Ethiopian Prelim Report
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safetypee
I don't see why. The preliminary report says
"At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units."
The crew could still wind the stab trim ANU using the manual electric trim 32 seconds before impact. The question has to be why didn't they use the manual electric trim to remove the elevator load completely then isolate the system using the two pedestal switches.
If so, then the basis of the certification is flawed.
"At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units."
The crew could still wind the stab trim ANU using the manual electric trim 32 seconds before impact. The question has to be why didn't they use the manual electric trim to remove the elevator load completely then isolate the system using the two pedestal switches.
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safetypee
I don't see why. The preliminary report says
"At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units."
The crew could still wind the stab trim ANU using the manual electric trim 32 seconds before impact. The question has to be why didn't they use the manual electric trim to remove the elevator load completely then isolate the system using the two pedestal switches.
I don't see why. The preliminary report says
"At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units."
The crew could still wind the stab trim ANU using the manual electric trim 32 seconds before impact. The question has to be why didn't they use the manual electric trim to remove the elevator load completely then isolate the system using the two pedestal switches.
1. The momentary trim inputs may be an artifact of the FDR data sampling algorithm, or the crew stopped trying when the trim wheels didn't turn after a few seconds. We don't really know for sure, and IMO you can't build an argument on such evidence.
2. The recorded increase in the stabiliser ANU position may also be a data artifact, or backlash in the cables and pulleys, or the trim motor locking mechanism. There are a number of suggestions from other sources (including EASA), that what you describe is physically impossible at high airspeeds and aerodynamic loads.
3. Re-enabling the trim switches after runaway trim, is not documented in any procedure, so cannot count towards the certification process. They could re-write the procedures, but currently it seems to fail certification under those extreme conditions.
Edit: Reuters has an excellent article, citing some very knowledgeable sources: https://www.reuters.com/article/us-e...-idUSKCN1RA0DP
The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said.
It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke.
It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke.
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Safety Pee, your point regarding the EASA document is well taken. I think we have to be careful of what we read into the data thus far; we really don't know what kind of effort was expended by the FO to move the manual trim wheel. But the possibility of excessive loads is clearly present, and that will be an immediately crucial question. Right now, there is no reason to believe that a similar effect would not occur on the NG, should an old-school runaway stabilizer occur, so the issue is going to need some prompt discussion.
To my mind as potential passenger it comes down to certification . It seems Boeing and the FAA are too cosy- its not unreasonable to delegate some things to the manufacturer because they have resources and expertise but perhaps thatc ulture became too ingrained and everyone got tripped up.
Is the MAX really still a 737 after 50 odd years; 3 engine types, new wings, new tail, vertical and horizontal , undercarriage changes due to engines, fly by wire, all digital flight deck no doubt a lot of structural changes internally to lighten weight - the only things that seem to still be 737 are the flight deck windows. Should the FAA really have accepted the grandfather premise or just told Boeing , especially after the 739 -sorry you are just going to have to call it 797. Its a great engineering feat , up to a point, but as MD found out when they went one step beyond with the MD90 you can go a stretch too far and not just in dimensions,
Is the MAX really still a 737 after 50 odd years; 3 engine types, new wings, new tail, vertical and horizontal , undercarriage changes due to engines, fly by wire, all digital flight deck no doubt a lot of structural changes internally to lighten weight - the only things that seem to still be 737 are the flight deck windows. Should the FAA really have accepted the grandfather premise or just told Boeing , especially after the 739 -sorry you are just going to have to call it 797. Its a great engineering feat , up to a point, but as MD found out when they went one step beyond with the MD90 you can go a stretch too far and not just in dimensions,
Goldenrivet, Gordon, Mansfield,
The critical parameter is the pitch trim position (units) on the FDR; above that are indications of electrical input. The top line being the pilots yoke switches command, when enabled, the lower the FCC output - MCAS. It is quite clear during the final stages of flight, that there is little or no trim change with pilot nose up input, if indeed that was an input vice noise, but for MCAS the nose down change is quite clear - dramatic.
Additional rationale is in Ethiopian airliner down in Africa
The argument is that excessive asymmetric control forces enable the tail trim to be repositioned nose down, but oppose all nose up movement.
