Indonesian aircraft missing off Jakarta

[Vessbot]

I wonder if the omission of this system from the manual, along with other manual dumbing-downs that we've complained about, are an overreaction to the notion that "we're not learning how to build the plane" about requirements to learn useless minutiae. The pendulum has swung too far.

[gums]

AHA!
They don't need to install a caution light, huh?
We can tell when the damned thing is working because the trim wheel moves all by itself!! Well, matbe, 'cause they say it is not part of the STS.
Gums wonders.. and this is getting scary.

[Lonewolf_50]

Strangely my original posts have been deleted... I cannot imagine what the PPrune police objected to!
I was making a general point, AoA is fundamental to everything we do as pilots, We must insist that - especially in aircraft where it is measured and used to influence systems - it is displayed to pilots. We must also fundamentally change our philosophy whereby there is an obsession with speed alone from day 1 of flight training, Lift (=G) varies with both speed AND AoA.
When, as it seems in this tragic case, an important sensor gives an erroneous value, and assuming that value is displayed (which it was not apparently) a trained pilot will quickly separate truth from lies. There was some discussion on this after AF 447, in re getting an AoA gage in the cockpit. (As I recall that system, one could get to a page somewhere in the displays that would show AoA, but there wasn't a gage in the cockpit in that model, nor in a number of (even most) other models ...) The industry apparently is of mixed opinion on that, for all that I agree with your core point.
I find that attitude to the AoA gage a little perverse: the various flight computers use and need AoA, there are AoA sensors, multiple, and yet somehow the pilots aren't deemed to need this information. (Nor is some space on the pilot displays allotted to it in many airliners, as I understand it).

[Concours77]

Lonewolf 50

I don’t recall the drill exactly, but AoA can be calculated on the A330 using fpv (the “bird”). “Flight Path Vector” in the FD. ACARS recorded a crew attempt to disable the Computer PrimSEC 1. This would have allowed the FPV to appear, then some mental math subtracting bird from AH.

This explains the crew fiddling with the FDs, turning them on, when they were prohibited by the book.

It led to some harsh criticism of the crew, “trying to fly the FD”. They weren’t looking for airdata, but AoA. Wouldn’t we all?

as I recall

[Vessbot]

AOA is the difference between bird vs. nose symbol, not bird vs. horizon

(and neither of those is counting the effect of wind)

[gums]

Salute Concours!

Yep, if you have a fixed reference in the display, HUD or FD, the AoA is the difference between the "bird" and that reference WRT the longitudianl axis of the jet. Sure, the wing chordline versus the fuselage axis must be allowed for, but on some planes this is a fairly easy thing to do.

I flew HUD planes with great velocity vector displays and those were extremely useful for establishing glide path or descent angle or..... The AoA display was only constantly presented on the HUD or "indexers" or even a steam gauge in two of the jets ( about 1400 hours). The biggest use of the AoA, besides knowing you were approaching stall, was approach to landing. The approach AoA took into account your weight!!! So a quick, rough calculation we used in other planes was good, but that AoA was the real deal. E.g. "approach speed is 175 kias plus 5 knots for every 1,000 pounds above 3,000 pounds", I leave it to the reader to figure out what that plane was, heh heh. I was USAF, but the USN used AoA a whole lot more for landing on a carrier deck, and USAF was more "airspeed" on approach than AoA.

All that being said, use of AoA by the crew for most flight regimes requires some training and rationale. My vote would be what I used it for - approach speed and stall warning. Oh well, enuf philosophy.

Gums...

[hec7or]

Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.

So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.

When they design these safety enhancements, why don't they tell us?

The inhibited "stall" aural warning at high AoA during high AoA on AF447 is another example of design engineers assuming pilots will behave like monkeys, where the warning was inhibited at high AoA and activated when the crew reduced the AoA. Also the Colgan Air flight 3407 Dash-8 Q400 at Buffalo would in it's final stages have generated GPWS aural alerts concerning the glideslope deviation together with stall/low airspeed warnings, one warning requiring a pitch up and the other requiring a pitch down, which could overload a tired flightcrew.

Clearly the safety systems require a rethink

[rideforever]

Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.
Should they not also disconnect STS and therefore MCAS ? during such a situation ?

[Concours77]

Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.

So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.

When they design these safety enhancements, why don't they tell us?

The inhibited "stall" aural warning at high AoA during high AoA on AF447 is another example of design engineers assuming pilots will behave like monkeys, where the warning was inhibited at high AoA and activated when the crew reduced the AoA. Also the Colgan Air flight 3407 Dash-8 Q400 at Buffalo would in it's final stages have generated GPWS aural alerts concerning the glideslope deviation together with stall/low airspeed warnings, one warning requiring a pitch up and the other requiring a pitch down, which could overload a tired flightcrew.

Clearly the safety systems require a rethink

On Colgan, the bugs were set high, the Stall Warn activated with twenty knots left of room. Pilot pulled, eventually wrestling with the pusher all the way down. GPWS? He was pulling with all his might as it was. Did he pull harder with GPWS command? Dunno.

[CurtainTwitcher]

So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.


Unfortunately this MCAS intervention is not a classic "runaway stabiliser", it stops for 5 seconds after trim application, then activates again. You could see how a crew dealing with an unreliable airspeed could end up with the MCAS progressively ratcheting in forward trim, sporadically counteracted by pilot back trim, or even holding a significant backstick force against the MCAS.

Imagine dealing with what you believe is the "bigger problem" (UA) and the elevator & trim act in a unexpected manner (due to the undocumented MCAS), that is what this crew probably faced. Would any of us have recognised we actually had a flight control system problem?

Ultimately, is is possible that the actual cause of this accident will turn out to be an undocumented flight control issue.

[hec7or]

"Should they not also disconnect STS and therefore MCAS ? during such a situation ?"

of course, but initially only the AP/AT would be disconnected in response to the airspeed disagree, the auto/main trim switch selection would come when you recognise the trim runaway, assuming you are not distracted by airspeed limit alerts and tactile warnings, ironically it would appear that the STS/MCAS is only active once the AP is disconnected.

"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

The point is, most pilots would assume that disconnecting the AP/AT would give you full manual control of the aircraft.

Clearly it doesn't.

[Smott999]

"Should they not also disconnect STS and therefore MCAS ? during such a situation ?"

of course, but initially only the AP/AT would be disconnected in response to the airspeed disagree, the auto/main trim switch selection would come when you recognise the trim runaway, assuming you are not distracted by airspeed limit alerts and tactile warnings, ironically it would appear that the STS/MCAS is only active once the AP is disconnected.

"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

The point is, most pilots would assume that disconnecting the AP/AT would give you full manual control of the aircraft.

Clearly it doesn't.

..,but did it in the NG? Wasn’t Hal / STS still enabled? Just not at mercy of bad AoA?

[CurtainTwitcher]

"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

That is exactly the point, the recognition of the actual problem is very subtle due to the intermit nature of the MCAS being disabled for 5 seconds after manual trim. The first step in the UA checklist is disconnect the autopilot...

Initially just setting the attitude & thrust will require lots of manual trimming, but once it is right, only then does the MCAS starts to intervene. Now the PM gets out the QRH performance and starts giving you target attitudes and thrust for you weight while you are trying to fly level at 5000 (Boeing provide this exact altitude for Flight with UA), it takes quite some time to let the speed stabilize the speed Straight and Level(S&L) for the crosscheck and to work out the erroneous indication(s).
However just flying S&L for the crosscheck is complicated by the MCAS/ pilot trim couple. As I said in an earlier post, it is possible that all three IAS indications were actually valid and in agreement, further confusing the issue.

[JPJP]

Regarding the AOA indication on the 737NG and MAX. It’s an ‘optional extra’ positioned at the top right on the PFD. The Flight Path Vector (FPV) is standard equipment and deselectable.

737 AOA Indicator.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/432x481/e7b83569_c06b_4d91_a0b7_2d397e0f608f_c460dc9136ee5730ca34bfc 573d8b923e98b11bd.jpeg

[gums]

Salute, JP!

That's an excellent AoA indicator you have shown. Has the critical areas of interest clearly shown.

IMHO, there's no need to monitor AoA second to second unless on approach or doing sone serious maneuvers down low and slow.

Where the indicator helps is when other thing are happening and that AoA could be a big player.

Gums...

[Longtimer]

With a lot of 737 Max8s flying, you have to wonder what was different about this bird? If an overall design problem, then why has there not been more incidents? If with only that bird, perhaps new suppliers or even a manufacturing flaw.

[PPRuNe Towers]

It is a good display Gums but as an extra cost option only - even on BBJs. Do I recognise it? Yes.

Have I flown it? Yes but only in the sim. Never seen one out in the wild.

Rob

[CurtainTwitcher]

With a lot of 737 Max8s flying, you have to wonder what was different about this bird? If an overall design problem, then why has there not been more incidents? If with only that bird, perhaps new suppliers or even a manufacturing flaw.
Not necessarily, it is entirely conceivable that a design flaw has been lurking there since day one of commercial operation. It just takes a specific sequence to cause an accident. I mentioned earlier the Lindy Effect (https://en.wikipedia.org/wiki/Lindy_effect), which is an observation that the longer something lasts, the higher the probability that it will continue to last.

The 737 STS design is very old, the MCAS is not. The STS dataset is enormous, it is well proven technology that has survived the test of time through huge numbers of potential failure modes. The MCAS is a new technology, with only 220 aircraft in service, it has a much smaller reliability dataset in the real world compared to the STS system. There haven't been a large enough sequences of failure modes to truely test the design of the MCAS, compared with the STS.

This is about a series of probability paths (multiple events in a unique sequence) that lead to an adverse outcome, STS has proven itself to be a solid design over an enormous range of these paths, and we can probably conclude it is unlikely to have a significant design flaw based on time in service. MCAS has not been around for long, and it appears that a unique series of events has demonstrated a flawed design early on in the service life.

[CaptainMongo]

AOA is great except when it isn’t.

Multiple air carrier incidents and accidents have been caused by a triggering event which was an inaccurate AOA input.

I, to, am a big fan of AOA, having flown the -16 on the back side of her years, but if the the sensors providing AOA are corrupted, AOA is worse than worthless, it is dangerous,

[gums]

Good point, Mongo, about corruption

OTOH, the big thing is that the Viper FLCS was gee command in pitch and rate command in roll. So the Aoa inputs were focused upon "limiter" functions and did not actively move control surfaces about nor affect your control wheel or stick "feel".
We had a troop fly the Viper for about ten minutes with no AoA and damaged pitot because a big pelican busted the radome off. The rate sensors and stby gains let him fly for those ten minutes, but then it got too hard and poof!

Airbus is closest FLCS we see that compares with the Viper. And it does not provide tactile feedback to the control sticks last I checked. And the bus is aero stable and not as "sensitive' to small changes in gee or AoA.

Gums ....

[wheels_down]

Ultimately, is is possible that the actual cause of this accident will turn out to be an undocumented flight control issue.

Everyone is talking about runaway trims and sensors.

What they fail to remember is this does not eliminate the option of pulling back on the column in a dive with high speed.

The conditions were clear. They could see the ocean nose on.

Why didn’t they pull back? They clearly could not.

[CurtainTwitcher]

Why didn’t they pull back? They clearly could not
Earlier in the thread it was mentioned that with an a very nose trim as speed increased it may become impossible to overcome the (large) trim with (small) elevator, even with both pilots using full force (h/t Centaurus (https://www.pprune.org/members/11325-centaurus), post #826 (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-21.html#post10305367), FCTM air load warning with nose down trim)

*** PURE SPECULATION No data to support ***: The contention would be that at some point the crew actually did use the stab trim cutout switches, leaving them with only manual wound (literally by hand, extremely slow), trim and elevator for pitch control, there was an IAS increase which reached an uncontrollable speed.

As I said, this possible chain of events is purely speculative based upon the previous sector having an AoA anomaly, the Boeing Bulletin, APA MCAS (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-26.html#post10308248) revelation and a little bit of supposition. Thus far the data does not contradict this speculation, but I reserve the right to be wrong as new facts emerge.

[Rated De]

Earlier in the thread it was mentioned that with an a very nose trim as speed increased it may become impossible to overcome the (large) trim with (small) elevator, even with both pilots using full force (h/t Centaurus (https://www.pprune.org/members/11325-centaurus), post #826 (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-21.html#post10305367), FCTM air load warning with nose down trim)

*** PURE SPECULATION No data to support ***: The contention would be that at some point the crew actually did use the stab trim cutout switches, leaving them with only manual wound (literally by hand, extremely slow), trim and elevator for pitch control, there was an IAS increase which reached an uncontrollable speed.

As I said, this possible chain of events is purely speculative based upon the previous sector having an AoA anomaly, the Boeing Bulletin, APA MCAS (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-26.html#post10308248) revelation and a little bit of supposition. Thus far the data does not contradict this speculation, but I reserve the right to be wrong as new facts emerge.


Perhaps what is most apparent is that the imbecile Byron Bailey ought be facing a conga line of litigation. This like most accidents are a sequence of events. That idiot is no professional pilot. Just another MSM windbag.

Given Boeing's post factum missives, this accident sequence may well have been far more complicated than the pilots had anticipated given the previous flight history.

[A Squared]

Multiple air carrier incidents and accidents have been caused by a triggering event which was an inaccurate AOA input.


The current Lion crash is the only accident I can think of where inaccurate AoA data was (may have been) a triggering event. What are some others?

[DaveReidUK]

*** PURE SPECULATION No data to support ***: The contention would be that at some point the crew actually did use the stab trim cutout switches, leaving them with only manual wound (literally by hand, extremely slow), trim and elevator for pitch control, there was an IAS increase which reached an uncontrollable speed.

Can somebody familiar with the engineering of the 737 comment on whether this is representative of the maximum speed that the stab trim moves?

rxPa9A-k2xY

I know from experience that jackscrews, by their nature, don't operate particularly rapidly, but 90 seconds (approximately) from full nose up to full nose down (and presumably vice versa) seems pretty slow.

If that's the case, the crew would have had little or no chance to reverse a THS runway in the height available.

[birdspeed]

A squared, I believe the A320 test flight at Perpignan, France, was caused by water freezing in an AOA probe. Struggling to
think of any others.

[gearlever]

A squared, I believe the A320 test flight at Perpignan, France, was caused by water freezing in an AOA probe. Struggling to
think of any others.

AOA sensors 1&2 were affected.

[bsieker]

Regarding the AOA indication on the 737NG and MAX. It’s an ‘optional extra’ positioned at the top right on the PFD. The Flight Path Vector (FPV) is standard equipment and deselectable.

737 AOA Indicator.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/432x481/e7b83569_c06b_4d91_a0b7_2d397e0f608f_c460dc9136ee5730ca34bfc 573d8b923e98b11bd.jpeg



I think all this is missing the point. The problems mostly occur when AoA data is faulty. So what is displaying the faulty data going to achieve?

As I see it there are two possibilities:

AoA data is reliable and accurate: are there any accidents that would have been prevented by the provision of raw AoA data to the pilots? How would you know?
AoA data is unreliable and/or erroneous: In this case displaying the wrong value(s) is going to add to any confusion.


For the first point: if you say "AF447", I find that highly doubtful. They already had (in their mind) the conflicting information of slightly high or neutral (only briefly slightly negative) pitch angle and very high rate of descent, and an intermittent, mostly counter-productive, stall warning. It is a hard argument to make that an AoA gauge would have reduced confusion and lead to the correct decision early enough. A displayed AoA of around 40 degrees would likely have been off the scale and/or be dismissed as "absurd", having never before been experienced by any A330 crew, either in the aircraft or the simulator.

The second point possibly applies here.

Bernd

[A Squared]

A squared, I believe the A320 test flight at Perpignan, France, was caused by water freezing in an AOA probe. Struggling to
think of any others.

Thanks. Perhaps it's splitting hairs, but I'm not sure that bad AoA data was a "triggering event" in that accident. In the Lion Air Accident, the current working theory is that bad AoA data caused the loss of control. In the Perpignan crash, If I understand it correctly, the crew caused the crash by commanding excessively slow flight at low altitude, and the bad AoA data caused the expected stall protections to fail to save the airplane from the crews control inputs. My understanding is that they were flying slow for the purpose testing the stall systems at an altitude much lower than the required 10,000 ft altitude required for that test.

[mross]

If each pilot could see his own AoA they could decide which was faulty whether it read FSD, zero or was erratic. Or the computer could flag one as 'out of range'

[bsieker]

The current Lion crash is the only accident I can think of where inaccurate AoA data was (may have been) a triggering event. What are some others?

Qantas Flight 72 (https://en.wikipedia.org/wiki/Qantas_Flight_72), as mentioned before.

Although the aircraft did not crash, it was nevertheless an accident, because there were several serious injuries. The pilots temporarily lost control.

Bernd

[bsieker]

If each pilot could see his own AoA they could decide which was faulty whether it read FSD, zero or was erratic. Or the computer could flag one as 'out of range'

If the computer could decide that one was "out of range", it should also decide not to use it as an input to any kind of stall-prevention or stability assistance function, or whatever it is called.

Bernd

[gearlever]

If each pilot could see his own AoA they could decide which was faulty whether it read FSD, zero or was erratic. Or the computer could flag one as 'out of range'

The stall warning on AF447 disappeared, because it was "out of range", 60 kts. The stall continued however......

[mross]

If the computer could decide that one was "out of range", it should also decide not to use it as an input to any kind of stall-prevention or stability assistance function, or whatever it is called.

Bernd

According to the AD bulletin, one of the effects of an AoA sensor failure is 'stick shaker on the affected side only'. So the computers can use 'logic' to determine this.

[bsieker]

According to the AD bulletin, one of the effects of an AoA sensor failure is 'stick shaker on the affected side only'. So the computers can use 'logic' to determine this.

The stick shaker is simply caused by a high AoA reading, regardless of its validity, which, in this particular case can be a symptom of a faulty sensor, when otherwise there are no indications of a stall. The stick shaker is not used as a fault-condition indicator by design.

So rather than deciding that it is faulty, the stick shaker activating on an erroneously high-reading AoA input means that the stick shaker logic does not decide that it is erroneous, but takes it at face value and activates the shaker. It's probably deliberately a very simple system: fewer things to go wrong.

Bernd

[mross]

Bsieker, thanks for the correction. Does stick shaker on one side only help the pilots to understand what's gone wrong? And is stick shaker only triggered by AoA?

[A Squared]

According to the AD bulletin, one of the effects of an AoA sensor failure is 'stick shaker on the affected side only'. So the computers can use 'logic' to determine this.

For clarity, there is no "AD Bulletin" There is an AD (Airworthiness Directive, AD #: 2018-23-51) and a "Flight Crew Operations Manual Bulletin" (TBC-19) The same-side stick shaker is mentioned in the manual bulletin, but not in the Emergency AD. Edit: Actually, it *is* mentioned in the AD, I apparently hadn't read the AD completely. Apologies for that.

That aside, I'm not convinced that this is the computer using logic to determine whcih data is bad. This seems more of a case of of how the problem is manifesting itself with an unintended symptom in a complex system, rather than a case of the Computer, in effect, saying : "Hmmmm, we've determined that the left AoA sensor is giving us bad data, so because we know this isn't really a stall AoA situation, we're only going to activate the stick shaker on the left side instead of both sides"

[bsieker]

Bsieker, thanks for the correction. Does stick shaker on one side only help the pilots to understand what's gone wrong? And is stick shaker only triggered by AoA?

I don't know if it has been changed for the MAX (probably not), so the following is for the NG:

The stick shaker has multiple inputs. Primarily AoA, but also other ADIRU inputs, thrust, flap/slat configuration, etc.

Although there are two shaker systems, either will shake both control columns because they are mechanically linked by a rather rigid (though frangible) connection.

Stick shaker activation will be recorded on the FDR and can probably be heard on the CVR as well.


Bernd

[Derfred]

A key question of the crew reaction is “were they following the Airspeed Unreliable checklist?”.

From the FR24 data, it appears that they were not. There is no attitude and thrust data in the QRH that provides level flight at 5000 feet at approx 300 knots. (Not on the NG anyway.)

So what were they doing?

