally Posted by RatherBeFlying View Post
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.
That's Design Assurance Level A. Does anyone here know what the DAL for this software actually is? As a previous poster said, it's no easy fix to make it so, if it's not DAL A.
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Old 3rd Apr 2019, 22:33
  #3013 (permalink)  
 
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Originally Posted by jagema View Post
This is correct
No it isn't. AFAIK

MCAS will operate regardless of the control column cutout switches.
It will not operate against the control column TRIM switches, or if the stab trim switches are cutout.
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Old 3rd Apr 2019, 22:38
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Originally Posted by Albino View Post
No it isn't. AFAIK

MCAS will operate regardless of the control column cutout switches.
It will not operate against the control column TRIM switches, or if the stab trim switches are cutout.
Precisely what I meant by "correct." We are saying the same thing. MCAS obeys all switches except for the aft and fwd motion of the column, essentially.

Edit: I thought I read control column cutout in my quoted post above. Corrected

Last edited by jagema; 3rd Apr 2019 at 22:49.
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Old 3rd Apr 2019, 22:45
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Originally Posted by Albino View Post
No it isn't. AFAIK

MCAS will operate regardless of the control column cutout switches.
It will not operate against the control column TRIM switches, or if the stab trim switches are cutout.
This is my understanding also, based on the schematic posted before:

737MAX Stab Trim architecture


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.
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Old 3rd Apr 2019, 23:24
  #3016 (permalink)  
 
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Mentour Pilot latest video

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...
>
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Old 3rd Apr 2019, 23:29
  #3017 (permalink)  
 
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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...
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Old 4th Apr 2019, 00:17
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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)...
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Old 4th Apr 2019, 00:55
  #3019 (permalink)  
 
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Ethiopian pilots followed Boeing procedure

(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...
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Old 4th Apr 2019, 01:11
  #3020 (permalink)  
 
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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.
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