Add to this that the electrical signal for trim acts directly on the trim motor at the rear of the aircraft, which can become ‘electrically stalled’ (or inhibited - under discussion). Whereas manual trim wheel from the flight deck involves cables which can stretch under high loads, this together with the unusual wheel to tail trim ratio, could appear to the pilots as having some wheel movement, but it being totally ineffective due to stretching the cables and not moving the tail trim mechanism, again due to high aerodynamic load restricting the nose - up sense.
With caveats, caution, and all reservations as above; also to be applied to Ethiopian airliner down in Africa
The critical parameter is the pitch trim position (units) on the FDR; above that are indications of electrical input. The top line being the pilots yoke switches command, when enabled, the lower the FCC output - MCAS. It is quite clear during the final stages of flight, that there is little or no trim change with pilot nose up input, if indeed that was an input vice noise, but for MCAS the nose down change is quite clear - dramatic.
Additional rationale is in Ethiopian airliner down in Africa
The argument is that excessive asymmetric control forces enable the tail trim to be repositioned nose down, but oppose all nose up movement.
Add to this that the electrical signal for trim acts directly on the trim motor at the rear of the aircraft, which can become ‘electrically stalled’ (or inhibited - under discussion). Whereas manual trim wheel from the flight deck involves cables which can stretch under high loads, this together with the unusual wheel to tail trim ratio, could appear to the pilots as having some wheel movement, but it being totally ineffective due to stretching the cables and not moving the tail trim mechanism, again due to high aerodynamic load restricting the nose - up sense.
With caveats, caution, and all reservations as above; also to be applied to Ethiopian airliner down in Africa
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I'm sorry, I can't accept an FDR "noise" explanation.
There are two electrical trim inputs coinciding with a change in trim position from 2.1 to 2.3 units.
Two separate FDR channels show trim input and trim position change.
A change of one parameter or the other could be noise, a change of both together cannot.
A trim change was commanded electrically, and the stab trim responded accordingly.
There are two electrical trim inputs coinciding with a change in trim position from 2.1 to 2.3 units.
Two separate FDR channels show trim input and trim position change.
A change of one parameter or the other could be noise, a change of both together cannot.
A trim change was commanded electrically, and the stab trim responded accordingly.
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The report mentions in passing that autothrottle was left engaged throughout but fails to acknowledge it should have been disengaged. It avoids the question of whether the pilots switched the stab cut-out back on even though the data shows that must have been the case. The Press Conference "was almost entirely focused on vindicating the actions of the pilots" according to the admittedly partisan Seattle Times - it is subjective but FWIW I agree.
Nonetheless it is abundantly clear the MCAS failed (as if we didn't know that by now). But it is a matter of fact that contrary to the Ethiopian Transport Minister's statement, the crew did NOT follow correctly the Boeing procedures and the Non Normal Checklist. The autothrottle was never disengaged (a significant oversight). Although the stab cut-out was belatedly used the trim was later re-engaged contrary to the Boeing advice to "ensure the CUTOUT switches remain at CUTOUT for the remainder of the flight".
Nonetheless it is abundantly clear the MCAS failed (as if we didn't know that by now). But it is a matter of fact that contrary to the Ethiopian Transport Minister's statement, the crew did NOT follow correctly the Boeing procedures and the Non Normal Checklist. The autothrottle was never disengaged (a significant oversight). Although the stab cut-out was belatedly used the trim was later re-engaged contrary to the Boeing advice to "ensure the CUTOUT switches remain at CUTOUT for the remainder of the flight".
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Hello all.
I'm not in the industry but have rudiments of aero.
So here is my silly question.
If the engines generate lift far forward pitched up this tends to assist further pitch up since the centre of lift moves far in front of the centre of gravity.
If the pitch becomes nose down, at some negative AOA the engines will similarly move the centre of negative lift forwards the air strikes the top of the nacelle. And tend to push nose down.
Is this a reasonable analysis?
David
I'm not in the industry but have rudiments of aero.
So here is my silly question.
If the engines generate lift far forward pitched up this tends to assist further pitch up since the centre of lift moves far in front of the centre of gravity.
If the pitch becomes nose down, at some negative AOA the engines will similarly move the centre of negative lift forwards the air strikes the top of the nacelle. And tend to push nose down.