If they were flying “an” attitude and thrust, that would automatically include manual trim, which would eliminate the MCAS issue, with a possible manual trim input every 5 seconds required.

[Hi_Tech]

Time no doubt will tell, but if correct no greater example of gross coporate negligence considering the fundamental change to the previously understood design and control!

I doubt it Sax R54
I will believe this statement and several similar ones posted by others that Crew are not briefed about this system, only if one B737 MAX certified pilot tells this forum that he was not aware of it. This mod in MAX is a significant change from previous -800 etc, to protect the aircraft exceeding AOA approaching a stall. I am sure there will also be an EICAS alert when the system is active.
So is there any one with B737 MAX training / Certified in this forum to clarify this matter?

[SpeedBurn]

737 elevator was considered as a cause for crash before also.

For example, Flydubai Flight 981 (Report:'Flydubai Flight 981 Crash' (https://en.wikipedia.org/wiki/Flydubai_Flight_981#Airline))

Even though it was filed as pilot error, there is still a controversy because, one dissenting commission member, a Rosaviatsiya representative, filed an alternative opinion report, claiming the commission ignored a possible mechanical malfunction of the Boeing 737's elevator controls.

As of now, the Interstate Aviation Committee has not released their final report of the crash.

Seems like there are some serious issued in elevator controls of 737's.

[Gary Brown]

737 elevator was considered as a cause for crash before also.

For example, Flydubai Flight 981 (Report:'Flydubai Flight 981 Crash' (https://en.wikipedia.org/wiki/Flydubai_Flight_981#Airline))

Even though it was filed as pilot error, there is still a controversy because, one dissenting commission member, a Rosaviatsiya representative, filed an alternative opinion report, claiming the commission ignored a possible mechanical malfunction of the Boeing 737's elevator controls.

As of now, the Interstate Aviation Committee has not released their final report of the crash.

Seems like there are some serious issued in elevator controls of 737's.

I think you're conflating two separate 737 crashes. Tatarstan 363 (https://en.wikipedia.org/wiki/Tatarstan_Airlines_Flight_363) which had the dissenting opinion you mention (and both the Official (https://skybrary.aero/bookshelf/books/3640.pdf) and Dissenting (http://mak-iac.org/upload/iblock/6e0/%D0%9E%D1%81%D0%BE%D0%B1%D0%BE%D0%B5%20%D0%BC%D0%BD%D0%B5%D0 %BD%D0%B8%D0%B5.PDF) Reports have been published), and FlyDubai 981, whose report has not yet been issued. There are some similarities with the present LionAir - but lots of dissimilarities too.

[MurphyWasRight]

I doubt it Sax R54
I will believe this statement and several similar ones posted by others that Crew are not briefed about this system, only if one B737 MAX certified pilot tells this forum that he was not aware of it. This mod in MAX is a significant change from previous -800 etc, to protect the aircraft exceeding AOA approaching a stall. I am sure there will also be an EICAS alert when the system is active.
So is there any one with B737 MAX training / Certified in this forum to clarify this matter?

Unless it is totally fake this line from the e-mail halfnut posted should answer the question. ( https://www.pprune.org/showthread.php?p=10307740 )

This is the first description you, as 737 pilots, have seen. It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM. It will be soon.
...
...
...
Captain XXXXXXX
DFW 737I
APA Safety Committee Chairman

[Concours77]

I doubt it Sax R54
I will believe this statement and several similar ones posted by others that Crew are not briefed about this system, only if one B737 MAX certified pilot tells this forum that he was not aware of it. This mod in MAX is a significant change from previous -800 etc, to protect the aircraft exceeding AOA approaching a stall. I am sure there will also be an EICAS alert when the system is active.
So is there any one with B737 MAX training / Certified in this forum to clarify this matter?

I do NOT meet your criteria for responding....

I would point out simply that your use of the word “briefed...” is not germane. The FAA has required addition to the TEXT of the Operators Manual.

It is required that there be written description, not “briefing...”

To be read, and understood by the Operators.

[Rananim]

Not necessarily, it is entirely conceivable that a design flaw has been lurking there since day one of commercial operation. It just takes a specific sequence to cause an accident
I prefer this theory..We dont know if this is MAX specific.A cursory glance at the NG AMM confirms that this will happen in a NG as well.There may well be differences between MCAS and STS but they both involve trim,they both require inputs that may be corrupted.....you will get unwanted trim if the holes in the swiss cheese line up whether youre sitting in an NG or MAX.

Because of the many variables involved(type of UAS/piloting ability/which FCC is controlling) this specific sequence just never occurred before.We know piloting ability is definitely a variable because this was the 4th flight in sequence.Which FCC is controlling the trim is just a throw of the die...software logic...the pilot wont know this in the heat of the moment(the data is buried in the AMM anyway).The type of UAS is a huge variable(ie multiple ADIRU inputs to FCC,single side vs both side etc).

Is it a design flaw?You cant design perfection...not yet anyway.They need the protection systems but unfortunately those systems may be fed with corrupt data.

Manual flight....you establish this as a recall item during a UAS event.What is manual flight?AP/AT/FD are de-selected.You dont de-select trim....now you may well de-select trim if you recognize STS (or MCAS) responding to bad side data(alertness good airmanship),or you may even disable the AP stab trim as a precaution before STS or MCAS even has a chance to trim(high level of airmanship)....or you may not recognize the insidious trim until its too late due startle factor/panic/inexperience/distraction of tactile aural warnings.So many variables...

Nobody has the data yet on MCAS but Im assuming it works via the AP stab trim motor(even though Boeing in their AD says to disconnect both trim motors) and its controlled by one FCC at a time...
What surprises me here is the total absence of any discussion on the need for pilots to disable loud tactile aural warnings being fed by corrupt data.This to me is the key.The need to establish calm and quiet in a confusing environment can not be over-stated.IMO,the 4 circuit breakers for left/right overspeed and stick shaker should be on the center pedestal right next to the pilots.Those warnings are life savers when correct....but they may be killers when corrupted by bad data.

[lomapaseo]

What surprises me here is the total absence of any discussion on the need for pilots to disable loud tactile aural warnings being fed by corrupt data.This to me is the key.The need to establish calm and quiet in a confusing environment can not be over-stated

In some regards I agree, however I have seen pilots who killed the noise and then took an action which they assumed corrected the cause and then continued on unaware that the problem was still there hiding in the background to catch them out later in the flight

[A Squared]

I prefer this theory..We don't know if this is MAX specific.

Boeing think that Boeing knows. The Boeing's manual bulletin and subsequent AD say that it is applicable to the 737MAX8 and 737MAX9 (But not the 737MAX7) They didn't choose that list of applicable models at random. I don't know what their reasons are, but for some reason they believe that this is limited to the -8 and -9. Is the MCAS installed only on the -8 and -9 but not on the -7, or any previous models? I don't know, just asking the question. It doesn't appear that there's a lot of information about this MCAS.

[GarageYears]

Boeing think that Boeing knows. The Boeing's manual bulletin and subsequent AD say that it is applicable to the 737MAX8 and 737MAX9 (But not the 737MAX7) They didn't choose that list of applicable models at random. I don't know what their reasons are, but for some reason they believe that this is limited to the -8 and -9. Is the MCAS installed only on the -8 and -9 but not on the -7, or any previous models? I don't know, just asking the question. It doesn't appear that there's a lot of information about this MCAS.

I think that is exactly the case. This MCAS system is something added for the MAX (but not documented...). There are lots and lots of NGs out there, and I think it safe to assume both Boeing and the FAA (and the APA for that matter) would have mentioned it, had it applied to the NG series.

The question "why?" is was added to the MAX doesn't seem to have been addressed? Was there some aspect of certification that required it? If so, how can the aircraft need it for certification, but not have it documented in the info that the folks in the pointy-end use?

- GY

[Concours77]

Boeing think that Boeing knows. The Boeing's manual bulletin and subsequent AD say that it is applicable to the 737MAX8 and 737MAX9 (But not the 737MAX7) They didn't choose that list of applicable models at random. I don't know what their reasons are, but for some reason they believe that this is limited to the -8 and -9. Is the MCAS installed only on the -8 and -9 but not on the -7, or any previous models? I don't know, just asking the question. It doesn't appear that there's a lot of information about this MCAS.

Trying not to be pedantic. If MCAS is “Manual Control Enhancement”, but is uncommanded, (automatic) does that make the nomenclature oxymoronic? To claim it is not oxymoronic is to require an “assumption” on the Operator’s part? In the absence of written reference, I would say it does. Isn’t that the basis for the AD?

[underfire]

There is some legacy stuff in the box that does get sorted out now and then. I noted a few when first getting into the RNP-AR coding. Smiths box would not allow an airport rwy endpoint over 10,000 feet. We were specifically always using flyby waypoints, even on tangent, tangent legs. There was a memorable waypoint which was erroneously coded flyover, when the ac passed over they waypoint, it porpused down about 300 feet and leveled off. Turns out, there was a legacy item the code looked up (in the sequence) that looked back to a simple radius of the Earth that was coded in. The ac went from the GPS geiod altitude to the simple radius altitude on the lookup.

Took a while to debug that one, but when you actually look through all of the code, you see all kinds of crap. No one is sure where/when to fix it, because not sure all of the myriad of sequencing possibilities in tracing the code.

Note: a long while back, there was a question on submerging the box in fresh water. Anything that has been submerged need to be soaked in fresh water, as the salt is so corrosive. Artefacts such as cannon from shipwrecks are soaked for years.

[PPRuNe Towers]

A word of warning. In the last 24 hours a video has started floating around supposedly of a flight sim flight with the FDR track running on it.

You'll know it when you see it as the sim user had a complete fetish for moving the external view position. It is unattributed but looks plausible, ignores things like the massive 'upward' FR24 spike prior to descent and shows repeated attempts to maintain control of attitude until impact.

Don't bother putting it on here due to the lack of provenance, the amateur use of the sim software and the building of confirmation bias with pretty pictures.

Rob

[Concours77]

There is some legacy stuff in the box that does get sorted out now and then. I noted a few when first getting into the RNP-AR coding. Smiths box would not allow an airport rwy endpoint over 10,000 feet. We were specifically always using flyby waypoints, even on tangent, tangent legs. There was a memorable waypoint which was erroneously coded flyover, when the ac passed over they waypoint, it porpused down about 300 feet and leveled off. Turns out, there was a legacy item the code looked up (in the sequence) that looked back to a simple radius of the Earth that was coded in. The ac went from the GPS geiod altitude to the simple radius altitude on the lookup.

Took a while to debug that one, but when you actually look through all of the code, you see all kinds of crap. No one is sure where/when to fix it, because not sure all of the myriad of sequencing possibilities in tracing the code.

Note: a long while back, there was a question on submerging the box in fresh water. Anything that has been submerged need to be soaked in fresh water, as the salt is so corrosive. Artefacts such as cannon from shipwrecks are soaked for years.

Hi. Aren’t the boxes constructed to keep sea water OUT of the interior? I can see washing the box to flush salt a couple times, but soaking? An example would be AF447? After TWO years in salt, the boxes were undisturbed by corrosion. At a depth of 4000 meters?

[slacktide]

The Boeing's manual bulletin and subsequent AD say that it is applicable to the 737MAX8 and 737MAX9 (But not the 737MAX7)

The 737-7 is not yet certified. You cannot write an airworthiness directive against a model that does not have a type certificate.

[CaptainMongo]

The current Lion crash is the only accident I can think of where inaccurate AoA data was (may have been) a triggering event. What are some others?

Accident on 27 November 2008 off the coast of Canet-Plage (66) to the Airbus A320-232 registered D-AXLA operated by XL Airways Germany

and

Lufthansa Airbus A321-200, registration D-AIDP performing flight LH-1829 from Bilbao,SP (Spain) to Munich (Germany) near Bilbao on Nov 5th 2014, loss of 4000 feet of altitude

are a couple examples

[FCeng84]

On the topic of stabilizer jack-screw speed I believe the video clip attached a few entries back is not showing the fastest available speed. 777 and 787 have stabilizer ranges of approximately 15 degrees with the maximum surface movement rate of 0.5 deg/sec. That gives 30 seconds to go from one end stop to the other. I believe that the 737 has a similar stabilizer motion range and may operate a bit slower. As a lower bound, I know that the 737 stabilizer can be moved at a rate of at least 0.25 deg/sec.
This is a slow surface and for that reason (among others) advertised piloting procedure is to always fly maneuvers and establish steady flight via the column (i.e., the elevator) using stabilizer trim only as a means of relieving steady column forces.

[Vessbot]

Trying not to be pedantic. If MCAS is “Manual Control Enhancement”, but is uncommanded, (automatic) does that make the nomenclature oxymoronic?

Sounds like an enhancement to manual control, that works automatically.

[CurtainTwitcher]

If you do a google search for "Maneuvering Characteristics Augmentation System" (must be in quotes to search fro the exact phrase, or MCAS, but limit the search to prior to 01 Nov 2018 there are no results.

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1808x808/screen_shot_2018_11_13_at_7_39_20_am_66a3d1f345b1097df79a0ba 54466423138fb7616.png

[Concours77]

Sounds like an enhancement to manual control, that works automatically.

Except that the “augmentation” works in opposition to manual control “feel”. Making it the opposite of enhancement or augmentation.

By the way, the Artificial Horizon locates the nose, and can be used to suss AoA from FPV, no?

cheers.

[infrequentflyer789]

If you do a google search for "Maneuvering Characteristics Augmentation System" (must be in quotes to search fro the exact phrase, or MCAS, but limit the search to prior to 01 Nov 2018 there are no results.

Yes, however if you search for "Maneuvering Characteristics" near "Augmentation System" with the date limit you do get a result.
The actual search is (with date before 1 Nov 2018),
["Maneuver Characteristics" AROUND(10) "Augmentation System"]

This throws up a PDF FCOM (http://http://www.newgenfltops.com/nw/images/manual/Combine_FCOM_800_Issue_1_Revision_2.pdf)for 737-800 from Newgen/Sabaidee. The reason it doesn't match your search is that the expansion of MCAS is split over two lines. The only match in the document is a glossary entry:
https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/600x250/80-untitled_picture_6cac9ae89a5132b05fbe9d87009f5c125c158b44.pn g
It is possible that the glossary is (or is intended to be) common to -800 and MAX documents, it is also possible that there are no other hits because other 737 FCOMs online predate the change and there are no MAX FCOMs online. It doesn't really tell us much except that there was a clear intent to document MCAS in some way - whether it actually was documented or how well we don't know. I'd be interested in what was in the AMM, if anything.

[Concours77]

“...what does this mean? Why is this search so difficult?”

Because the system is proprietary?

wiedehopf: Are we not discussing the accident? If the tailplane is automatically trimming nose down due to (erroneous) and high AoA, and making nose up more difficult, until the Pitch cannot be overcome by elevator, that seems to me to be opposite to operators’ inputs. The “expected” feel is not “heavy”, making it opposite to (expectation of) manual control feel, no?

Explain your reference to “stall”? That was not the pilots problem. Their issue was inability to climb, not stall recovery. Right?

[CurtainTwitcher]

Thank you infrequentflyer789, it's a great day when I can learn a new trick from somebody, particularly enhancing knowledge of advanced google search syntax. A cheat sheet covering all the search operators including the "near" function: Every Google Search Operator You’ll Ever Need (https://www.hbagency.com/every-google-search-operator-youll-ever-need/).

I agree with your conclusion about finding a reference in the -800 series documentation being a commonality hangover with the MAX. Has anyone else found a reference to the MCAS in a NG only manual?

[leighlight]

When flying a 737 heritage and a NG, pilots would be used to seeing the wheels spin at take-off and landing, during the manual parts of the flight.
They were also used to the wheels spinning in apparent opposite direction to the expected, because it was a "Speed Trim" system... so with the new "Max" which, " was not an upgrade that required additional training" the spinning trim wheels are normal....except it would seem, they are not

[JPJP]

Thank you infrequentflyer789, it's a great day when I can learn a new trick from somebody, particularly enhancing knowledge of advanced google search syntax. A cheat sheet covering all the search operators including the "near" function: Every Google Search Operator You’ll Ever Need (https://www.hbagency.com/every-google-search-operator-youll-ever-need/).

I agree with your conclusion about finding a reference in the -800 series documentation being a commonality hangover with the MAX. Has anyone else found a reference to the MCAS in a NG only manual?

MCAS is only installed and certified in the MAX8 and MAX9 aircraft. It does not exist in the NG.

There is no reference to the system in the Boeing FCOM. Therefore, there is no reference to it in any airlines operating manuals. The system will only drive the trim nose down.

[SeniorDispatcher]

https://www.marketwatch.com/story/boeing-withheld-crucial-safety-information-on-new-737-models-experts-say-2018-11-12

https://www.dallasnews.com/business/airlines/2018/11/12/southwest-american-pilots-unions-question-737-max-documentation-after-indonesia-crash

[Rated De]

https://www.marketwatch.com/story/boeing-withheld-crucial-safety-information-on-new-737-models-experts-say-2018-11-12

https://www.dallasnews.com/business/airlines/2018/11/12/southwest-american-pilots-unions-question-737-max-documentation-after-indonesia-crash

Thank you for this..
This is precisely why the families of all those onboard ought be litigating both Boeing and that imbecile Byron Bailey ' the Captain" who without any fact simply blamed the pilots.

That reasoning doesn't make sense, said Roger Cox, a retired investigator with the U.S. National Transportation Safety Board and a former airline pilot. Flight crews have a right to be concerned that details about the new system weren't included in manuals and the short training courses they were required to take before flying the upgraded 737, Cox said."I would be pretty pissed" about not being told, he said. "This is important systems information that pilots should know about."



A more forthright and succinct statement is that a non-industry captured regulator would now ground the aircraft, sort the anomaly and let all pilots completed mandated differences training.

[CONSO]

https://www.wsj.com/articles/boeing-withheld-information-on-737-model-according-to-safety-experts-and-others-1542082575?mod=hp_lead_pos2

may be behind pay wall ...

ByAndy Pasztor andAndrew TangelNov. 12, 2018 11:16 p.m. ET Boeing (http://quotes.wsj.com/BA) Co. BA -3.33% (http://quotes.wsj.com/BA?mod=chiclets) withheld information about potential hazards associated with a new flight-control feature suspected of playing a role in last month’s fatal Lion Air jet crash, according to safety experts involved in the investigation, as well as midlevel FAA officials and airline pilots.The automated stall-prevention system on Boeing 737 MAX 8 and MAX 9 models—intended to help cockpit crews avoid mistakenly raising a plane’s nose dangerously high—under unusual conditions can push it down unexpectedly and so strongly that flight crews can’t pull it back up. Such a scenario, Boeing told airlines in a world-wide safety bulletin roughly a week after the accident, can result in a steep dive or crash—even if pilots are manually flying the jetliner and don’t expect flight-control computers to kick in.

Power point rangers strike again- a few heads should roll in Commercial ---(IMHO)

[Rated De]

https://www.wsj.com/articles/boeing-withheld-information-on-737-model-according-to-safety-experts-and-others-1542082575?mod=hp_lead_pos2

may be behind pay wall ...



Power point rangers strike again- a few heads should roll in Commercial ---(IMHO)

Not just in commercial. This reeks of regulatory capture.
The old Boeing was a very different beast, now captured by self interested corporate types, like every other business, spread sheet driven modelling, with an eye on the price of short term incentives and longer term share vesting dates is pretty much how the west was lost.

[GlueBall]

"Manual flight....you establish this as a recall item during a UAS event.What is manual flight?AP/AT/FD are de-selected.You dont de-select trim....now you may well de-select trim if you recognize STS (or MCAS) responding to bad side data(alertness good airmanship),or you may even disable the AP stab trim as a precaution before STS or MCAS even has a chance to trim(high level of airmanship)....or you may not recognize the insidious trim until its too late due startle factor/panic/inexperience/distraction of tactile aural warnings.So many variables..."

How about grabbing the trim wheel with your hand? Not part of MANUAL inputs?

[sSquares]

Not just in commercial. This reeks of regulatory capture.
The old Boeing was a very different beast, now captured by self interested corporate types, like every other business, spread sheet driven modelling, with an eye on the price of short term incentives and longer term share vesting dates is pretty much how the west was lost.