Is this a reasonable analysis?
David
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I think your assertion contains several logical inconsistencies:
1. The momentary trim inputs may be an artifact of the FDR data sampling algorithm, or the crew stopped trying when the trim wheels didn't turn after a few seconds. We don't really know for sure, and IMO you can't build an argument on such evidence.
2. The recorded increase in the stabiliser ANU position may also be a data artifact, or backlash in the cables and pulleys, or the trim motor locking mechanism. There are a number of suggestions from other sources (including EASA), that what you describe is physically impossible at high airspeeds and aerodynamic loads.
3. Re-enabling the trim switches after runaway trim, is not documented in any procedure, so cannot count towards the certification process. They could re-write the procedures, but currently it seems to fail certification under those extreme conditions.
Edit: Reuters has an excellent article, citing some very knowledgeable sources: http://www.reuters.com/article/us-et...-idUSKCN1RA0DP
1. The momentary trim inputs may be an artifact of the FDR data sampling algorithm, or the crew stopped trying when the trim wheels didn't turn after a few seconds. We don't really know for sure, and IMO you can't build an argument on such evidence.
2. The recorded increase in the stabiliser ANU position may also be a data artifact, or backlash in the cables and pulleys, or the trim motor locking mechanism. There are a number of suggestions from other sources (including EASA), that what you describe is physically impossible at high airspeeds and aerodynamic loads.
3. Re-enabling the trim switches after runaway trim, is not documented in any procedure, so cannot count towards the certification process. They could re-write the procedures, but currently it seems to fail certification under those extreme conditions.
Edit: Reuters has an excellent article, citing some very knowledgeable sources: http://www.reuters.com/article/us-et...-idUSKCN1RA0DP
[/QUOTE]
www.reuters.com/article/us-ethiopia-airplane-regulator-insight/regulators-knew-before-crashes-that-737-max-trim-control-was-confusing-in-some-conditions-document-idUSKCN1RA0DP[/QUOTE]I would say that the majority of pilots who have been trained on the 737 (I have flown the Jurassic, old EFIS and NG series) see the "stab manual trim wheels" (located on either side of both pilots center pedestal),rotating, indicating that the use of the control column switches "are trimming the stab" (no autopilot engaged). Due to air loads and the fact that MCAS had activated, this lead the crew to believe that the STAB trim wasn't working properly (manually or electrically). Again, we are not privy to the CVR and all of the distractions the crew were facing at this crucial time, low to the ground.
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Until a point is reached where multiple AoA vane failures can be dealt with safety, accounting for MCAS, it’s not an airworthy design IMHO.
Large multiple bird strikes to the radome can cause issues to airflow which make AoA readings erroneous. It was a known issue on a previous fleet. You simply need to be able to turn off MCAS or have an override switch that takes tabular data of airspeed and flap settting and resets the stab appropriately. Either this, or a stab that hasn’t been designed to be uncontrollable manually at high speed.
Large multiple bird strikes to the radome can cause issues to airflow which make AoA readings erroneous. It was a known issue on a previous fleet. You simply need to be able to turn off MCAS or have an override switch that takes tabular data of airspeed and flap settting and resets the stab appropriately. Either this, or a stab that hasn’t been designed to be uncontrollable manually at high speed.
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Additional training in sims to cover “startle events” above and beyond the “minimum required by regulators” should certainly help with the last slice of Swiss cheese in a scenario not conceived as a mandatory training item.
Basic thrust + attitude + trim training to cover startle effects together with basic flight path monitoring could possibly have saved many recent Boeing fatalities.
I’ve added “trim” to basic attitude flying because for a well-trained pilot, that should go without saying, but maybe in light of the two recent B737 fatalities it needs to be said.
The question is whether the regulators and/or airlines can be persuaded of this, which might increase their costs compared to their minimalist competitors.
Of course, the airlines would prefer to buy fool-proof aircraft so they can employ fools for peanuts, but we’re obviously not there yet. I think the message is we still need good pilots, and the industry needs to suck that up and pay and train accordingly.
For example (Boeing only): Lion Air, Ethiopian, Atlas, Asiana, Emirates, Air Niugini, FlyDubai, Tatarstan, Adam Air, Garuda etc (off the top of my head) were all events that could have been survivable with a different crew at the controls. To say it’s impossible to train crews to deal with difficult events is simply not true. It just might involve higher standards, more training, and inevitably more money. The industry will need to decide where the economic trade-off should lie.
Is Boeing complicit in the 737MAX events? Undoubtedly. But could better trained pilots have saved the day and become heroes? I suggest yes. One crew did (with a bit of help from a jumpseater).
I’m not necessarily suggesting I would personally have done any better - I could also be a victim of insufficient training. But I do spend a bit of my spare time trying to learn from others mistakes, including reading this website, which contains an enormous wealth of knowledge amidst the noise. I’m certainly not prepared to just throw my hands in the air and say “this was unsurvivable”. In fact, I put to any professional 737MAX pilot: if you studied the Lion Air preliminary report, you should have been well prepared for this event - it should not have been startle factor at all. If, in fact, you didn’t study the report and arm-chair fly it so as to be so-prepared, then maybe this isn’t the job for you.
As a professional pilot, of course I want my aircraft to be designed as safely as possible, but I need to be always ready to perform when it decides not to. That’s my job.
Last edited by Derfred; 6th Apr 2019 at 10:33.
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NRK ,the Norwegian news , is now quoting Bjørn Ferm and Mentour Pilot that it was impossible to recover for the poor crew.
I feel somewhat sorry for Boeing.
What ever these two chaps have said, the news picks the part that makes most headlines!
Gone Fishing
I feel somewhat sorry for Boeing.
What ever these two chaps have said, the news picks the part that makes most headlines!
Gone Fishing
Derfred, one of the best posts I've read on Prune for a while. 
If only... they'd kept trimming back after they switched the stab trim back on... if only. RIP you poor fellows.
NRK ,the Norwegian news , is now quoting Bjørn Ferm and Mentour Pilot that it was impossible to recover for the poor crew.
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I’m certainly not prepared to just throw my hands in the air and say “this was unsurvivable”. In fact, I put to any professional 737MAX pilot: if you studied the Lion Air preliminary report, you should have been well prepared for this event - it should not have been startle factor at all. If, in fact, you didn’t study the report and arm-chair fly it so as to be so-prepared, then maybe this isn’t the job for you.
Derfred, Bloggs,
See post Ethiopian Prelim Report and previous via links; particularly https://www.pprune.org/10439906-post.html
Unsurvivable - at some point in time; probability after selecting stab trim off !
And that using yoke elect trim before that would loose the battle with MCAS 10 to 5 against; or that using manual wheel trim after that point, the single crew member might be unable to restore trim to an acceptable stick force to enable the other pilot to control altitude and speed whilst stick forces increase and trim is even less effective.
Startle - nothing worse than following procedures and finding that they don’t work !
The line which the industry has crossed, as in this accident, is the assumption that pilots will always be able to manage … ,
………………………………………………………but only with hindsight
See post Ethiopian Prelim Report and previous via links; particularly https://www.pprune.org/10439906-post.html
Unsurvivable - at some point in time; probability after selecting stab trim off !
And that using yoke elect trim before that would loose the battle with MCAS 10 to 5 against; or that using manual wheel trim after that point, the single crew member might be unable to restore trim to an acceptable stick force to enable the other pilot to control altitude and speed whilst stick forces increase and trim is even less effective.
Startle - nothing worse than following procedures and finding that they don’t work !
The line which the industry has crossed, as in this accident, is the assumption that pilots will always be able to manage … ,
………………………………………………………but only with hindsight
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These results should not have any bearing on the future safety of the MAX, since MCAS should never cause that fault again. Whether it is relevant to runaway stabiliser trim, and apply to the existing NG models, is a deeper issue.
alf5071h
The line which the industry has crossed, as in this accident, is the assumption that pilots will always be able to manage … ,
………………………………………………………but only with hindsight
………………………………………………………but only with hindsight
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This is possibly another naiive question, but is it even possible for a MAX at the high AOA for which MCAS was designed to detect to approach maximum airspeed? What is the scenario where you can actually pull yourself into a stall at this speed, and would any non-suicidal pilot try it? There should have been some sort of "sanity" cut out in the design -- keep in mind that the Captain's airspeed was reporting as even higher than the actual speed. Was there any speed limit, or would MCAS have activated at a speed that would have torn off the stabilizer?