Do these companies not realise that they have the pension money of people invested in them that are there for the long haul and not for quick gains?

[Derfred]

"Manual flight....you establish this as a recall item during a UAS event.What is manual flight?AP/AT/FD are de-selected.You dont de-select trim....now you may well de-select trim if you recognize STS (or MCAS) responding to bad side data(alertness good airmanship),or you may even disable the AP stab trim as a precaution before STS or MCAS even has a chance to trim(high level of airmanship)....or you may not recognize the insidious trim until its too late due startle factor/panic/inexperience/distraction of tactile aural warnings.So many variables..."

How about grabbing the trim wheel with your hand? Not part of MANUAL inputs?

Or simply using main electric trim, which is a normal part of manual flight.

[fdr]

Or simply using main electric trim, which is a normal part of manual flight.

Which is fine, unless you are cognitively overloaded.

MCAS sounds like a SAS to maintain adequate stick force gradient at high alpha. The earlier B737s had a speed trim that worked for a short window of operation in the acceleration phase to ensure that when hand flown the stick force gradient was acceptable, This didn't function at high AOA. The longer fuselage will result in less downwash at the stabiliser than the earlier aircraft, and would otherwise need a larger span tailplane to be incorporated, or strakes etc to be installed. There are other fixes for that, but the easy one is to have an AOA triggered trim bias to offset an unacceptably low inherent stick g, or inadequate positive gradient of the curve.

Presumably the trim system still is observed to move and the trim in motion clacker has not been removed from the SLUF. Forewarned, a crew would be expected to isolate the trim, but that all depends on the remaining cognitive capacity. The CVR will show a saturated crew almost certainly.

As an industry we don't cope well with sensor failures, and as systems become more integrated the problem will grow.

Initially it sounded like an enhancement as it could be done... after some thought, I doubt that is the case at all.

The normal B737 handles high AOA pretty well. I have fully stalled, to the break the 3,4,500's at mid altitude, and at cruise altitude, The mid altitude was in all configurations, and both cases in accelerated and unaccelerated stalls. The Classic at least had straight forward control through the stall, it can have a modest wing drop, but quite manageable, slat rigging makes a difference. The longer bodies have slightly higher effectiveness in pitch control, but the stick force gradient would reduce. Haven't stalled them so cannot comment.

Next crew to have this sort of issue will have some prior knowledge that this crew did not have.

[Airbubba]

https://www.wsj.com/articles/boeing-withheld-information-on-737-model-according-to-safety-experts-and-others-1542082575

Wall Street Journal article on the MAX flight control system.

[Rated De]

Do these companies not realise that they have the pension money of people invested in them that are there for the long haul and not for quick gains?

When the NYSE wants monthly updates and quarterly statements, when the fund managers make money long, short up and down it matters little.
AOC responsibility isn't vested with CEOs, it rests with underlings.
Teams of legal ensure the corporation is protected. The Warsaw Convention limits payouts, meaning that with the appropriate insurance the 'corporation' continues on.
To truly understand the disconnect in the modern corporation is to look right into the eye of the MBA. It knows how to tell you the price of everything (which is then minimised by outsourcing or contracting) but is not in the slightest concerned with the cost of anything. When incentive is structured, these managers cut for the short term, as their incentives are short term.
As the Esso Longford gas explosion Royal Commission demonstrated managers hoped it didn't happen on their shift.
The modern day airline management know the price of everything, they cut until it breaks hoping that the point of failure happens when the other guy is in charge.
As NASA found out with the Challenger, the Normalisation of Deviance is costly.

That Boeing withheld pertinent information ought concern all pilots. It would appear the unions have noticed the significance.
The FAA must act but will continue twiddling its thumbs hoping that nothing else goes wrong...
After all there is a lot riding on the commercial success of this product. You could almost bet the airlines are lobbying as are Boeing not to do anything as the 'commercial impact' will be substantial. The FAA ought be focused on the impact of that aircraft in the sea and the lost souls.

[RickNRoll]

They put in MCAS in case the plane is not going to behave in manual flight the way pilots will expect it to. Now, if you have to disable the MCAS, the plane is not going to respond the way you have expected a 737NG to respond when flying manually and it will be more prone to stalling.

[Deejaypee]

Rated De.
Spot on! What a ripper post. The New World under the Corporate reign of the cut and run MBA👍.

[glad rag]

Rated De.
Spot on! What a ripper post. The New World under the Corporate reign of the cut and run MBA👍.

​​​​​Absolutely.

How did Boeing push these aircraft through certification without through oversight.

Do they get dispensation to do it themselves??

[gearlever]

I think it's fair to say this tragedy will become a battlefield for many lawyers. Oh BO.....

[phil gollin]

.

Going back a bit ...... the problem SEEMS to have arisen due to a single (?) AOA sensor failure. This then SEEMS to have activated the stick shaker on the side of the aircraft associated with that failed sensor , which happened to be the PF's side (correct ?) In addition this single failure then gave rise to the MCAS starting the POSSIBLE disastrous changes in trim.

IF that is basically correct.

1: It has been stated on here that there were two AOA sensors on the plane, so how did the FCS decide which one was at fault ? IF they had had, say, three AOA sensors, then at least the electronics could have "voted" two-to-one which were correct and which "failed".

2: Whilst the stick-shaker seems to have been a warning there has been no mention (I may have missed it) of any particular flashing button, or message on a glass screen coming up to note that a particular AOA sensor had failed. I note that there will always be a problem with overloading pilots with information when there is an emergency but asking overloaded pilots to remember the possibility of a failed AOA sensor causing one-sided stick-shaker operation seems optimistic.

[A Squared]

. This then SEEMS to have activated the stick shaker on the side of the aircraft associated with that failed sensor , which happened to be the PF's side (correct ?)

I haven't seen anything to suggest that the stick shaker activated on one side or another in this accident. We don't have that kind of detail on what went on, or I haven't seen it anyway. Boeing listed activation of stick shaker on one side as a possible symptom of this failure, in their list of other possible symptoms. Whether any one of the listed possible symptoms manifested itself on the accident flight is speculative at best.

[bsieker]

[...] a single (?) AOA sensor failure [...] SEEMS to have activated the stick shaker

[...] there were two AOA sensors on the plane, so how did the FCS decide which one was at fault ?

[...] a failed AOA sensor causing one-sided stick-shaker operation seems optimistic.

This misconception has been floating around for long. The stick shaker is not an annunciation of a sensor failure.

It is the normal approach-to-stall warning triggered by a high AoA value. It does not know that that high AoA value is wrong, but for stall warning it is better to be safe than sorry, so the two stick shakers apparently are independent, and each each rely on the AoA probe on their respective side of the aircraft. Which makes perfect sense from a redundancy and failsale-design standpoint. If two AoA sensors disagree, don't try to be smart and figure out which might be wrong. Just activate a stick shaker on a high value. In most cases it is preferable to have a spurious stall warning when there is no stall, than not to have a stall warning on a real approach to stall.


TL;DR: A "failed sensor" did not activate the stick shaker. A high AoA value did.

Bernd

[DaveReidUK]

IF they had had, say, three AOA sensors, then at least the electronics could have "voted" two-to-one which were correct and which "failed".

The A320 has three AoA sensors.

That just meant, in the Perpignan crash, that the two frozen ones outvoted the working one ...

[sAx_R54]

A key question of the crew reaction is “were they following the Airspeed Unreliable checklist?”.

From the FR24 data, it appears that they were not. There is no attitude and thrust data in the QRH that provides level flight at 5000 feet at approx 300 knots. (Not on the NG anyway.)

So what were they doing?

If they were flying “an” attitude and thrust, that would automatically include manual trim, which would eliminate the MCAS issue, with a possible manual trim input every 5 seconds required.




Presumably you could only have arrived at that conclusion on the latter point to eliminate MCAS, if you believe in manual reversion that they had set the stab trim cutout to CUTOUT as per the AD?

[aterpster]

​​​​​Absolutely.

How did Boeing push these aircraft through certification without through oversight.

Do they get dispensation to do it themselves??
They essentially self-certify. The FAA wouldn't know how.

[lomapaseo]

​​​​​Absolutely.

How did Boeing push these aircraft through certification without through oversight.

Do they get dispensation to do it themselves??

Without a complete investigation we don't have the facts to point to an inadequate certification.

We should revisit this a year from now. Meanwhile make sure the AD reasonably protects us while we answer this question

[Rananim]

The automated stall-prevention system on Boeing 737 MAX 8 and MAX 9 models—intended to help cockpit crews avoid mistakenly raising a plane’s nose dangerously high—under unusual conditions can push it down unexpectedly and so strongly that flight crews can’t pull it back up. Such a scenario, Boeing told airlines in a world-wide safety bulletin roughly a week after the accident, can result in a steep dive or crash—even if pilots are manually flying the jetliner and don’t expect flight-control computers to kick in.

Boeing doesnt design its FCCs with a psycho mode.Qantas 72 isnt possible in a Boeing.The poor travelling public probably think the FCC forced the Lionair into a dive.It didnt.It trimmed the aircraft nose down in response to faulty AoA....the report from the previous commander clearly says "STS trimming the wrong way".The pilots just have to recognize that they are facing unreliable data,disengage the automation,disengage the AP stab trim motor and fly the plane using basic attitude/thrust combinations.
Unfortunately,it appears that the accident crew became a bit saturated and just played tug of war with the opposing trim until time ran out.Startle factor/panic reduces cognitive faculties.The continuous stick shaker activation will contribute greatly to that startle factor.

The A320 has three AoA sensors.

That just meant, in the Perpignan crash, that the two frozen ones outvoted the working one ...
See how difficult it is to design perfection?

[Zeffy]

As an industry we don't cope well with sensor failures, and as systems become more integrated the problem will grow...

Tightly integrated systems can overwhelm a crew when a sensor failure occurs -- especially when the package of simulator anomalies used in training hasn't included the (unanticipated and poorly documented) condition.

Should "better/more training" remain the sole remedy?

[JRBarrett]

.

Going back a bit ...... the problem SEEMS to have arisen due to a single (?) AOA sensor failure. This then SEEMS to have activated the stick shaker on the side of the aircraft associated with that failed sensor , which happened to be the PF's side (correct ?) In addition this single failure then gave rise to the MCAS starting the POSSIBLE disastrous changes in trim.

IF that is basically correct.

1: It has been stated on here that there were two AOA sensors on the plane, so how did the FCS decide which one was at fault ? IF they had had, say, three AOA sensors, then at least the electronics could have "voted" two-to-one which were correct and which "failed".

2: Whilst the stick-shaker seems to have been a warning there has been no mention (I may have missed it) of any particular flashing button, or message on a glass screen coming up to note that a particular AOA sensor had failed. I note that there will always be a problem with overloading pilots with information when there is an emergency but asking overloaded pilots to remember the possibility of a failed AOA sensor causing one-sided stick-shaker operation seems optimistic.

I am an avionics maintenance engineer. I have not worked on any model of the 737, but am very familiar with AOA systems on a variety of other transport category aircraft.

If the probe dropped offline completely - stopped sending any position data to the aircraft systems - that would usually trigger some kind of CAS message to indicate a failure of the probe has occurred. On all aircraft I am familiar with, this would also trigger a warning that the stall warning and protection system is degraded. A single AOA probe can trigger the stick shaker if it senses high alpha, but normally both probes have to be in agreement as to the high alpha to trigger the stick pusher. With a failed probe, you would not have the pusher function available.

The MCAS system is a new concept to me. I would think that in a well-designed system, you would want to have agreement as to current AOA from BOTH probes before automatically applying nose down trim, just as with the voting required to activate the pusher on every aircraft model I have worked on.

In this incident, it appears that the left probe was still sending data, thus no “AOA fail” message - but the AOA data from the left probe appears to have been massively different than that from the right side.

I do not have access to a 737 Max AMM, so do not know the post-installation functional checks required after replacing a faulty probe. Most probes contain a guide pin which inserts into a matching hole in the fuselage to insure that the probe body is physically oriented correctly in relation to the airframe, but on all aircraft models I work on, installing a new probe also requires a post-install rigging check to insure that the output position data is correct. Typically this is done with a rigging adapter that attaches to the probe and fuselage, containing a calibrated angle scale and pointer, and a mount to attach a calibrated digital protractor. The check usually involves rotating the probe to specific angles, while checking the generated data to insure it is accurate.

IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.

An AOA rigging adapter is a precision piece of test equipment that has to be calibrated and certified. A large airline would likely have one on hand, a smaller airline not so likely.

[Gysbreght]

Originally posted by bsieker: If two AoA sensors disagree, don't try to be smart and figure out which might be wrong. Just activate a stick shaker on a high value. In most cases it is preferable to have a spurious stall warning when there is no stall, than not to have a stall warning on a real approach to stall.

Agreed, except that perhaps the authority of the MCAS should be limited if only one of the two AoA systems outputs a high value.

[Concours77]

Tightly integrated systems can overwhelm a crew when a sensor failure occurs -- especially when the package of simulator anomalies used in training hasn't included the (unanticipated and poorly documented) condition.

Should "better/more training" remain the sole remedy?

Good question, but we know the answer before it was asked. The answer? Yes.

Case in point? Batteries in a to-go box. Duct tape and twine, simples.

The alternative: grounding, refit, reprise certification, punish those who deliberately isolated safety critical data from operator group.

Heads will not roll. Boeing owns the guillotine, and it is placarded “Inop”.

just sayin’

[bsieker]

Boeing doesnt design its FCCs with a psycho mode.Qantas 72 isnt possible in a Boeing.

That's really the epitome of double-think: "This airplane crashed and killed almost 200 people because of a Boeing system idiosyncrasy, but Boeing is better than Airbus, because this other accident of an Airbus which only injured a few people."

You do realise that this Boeing aircraft behaviour is deliberate, whereas the problem with Qantas Flight 72 was a software error (what some people call a bug: i. e. the opposite of "designed")?

You realise further, I hope, that there are good reasons for having the system put in nose-down commands when approaching the stall angle of attack?

I'm not normally one to pit the aircraft manufacturers against each other, I know they all have their problems, but this kind of fanboyism has gone too far.

The pilots just have to recognize that they are facing unreliable data,disengage the automation,disengage the AP stab trim motor and fly the plane using basic attitude/thrust combinations.
(my emphasis)

Right. "Just ...". That's easy.

P. S. Your posts would be a lot more readable if you put a blank space behind punctuation marks (https://en.wikipedia.org/wiki/Klempen).

[bsieker]

Agreed, except that perhaps the authority of the MCAS should be limited if only one of the two AoA systems outputs a high value.

Perhaps. I'm not judging the MCAS either way, I was just trying to clear up the misconception that the stick shaker was designed to be used as a failure indicator, whereas here it was just an inadvertent symptom among many.

[Vessbot]

Except that the “augmentation” works in opposition to manual control “feel”. Making it the opposite of enhancement or augmentation.
Sounds like low stick force isn't the quality they wanted enhanced, but rather the opposite. It's not an aerobatic plane, but an airliner; so high stick force at high AOA to prevent stall is desired.

By the way, the Artificial Horizon locates the nose, and can be used to suss AoA from FPV, no?

cheers.
I don't know what you mean by "locates."
The difference between nose and horizon is attitude.
The difference between nose and FPV (wind notwithstanding) is AOA.

[Towhee]

Boeing Co. withheld information about potential hazards associated with a new flight-control feature suspected of playing a role in last month’s fatal Lion Air jet crash, according to safety experts involved in the investigation, as well as midlevel FAA officials and airline pilots.

The automated stall-prevention system on Boeing 737 MAX 8 and MAX 9 models—intended to help cockpit crews avoid mistakenly raising a plane’s nose dangerously high—under unusual conditions can push it down unexpectedly and so strongly that flight crews can’t pull it back up. Such a scenario, Boeing told airlines in a world-wide safety bulletin roughly a week after the accident, can result in a steep dive or crash—even if pilots are manually flying the jetliner and don’t expect flight-control computers to kick in.



---Wall Street Journal

https://www.wsj.com/articles/boeing-withheld-information-on-737-model-according-to-safety-experts-and-others-1542082575

[A Squared]

---Wall Street Journal " Boeing Co. withheld information about potential hazards associated with a new flight-control feature suspected of playing a role in last month’s fatal Lion Air jet crash, ..."

Point of order; they didn't just withhold information on the "potential hazards associated with a new flight-control feature", they withheld information of the very existence of the "new flight-control feature".

[Vessbot]

Well it looks like my question: "is there another system that's supposed to be automatically moving the trim prior to 10 seconds?" has been answered. Thanks Boeing!

[underfire]

Hi. Aren’t the boxes constructed to keep sea water OUT of the interior? I can see washing the box to flush salt a couple times, but soaking? An example would be AF447? After TWO years in salt, the boxes were undisturbed by corrosion. At a depth of 4000 meters?

You dont know if the box has been damaged and salt water has gotten in.

[CONSO]

After TWO years in salt, the boxes were undisturbed by corrosion. At a depth of 4000 meters?

At 4000 meters depth-little- no oxygen-Corrosion is related or equal to OXIDATION which requires . . . Oxygen...

[Concours77]

At 4000 meters depth-little- no oxygen-Corrosion is related or equal to OXIDATION which requires . . . Oxygen...

Think “electrolysis”. Corrosion is not the only hazard.

[Concours77]

Sounds like low stick force isn't the quality they wanted enhanced, but rather the opposite. It's not an aerobatic plane, but an airliner; so high stick force at high AOA to prevent stall is desired.


I don't know what you mean by "locates."
The difference between nose and horizon is attitude.
The difference between nose and FPV (wind notwithstanding) is AOA.

Trying to make a point here. Attitude is displayed as degrees above/below Horizon. FPV displays degrees above/below horizon. Consolidating both elicits AoA. No? You brought up wind, I didn’t.

ALSO. “Augmentation” does not assist input. It works independently of crew handling.

It is not installed to assist manual handling. It is installed to protect the aircraft from the crew, just as shaker/pusher does. Except, as here in Lion Air, it loses the plot and destroys the aircraft, crew and all the souls on board.

[silverstrata]

It didnt.It trimmed the aircraft nose down in response to faulty AoA....the report from the previous commander clearly says "STS trimming the wrong way".The pilots just have to recognize that......

How do you know this is a genuine fault? The 737 STS system ALWAYS trims the wrong way - that is what it is ‘suposed’ to do. And the pilot response on every flight is to trim back the right way - which overrides the STS. Which is why the STS is widely cursed - it may be trimming into a ‘safer’ regime, but pilots like an aircraft to be in trim, not trimmed nose down - so it is still a pain when hand flying.

Silver

[Concours77]

Silverstrata. The theory/design is not misunderstood. It is straightforward and subject to a separate discussion relative to design and functionality?

The issue is and remains: A flight critical controls mode is installed on a fleet. It screws up, and we find out no one knew of its existence. Airbus has been subject to extreme criticism for doing similar with mode confusion the result. Boeing’s turn in the barrel. Disclosure by manufacturer is a (mandatory) minimum, understanding is up to the operator.

”handling” is done with controls. Maneuvering.

Long term stability is for TRIM.

Using a slow but immensely powerful control on climbout involves the risk of getting stuck in dangerous area of migrating neutral stick position and needing to overcome trim induced bias attitude with what amounts to a trim tab (elevator). Just my opinion. Is it fatally wrong?

[underfire]

At 4000 meters depth-little- no oxygen-Corrosion is related or equal to OXIDATION which requires . . . Oxygen...

Exactly, but once to the surface..oxygen!

News is now saying the lazy B knew of the issue. (WSJ at least)

[CHfour]

@Vessbot
The difference between nose and FPV (wind notwithstanding) is AOA
If only life were so simple! I think you need to factor in the angle on incidence. The 738 cruises at around 2 1/2 nose up but the AoA is more like 5-6 degrees? Apparently the slightly nose up attitude was to allow the fuselage to contribute some lift. Airbus, by contrast, rig their products to cruise with level cabins for passenger comfort. I've never operated the Bus though so the last bit could be B/S?

[Vessbot]

Trying to make a point here. Attitude is displayed as degrees above/below Horizon. FPV displays degrees above/below horizon. Consolidating both elicits AoA. No?