I have a very strong feeling that the designers of the MCAS programming had no idea of the effect of speed upon the forces required to move the stabilizer trim, indicated by the fact that they slightly changed the algorithm to account for speed sometime during the certification process.
I have a very strong feeling that the designers of the MCAS programming had no idea of the effect of speed upon the forces required to move the stabilizer trim, indicated by the fact that they slightly changed the algorithm to account for speed sometime during the certification process.
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The question of what stage of the flight it was, which may be grounds for legal arguments.
I am not a lawyer, but I don't think you ever want to be in a civil trial where the jury is wondering why the executives are not in jail yet.
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I doubt there is going to be much in the way of legal arguments on this one, Boeing does not want to be anywhere near a jury. You can talk about the pilot's actions all that you want to, but ultimately you have two aircraft that were pointed to the ground at low altitude by a flight control system that was not disclosed to the pilots and that was (according to media reports) significantly different than the certification documentation described. Add in the cosy relationship with the FAA along with the whistleblower report and this is not something that goes to trial if Boeing has any competent lawyers left.
I am not a lawyer, but I don't think you ever want to be in a civil trial where the jury is wondering why the executives are not in jail yet.
I am not a lawyer, but I don't think you ever want to be in a civil trial where the jury is wondering why the executives are not in jail yet.
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SafetyPee, with respect to the erroneous angle of attack issue that I assigned elephant in the room status too…my thinking is that we have two airplanes, and three AoA vanes, all on the left side, that have generated faulty data in a very short fleet history. My concern is that there is a problem in either the hardware or software that is processing the AoA inputs. As the NG does not, to my knowledge, have an extensive history of this, I have to wonder what else is different in the air data systems of the Max, and how changes in design at that location may also have been poorly vetted...which of course leads to lots of other questions about the design process.
There is no doubt that Boeing’s MCAS stability augmentation idea has failed miserably. It is another case of a safety system introducing more risk than it was installed to prevent. The unscheduled operation of this system , particularly as it was modified following flight test (from 0.6 degrees per actuation to 2.5 degrees), introduces a very serious threat that clearly was not contemplated by the system safety analysis. Further, Boeing’s contention that the runaway stabilizer procedure is an adequate mitigation of this threat is not valid. Since the 707, Boeing aircraft have incorporated cutout switches in the control column designed to interrupt the pitch trim system if it is trimming in the opposite direction to which the pilot is pushing or pulling. These switches are intended to stop the system from moving the horizontal stabilizer too far until the pilot has the time to manually disable the system entirely. The MCAS bypassed these switches, which is a logical design when looked at solely from the purpose of the MCAS itself. Since MCAS is intended to increase the control force as the angle of attack approaches the stall, it doesn’t make much sense to allow MCAS to be cut out as the pilot inadvertently pulls the nose up toward that stalling angle.
But when looked at from the standpoint of the threats posed by a powerful stabilizer running away at high airspeed, this design is absurd. And since the first step in the runaway stabilizer procedure is to hold the control column firmly, thus utilizing those column cutout switches to inhibit further trimming, the risk mitigation assured by that procedure is severely diminished.
That all said, Muillenberg was appropriately diplomatic when he said that Boeing owns one link in the chain, but not the whole chain.
When crew of ET-302 lifted the airplane off the runway at Addis Ababa, the first thing that happened was the captain’s stick shaker activated. This is pretty much the same thing that happened to LNI610, the Lion Air flight that crashed on Oct 29, and LNI043, the flight to Jakarta on Oct 28 . It may well have occurred to that airplane on Oct 26 and 27 as well. The crew operating LNI043 did not record the stick shaker operation in the aircraft maintenance log; perhaps others did not as well.
The captain of LNI043 was able to ascertain pretty quickly that the left side instruments were unreliable by comparing with the standby flight instruments. The crew of LNI610, on the other hand, did not believe they had any reliable altitude information, and asked ATC for groundspeed information. To date, I have not seen any information regarding discussion on the ET-302 flight deck about standby instruments, or comparisons between left and right instruments.
The captain of LNI043 retracted the flaps after he had determined which instruments were malfunctioning, and he had verified the airspeed. In the ET-302 case, that does not appear to have happened; instead, it looks to me like they retracted the flaps because they were task saturated and clinging to normal procedure.