Yes, as long as long as you're clear about what you're adding/subtracting.

You brought up wind, I didn’t.
Yes I brought it up for clarity.

It is not installed to assist manual handling. It is installed to protect the aircraft from the crew, just as shaker/pusher does.

This protection is the relevant assistance.

@Vessbot

If only life were so simple! I think you need to factor in the angle on incidence. The 738 cruises at around 2 1/2 nose up but the AoA is more like 5-6 degrees?

True, but the difference between nose and wing chord is fixed, and small, and can therefore be disregarded when talking about understanding changes, i.e., if you do this with the AOA, the airplane will respond like that. The nose vs. wing difference is just the adjustment of a fixed datum, for which difference us pilots don't have any data to base on anyway. So nose symbol vs. FPV is good for a basic conception.

Plus (and I'm perfectly willing to be shown wrong about this) I'd put money on that the axis of the nose symbol is the datum in use by all the engineering anyway. Because, as you yourself pointed out, there are more components in addition to the wing that contribute to the overall AOA-dependent behavior.

Apparently the slightly nose up attitude was to allow the fuselage to contribute some lift. Airbus, by contrast, rig their products to cruise with level cabins for passenger comfort. I've never operated the Bus though so the last bit could be B/S?

I would be shocked if this is true. The performance value of note is not lift (we can easily get much more lift than we need by simply hauling back on the stick) but rather lift/drag ratio. And I cannot imagine why they would want to use any lift tied with the drag penalty of flying the tube askew to the air, as opposed to the wing, which is exquisitely designed specifically for the purpose of high L/D in the cruise condition.

If indeed the fuselage is pitched up (note I said the fuselage and not the nose symbol, which can be programmed to be displayed anywhere wrt. the fuselage axis) I would bet that it's because the planes are cruised slower than originally intended, or some other compromise reason... but not because the original designers wanted it.

[KenV]

The issue is and remains: A flight critical controls mode is installed on a fleet. It screws up, and we find out no one knew of its existence. It's one thing to mince rumors. Another to spout abject falsehoods. FAA, CAA, and lots of other regulatory folks were well aware of the existence of the MACS and tested it thoroughly. What Boeing did not do is include it in the pilot's flight manual. There is a vast vast difference between "not included in the flight manual" and "no one knew of its existence." VAST.

[DaveReidUK]

True, but the difference between nose and wing chord is fixed, and small, and can therefore be disregarded when talking about understanding changes, i.e., if you do this with the AOA, the airplane will respond like that. The nose vs. wing difference is just the adjustment of a fixed datum, for which difference us pilots don't have any data to base on anyway. So nose symbol vs. FPV is good for a basic conception.

Yes, for the record the 737's wing root incidence is about +1°, so the difference between fuselage and wing AoA will be negligible.

[safetypee]

Ken, #1115, whist agreeing with the sentiment of your post, there is as yet no public evidence of the authorities’ knowledge.
It would be very surprising that there was none; however it is more important to understand their process of approval, the depth of knowledge, assessment, and validation of safety cases.
Something slipped through the net; the safety process appears to have failed us. This is a key aspect of our trust in the airworthiness of the aircraft we fly, hence the strength of the operational concern.

[gums]

Salute!

Thanks, Dave, suspected as much about wing chord line. OTOH I wouldn't be using the bird versus fuselage reference for anything but gross maneuvers as we saw in AF447. That plane had the "bird" out of sight well berlow the fuselage/wing chord reference symbol

And @ Ken........ Good friggin grief, PLZ explain what good is a system that is suppoded to "help" we mortal pilots and has a fault that results in uncommanded and large trim changes at exactly the wrong time - like just after takeoff and flaps are coming up with associated trim change ( unlike Airbus, best I can see). We could care less how many "other folks" knew about MCAS, and considering that their testing and such was based on their kniowledge of how the feature was supposed to work. And did they flight test the feature with AoA sensor failure or other sensor inputs to the FCC? But the poor sob that was supposed to save the day by going "not so" manual, didn't have a chance. I especilly like the part that gives you a few seconds to get all the switches off and/or circuit breakers pulled before you get the full nose down trim once again. GASP!

Whole thing should be very scary to mortal pilots, and I am very upset with Boeing, as I always thot that made very good planes and they were pilot-friendly.

Gums sends...

[gearlever]

IMHO BOEING has lost its virginity.

[sgs233a]

It trimmed the aircraft nose down in response to faulty AoA....the report from the previous commander clearly says "STS trimming the wrong way".
It didnt.It trimmed the aircraft nose down in response to faulty AoA....the report from the previous commander clearly says "STS trimming the wrong way".The pilots just have to recognize that......How do you know this is a genuine fault? The 737 STS system ALWAYS trims the wrong way - that is what it is ‘suposed’ to do. And the pilot response on every flight is to trim back the right way - which overrides the STS. Which is why the STS is widely cursed - it may be trimming into a ‘safer’ regime, but pilots like an aircraft to be in trim, not trimmed nose down - so it is still a pain when hand flying.

Silver

Not a Boeing driver, frequent lurker here, but:

From reading this thread and the Boeing alert, it seems that a big part of the issue is that the crew was not aware of MCAS. The writeup for STS "trimming the wrong way" may very well have been written by a captain that thoroughly understood STS and was aware of which way it should normally trim....but what he was seeing was the undocumented MCAS feature trimming opposite to the direction STS would be expected to trim in a given flight regime. The writeup mentioning STS may just be because this was the only feature of which he was aware that would cause the trim to run in manual flight.

[lurkingpax]

Some Google Fu reveals a document called "OPERATIONAL EVALUATION REPORT – BOEING 737" "ORIGINAL – JANUARY 10, 2018" from ANAC (Brazilian civil aviation authority). In "Appendix 2 – OPERATOR DIFFERENCE REQUIREMENTS (ODR) TABLES" MCAS is listed as "No" on both flight characteristics (FLT CHAR) and change of procedures (PROC CHNG).

[Jwscud]

Having flown the 737 and the 777 several,things are noticeable:

Firstly, Boeing are getting more and more reticent about what information is provided in the FCOM - the exact info about TOGA at low alt/touchdown that caught the EK crew out was present in older editions of the manual but not recent ones. I often learn things that aren’t in the manuals from people who were trained on the aircraft some time back.

The 737 is a 60s design out of which Boeing have wrung every single knot and .02% fuel efficiency increase. They have done so while being forced to keep a common type rating with the Classic. They have added a lot of stuff in the background while maintaining a flight deck with an archaic and complex Caution and Warning system that has zero prioritisation and requires much more cognitive processing simply to identify the correct checklist than the EICAS that Boeing have been supplying with newer aircraft since the 80s.

This is not a happy collection of circumstances, partially driven by customers (SWA I’m looking at you...) but also a consequence of what seems to be a new philosophy at Boeing.

[FCeng84]

The shape of a commercial airplane wing (twist, sweep, variation in cord and camber along the span, etc.) is very complex. As such it would be quite a trick to define a true "zero AOA" reference. What ever metrics used for such a calculation would undoubtedly be impacted by flap position and wing spanwise droop that is a function of payload/fuel distribution and wing lift loading. AOA as determined from the body mounted vanes (with correction for known local flow distortion) is presented with respect to the fuselage.

As has been mentioned, transport aircraft that are optimized to maximize L/D are designed to fly with a small, positive pitch attitude. To design an airplane to fly with deck angle of exactly zero in cruise would be to leave unrealized performance on the table! Separately, optimization of takeoff, landing, and ground operations usually results in a slightly negative deck angle when taxiing. I strongly doubt that any commercial transport airplane has ever been designed to have exactly zero deck angle either during ground taxi or cruise.

[JPJP]

It's one thing to mince rumors. Another to spout abject falsehoods. FAA, CAA, and lots of other regulatory folks were well aware of the existence of the MACS and tested it thoroughly. What Boeing did not do is include it in the pilot's flight manual. There is a vast vast difference between "not included in the flight manual" and "no one knew of its existence." VAST.

Correct. During Part 25 Certification a requirement for MCAS was identified. It was then added. As you stated, the current brouhaha relates to the lack of any form of reference material that’s visible to pilots.

[CHfour]

Yes, for the record the 737's wing root incidence is about +1°, so the difference between fuselage and wing AoA will be negligible.

So the AoA is around 3.5 degrees in the cruise? (seems a bit low). BTW (and off topic) I've just googled for some Airbus PFD images and it would seem that the Bus cruises at 2.5 just like the 738.

[Vessbot]

The shape of a commercial airplane wing (twist, sweep, variation in cord and camber along the span, etc.) is very complex. As such it would be quite a trick to define a true "zero AOA" reference. What ever metrics used for such a calculation would undoubtedly be impacted by flap position and wing spanwise droop that is a function of payload/fuel distribution and wing lift loading. AOA as determined from the body mounted vanes (with correction for known local flow distortion) is presented with respect to the fuselage.

Exactly, and this is the flip side of all the non-wing "total airframe" components I mentioned that make any zero-AOA datum, arbitrary. So you might as well choose a convenient one, that is not separated, as you note, by the AOA vanes by a floppy piece of structure (and, I'll add, from the IRS).

As has been mentioned, transport aircraft that are optimized to maximize L/D are designed to fly with a small, positive pitch attitude. To design an airplane to fly with deck angle of exactly zero in cruise would be to leave unrealized performance on the table!

This I disagree with. Why can't a plane be designed for the cruise condition with an exactly-level fuselage attitude (least drag) and wing incidence set for an AOA that yields best performance?

(I understand that after the basic design is "locked in" and it would be too time consuming and expensive to change the incidence, customer requirements or fuel prices or max gross weight or many other factors could change the cruise condition, and therefore the AOA, and therefore fuselage attitude; but I'm talking about the design process at the very beginning, before it's molested by such changes.)

[Concours77]

Correct. During Part 25 Certification a requirement for MCAS was identified. It was then added. As you stated, the current brouhaha relates to the lack of any form of reference material that’s visible to pilots.





Will you elaborate? “A requirement for MCAS was identified. It was then added.” What is not included in the statement is:

“It was then certified to the C of A....” How? By Boeing memo? Or did it just sound like a good idea?

In the AD, “After manufacturer’s research, it was ‘discovered’ (sic) that Trim could continue Nose Down after receiving sensor data related to a high and erroneous AoA value...” Can we review the research? Because I am not buying that it was ever “unknown”.

[Vessbot]

So the AoA is around 3.5 degrees in the cruise? (seems a bit low). BTW (and off topic) I've just googled for some Airbus PFD images and it would seem that the Bus cruises at 2.5 just like the 738.
Given what me and FCeng84 have written, it seems especially fruitless to compare displayed AOA across not only different airframes, but different avionics systems.

[Concours77]

Exactly, and this is the flip side of all the non-wing "total airframe" components I mentioned that make any zero-AOA datum, arbitrary. So you might as well choose a convenient one, that is not separated, as you note, by the AOA vanes by a floppy piece of structure (and, I'll add, from the IRS).



This I disagree with. Why can't a plane be designed for the cruise condition with an exactly-level fuselage attitude (least drag) and wing incidence set for an AOA that yields best performance?

(I understand that after the basic design is "locked in" and it would be too time consuming and expensive to change the incidence, customer requirements or fuel prices or max gross weight or many other factors could change the cruise condition, and therefore the AOA, and therefore fuselage attitude; but I'm talking about the design process at the very beginning, before it's molested by such changes.)

No airplane can fly with zero AoA. Let’s don’t do Bernoulli. If the centerline of chord is the same as the fuselage longitudinal line, and the fuselage is level, the airplane will sink. The angle of incidence of the wing allows less nose up when close to level. It is “baked in” AoA. No?

There was an aircraft with “variable incidence wing”. It allowed Nose Down level flight, or even very Nose Down glide slope. It was useful for seeing the deck on the aircraft carrier when landing. Vought F-8 Crusader.

[Vessbot]

No airplane can fly with zero AoA. Let’s don’t do Bernoulli. If the centerline of chord is the same as the fuselage longitudinal line, and the fuselage is level, the airplane will sink. The angle of incidence of the wing allows less nose up when close to level. It is “baked in” AoA. No?
Correct on all counts. I think you read something into my post that wasn't there.

There was an aircraft with “variable incidence wing”. It allowed Nose Down level flight, or even very Nose Down glide slope. It was useful for seeing the deck on the aircraft carrier when landing. Vought F-8 Crusader

Yes, and thanks for characterizing it correctly as being for visibility. I want to give up on the world and cry myself to sleep, every time I see it written that it's for increasing the AOA. Hello McFly, every airplane already has a system for doing that, it's called the elevator!

[Concours77]

Correct on all counts. I think you read something into my post that wasn't there.

Meant as “I agree”. Sorry.

[Vessbot]

Meant as “I agree”. Sorry.
Gotcha! I thought you were disagreeing with me, where I couldn't find any part that actually does ;) Look who's reading into things now...

[silverstrata]

[left]


From reading this thread and the Boeing alert, it seems that a big part of the issue is that the crew was not aware of MCAS. The writeup for STS "trimming the wrong way" may very well have been written by a captain that thoroughly understood STS and was aware of which way it should normally trim....but what he was seeing was the undocumented MCAS feature trimming .

Ok, I did not know the Max has a new stall assistance system. As mentioned before, the STS normally trims down after take off, so it would be operating in the same manner as the new stall system. So again the tech log entry about STS appears incorrect.

And why did Boeing incorportate a stall assistance system that trims, instead of using a stick pusher? This seems to go against the normal rules for flying. You fly the aircraft with the control column, not the trimmer.

A stick-pusher acts on the control columm, to increase speed, and withdraws its influence instantaneously to allow you to recover from the dive. A stall trimmer will trim you down, but leave that nose-down pressure on and impede your recovery from the dive.

A stick pusher is flying the control column, in the normal manner; while a stall trimmer is flying the trimmer. Is this how the new stall system works? I cannot find details on-line.

Silver

[infrequentflyer789]

I do not have access to a 737 Max AMM, so do not know the post-installation functional checks required after replacing a faulty probe. Most probes contain a guide pin which inserts into a matching hole in the fuselage to insure that the probe body is physically oriented correctly in relation to the airframe, but on all aircraft models I work on, installing a new probe also requires a post-install rigging check to insure that the output position data is correct. Typically this is done with a rigging adapter that attaches to the probe and fuselage, containing a calibrated angle scale and pointer, and a mount to attach a calibrated digital protractor. The check usually involves rotating the probe to specific angles, while checking the generated data to insure it is accurate.

IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.

Based on an old 737 AMM and zero personal hands-on experience, it looks like 737 has something very similar. AMM refers to a "calibrator" which is to used in system performance test after installation. Stall warning system is to be tested with various flap settings rotating the vane until shaker triggers.

I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).

It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.

Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.

[Gipsy Queen]

No airplane can fly with zero AoA. Let’s don’t do Bernoulli. If the centerline of chord is the same as the fuselage longitudinal line, and the fuselage level, the airplane will sink. The angle of incidence of the wing allows less nose up when close to level. It is “baked in” AoA. No?

I agree - the subject is sufficiently complicated without introducing Daniel Bernoulli! The above is correct in the case of the x-axis of the fuselage being coincident with that of the wing chord but Vessbot's was wondering why a wing AoA (disregarding wash-out) necessarily has to be related to the angle of incidence of the fuselage. The answer is, of course, that it doesn't.

[A Squared]

And why did Boeing incorportate a stall assistance system that trims, instead of using a stick pusher? This seems to go against the normal rules for flying.


My guess: Because that's what was easiest. To add a stick pusher requires adding hardware. You have to add an actuator and linkage to the control system. Adds weight and cost and parts count, and I have to think that the certification process required for a new, previously non-existent system whcih is attached to and moves the primary control system is not trivial.

On the other hand to do it via stabilizer trim is a lot simpler, you've already got all the hardware you need installed, the jack screw and it's actuator. About all it takes is adding some code to the software for the control computer.* So, there you go, no new parts, no additional weight, and no regulator looking over your shoulder saying "Whoa, you just added a mechanical system that applies control inputs to the primary control system. Lets take a look at all the specs for such systems"



* I'm kinda speculating on that, details are still a little fuzzy on what exactly the MCAS 'is" but so far it doesn't seem that there are any physical parts to it that aren't already there and used by other systems.

[KenV]

Why can't a plane be designed for the cruise condition with an exactly-level fuselage attitude (least drag) and wing incidence set for an AOA that yields best performance?Why? Because there are numerous "cruise conditions", even on the same flight. How so? The required AOA depends on a variable that is constantly changing: weight. Do you set "best performance" for the conditions at top of climb? Half way to destination? Just before descent? With a completely full passenger load? With a partial passenger load? With full tanks? Partially filled tanks? There are multiple variables that drive weight.

[Vessbot]

Why? Because there are numerous "cruise conditions", even on the same flight. How so? The required AOA depends on a variable that is constantly changing: weight. Do you set "best performance" for the conditions at top of climb? Half way to destination? Just before descent? With a completely full passenger load? With a partial passenger load? With full tanks? Partially filled tanks? There are multiple variables that drive weight.

Yes, I've already pointed out that variables can change the day to day (or even minute to minute) condition from the design one. The design condition is the baseline (full of assumptions about altitude, weight, speed, etc.) that all those changes are relative to. (Kinda like air pressure changes all the time and 29.92 is arbitrary, but we still need some baseline for the calculations.) If oil gets too expensive, or New York TRACON is too congested, you'll be cruising at a higher AOA than designed.

But the way I read FCeng's post (maybe I misread, or he'll elaborate) is that even in the original baseline design, which would be reflected in the average of the day to day, it's advantageous to have the fuselage pitched up. That's what I'm questioning.

[A Squared]

But the way I read FCeng's post (maybe I misread, or he'll elaborate) is that even in the original baseline design, it's advantageous to have the fuselage pitched up. That's what I'm questioning.

I'm curious about that as well.

[vrb03kt]

A lot has been said today in the media about pilots not knowing about the new system and sure enough I can’t find anything about MCAS in the manuals. However, the max and NG FCOM both state that the speed trim system will trim the stabiliser nose down as the airspeed approaches the stall speed. Neither alludes to what the exact inputs are to this system (eg. AoA vanes), nor that it can erroneously occur if x y z fails but the actual function of the system that is suspected to have caused the problem isn’t new to the 737 max? Or have I missed something?

[glad rag]

Based on an old 737 AMM and zero personal hands-on experience, it looks like 737 has something very similar. AMM refers to a "calibrator" which is to used in system performance test after installation. Stall warning system is to be tested with various flap settings rotating the vane until shaker triggers.

I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).

It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.

Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.

From my "limited" experience the "pin" locates/datums the unit physically: you then index the sensor to the extremes check output against manual figures then calibrate (https://www.google.co.uk/search?rlz=1C1PRFI_enGB777GB777&q=calibrate&spell=1&sa=X&ved=0ahUKEwiVgZe0tdLeAhWqDsAKHaRCBZ4QBQgrKAA) using a measuring device degrees to output. On a rotary sensor you use a protractor that is "zeroed" at a specified end stop.On a flapper once again end output checks followed by precise angle/output performance.

Just don't forget to take the kit off for the heater checks....:ugh:

[Rated De]

Whole thing should be very scary to mortal pilots, and I am very upset with Boeing, as I always thot that made very good planes and they were pilot-friendly.

Precisely Gums.

Boeing once were a company that understood the inherent risks. They were a manufacturing company, their emphasis was rightly on risk minimisation.
Now it is simply another corporate entity, driven by MBA and financial statement manipulation. These idiots know little of the industry they financially engineer and care less, the entire focus is the next quarter.

[DaveReidUK]

No airplane can fly with zero AoA.

That's true, though of course most wings, being cambered, do still produce some lift at 0° AoA.

[Alwaysairbus]

It’s interesting that such a single point system failure may have cause the stab trimming and that differences wise it’s not contained in the training if media reports are correct. Something to look at when updating similar aircraft types and operating aircraft with a common flight deck but updated systems.