The ET-302 crew did not comply with the Boeing procedure for a runaway stabilizer. As far as I can tell, no one actioned any item on the procedure except the most important step, disabling the master trim cutout switches. The procedure calls for the autopilot and autothrottles to be turned off. This crew made several attempts early on to get the autopilot engaged; it finally engaged and then disengaged itself 33 seconds later. They selected level change and set the speed to 238 knots. No one ever made any attempt to actually control the airspeed. My hunch is that they assumed the autothrottle would control the airspeed, which is a pretty standard response for automation-dependent flight crew. However, level change controls the airspeed with pitch; without being able to get the nose up, the airspeed is headed for the barber pole.
If they still had 238 knots in the MCP window and were actually doing well over 300, the FD command bar was probably pasted to the top of the PFD; that may be what the captain meant when he said the “pitch is not enough”. I can’t see any other reason. They were climbing. In the final 3 minutes, with the pitch trim selected off, they had climbed about 4000 feet.
It appears that the first officer attempted to crank the manual trim wheel after they had shut off the electric trim system. Only eight seconds after the captain told him to try the manual trim, the first officer reported that it wasn’t working. It is impossible to know exactly how he reached that conclusion; at the speed they were moving at, I’m pretty sure the trim would have been difficult to move. I’m skeptical toward the claim that it was impossible to move; I would be more inclined to believe that if the first officer had said more or taken more time attempting to crank the wheel. The two ANU trim inputs shortly before the end indicate that a) the master trim switches were back on, and b) the trim motor was capable of moving the stabilizer. I really don’t know whether the manual trim can be frozen while the trim motor still works, but I suspect it would be the other way around. Bear in mind that it takes a couple of dozen revolutions of the manual wheel to make a dent in the nose down trim forces. I’d certainly love to see some flight test data on this…my visit to recurrent training next month will be interesting and possibly colorful.
I really don’t know whether the captain knew that the master trim switches had been selected back on. Prior to this, the captain had successfully countered MCAS inputs with nose up electric trim, just as the Lion Air captain did. When the MCAS operates the final time, the captain does not appear to attempt to counter it with the electric trim switches. I cannot help wondering if he still believed the electric trim was off; if so, he may have thought there was nothing he could do except try to pull up with the elevator control.
We have what appear to be three flights with very, very similar attributes. The crew of LNI043 managed to resolve the entire problem, even without knowing of the existence of MCAS (possibly with help from the jumpseater). They resolved it so successfully that they flew all the way to Jakarta with the autopilot off and manually trimming. They then failed to accurately record all of the faults (like the stick shaker) in the AML, a fact they will have to live with for the rest of their lives. The crew of ET-302 knew about MCAS, disabled it, and were still unable to get the situation under control…despite gaining what appears to be adequate altitude.
I suspect that there are huge issues of automation dependency, systems knowledge (standby instruments), and workplace cultural issues (repetitive writeups that are not resolved and turn out to be incomplete) in play here.
There is no doubt that Boeing’s MCAS stability augmentation idea has failed miserably. It is another case of a safety system introducing more risk than it was installed to prevent. The unscheduled operation of this system , particularly as it was modified following flight test (from 0.6 degrees per actuation to 2.5 degrees), introduces a very serious threat that clearly was not contemplated by the system safety analysis. Further, Boeing’s contention that the runaway stabilizer procedure is an adequate mitigation of this threat is not valid. Since the 707, Boeing aircraft have incorporated cutout switches in the control column designed to interrupt the pitch trim system if it is trimming in the opposite direction to which the pilot is pushing or pulling. These switches are intended to stop the system from moving the horizontal stabilizer too far until the pilot has the time to manually disable the system entirely. The MCAS bypassed these switches, which is a logical design when looked at solely from the purpose of the MCAS itself. Since MCAS is intended to increase the control force as the angle of attack approaches the stall, it doesn’t make much sense to allow MCAS to be cut out as the pilot inadvertently pulls the nose up toward that stalling angle.
But when looked at from the standpoint of the threats posed by a powerful stabilizer running away at high airspeed, this design is absurd. And since the first step in the runaway stabilizer procedure is to hold the control column firmly, thus utilizing those column cutout switches to inhibit further trimming, the risk mitigation assured by that procedure is severely diminished.