[CurtainTwitcher]

Some Google Fu reveals a document called "OPERATIONAL EVALUATION REPORT – BOEING 737" "ORIGINAL – JANUARY 10, 2018" from ANAC (Brazilian civil aviation authority). In "Appendix 2 – OPERATOR DIFFERENCE REQUIREMENTS (ODR) TABLES" MCAS is listed as "No" on both flight characteristics (FLT CHAR) and change of procedures (PROC CHNG).

Good pickup lurkingpax, nice crowdsourcing info.
For those interested in following the document trail that lurkingpax has added to, the original document is here:
OPERATIONAL EVALUATION REPORT THE BOEING COMPANY BOEING 737 (http://www.anac.gov.br/assuntos/setor-regulado/profissionais-da-aviacao-civil/avaliacao-operacional-1/Boeing_737_OE_Report_Revoriginal.pdf)
Archived copy OPERATIONAL EVALUATION REPORT THE BOEING COMPANY BOEING 737 (https://web.archive.org/web/20181113231440/http://www.anac.gov.br/assuntos/setor-regulado/profissionais-da-aviacao-civil/avaliacao-operacional-1/Boeing_737_OE_Report_Revoriginal.pdf)
Page 18 is the only listing for the MCAS
Appendix 2 – OPERATOR DIFFERENCE REQUIREMENTS (ODR) TABLES This Design Differences tables, from the Boeing 737-800 to the Boeing 737-8, were proposed by The Boeing Company and validated by ANAC. They list the minimum differences levels operators must use to conduct differences training, checking and currency of flightcrew members

[locblue]

How’s this for arrogance and obfuscation! Boeing and FAA are “evaluating” the “need” for a software update and updating training procedures. So 189 deaths are not sufficiently compelling?!

Boeing, U.S. Regulator Weigh Software Fix on 737 Max After Crash
https://www.bloomberg.com/news/articles/2018-11-13/boeing-u-s-regulator-weigh-software-fix-on-737-max-after-crash

[CurtainTwitcher]

original #978 (https://www.pprune.org/10307740-post978.html) (permalink (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-post10307740.html))
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
Just to clarify & simplify the above statement. The MCAS is active when:

Flaps UP
High Angle of Attack signal (real or spurious)
MCAS inactivation

Flaps extended
Reduced AoA
Pilot Electric trim
unknown:

Stabilizer trim cutout switches to cutout - Does this prevent the MCAS operating.

When active, its rate of change is dependant upon mach number. At low level, low mach number, highest rate of MCAS trim is commanded. At 5000 feet, we could expect the trim to run at or close to the full rate.


From the extremely brief description of the MCAS, we can possibly conclude:

JT 610 retracted flaps to the clean configuration (MCAS becomes active)
Low mach number, therefore maximum rate of horizontal stabilizer of 0.27 degrees per second, and a forward deflection to the limit with a spurious AoA input.

[FCeng84]

A couple of thoughts on cruise deck angle and maximum performance (i.e., L/D):

1. At a given airplane configuration (read flap/gear position and payload/fuel distribution) the deck angle that generates lift = 1g will be a function of Mach number and airspeed. You can fly faster at a lower deck angle or slower at a higher deck angle. As fuel is burned during the flight, the lift required to balance weight will go down and the deck angle at a given speed will reduce. This is partially compensated during a flight by increasing altitude along a constant Mach line that yields lower impact pressure and thus less lift. All of this is factored into the airplane design.

2. While I am not an aerodynamicist and may be going out on a limb here, I believe that a configuration with the body at a slight positive deck angle will realize greater incremental benefit from the small amount of lift generated by the body vs the slight increase in body drag going from zero AOA to a small positive AOA. It is quite clear that negative AOA for the body would be a loser as it would result in increased drag and decreased lift.

3. Another variable in the mix is engine angle with respect to both the fuselage (left/right) and wing (up/down). Some of that is based on inlet flow distortion due to the forebody and considerations for engine out characteristics, but I believe that pitch/lift trim for cruise also factors in.

Standing by for someone with more aero background than me to comment.

[tscell7]

Not an aerodynamicist either but the fuselage most definitely does have a lift coefficient (extreme example: SR-71). Would love it if someone with deep experience could add some commentary here.

[FCeng84]

Per https://twitter.com/jonostrower the 737 MAX MCAS system runs the stabilizer at 0.27 deg/sec regardless of speed/mach. The amount of nose down stab motion provided by MCAS is listed as "up to 2.5 degrees. MCAS uses the stab trim motors and is disabled via selecting stab cutout.

[hans brinker]

Ok, I did not know the Max has a new stall assistance system. As mentioned before, the STS normally trims down after take off, so it would be operating in the same manner as the new stall system. So again the tech log entry about STS appears incorrect.

And why did Boeing incorportate a stall assistance system that trims, instead of using a stick pusher? This seems to go against the normal rules for flying. You fly the aircraft with the control column, not the trimmer.

A stick-pusher acts on the control columm, to increase speed, and withdraws its influence instantaneously to allow you to recover from the dive. A stall trimmer will trim you down, but leave that nose-down pressure on and impede your recovery from the dive.

A stick pusher is flying the control column, in the normal manner; while a stall trimmer is flying the trimmer. Is this how the new stall system works? I cannot find details on-line.

Silver



The way I understand it from reading here:
On a normal flight you would expect the STS to trim ANU during acceleration as it is trying to keep the aircraft at the previous speed. The previous captain said the STS was trimming incorrectly, so different from normal, so AND. The MAX has a new MCAS system, that will trim AND if it senses a low speed situation. There were reports the AOA on the captain side was giving erroneous high AOA values, leading to stick shaker as well, if the MCAS used these faulty values it could have kept on trying to trim AND every time the pilots stopped using elevator trim (if the even tried to override the airplane trim inputs manually). This could have resulted in the stabilizer being full AND, and the elevators not having enough authority to keep the nose up. The aircraft trim input could be stopped by using the elevator trim or, more permanentely, by the cutout switches (apparently is has been confirmed this would stop the MCAS from trimming.

TLDR: I think the STS trim being wrong write up was "correct", because the STS will during acceleration trim ANU not AND.

[LaissezPasser]

Per twitter com/ jonostrower the 737 MAX MCAS system runs the stabilizer at 0.27 deg/sec regardless of speed/mach. The amount of nose down stab motion provided by MCAS is listed as "up to 2.5 degrees. MCAS uses the stab trim motors and is disabled via selecting stab cutout.
Just to clarify, the MCAS function "moves the horizontal stabilizer trim upward at 0.27 degrees per second up to 2.5 degrees and 9.26 seconds at a time." So, each time the MCAS function runs, that's another 2.5 degrees.

[BuzzBox]

The B737 FCOM states:

As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane.

How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?

[CurtainTwitcher]

The B737 FCOM states:
How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?
Complete the Runaway Stabiliser NNC checklist memory items prior to the flaps being retracted. I just so happens that the first 2 items of the runaway checklist ALREADY covered by the Airspeed Unreliable NNC memory items.

Net result is actually very simple: Airspeed Unreliable NNC memory items, THEN stab trim cutoff switches.....CUTOFF

I’m still very curious as to why a system that is designed to automatically change the aircraft configuration without pilot input (via stab trim) would act based on the output data of only one of the two AOA probes. The stick pusher, which also activates based on AOA, requires both probes to agree that a stall is imminent before it will trigger, specifically because AOA probes can and do malfunction.
$$$$ - Saved some development or certification costs. Rated De covered this succinctly in #1083 (https://www.pprune.org/showpost.php?p=10309768&postcount=1083)

[JPJP]

I assume that operation of the pusher is well covered in sim and classroom training, but it appears that Max flight crews have been kept completely in the dark about the MCAS system and what it does (until now).




There’s an old saying about the word “assume”, and there is no ‘pusher’ on any 737 model. Including the MAX8.

[neville_nobody]

Pilots were kept in the dark. One has to ask why? What reason would you have to not disclose an additional system? https://www.seattletimes.com/business/boeing-aerospace/u-s-pilots-flying-737-max-werent-told-about-new-automatic-systems-change-linked-to-lion-air-crash/

One question worth asking is does this additional system affect the type certificate? If the answer to that is yes then it opens a whole new can of worms.

[LEOCh]

Now that the new MCAS is firmly implicated, I wonder why it was implicated on the MAX. One possible reason was that it was considered to be required by transport airworthiness certification via section 25. But looking through section 25 it is hard to find which exact requirement. The full regs can be easily googled (can't post as am probationary)

The previous requirement for STS is fairly easy to understand, as section 25.173 calls for "(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots". Effectively the certification standard demands the aircraft emulate (some!) characteristics of a theoretical longitudinally stable aircraft which would have:
a) COG significantly ahead of the Neutral point
b) generally have the trimmed horiz stab flying at significantly lower AoA than the wing (probably to the point of downward lift at the tail)

However for modern air transports, which are desired to have optimal efficiency, it should be best to operate at near neutral longitudinal stability (i.e. just stable). In this configuration the tail is generating very little lift in either direction and hence very little drag. "Manual flight" is really "Simulated longitudinally stable manual flight necessary to conform to 25.173", which is confusing because as other posters have noted, this is really a less-automated mode instead of a manual mode. It's not so relevant to this thread anymore, but the STS seems more regulatory than useful really...changing the stab trim position via STS does not really create a more longitudinally stable aircraft and the pilot is likely to respond by removing the trim input anyway to avoid unwanted altitude excursions. This leaves him back with a slightly stable aircraft which would probably cause fatigue if pilots were expected to fly whole sectors in "manual mode", but of course they are not.

So why MCAS? It is not a speed stability system, it appears to be a beefed up stall prevention system. If the MAX COG has crept further back, it's stability margin has decreased but I don't see why an antistall stab trimmer becomes a good idea or a regulatory requirement. Looking through the stall/stall warning sections (25.201, 25.203, 25.207) there is a preference for aggressive pitch down at stall which may have something to do with it:The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—
(1) A nose-down pitch that cannot be readily arrested;
(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or
(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. If anyone has a better understanding of how MCAS is dictated by section 25 airworthiness would be very interesting to hear.

[sAx_R54]

The B737 FCOM states:



How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?

Explanation of sorts here: https://theaircurrent.com/aviation-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas-jt610/

[Rananim]

I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.

[DaveReidUK]

I love the way that, after 1000+ posts, we're back at Aerodynamics 101. :O

No airplane can fly with zero AoA.Wow! I don’t see a lot of aircraft falling from the sky. Could you be incorrect?
No, not incorrect, just badly worded.

[omnis]

Originally Posted by Concours77 https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-post10310346.html#post10310346)
No airplane can fly with zero AoA.

I love the way that, after 1000+ posts, we're back at Aerodynamics 101. https://www.pprune.org/images/smilies/embarass.gif[
No, not incorrect, just badly worded.


Badly worded, and certainly not correct. Clear need to define terms and check textbooks.

[roundsounds]

I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.
finally a post making sense!

[gearlever]

I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.

Spot on. You nailed it:ok:

[A Squared]

I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.

As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . There's a lot of evidence which suggests that this theory is fanciful. Not the least of whcih is the fact that Boeing, after realizing that they had a serious problem that was killing entire planeloads of Boeing passengers at a whack, and after careful consideration, issued a Manual bulletin, and subsequent Emergency Airworthiness Directive which describes the problem as being exclusive to the 737 MAX models, and not applicable to 737NG or prior. Even given that Boeing hasn't behaved admirably leading up to the accident, It is very difficult to imagine the folks at Boeing, after realizing what likely led to the accident, deciding: "Hey, even though we realize there's a serious problem, and even though this problem could just as likely cause an NG to crash through the same sequence of events, we're going to publicly pretend that this is limited to MAX models only. Point being, Boeing probably had pretty sound reasons for saying that the problem was exclusive to the 737 MAX -8 and -9, and not including the NG and prior models.

[silverstrata]

The way I understand it from reading here:
On a normal flight you would expect the STS to trim ANU during acceleration as it is trying to keep the aircraft at the previous speed. TLDR: I think the STS trim being wrong write up was "correct", because the STS will during acceleration trim ANU not AND.

The STS normally trims nose down after take-off.
It is safer that way....

Silver

[Gysbreght]

Fuselage contributing to lift

The wing lift is associated with drag, called lift-dependent drag or 'induced' drag. It is a function of the wing aspect ratio and the spanwise lift distribution. To minimize induced drag you want to have a high aspect ratio and ideally an elliptical spanwise lift distribution. The drop-off of lift near the tip causes the tip trailing vortices.

If the lift dropped to zero at the wing roots you would effectively have two wings with half the aspect ratio and twice the induced drag as the complete wing, and 'tip' vortices at the tips and wing roots.

That doesn't happen. The spanwise lift distribution is continuous across the fuselage except perhaps a slight dip. In other words, the fuselage contributes to the overall lift.

[silverstrata]

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

That seems a lot of trim to me. Try flying at 5,000 and 250 kt, and then mis-trimming forward by 2.5 degrees. You will be struggling.

Strikes me that Boeing designed this stall-trim system assuming that the aircraft would be slow and approaching the stall, when 2.5 degrees of trim would be acceptable. But at 250 kt in a normal climb, 2.5 degrees of trim becomes a liability. And if it gives you another 2.5 degrees of trim, you are uncontrollable.

Sounds like Boeing made this system on the cheap. The reason aircraft have stick pushers, is because the push is instantaneous, on the stick, and its withdrawal is instantaneous too - so you can pull out of the dive. But if you ‘push’ via a trim input, you cannot easily undo that trim - and so you cannot easily recover from the dive. And as a basic flying principle, we always fly with the stick, and trim out the stick pressure. You never fly with the trimmer, which is what this system is trying to do.

Furthermore, an erroneous stall-trim input at 250 kt (due faulty AoA indication) will give huge stick forces that the system did not intend - and perhaps the designer and test pilots never considered. A faulty stick push can be easily overcome with stick pressure - it was designed that way, and haveing already got the T-shirt on that one, the aircraft is easily flyable. But if a system gives you 2.5 of trim, and then another 2.5 of trim, you are going down, because the stab-trim is much more powerful than the elevator. Did anyone ever test this...?

Silver

[gearlever]

- degrees
- units
- % MAC

Where does a 737 jock read "degrees" of the STAB ?

[silverstrata]

That's true, though of course most wings, being cambered, do still produce some lift at 0° AoA.

I wish you would all stop this angle of attack nonsense. You are all confusing fuselage pitch-angle (on the PFD) and the wing angle (which can be changed with the rigging angle). Of course you can fly an aircraft with zero fuselage angle, as long as the wing still has a positive rigging angle, to maintain a positive wing angle of attack. Which is how most aircraft are built.

The reason most aircraft fly with 2.5 degrees pitch up, is that we do not fly at max speed. If you flew at MMO plus 3%, which is the max testing design speed, you would probably find that the fuselage had a zero pitch angle. (A negative fuselage angle would induce some efficiency and stability problems.)

Silver

[silverstrata]

- degrees
- units
- % MAC

Where does a 737 jock read "degrees" of the STAB ?

That is a 737 colloquialism, using ’degrees’ in the generic sense - eveyone says ‘degrees’ but we mean ‘units’. As you say, they are not real degrees - you would have to look on the trim-sheet to see the conversion between MAC and trim, as it is not linear.

And regards rhe STS, people here saying it is controlled by speed. It is a bit more complex than that, as the inputs to the STS are:

Stab position.
Thrust lever position.
Airspeed.
Vertical speed.
Time after take-off (more than 10 seconds)
Time after manual trim (more than 5 seconds)
N1 more than 60%.
No autopilot.
Elevator trim requirements

Silver.

[A Squared]

That is a 737 colloquialism, using ’degrees’ in the generic sense - eveyone says ‘degrees’ but we mean ‘units’. As you say, they are not real degrees - you would have to look on the trim-sheet to see the conversion between MAC and trim, as it is not linear.

I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.

[Capn Bloggs]

As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . There's a lot of evidence which suggests that this theory is fanciful.
I agree ("fanciful" is a bit harsh though ;)). Rananim, the STS and the MCAS appear to be different. This 737 hit the water going very fast. The STS should have been trimming back. The MCAS, if the AoA was erroneously high, would have been trimming forward. The MCAS is only fitted to the Max.

[MickG0105]

...
And regards rhe STS, people here saying it is controlled by speed. It is a bit more complex than that, as the inputs to the STS are:

Stab position.
Thrust lever position.
Airspeed.
Vertical speed.
Time after take-off (more than 10 seconds)
Time after manual trim (more than 5 seconds)
N1 more than 60%.
No autopilot.
Elevator trim requirements

Silver.

Can I ask where you got the 'N1 more than 60%' condition for the STS from please?

[A Squared]

I agree ("fanciful" is a bit harsh though ;)). Rananim, the STS and the MCAS appear to be different. This 737 hit the water going very fast. The STS should have been trimming back. The MCAS, if the AoA was erroneously high, would have been trimming forward. The MCAS is only fitted to the Max.

I agree with your comments about the MCAS, I disagree that "fanciful" is unnecessarily harsh. Boeing knows what caused the crash and how their automated stabilizer trim systems contributed to that. That's why they published the Bulletin, which spawned the Emergency Airworthiness Directive. Boeing has delivered 200-ish MAX's They have delivered 7,000-ish NG's. Given that the Bulletin and Emergency AD are very clearly specific to the MAX's and not applicable to the NG's, only 2 possibilities exist:

1.) Boeing knows that the malfunction that caused the Lion Air Crash is something that could occur in all 7200 737 NGs and MAXs, but for reasons known only to themselves, limited the applicability of the Emergency Airworthiness Directive to the 200 MAXs, and not to the other 7,000 NGs whcih could also experience the same malfunction.

2.) Boeing, the designer and manufacturers of both the NGs and MAXs, has very good reasons for believing that the malfunction will only happen in the 200 MAXs and not in the 7,000 NGs in service.

I would say "fanciful" is a fairly restrained characterization of thinking which considers 1 to be more plausible than 2.

[silverstrata]

Can I ask where you got the 'N1 more than 60%' condition for the STS from please?

From the FCOM (737 classic). OMB ll 9.20.9. STS requires 60% N1 before operation.
I presume that limits STS to the climb.

Silver

[silverstrata]

I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.

Good point.

The trim sheet has: Indexes, MACs, and Units. The FCOM has Units. Nowhere that I can see mentions trim degrees. If Boeing is quoting real degrees here, and not trim units, then that is a whole different ballgame. What is the conversion between degrees and units?

However, if the aircraft was slow, nearing the stall, you would indeed need 2.5 units (or degrees?) of forward trim. The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...

Silver

[gearlever]

I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.

Yep.
That's how I feel as well, but may be wrong.

[A Squared]

Good point.

The trim sheet has: Indexes, MACs, and Units. The FCOM has Units. Nowhere that I can see mentions trim degrees.

If Boeing is quoting real degrees here, and not trim units, then that is a whole different ballgame. What is the conversion between degrees and units? Besides, if the aircraft was slow, nearing the stall, you would indeed need 2.5 units (or degrees?) of forward trim. The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...

Silver


You're probably correct that "degrees" doesn't appear in *operational* publications, But I would speculate that you would find "degrees" in design/engineering/maintenance publications, and that's probably where the numbers in the AD come from; drawn directly from the design specs for the MCAS.

The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...

..or *another* 2.5 degrees/units 5 seconds later ...

[infrequentflyer789]

As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . [...] Point being, Boeing probably had pretty sound reasons for saying that the problem was exclusive to the 737 MAX -8 and -9, and not including the NG and prior models.

Boeing probably had pretty sound (to Boeing) reasons for not telling anyone about MCAS, those reasons probably look like "WTF" to everyone else, but that's with the benefit of hindsight.

I would caution that the AD is quite specific that it addresses an issue caused by erroneously high AOA input, if AOA is not an input to STS on previous models then this issue can't happen on them. That doesn't mean that STS on NG doesn't have the capability to dump you into the ocean, just that it cannot do it because of one dud AOA input, and the runaway stab trim procedure is there for the other cases. It is also possible that STS runaway on NG and earlier would require multiple sensor or ADIRU failures making it a lot less likely - I still don't get how only one sensor failure can upset MCAS, I can't (or don't want to) believe they took a direct output from one sensor into the stab trim.

My personal theory on STS/MCAS naming is that MCAS was invented and named in engineering/design to meet Part 25, and the existing STS was the cheapest/simplest implementation option (possibly just software change and an additional input on the same unit). The MCAS acronym (definition only) sneaked out into some versions of the documentation but then someone decided that if it was effectively just an existing system, STS, why document it separately - it's all STS which is already covered. Further down the line someone decided that if the additional STS documentation was about an area of the envelope where only test pilots would go, why worry anyone else with it. So it became undocumented.

Add to the above a very slight suspicion that treating it as a separate new control system just might have opened a certification can of worms that Boeing wanted to avoid (as someone else suggested earlier in thread).

[A0283]

I think many of those on Pprune are not only interested in the WHAT but also the WHY. Why did Boeing design it this way and communicate it this way.

A real life option of what might have happened in the WHY field is that during design and test they stumbled on some issues in the envelope with respect to stall in certain regimes. As a designer/manufacturer, WHATEVER you run into, you don't want to 'break the Type Certificate'. Breaking the TC is not only very costly but will also cause substantial delay. (I change the sequence of the items compared to what A Squared posted earlier in a reply to Silverstrata - changed it because it is meaningful). So they generated a number of possible solutions. They selected the 'MCAS' solution for implementation which did not require breaking the TC. What we don't know, next to knowing surprising little about MCAS at this stage, is how it was tested and under what assumptions and rules. And if anyone rang a bell (would surprise me if not, but ..).

A proper solution requires proper communication to all stakeholders, so not only to pilots, trainers but also to maintainers. It is hard to understand why that communication was not up to the expected level. Especially as a number of consequences of the MCAS design decision will appear counter-intuitive to quite a few people.

In another 737 case I was surprised that 737's did not have a certain well proven system. The published reason for that was documented as being to stop short from breaking the TC. Will try to dig up that information.

When there are answers on the issues above, it would not surprise me if there will be other questions that will zoom in on the history of the envelope issue. Was it there before the MAX, and how much margin was there for earlier solutions.

[A Squared]

That doesn't mean that STS on NG doesn't have the capability to dump you into the ocean, just that it cannot do it because of one dud AOA input,

No disagreement. Any time you're changing the angle of a stabilizer, the potential exists for loss of control of the aircraft if you lose control of the system adjusting the stabilizer. I doubt anyone disagrees on that point. The context of the discussion at hand is the specific malfunction whcih (apparently) led to loss of control of Lion's 737MAX as the result of bad data from a single AoA sensor.

[A Squared]

The MCAS acronym (definition only) sneaked out into some versions of the documentation but then someone decided that if it was effectively just an existing system, STS, why document it separately - it's all STS which is already covered.

Not to say that you're wrong, that may well have been the thinking. But the objections to that rationale is that the MCAS does something pretty different than the STS (almost opposite) for different reasons and in a very different flight regime ,so saying that "it's just the same "system"" is a weak reason for not disclosing it.

[Concours77]

MCAS is different, and was developed to accomodate the new powerplants, and their re orientation on the airframe.

Further forward, lifted, and contributing to 14% improvement in SFC.

Some would claim this was a challenge to all sorts of previously certified characteristics (sic). Handling changed, controls response changed, and required a new protection against NU getting too aggressive.

Data was kept from pilots Boeing considered “average”. Too sophisticated to clue in the operator? Interesting? Because in Boeing’s response, it was claimed “new research” uncovered the problem, that is ridiculous.

FAA reserved the right to “augment” the AD at a later date. I will repeat my comment re the CofA. If handling is markedly different sufficient to add a new trimming feature, should the certification be amended? Boeing obviously thought it was subtle enough to not mention in the FCOM. If the handling is different, to what degree should it remain ignored? (Purposely ignored, and not acknowledged to the operators.)

[A Squared]

If handling is markedly different sufficient to add a new trimming feature, should the certification be amended? Boeing obviously thought it was subtle enough to not mention in the FCOM. If the handling is different to what degree should it remain ignored?


Or to put it differently: When is a 737 no longer a 737?

[MickG0105]

From the FCOM (737 classic). OMB ll 9.20.9. STS requires 60% N1 before operation.
I presume that limits STS to the climb.

Silver
Thanks for that, that is interesting, that engine parameter is not listed in the conditions for speed trim for the NG.

Conditions for speed trim operation are listed below:
• STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68
• 10 seconds after takeoff
• 5 seconds following release of trim switches
• Autopilot not engaged
• Sensing of trim requirement

[DaveReidUK]

You're probably correct that "degrees" doesn't appear in *operational* publications, But I would speculate that you would find "degrees" in design/engineering/maintenance publications, and that's probably where the numbers in the AD come from; drawn directly from the design specs for the MCAS.

If it's any help, typical values for older 737s (may vary for the NG/Max, but unlikely to be much different) are:

Max downwards stab (i.e. a/c nose up) angle: -12.9°
Max upwards stab (i.e. a/c nose down) angle: +4.2°

A stab angle of 0° corresponds to 4 units of nose-up (ANU) trim at the lever. Presumably the purpose of ANU units is simply to avoid confusion between positive and negative values, so it may well be that each ANU unit increment = 1°.

[Gysbreght]

Originally posted by AO283: "As a designer/manufacturer, WHATEVER you run into, you don't want to 'break the Type Certificate'. Breaking the TC is not only very costly but will also cause substantial delay. "

I would think that virtually ANY change to an approved type design requires an amendment to the Type Certificate. See the B737 Type Certification Data Sheet (TCDS) on the FAA website:

TCDS A16WE Rev 64 Boeing Company, The (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/179CDACD213801658625832A006B2E37?OpenDocument)

[Concours77]

Or to put it differently: When is a 737 no longer a 737?

Affirm. But then that’s just an opinion. Once more for good measure: Disclosure is a mandatory feature of change. Without it, airplanes can start to become “What’s it doing now”. Ask Bernard.

Compared to the Classic, the MAX is “hot”. Airbus builds wildly different airframes under the same TC, ?No? Boeing afforded the same discretion? Sim only, no airtime to upgrade? Sisters?

[A Squared]

If it's any help, typical values for older 737s (may vary for the NG/Max, but unlikely to be much different) are:

Max downwards stab (i.e. a/c nose up) angle: -12.9°
Max upwards stab (i.e. a/c nose down) angle: +4.2°

A stab angle of 0° corresponds to 4 units of nose-up (ANU) trim at the lever. Presumably the purpose of ANU units is simply to avoid confusion between positive and negative values, so it may well be that each ANU unit increment = 1°.

So, assuming the MAX is not terribly different, you may only be 2 operation cycles of the MCAS away from having your stabilizer at full stab-up (nose down) travel.

[DaveReidUK]

I would think that virtually ANY change to an approved type design requires an amendment to the Type Certificate. See the B737 Type Certification Data Sheet (TCDS) on the FAA website:

Yes, of course.

But the poster's point was that having to amend an existing TC is greatly preferable to the FAA/EASA saying that the manufacturer has changed so much that it's essentially a new type and has to be certificated from scratch.

[Concours77]

So, assuming the MAX is not terribly different, you may only be 2 operation cycles of the MCAS away from having your stabilizer at full stab-up (nose down) travel.

HolyCrap. That is a chilling way to see it.

The question. Will the inclusion of the OMB solve the problem? Or is the design of MCAS insufficient, relying evidently on one AoA input. Seemingly, other sensor input did not mitigate the one vane from overruling the other parameters? Flaps, Mach, etc.?

[safetypee]

locblue, #1146. :ok:

The linked article discusses hardware/ software changes; the option for training, human intervention and mitigation, given this event, should not be considered.
It is also questionable if the crew actions and description given in the AD will be adequate in the short term.
The regulators might judge otherwise, they must be under considerable internal pressure due to apparent weaknesses in their safety oversight. Also pressure from the wider industry, concerns over disruption and economic upset if 737MAX flying was interrupted.

A cynical view might point to overt support at high level; economically too disastrous to contemplate, so take the risk. But ensure that the risk is passed down the line; if there is an event the operator / crew will be in the firing line - ‘didn’t follow procedures’, without thought whether crews would be able to act or not.
If so then the regulatory decision process sheds all responsibility.
If the unlikely occurs then the enormity of the effect on the industry (including regulators) will be unimaginable.

[Concours77]

safetypee....

”...A cynical view might point to overt support at high level; economically too disastrous to contemplate, so take the risk. But ensure that the risk is passed down the line; if there is an event the operator / crew will be in the firing line - ‘didn’t follow procedures’, without thought whether crews would be able to act or not.
If so then the regulatory decision process sheds all responsibility.
If the unlikely occurs then the enormity of the effect on the industry (including regulators) will be unimaginable...”

It would only be cynical if the reality was other than blatantly apparent. I would use the word “rational”...

Who guards the guardians? “Self Certification?” Sole source sensor? Economically too disastrous? At ten million per plaintiff....

Let’s see, that’s two Billion dollars, plus another Billion for legal fees. Plus re-certification. It’s a standout airplane. The corporation not so much.

[KenV]

Any time you're changing the angle of a stabilizer, the potential exists for loss of control of the aircraft if you lose control of the system adjusting the stabilizer. I doubt anyone disagrees on that point.Don't disagree per se, but this statement requires clarification. Runaway trim (from whatever cause) can cause loss of control only in a very small corner of the flight envelope (specifically, when slow, at high AOA, and with the engines at high power.) The accident aircraft was not in this corner of the envelope so the pilots had sufficient elevator authority to over ride the trim. Assuming (and yes this remains a BIG assumption) the trim ran away due to the MCAS getting erroneous AOA sensor data, the pilots should have been able to maintain control and overcome the mis trim. The mystery is why they did not.

Further, on the subject of why Boeing did not include the MCAS in the flight manual, could it be because the existing procedures for runaway trim are applicable whether the cause is a stuck switch, stuck sensor, misbehaving STS, or misbehaving MCAS. Troubleshooting the systems to determine what the root cause is for a runaway trim condition just delays the pilot from executing the runaway trim procedures. Further still, this could also explain why Boeing chose to implement MCAS the way they did, using the trim system vs a stick pusher. It makes operation of the system and any emergency procedures related to malfunction of the system completely transparent to the pilots.

[Concours77]

If MCAS was active, by definition your parameters were met. TRIM AND at altitude should be recoverable? Changing the focus again?

[infrequentflyer789]

Not to say that you're wrong, that may well have been the thinking. But the objections to that rationale is that the MCAS does something pretty different than the STS (almost opposite) for different reasons and in a very different flight regime ,so saying that "it's just the same "system"" is a weak reason for not disclosing it.

Just to clarify, I wasn't saying that I agree with the rationale either, just speculating as to how it could have come about, whilst also aware that I do so with the benefits of hindsight, and without the pressure of needing to design a new plane because the 320neo is eating our lunch whilst not having the time or resources to actually design a new plane.

Sometimes (in any field) the answer to "how the :mad: did we get so far of course?" is "one small step at a time, each step logical and justifiable considered in isolation".

New flight control system -->
New control system implemented "efficiently" by using existing system -->
Modification to existing system -->
Undocumented modification to existing system

Overall result: undocumented new flight control system. And crash, of course, or we wouldn't be here:(.

[Capn Bloggs]

Runaway trim (from whatever cause) can cause loss of control only in a very small corner of the flight envelope (specifically, when slow, at high AOA, and with the engines at high power.)
You've said this before. Have you any proof, or is it a certification requirement? I am sure (never having actually tried it) that if I was doing > 250 KIAS (as they appear to have been doing when level and more in the dive) there is no way (not 737 but medium jet) I could pull enough backstick to hold the nose up if the stab trim was wound full nose-down.

[WillowRun 6-3]

From the reporting in the Wall Street Journal (by A. Pasztor and A. Tangel) yesterday, Nov. 13:

"....company had decided against disclosing more details to cockpit crews due to concerns about inundating average pilots with too much information - and significantly more technical data - than they needed or could digest." (emphasis added)

First, what and who provided the definition of an "average pilot"? (and various extension questions, such as, is this construct of an average aviator one that is subject to refinement accounting for air carrier, extent or absence of military flying career, age and education, country of licensing, just to iterate a selection). And aren't the abilities and attributes expected of aviators for sorting through sudden, severe onsets of high spikes in information the business of other authorities in the safety system of aviation - not the airframers?

Second, have we - the global civil international aviation system - just witnessed the otherwise unheralded start of autonomous aircraft? Understanding the instrumentation, flight control and avionics components of the accident aircraft did not constitute a complete autonomous capacity - but it reportedly was autonomous enough so that the first step of the Holy Trinity seems to have been voided. [ Aviate, Navigate.....]

Third, attention General Counsel of Boeing: Remember, one central ability and attribute of a proper General Counsel is to be steadfast as "the calm presence in the room."

WillowRun 6-3

[filejw]

You've said this before. Have you any proof, or is it a certification requirement? I am sure (never having actually tried it) that if I was doing > 250 KIAS (as they appear to have been doing when level and more in the dive) there is no way (not 737 but medium jet) I could pull enough backstick to hold the nose up if the stab trim was wound full nose-down.

I've been in the sim when both pilots needed to fly the a/c because the runaway wasn’t recognized in a timely manner . Doable but not pretty.

[KenV]

You've said this before. Have you any proof, or is it a certification requirement?This has been a basic design philosophy at both Boeing and Douglas since forever. And because that is the way it is designed, it is tested to ensure it behaves as designed and is part of the type certification. Indeed, when Boeing provides the flight training, that training includes flying the aircraft (in the sim) with a hardover trim condition. No it's not easy and may require both pilots. But yes it is very much possible.

Remember the 737 flight control system was designed in the 60s and is fully mechanical. The 737 has a measure of SCAS (Stabililty Control Augmentation System) with STS and MCAS, but those systems act entirely thru the mechanical flight control system (more specifically, the trim portion of the mechanical flight control system). Further, the exact action (though not the reason why) of these systems is easily discernible by looking at the trim wheel, the action is over ridden/cancelled with the push of the trim switch on either pilot's yoke, can be cut out completely with a cutout switch, and if all else fails, can be overpowered by applying high stick forces. The Boeing and Douglas design philosophy is to always give the pilots the ultimate say and the ability to overpower whatever an electronic system wants to do. Indeed that is why these systems only operate through the trim system, because the base flight control system can overpower trim. Yes, this means that unlike many Airbus aircraft, the pilots of Boeing/Douglas aircraft can do stupid things and endanger the aircraft. It's not like the A320 where if you have the wrong flight control mode selected the pilots cannot simply overpower the system by pushing on the stick and/or the throttle to over ride envelope protection, and must instead deselect the current mode and select another. I'm not saying or even suggesting one design approach is "better" or "safer" than the other, I'm saying the approaches are fundamentally and significantly different.

[KenV]

From the reporting in the Wall Street Journal (by A. Pasztor and A. Tangel) yesterday, Nov. 13:

"....company had decided against disclosing more details to cockpit crews due to concerns about inundating average pilots with too much information - and significantly more technical data - than they needed or could digest." (emphasis added)

First, what and who provided the definition of an "average pilot"? (and various extension questions, such as, is this construct of an average aviator one that is subject to refinement accounting for air carrier, extent or absence of military flying career, age and education, country of licensing, just to iterate a selection). And aren't the abilities and attributes expected of aviators for sorting through sudden, severe onsets of high spikes in information the business of other authorities in the safety system of aviation - not the airframers?You appear to be very ignorant of the certification process. Yes, the airframers decide that, and no the regulators do not. Airbus for example assumes a significantly lower skill level pilot than either Boeing or Douglas, provides more automation, and provides/imposes more protections that cannot be over ridden by the pilots. Further, I've already said it multiple times. The emergency procedure for a 737 runaway trim condition (whatever the root cause be it STS, MCAS, or magical gremlins) is the same. Inundating the pilots with checklists and troubleshooting procedures to determine root cause is pointless and just delays execution of the procedure. Perform the procedure, fly the airplane, get it on the ground, and let the maintainers do the trouble shooting. As it is, the vast majority of pilots don't understand STS and indeed don't understand what it does nor why even after hundreds of thousands of pilots have flown the aircraft for multiple decades and millions upon millions of safe flight hours. The point is they don't really need to understand STS. They only need to recognize a runaway trim condition, need to be trained to recognize it in a timely manner, and need to know the procedures to overcome it. The same applies to MCAS.

[safetypee]

Concours, #1194, OK rational.
The economic view considered aircraft grounding.
Add dented corporate and aircraft images, effect on all sales.

“through inaction, do not allow a human being to come to harm” Asimov

[WillowRun 6-3]

KenV

I might be less knowledgeable than you are but I'm not making broad assumptions in these questions. It remains to be determined whether disclosure of the MCAS subsystem would or would not have made a substantial difference in this crash. Yeah, I don't claim expertise in type certification processes but then, I'm also not asserting clairvoyance as to what the CVR - and probable cause analysis of the AIB - will show.

As to differences in Boeing and Airbus, this still begs the question. You didn't get any sense from statements by officials of pilots' unions that references to an "average pilot" were a bit objectionable?

As for the definition itself, read my post and see if this time you can spot where I said this point is up to regulators... ( hint, I didn't).

It's still a valid question: how is this baseline defined, based on what data, and with what level of transparency.

[silverstrata]

Max downwards stab (i.e. a/c nose up) angle: -12.9°
Max upwards stab (i.e. a/c nose down) angle: +4.2°

A stab angle of 0° corresponds to 4 units of nose-up (ANU) trim at the lever. Presumably the purpose of ANU units is simply to avoid confusion between positive and negative values, so it may well be that each ANU unit increment = 1°.

Ah, well that clears up the small mystery.
The Classic has 17 units of trim, which equates very nicely to your 17 degrees of trim.

Silver.

[silverstrata]

Don't disagree per se, but this statement requires clarification. Runaway trim (from whatever cause) can cause loss of control only in a very small corner of the flight envelope (specifically, when slow, at high AOA, and with the engines at high power.) .

Disagree entirely, the mighty stab trim can overpower the piddling elevator.
Especially if you were on the front of the flight envelope to start with.
Have you ever tried giving max nose down trim at 250 kt...?

Silver

[Sriajuda]

A layman's question: As this MCAS trim behaviour seems to be designed as an emergency measure in lieu of a stick pusher, is there any information feedback to the pilots that it's been activated? Visual, auditory? Apart from the normal feedback of the trim wheels turning and clacking?

[silverstrata]

So, assuming the MAX is not terribly different, you may only be 2 operation cycles of the MCAS away from having your stabilizer at full stab-up (nose down) travel.

Which is why you want a stick push, rather than a stall trim.

A pusher can give you full stick forward, but as soon as you reach flying speed it will relent and give you normal control with normal trim. So you fly away with a few beads of sweat, and get some tea and bisuits with chiefy. But a stall-trimmer on the forward stop is going to take an age and a half to retrim - even if you recognise what has happened - resulting in salty tea and soggy biscuits with Davey Jones...!

Silver

[silverstrata]

A layman's question: As this MCAS trim behaviour seems to be designed as an emergency measure in lieu of a stick pusher, is there any information feedback to the pilots that it's been activated? Visual, auditory? Apart from the normal feedback of the trim wheels turning and clacking?

One imagines that the stick-shaker might also operate (there is no stall alert-tone or warning-lamp). Depends how close to the stall this stall-trimmer system starts to operate. Other than that, I presume the new system is like the STS, and all you get is the clunking and clanking of the trim-wheel. If it had a new warning tone or light, it would have needed a separate section in the manual.

And bearing in mind that this was a spurious warning, would the stick shaker even operate..?

Silver.

[hans brinker]

The STS normally trims nose down after take-off.
It is safer that way....

Silver


From you posts it is obvious you know more about the 737 than I, so apologies for going on about this, but I feel it is an important point:
The main reason for the STS is to make the airplane more speed stable, so it artificially flies the airplane back to a reference speed with trim inputs. If the STS trims AND after T/O your speed must have been below the reference speed, and the STS trims to get the nose in the direction of the speed change back to reference speed.
It does not trim AND just because it is safer, it is flying the aircraft with the trim to get the aircraft back to reference speed. Most pilots seem to blip the trims to set a new reference speed, and stop the STS from trimming in that direction. As the pilot starts to accelerate the STS will try to keep the reference speed so it will trim ANU, while the pilot wants to accelerate and trims AND.
What I think happened with the write-up is that after they raised the flaps and were accelerating the MCAS kept trimming AND and that is the unexpected trim direction the previous captain was referring to, because the STS would still be trying to trim ANU normally. I am inclined to believe he knew what the expected trim direction was in this situation based on the fact he was able to continue without crashing.
Again, all just based on an A pilot reading about B planes, so if I am incorrect about the technology please correct me.