That all said, Muillenberg was appropriately diplomatic when he said that Boeing owns one link in the chain, but not the whole chain.
When crew of ET-302 lifted the airplane off the runway at Addis Ababa, the first thing that happened was the captain’s stick shaker activated. This is pretty much the same thing that happened to LNI610, the Lion Air flight that crashed on Oct 29, and LNI043, the flight to Jakarta on Oct 28 . It may well have occurred to that airplane on Oct 26 and 27 as well. The crew operating LNI043 did not record the stick shaker operation in the aircraft maintenance log; perhaps others did not as well.
The captain of LNI043 was able to ascertain pretty quickly that the left side instruments were unreliable by comparing with the standby flight instruments. The crew of LNI610, on the other hand, did not believe they had any reliable altitude information, and asked ATC for groundspeed information. To date, I have not seen any information regarding discussion on the ET-302 flight deck about standby instruments, or comparisons between left and right instruments.
The captain of LNI043 retracted the flaps after he had determined which instruments were malfunctioning, and he had verified the airspeed. In the ET-302 case, that does not appear to have happened; instead, it looks to me like they retracted the flaps because they were task saturated and clinging to normal procedure.
The ET-302 crew did not comply with the Boeing procedure for a runaway stabilizer. As far as I can tell, no one actioned any item on the procedure except the most important step, disabling the master trim cutout switches. The procedure calls for the autopilot and autothrottles to be turned off. This crew made several attempts early on to get the autopilot engaged; it finally engaged and then disengaged itself 33 seconds later. They selected level change and set the speed to 238 knots. No one ever made any attempt to actually control the airspeed. My hunch is that they assumed the autothrottle would control the airspeed, which is a pretty standard response for automation-dependent flight crew. However, level change controls the airspeed with pitch; without being able to get the nose up, the airspeed is headed for the barber pole.
If they still had 238 knots in the MCP window and were actually doing well over 300, the FD command bar was probably pasted to the top of the PFD; that may be what the captain meant when he said the “pitch is not enough”. I can’t see any other reason. They were climbing. In the final 3 minutes, with the pitch trim selected off, they had climbed about 4000 feet.
It appears that the first officer attempted to crank the manual trim wheel after they had shut off the electric trim system. Only eight seconds after the captain told him to try the manual trim, the first officer reported that it wasn’t working. It is impossible to know exactly how he reached that conclusion; at the speed they were moving at, I’m pretty sure the trim would have been difficult to move. I’m skeptical toward the claim that it was impossible to move; I would be more inclined to believe that if the first officer had said more or taken more time attempting to crank the wheel. The two ANU trim inputs shortly before the end indicate that a) the master trim switches were back on, and b) the trim motor was capable of moving the stabilizer. I really don’t know whether the manual trim can be frozen while the trim motor still works, but I suspect it would be the other way around. Bear in mind that it takes a couple of dozen revolutions of the manual wheel to make a dent in the nose down trim forces. I’d certainly love to see some flight test data on this…my visit to recurrent training next month will be interesting and possibly colorful.
I really don’t know whether the captain knew that the master trim switches had been selected back on. Prior to this, the captain had successfully countered MCAS inputs with nose up electric trim, just as the Lion Air captain did. When the MCAS operates the final time, the captain does not appear to attempt to counter it with the electric trim switches. I cannot help wondering if he still believed the electric trim was off; if so, he may have thought there was nothing he could do except try to pull up with the elevator control.
We have what appear to be three flights with very, very similar attributes. The crew of LNI043 managed to resolve the entire problem, even without knowing of the existence of MCAS (possibly with help from the jumpseater). They resolved it so successfully that they flew all the way to Jakarta with the autopilot off and manually trimming. They then failed to accurately record all of the faults (like the stick shaker) in the AML, a fact they will have to live with for the rest of their lives. The crew of ET-302 knew about MCAS, disabled it, and were still unable to get the situation under control…despite gaining what appears to be adequate altitude.
I suspect that there are huge issues of automation dependency, systems knowledge (standby instruments), and workplace cultural issues (repetitive writeups that are not resolved and turn out to be incomplete) in play here.