[Sriajuda]

One imagines that the stick-shaker might also operate (there is no stall alert-tone or warning-lamp). Depends how close to the stall this stall-trimmer system starts to operate. Other than that, I presume the new system is like the STS, and all you get is the clunking and clanking of the trim-wheel.

Apart from the discussion if a ND trim automatism on a detected impeding stall (false or not) is a good choice, I think that any autonomous, severe intervention into the flight controls should always be clearly announced to the pilots. The stick shaker cannot replace that as it is only reporting on a condition of the AC, not on actions taken by it's (autonomous) systems.

Just my 2 cents as a systems engineer in other fields than aviation.

[hans brinker]

That seems a lot of trim to me. Try flying at 5,000 and 250 kt, and then mis-trimming forward by 2.5 degrees. You will be struggling.

Strikes me that Boeing designed this stall-trim system assuming that the aircraft would be slow and approaching the stall, when 2.5 degrees of trim would be acceptable. But at 250 kt in a normal climb, 2.5 degrees of trim becomes a liability. And if it gives you another 2.5 degrees of trim, you are uncontrollable.

Sounds like Boeing made this system on the cheap. The reason aircraft have stick pushers, is because the push is instantaneous, on the stick, and its withdrawal is instantaneous too - so you can pull out of the dive. But if you ‘push’ via a trim input, you cannot easily undo that trim - and so you cannot easily recover from the dive. And as a basic flying principle, we always fly with the stick, and trim out the stick pressure. You never fly with the trimmer, which is what this system is trying to do.

Furthermore, an erroneous stall-trim input at 250 kt (due faulty AoA indication) will give huge stick forces that the system did not intend - and perhaps the designer and test pilots never considered. A faulty stick push can be easily overcome with stick pressure - it was designed that way, and haveing already got the T-shirt on that one, the aircraft is easily flyable. But if a system gives you 2.5 of trim, and then another 2.5 of trim, you are going down, because the stab-trim is much more powerful than the elevator. Did anyone ever test this...?

Silver


Absolutely agree

[hans brinker]

This has been a basic design philosophy at both Boeing and Douglas since forever. And because that is the way it is designed, it is tested to ensure it behaves as designed and is part of the type certification. Indeed, when Boeing provides the flight training, that training includes flying the aircraft (in the sim) with a hardover trim condition. No it's not easy and may require both pilots. But yes it is very much possible.

Remember the 737 flight control system was designed in the 60s and is fully mechanical. The 737 has a measure of SCAS (Stabililty Control Augmentation System) with STS and MCAS, but those systems act entirely thru the mechanical flight control system (more specifically, the trim portion of the mechanical flight control system). Further, the exact action (though not the reason why) of these systems is easily discernible by looking at the trim wheel, the action is over ridden/cancelled with the push of the trim switch on either pilot's yoke, can be cut out completely with a cutout switch, and if all else fails, can be overpowered by applying high stick forces. The Boeing and Douglas design philosophy is to always give the pilots the ultimate say and the ability to overpower whatever an electronic system wants to do. Indeed that is why these systems only operate through the trim system, because the base flight control system can overpower trim. Yes, this means that unlike many Airbus aircraft, the pilots of Boeing/Douglas aircraft can do stupid things and endanger the aircraft. It's not like the A320 where if you have the wrong flight control mode selected the pilots cannot simply overpower the system by pushing on the stick and/or the throttle to over ride envelope protection, and must instead deselect the current mode and select another. I'm not saying or even suggesting one design approach is "better" or "safer" than the other, I'm saying the approaches are fundamentally and significantly different.

Not the only one saying this already, but I think you are wrong on how easy it would be to overpower the stabilizer. There is a thread about a Ryanair 737 losing control in the Go Around in Eindhoven a few years ago, I think they managed to get to 42 degrees ANU at a speed of 80 KIAS while pushing all the way forward, before they started trimming AND and were able to regain control (I think since added to the checklist is something about using trim in unusual attitudes). Also the 737 in Rostov-on-Don was likely trimmed full AND and the pilots were unable to pull out at 300+ KIAS, so very different sides of the envelope, but similar problem.

[Rananim]

the STS and the MCAS appear to be different. This 737 hit the water going very fast. The STS should have been trimming back. The MCAS, if the AoA was erroneously high, would have been trimming forward. The MCAS is only fitted to the Max.

No I dont concur...the plane was going fast but HAL (ie FCC A) thought it was near to a stall so....it trimmed down...hence the previous commanders observation of " STS trimming the wrong way"..AMM NG data implies that providing the controlling FCC is being fed corrupt data(CAS/AoA etc) and the crew dont disable AP stab trim(or ALL trim as per Boeing AD),then it will happen in a NG as well.....Boeing admit that it is MCAS that caused the unwanted trim in 610...ergo,MCAS is STS with some differences(software tweaks) perhaps...it cant be too different due type certificate commonality concerns as discussed.Maybe the exact sequence of 610 cant happen in a NG we need more data.

Design?As stated you cant design perfection so you have to design the best available considering the fallibilities of pitot-static system....somebody pointed out that Airbus have 3 AoA vanes and use a voting system but this was proven fallible at Perpignan...Bernd said its better to have a false stall warning than no stall warning...agreed. Airbus PRIM on Qantas 72 provided an elevator nose down input...here with 610 Boeing provides a nose down trim.Silver has argued quite well that perhaps Airbus is right in this matter.Fokker put the circuit breakers for these tactile/aural warnings close to the pilots,others bury them on the panels behind the pilots.Whatever the nuances and differences are....no design is perfect and the crew remain the last and best(hopefully) line of defense.

[Sriajuda]

Found this simulator video of a 737 trim runaway, what is the alarm at 2:57? And are the switches to turn off the autotrim still there in the newer 737s, and would they disable the MCAS?

[Organfreak]

I'm only a lowly SLF and yet, from reading the thread, it's quite clear to me that you can turn off the autotrim on the MAX, which is what they do in response to the alarm @ 2:57! Then you can plainly see them trimming by hand. :eek:

[A0283]

Originally posted by A0283: "As a designer/manufacturer, WHATEVER you run into, you don't want to 'break the Type Certificate'. Breaking the TC is not only very costly but will also cause substantial delay. "

Gysbrecht's post: I would think that virtually ANY change to an approved type design requires an amendment to the Type Certificate.

DaveReidUK's post:Yes, of course. But the A0283's point was that having to amend an existing TC is greatly preferable to the FAA/EASA saying that the manufacturer has changed so much that it's essentially a new type and has to be certificated from scratch.


In general terms there are many different types and levels of 'changes'. Some - listed in decreasing level of certification impact - would require a new type certificate (breaking the TC), some a type certificate amendment, others neither.

Generally, the lower its certification impact, the easier it is to get a change implemented. In quite a few cases certification impact is estimated by the manufacturer and can be enough reason to select a design option with less certification impact. You would of course go to EASA/FAA when you to be sure of an estimate or get a process going.

On the other hand. You could say that having a large number of changes that are blocked on account certification impact are part of the reason for manufacturers to move up to a new model or even new type.

[IcePack]

Something to try in the sim which was used on the 737-300 in the 1980’s. In the event of a full nose down stabiliser jam. Try getting the pnf to put his lap strap around the control column. ( pnf will have to move seat fwd & possibly extend the belt) Pnf then moves their seat back to take the “weight” from the column. PF should find the aircraft will fly virtually hands free. PF will find he has some fwd & aft movement of the control column to enable normal flight, but a little bit more adjustment can be made by the pnf by moving the belt either up or down the control column. Once all QRH drills etc have been done at about 500ft on finals PF prepares to take the weight & gets PNF to release the belt & strap themselves in for landing.
(It works & takes the effort out of the excercise some trainers dislike it for various reasons. Imho make use of anything that can help the situation)

[A Squared]

I'm only a lowly SLF and yet, from reading the thread, it's quite clear to me that you can turn off the autotrim on the MAX, which is what they do in response to the alarm @ 2:57! Then you can plainly see them trimming by hand. :eek:


I'm not sure it's quite as clear as you think it is. The guidance in the Emergency Airworthiness Directive states:

Disengage autopilot and control airplane pitch attitude with control column and main electric trim as required. If relaxing the column causes the trim to move, set stabilizer trim switches to CUTOUT. If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually.

To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence.

[jimtx]

Now that the new MCAS is firmly implicated, I wonder why it was implicated on the MAX. One possible reason was that it was considered to be required by transport airworthiness certification via section 25. But looking through section 25 it is hard to find which exact requirement. The full regs can be easily googled (can't post as am probationary)

The previous requirement for STS is fairly easy to understand, as section 25.173 calls for "(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots". Effectively the certification standard demands the aircraft emulate (some!) characteristics of a theoretical longitudinally stable aircraft which would have:
a) COG significantly ahead of the Neutral point
b) generally have the trimmed horiz stab flying at significantly lower AoA than the wing (probably to the point of downward lift at the tail)

However for modern air transports, which are desired to have optimal efficiency, it should be best to operate at near neutral longitudinal stability (i.e. just stable). In this configuration the tail is generating very little lift in either direction and hence very little drag. "Manual flight" is really "Simulated longitudinally stable manual flight necessary to conform to 25.173", which is confusing because as other posters have noted, this is really a less-automated mode instead of a manual mode. It's not so relevant to this thread anymore, but the STS seems more regulatory than useful really...changing the stab trim position via STS does not really create a more longitudinally stable aircraft and the pilot is likely to respond by removing the trim input anyway to avoid unwanted altitude excursions. This leaves him back with a slightly stable aircraft which would probably cause fatigue if pilots were expected to fly whole sectors in "manual mode", but of course they are not.

So why MCAS? It is not a speed stability system, it appears to be a beefed up stall prevention system. If the MAX COG has crept further back, it's stability margin has decreased but I don't see why an antistall stab trimmer becomes a good idea or a regulatory requirement. Looking through the stall/stall warning sections (25.201, 25.203, 25.207) there is a preference for aggressive pitch down at stall which may have something to do with it:The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—
(1) A nose-down pitch that cannot be readily arrested;
(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or
(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. If anyone has a better understanding of how MCAS is dictated by section 25 airworthiness would be very interesting to hear.

I'm guessing but: So, level flight autopilot on, but speed decays because of whatever, auto pilot trims stab up while pilots don't notice? When approaching stall, because of stab trim position, pilots don't have enough down elevator control to offset current stabilizer trim so Boeing trims down for them? Is this one of the things that Boeing added the MCAS for?
I also heard some reference to steep turns. If you trim during steep turns (my mantra learned during pilot training was "trim, trim, trim", but who knows now with the current aircraft) but let airspeed decay would the elevator not be able to unload the aircraft and Boeing wanted to help again.
Or is there some other reason for adding the MCAS other than the unlikely situation of the pilots trimming while slowing to that angle of attack that would trigger the MCAS. I could see something like that happening at altitude though but it could happen on earlier models also. Did the stretch change things where the stab might not allow elevator control? Of course, the older 737s ( I think) and the 727 could be put in a rare situation where you couldn't even trim electrically if you had opposite elevator input until you relaxed the elevator input and air load.

[LDRA]

Rananim;

MCAS pitch control authority might be different than STS, especially if MCAS false activate at high speed using high trim rate.

Existing runaway trim procedure may or may not be considered enough to mitigate hazard from MCAS false activate failure

[Organfreak]

I'm not sure it's quite as clear as you think it is. The guidance in the Emergency Airworthiness Directive states: [snip]

To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence.

Thanks, A Squared. I was responding mostly to what I saw in the video, which wasn't even a MAX. That'll teach me to be a know-nothing know-it-all! [maybe]

[Concours77]

A Squared.....”......To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence...”

If it is installed to provide Stall recovery, would it be reasonable to allow it to be disabled? I’m thinking there is more to Boeing’s Logic re non disclosure...

[tdracer]

Who guards the guardians? “Self Certification?”
I've been staying out of this one for the most part - I'm not a pilot, and my expertise is Propulsion, not Flight Controls.
But "Self Certification" is a serious miss-representation of the process. Boeing is "Delegated", not self certifying. The FAA is intimately involved in the a new Amended Type Certificate (ATC) (which is the case for the MAX). I was a DER or the delegated equivalent AR for nearly 30 years and was quite involved in the certification process.

A new ATC is a long, complicated process, with countless meetings between Boeing and the FAA. There would be a high level 'Tech Fam' where Boeing described all the changes planned for the MAX relative to the NG, then a series of specialist meetings where the certification basis is negotiated (this somewhat unique for an ATC (rather than a new TC) and gets into something called the Change Product Rule or CPR - CPR basically says any system that's changed as part of the new ATC has to step up to the latest regulations and be re-certified, but there are exceptions and these are what get negotiated). Then there would be another set of specialists meetings where the planned changes are discussed in detail, and the FAA would determine which items will be FAA retained and what will be delegated. In the Propulsion world, precious few changes got delegated - although many of the documenting deliverables were delegated (in my case, the FAA would routinely delegated the flight test reports, since the FAA would nearly always participate in the flight test and the 'report' was simply my documentation of what they'd already witnessed).
I don't know what happened with MCAS - I'd be somewhat surprised if MCAS was fully delegated but it's certainly possible - but to say the FAA was unaware or uninvolved in the certification of MCAS doesn't reflect the way Boeing and the FAA work.

[A Squared]

A Squared.....”......To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence...”

If it is installed to provide Stall recovery, would it be reasonable to allow it to be disabled? I’m thinking there is more to Boeing’s Logic re non disclosure...

I don't know. I'm just saying that Boeing's words in the AD seems to indicate that moving the switches to "CUTOUT" is not a 100% guaranteed clean kill on stabilizer trimming. Why that is, or how it happens, I can only speculate. I have a hard time imagining designing a system that moves your flight control surfaces, and you can't turn off completely. Perhaps Boeing is alluding to the possibility that the contacts in a STAB TRIM switch become welded and don't open the circuit. I'm grasping at straws here.

[A Squared]

I've been staying out of this one for the most part -

I was just thinking the other day ..."Hmmmm Haven't seen much of tdracer lately ... " Not that I blame you.

[Concours77]

I don't know. I'm just saying that Boeing's words in the AD seems to indicate that moving the switches to "CUTOUT" is not a 100% guaranteed clean kill on stabilizer trimming. Why that is, or how it happens, I can only speculate. I have a hard time imagining designing a system that moves your flight control surfaces, and you can't turn off completely. Perhaps Boeing is alluding to the possibility that the contacts in a STAB TRIM switch become welded and don't open the circuit. I'm grasping at straws here.

Then it must be an informal, quasi optional Stall Recovery tool. Pilot’s discretion. (I kid).

rgds.

[pax2908]

I am just an electronics engineer (non-aero) _and_ I may well have missed some important information from this discussion.
But earlier, I think it was mentioned that either one or the other of two possible sources of AoA information is selected "automatically" and remains throughout the flight, then switch to the other one for the next flight. Is this more or less correct? What is the rationale behind this? To me it appears as "we did not quite know what to do with two AoA sensors here". Presumably this also went through some review by the FAA ... who were happy with the explanation?

[DaveReidUK]

A new STC is a long, complicated process, with countless meetings between Boeing and the FAA. There would be a high level 'Tech Fam' where Boeing described all the changes planned for the MAX relative to the NG, then a series of specialist meetings where the certification basis is negotiated (this somewhat unique for an STC (rather than a new TC) and gets into something called the Change Product Rule or CPR - CPR basically says any system that's changed as part of the new STC has to step up to the latest regulations and be re-certified, but there are exceptions and these are what get negotiated).

The 737 MAX 8 was certificated (as the "737-8") in Revision 58 of the Boeing 737 Type Certificate (A16WE) on 8th March 2017.

While there are indeed a handful of STCs (16 in fact) that are applicable to the MAX 8, they do not form part of the TC.

[gums]

Question rom a "light" pilot.

I am having a hard time with the procedure for runaway trim in the matter under discussion.

The AD and such indicate that manual trim using the switches on the yoke will work, but then the system runs the trim again unless the whole trim sytem is turned off. So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?

Jez asking...
Gums

[gearlever]

Question rom a "light" pilot.

I am having a hard time with the procedure for runaway trim in the matter under discussion.

The AD and such indicate that manual trim using the switches on the yoke will work, but then the system runs the trim again unless the whole trim sytem is turned off. So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?

Jez asking...
Gums

Just pull out the handles inside wheel and trim it like a Cessna.

[wiedehopf]

Question rom a "light" pilot.

I am having a hard time with the procedure for runaway trim in the matter under discussion.

The AD and such indicate that manual trim using the switches on the yoke will work, but then the system runs the trim again unless the whole trim sytem is turned off. So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?

Jez asking...
Gums

Check the video posted a few posts above.
You can see the trim wheel in action. It can be turned by hand and even has a crank that is not used on the video (not sure why)
(This is the video in question https://tinyurl.com/ycah3ww)
A picture of the trim wheel with handle extended:
https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/290x320/main_qimg_7efd9e28308cc52c39e2ead71d7987eb_ed21aba720d8985c7 2e4e243a31db25f42b77561.jpeg

[tdracer]

I don't know. I'm just saying that Boeing's words in the AD seems to indicate that moving the switches to "CUTOUT" is not a 100% guaranteed clean kill on stabilizer trimming. Why that is, or how it happens, I can only speculate. I have a hard time imagining designing a system that moves your flight control surfaces, and you can't turn off completely. Perhaps Boeing is alluding to the possibility that the contacts in a STAB TRIM switch become welded and don't open the circuit. I'm grasping at straws here.

There are several failures that can cause a stab trim runaway - and not all are solved by turning the system 'OFF". In addition to welded switch contacts, the motor can fail in such a way that you can't turn it off. These failures are rare, but not impossible - the only way to 100% disable the system is to pull the circuit breaker(s) (and that's not something they like to tell the pilots to do).

[gums]

Thanks TDracer and others,

My poor copy of the 737 manual is not all that clear about the mechanical connection of the trim wheel with the stab. Looks to me like it still uses some electrical thing and does not physically rotate the jackscrew or have gears and such. So a frozen/welded electrical component might result in frozen stab angle and all that it entails.

After all this great discussion, I have a feeling that the Rostov crash investigation may not be over. Go around, steep climb and decreasing speed, push over and then can't recover. That is the scariest video I have ever seen other than ......

I then place myself in the cockpit of this plane and short time after gear gear up we get the nose down upset and finally recover and continue on up to 5,000 feet. And then we have ten minutes of porpoising and I wonder if it was MCAS cycles and the main electric trim switches were still not turned off simply because every time the pilot stopped using the yoke switch the damned plane started nosing over again.

later, Gums wonders....

[A0283]

Hard to keep up gents. Wonder if this description has been posted before. Pity there is no source and date on it.

The MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated AOA. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors, and during flaps up flight at airspeeds approaching stall.

MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control Computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the AOA exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 deg's and are provided at a rate of 0.27 deg's per second. The magnitude of the stabilizer input is lower at high Mach number, and greater at low Mach numbers. The function is reset once the AOA falls below the AOA threshold, or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

[CONSO]

RE GUMS 1233 cable/mechanical connection of trim wheel with styab ..
This SLF happend via google to find this link FWIW re 737 NG

https://www.slideshare.net/theoryce/b737-ng-flight-controls
Which in part says ( emphasis added )



4. (https://image.slidesharecdn.com/flightcontrols-131003221914-phpapp01/95/b737-ng-flight-controls-4-638.jpg?cb=1443782442) Fully powered rudder Hydraulic systems A-B and Stby. No tab Power assisted Elevators Hydraulic systems A and B Manual reversion Aerodynamic balance tab. Power assisted Ailerons. Hydraulic systems A and B. Manual reversion Aerodynamic balance tab. Primary Flight Controls
5. (https://image.slidesharecdn.com/flightcontrols-131003221914-phpapp01/95/b737-ng-flight-controls-5-638.jpg?cb=1443782442) Movable horizontal stabiliser powered by an electric motor for manual trim or by the Autopilot. Manual via cables from the trim wheels in the cockpit. Four flight spoilers on each wing symmetrical pairs powered by hydraulic systems A and B Secondary Flight Controls Inboard and outboard double slotted T.E Flaps Normally Hydraulic system B Alternate Electric motor Outboard and Inboard spoilers Ground only Hydraulic system A 4 slats on each wing . Hydraulic system B Normal Standby Hydraulic Alternate 2 Krueger flaps On each wing inboard of the engines move with the slats

[svhar]

I wonder why they did not make a 757 MAX instead of a 737. The 737 has lots of flaws and the MAX label has not changed anything. I don't remember any flaws on the 757.

[SLFstu]

Non-flyer here, sticking my neck out.

I note that tdracer and DaveReidUK provided clout about certification methodology in recent posts.

My source docs,
* B737 TC (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/179cdacd213801658625832a006b2e37/$FILE/A16WE_Rev_64.pdf) - an FAA doc actually named “Type Certificate Data Sheet A16WE” (page with link originally provided by Gysbreght in post #1187)
* Section 25.672 (https://www.ecfr.gov/cgi-bin/text-idx?SID=354c7d822807db97db0eec85c5962713&mc=true&node=se14.1.25_1672&rgn=div8) of Part 25 Airworthiness Standards (FAA)

Firstly, note that the B737 MAX-8 and -9 have both been methodically added to the B737 TC, as shown on the current B737 Type Certificate Data Sheet. Application for MAX-8 certification was first put to the FAA in June, 2012; design was finalized mid 2013; and its specific TC was issued in March 2017, (10 weeks before being flown by the first operator). In doing so, these new MAX a/c append in time sequence to a long list of all previous 737 variants that the TC Data Sheet covers. There are 2 more of the 737 MAX family to come yet.

So, digging into the TC, towards the end where 737-8 and 737-9 certification is summarized. On page 74 of the doc we find that in the CONTROL SYSTEMS section, the included item Part 25.672 Stability Augmentation and Automatic and Power-Operated Systems (an FAA named item) gets its first and only mention. This in a certification data sheet which includes every model since the very first 737, so logically the item applies only to the MAX-8 and -9. Apparently none of the prior B737 variants were certified for this.

Is it too much to conclude that this is MCAS first seeing the regulatory light of day?

If so, in the second source doc above, the FAA sets out the regulatory requirements for systems having this class of functionality. If Boeing had the MCAS in mind in citing this section for certification, there may be some pertinent questions to be answered.

[tdracer]

The 737 MAX 8 was certificated (as the "737-8") in Revision 58 of the Boeing 737 Type Certificate (A44WE) on 8th March 2017.

While there are indeed a handful of STCs (16 in fact) that are applicable to the MAX 8, they do not form part of the TC.
Yes. I stand corrected - I'm going to claim a senior moment Mulligan :O. I was using the term STC in that post, I should have been using ATC (Amended Type Cert) - STC is different (although related). I'll edit it accordingly to avoid the most serious confusion.

[WillFlyForCheese]

Question rom a "light" pilot.

I am having a hard time with the procedure for runaway trim in the matter under discussion.

The AD and such indicate that manual trim using the switches on the yoke will work, but then the system runs the trim again unless the whole trim sytem is turned off. So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?

Jez asking...
Gums

Here ya go . . . all you wanted to know about spinning wheels . . .

Watch Videos Online Aircraft trim system, 737-200 Veoh.com (http://www.veoh.com/watch/v382244KFJh32DJ)

[LDRA]

Firstly, note that the B737 MAX-8 and -9 have both been methodically added to the B737 TC, as shown on the current B737 Type Certificate Data Sheet. Application for MAX-8 certification was first put to the FAA in June, 2012; design was finalized mid 2013; and its specific TC was issued in March 2017, (10 weeks before being flown by the first operator). In doing so, these new MAX a/c append in time sequence to a long list of all previous 737 variants that the TC Data Sheet covers. There are 2 more of the 737 MAX family to come yet.

So, digging into the TC, towards the end where 737-8 and 737-9 certification is summarized. On page 74 of the doc we find that in the CONTROL SYSTEMS section, the included item Part 25.672 Stability Augmentation and Automatic and Power-Operated Systems (an FAA named item) gets its first and only mention. This in a certification data sheet which includes every model since the very first 737, so logically the item applies only to the MAX-8 and -9. Apparently none of the prior B737 variants were certified for this.

Is it too much to conclude that this is MCAS first seeing the regulatory light of day?

If so, in the second source doc above, the FAA sets out the regulatory requirements for systems having this class of functionality. If Boeing had the MCAS in mind in citing this section for certification, there may be some pertinent questions to be answered.

Good find

I just read through section 25.672, if MCAS falls under this section, Boeing is in deep trouble. MCAS fails every.single.clause in this section! There wouldn't be simple fix...

[Gipsy Queen]

That's true, though of course most wings, being cambered, do still produce some lift at 0° AoA.

Actually, depending upon design, a wing can produce a lot of lift at 0deg AoA, certainly more than AoA alone with a conventional wing profile. But I thought we had agreed to leave Bernoulli out of it!

[George Glass]

It has been noted by others that differences training from B737-800 to B737-8 does not require emphasis on the MCAS.Boeing wants to keep it simple. If you get a hold of the Runaway Stabilizer Non Normal Checklist you can see why.If correctly followed it CAN resolve the issue. But like all non-normals it requires a well trained Pilot and the correct response to the condition. Having personally experienced an Airspeed Unreliable event in-flight I can only say that the startle factor is massive.Simultaneous stall and overspeed warnings is a challenge to say the least. You dont have long to make the right call.Same for runaway stab.Still the most likely cause is incorrect response to the condition.Having said that I have great sympathy for the crew.Very easy to play monday morning quarterback.Why the aircraft was cleared to line after 3 sectors with a know problems is another question.

[Capn Bloggs]

So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?
You can trim it by manually turning the trim wheel.

Having personally experienced an Airspeed Unreliable event in-flight I can only say that the startle factor is massive.Simultaneous stall and overspeed warnings is a challenge to say the least.
And then you're supposed to notice the trim quietly winding itself full nose down...

It should be one or the other. As mentioned previously, if there is a chance that the trim will runaway during a Unreliable Airspeed scenario, then "Stab Trim OFF" must be one of the memory items.

[Loose rivets]

CONSO's link has an interesting item on page 52. (Quick access via strip on top edge of black bar.)

Cam switches under the flight-deck floor disable the trim when the control column is moved in the opposite direction to the stabilizer trim direction. I'd struggling to cope with the technical issues, but if this has been mentioned, I missed it.

https://www.slideshare.net/theoryce/...light-controls (https://www.slideshare.net/theoryce/b737-ng-flight-controls)

[neville_nobody]

If correctly followed it CAN resolve the issue. But like all non-normals it requires a well trained Pilot and the correct response to the condition.

Additional to this you need to identify the problem. I could imagine this being quite a difficult task if you had unreliable airspeed announciations additional to stick shakers going off, even hearing the trim running may not be possible. The only way to confirm would be to look down but not sure who would be doing that when you are under the impression the aircraft is stalled.

[jimtx]

Here ya go . . . all you wanted to know about spinning wheels . . .

I can't post the OP's video

That mechanical stab trim brake system was changed to some type of column activated electrical cutout system some time in the later models. The brake release knob on aft pedestal normally used to secure the garbage bag on the 727 and, I guess the 737, was replaced by a switch to override the stab trim cutout system. If the Max still has the latest cutout system they must have made a bypass around it for the MCAS. When I was a 727 plumber we checked the brake, as seen on the video, on first flight of the day. 737 drivers please chime in on whether this system is checked/installed on your aircraft.

[megan]

Article on AVweb. Closing para,I don’t often speculate on such things but if I were a betting man—and I’ve been known to dabble at the tables—my bet is that the accident investigation will once again revisit a familiar theme: automation bias. In the name of safety shaped by consistent procedures driven by automation in the cockpit, pilots are increasingly losing their basic feel for putting hands on controls and throttles and actually flying a flyable airplane. A second focus will be the utter perversity of how pilots now have to be trained to keep the automation that’s supposed to help them from killing them instead.https://www.avweb.com/eletter/archives/101/4190-full.html

[b1lanc]

There are a number of questions surrounding the certification process which as far as I can tell revolve around what documentation and/or training of a 'new' flight control automation capability was not documented to airlines let alone crew by the manufacturer. But, do we even know;
1) If the sensor was faulty or was there a problem further down the line, AoA sensor was replaced (1 or 2 sensors depending upon source cited) and it either didn't fix one "symptom" (airspeed disagree) or make matters a whole lot worse, but still allowed the next segment to complete, albeit more harrowing then the prior 2 segments (that may have been pilot skill - refer back to Golden Rivit's post #956 and listen to Capt VanderBurgh's last 10 minutes, including China Airlines 140 and the Delta L-1011 San Diego incident? Ok you can argue whether the unfortunate toga press caused 140, but I'd suggest that happened because of stab trim gone rogue.)
2) Was the correct part installed correctly?

Interesting read on why MCAS allegedly was created for the MAX - https://theaircurrent.com/aviation-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas-jt610/. Take it for what it is worth as the site content was written by a media type.

[mnttech]

The 737 MAX 8 was certificated (as the "737-8") in Revision 58 of the Boeing 737 Type Certificate (A44WE) on 8th March 2017.
I'm confused, A44WE is not the FAA TCDS, comes up as not a valid number, A16WE is, or at least per the FAA RGL.EASA number?
14 CFR 25.672 came out with Amdt. 25-23, Eff. 5/8/70, so it shows up (somewhat) in Type Certification Basis, (737-300 Series Airplanes), Type Certification Basis, (737-400 and 737-500 Series Airplanes),
*Applicable only to new or major modified structure or to new systems and components unique to the 737-400, and 737-500 series airplane with respect to the existing Model 737-200 Series airplane.
737-600, -700, -800 (Cont’d.) Certification Basis: A. 14 CFR Part 25 of the Federal Aviation Regulations as amended by Amendments 25-1 through 25-77 with the exceptions listed below
So for those aircraft it is a valid rule

[tdracer]

A bit surprised this hasn't been posted previously - it's from the Monday Seattle Times:
https://www.seattletimes.com/business/boeing-aerospace/u-s-pilots-flying-737-max-werent-told-about-new-automatic-systems-change-linked-to-lion-air-crash/A former Boeing executive, speaking on condition of anonymity because discussion of accident investigations is supposed to be closely held, said that Boeing engineers didn’t introduce the change to the flight-control system arbitrarily.
He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.

Side note - I'm a bit surprised even a 'former' Boeing exec was willing to talk about this to the press - this sort of information release during an active investigation could get them in some serious hot water...

[silverstrata]

A bit surprised this hasn't been posted previously - it's from the Monday Seattle Times:
https://www.seattletimes.com/business/boeing-aerospace/u-s-pilots-flying-737-max-werent-told-about-new-automatic-systems-change-linked-to-lion-air-crash/

Quote: He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.


What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.

To alleviate this nose up pitch, you need to either:.. a. reduce power, which is a counter-intuitive action during an incipient stall; or b. trim forward. The trim operates on the stab, not the elevator, and can give you a lot more control authority to oppose the engine pitch-up moment.

But here is the problem - the 737-max has much bigger engines, and a larger thrust-pitching moment. So at slow speed approaching the stall, giving too much recovery power may simply flip the aircraft over backwards. Which would really spoil your day. To (partially) mitigate this, you want an auto-forward stab-trim, to give the pilot more nose-down control authority at slow speeds.

Enter the new MCAS system.

However, MCAS can generate its own litany of problems and failure modes. If it gives full nose-down pitch during a stall, you still have that forward trim applied during the dive-recovery, which will hamper the recovery. (Which is why a stick-pusher combined with a thrust limiter might be a better option.). Likewise, if MCAS gets false information and operates at 250 kts, it is possible it will overpower the elevator and pitch you down. And if the auto-trim was not noticed, that trim will still be applied while you are trying to recover from a dive, hampering any recovery.

So in attempting to alleviate one problem, you have generated several others. MCAS looks like design firefighting, rather than a clean-sheet holistic design philosophy. And in making this fix, nobody went through all the possible failure scenarios - including a failure giving full forward trim at 250 kt.

I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.

Silver

[Nomad2]

Thing is, if you look up the "power and pitch" settings, for UAS, they tell you to set figures that are almost exactly what you need to continue 'as you are'.
So, if you are at FL350, doing .78 and set the pitch and power in the table, you will still be going along much as before.
Now, while we can all see the value in this, I'd suggest you need a power setting that is safe, and start on down straightaway if practible.
I'd set a medium power setting, one that I know would prevent stall and overspeed. This would work nicely at low altitude.
If it happened upstairs, my choice would be the same, and descend.
If for some reason, you couldn't descend, then the numbers in the table are what you need.

[silverstrata]

Thing is, if you look up the "power and pitch" settings, for UAS, they tell you to set figures that are almost exactly what you need to continue 'as you are'. So, if you are at FL350, doing .78 and set the pitch and power in the table, you will still be going along much as before.

Nomad ... we left UAS behind some 20 pages ago.
This incident is likely nothing to do with UAS.
Please read some back-pages.

Silver

[jimtx]

What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.

To alleviate this nose up pitch, you need to either:.. a. reduce power, which is a counter-intuitive action during an incipient stall; or b. trim forward. The trim operates on the stab, not the elevator, and can give you a lot more control authority to oppose the engine pitch-up moment.

But here is the problem - the 737-max has much bigger engines, and a larger thrust-pitching moment. So at slow speed approaching the stall, giving too much recovery power may simply flip the aircraft over backwards. Which would really spoil your day. To (partially) mitigate this, you want an auto-forward stab-trim, to give the pilot more nose-down control authority at slow speeds.

Enter the new MCAS system.

However, MCAS can generate its own litany of problems and failure modes. If it gives full nose-down pitch during a stall, you still have that forward trim applied during the dive-recovery, which will hamper the recovery. (Which is why a stick-pusher combined with a thrust limiter might be a better option.). Likewise, if MCAS gets false information and operates at 250 kts, it is possible it will overpower the elevator and pitch you down. And if the auto-trim was not noticed, that trim will still be applied while you are trying to recover from a dive, hampering any recovery.

So in attempting to alleviate one problem, you have generated several others. MCAS looks like design firefighting, rather than a clean-sheet holistic design philosophy. And in making this fix, nobody went through all the possible failure scenarios - including a failure giving full forward trim at 250 kt.

I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.

Silver
So the MCAS is needed on all 737s starting with the 737-300?

[Mad (Flt) Scientist]

A question was posted a couple of pages ago, asking who defined an "average pilot", taking issue with the "tone" of the Boeing statement about not giving too much info to said "average pilot" and implying the airframer was somehow in favour of the idea of a "lowest common denominator" pilot.

Nothing, from my perspective, could be further from the truth than that last item. But to address the points raised:

The concept of the "average pilot", or terms very similar to that, is found in many places in part 25. Picking a regulation at not-quite-random, 14CFR25.143 (b) states (my emphasis)
(b) It must be possible to make a smooth transition from one flight condition to any other flight condition without exceptional piloting skill, alertness, or strength, and without danger of exceeding the airplane limit-load factor under any probable operating conditions, including
You'll find similar wording throughout the regulations where the ability of the pilot to do a task will influence whether you pass or fail the requirement. The purpose of these words is to prevent the Chuck Yeager of any specific company going out, flying some ridiculously perfect and intricate manoeuvre, and saying "it's fine, I can fly it, must be ok". Test pilots are, for the purpose of assessing the piloting demands of the aircraft, not allowed to use their innate flying skills (they can use them for the "academic data gathering manoeuvres, where pilot workload and human factors are not a criteria) - instead they have to use their TEST PILOT skills - and specifically the one which says "I can determine, from my knowledge of how pilots in general interact with aeroplanes, and what they can and cannot be expected to manage, that this is/is not acceptable". The term frequently used for this is "average pilot".

Who decides exactly what the average pilot can do - collectively, the Test Pilot community; it is ultimately a judgement, but one made by a group, including not just the company's own pilots but also the Test Pilots from the various regulatory agencies. And they will indeed base that judgement on an understanding of the target pilot for the machine in question - so a private pilot won't be assumed able to cope with as much as an airline one. And as awareness builds of shifts in the pilot population and their skillset - as in, for example, many fewer with a military/fighter background, the skill level assumed may be degraded or amended, OR it may be identified that remedial action in training etc is required to bridge the gap. Usually it's all of these things.

And why do I say the OEM has no interest in downgrading the average pilot? Look at the first point - the smarter the "average pilot", the easier it is to pass the cert tests, even with a really clunky and horrible human interface or terrible flying characteristics. We'd love to assume every pilot was the hero who saves the day - every failure would be classed as benign, and certification would be a breeze. But that's not realistic, nor frankly fair to the people we hand the machine over to when the cert is done.

[rmac2]

If MCAS only activates with no flap deployed, and activates more aggressively at slower speed and therefore likely lower altitude, then I think that my first action on intermittent runaway stab trim close to the ground might be to pop out flaps 1 and see if that stops it immediately. Just a thought.

Of course the pilot would have to actually know of the existence of the system and its limitations first in order to be able to do that. The Lion Air crew didn't

[tdracer]

What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.
<snip>
I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.

Silver




Silver, not really disagreeing with anything you wrote. I doubt you'd get much argument that MCAS needs a serious re-think.
Sadly, this wouldn't be the first time that the 'fix' for an accident cause was a main contribution in another accident. I rather doubt it'll be the last...
Cranbrook 737 crashed because - as a safety enhancement - Boeing disabled the thrust reverser in-flight by closing the T/R hydraulic isolation valve, which allowed the T/R to re-deploy when they rejected the landing to miss the snowplow - https://aviation-safety.net/database/record.php?id=19780211-0
Lauda 767 crashed because a reverser deployed in-flight when a mis-rigged sensor caused the auto-restow system - implemented to prevent a repeat of Cranbrook - allowed the isolation valve to open in flight. (Lauda is a bit personal and a very bad memory for me - auto-restow wasn't my system, but I was deeply involved in the investigation - I was half-sick for weeks - made worse because the effective gag order in effect during an accident investigation meant I couldn't talk to anyone about what was going on.)
I'm reasonably sure MCAS was properly certified, with the associated FMEA (Failure Modes and Effects Analysis), and perhaps a SSA (System Safety Assessment) - at least in the Propulsion world FMEAs are not probabilistic (basically shows no single failure is unsafe) and the SSA covers the probabilities for multiple or combinations of failures.
Again, I'm not involved and have no direct knowledge of what happened, but I can guess: Someone did an FMEA of MCAS - determined that the worse case failure was no worse than a stab trim runaway, which has a procedure - and decided it was acceptable. The people that reviewed it (including, in all likelihood, an FAA specialist) didn't dispute that - not recognizing how bad it might be if an overloaded crew didn't figure out what was happening.
I'm also reasonably sure there are some 737 flight control types who are pretty sick about it right now. I've never had an accident or serious incident attributed to a system that I was responsible for (and I pray that remains the case). But I know how the Lauda investigation affected me and trust me, it wasn't pretty. I can only imagine how much worse it would be if it was my system...
When it's all said and done, and a fix is certified and implemented, I have little doubt there will be some retirements and/or resignations among the 737 flight controls ranks - perhaps worse.

[A Squared]

If MCAS only activates with no flap deployed, and activates most positively at slower speed and therefore likely lower altitude, then I think that my first action on intermittent runaway Stab trim close to the ground might be to pop out flaps 1 and see if that stops it. Just a thought.

It's worth pointing out that only a pilot who was aware of MCAS and knew knew it's operation parameters might think to try that. Until a few days ago there were no airline pilots who fit that description.

[Wannabe Flyer]

Any News on the CVR?

[underfire]

Quote: He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.
So the aircrafts design was mechanically and aerodynamically pushed too far, so software protection was necessary.to certify it as safe....

Pushed the 737 too far? Max or maxxed out?