Ethiopian airliner down in Africa
Salute1
@FCeng I got the 40 seconds and it is what I posted - 10 ,seconds without pilot trim switch,5 second rest, 10 seconds of trim with no pilot trim, 5 second rest and then 10 seconds trim without pilot trim. That would move the stab well over 5 degrees if stab was trimmed just above zero, but could max it out if trim was zero or more nose down.
One thing all 737 pilots need to understand is this
And then tell them that if the trigger event/value is still present that the process repeats.
Those Lion troops did a good job but for treating the problem as an MCAS quirk or STS working backwards as previous crew asserted, or a "runaway trim" that was not continuous. And besides, if they had heard about MCAS, then it was suposed to work making steep turns or slowing way down at cruise altitude or whatever and not be a stall prevention gizmo, and not activating just after flap retraction on takeoff.
Hoping new computer folks at Boeing do a better fault tree analysis this time.
Gums sends...
@FCeng I got the 40 seconds and it is what I posted - 10 ,seconds without pilot trim switch,5 second rest, 10 seconds of trim with no pilot trim, 5 second rest and then 10 seconds trim without pilot trim. That would move the stab well over 5 degrees if stab was trimmed just above zero, but could max it out if trim was zero or more nose down.
One thing all 737 pilots need to understand is this
One of the key elements to the baseline MCAS logic is that it will only put in a single increment of stabilizer motion as long as no pilot trim command is given.
Those Lion troops did a good job but for treating the problem as an MCAS quirk or STS working backwards as previous crew asserted, or a "runaway trim" that was not continuous. And besides, if they had heard about MCAS, then it was suposed to work making steep turns or slowing way down at cruise altitude or whatever and not be a stall prevention gizmo, and not activating just after flap retraction on takeoff.
Hoping new computer folks at Boeing do a better fault tree analysis this time.
Gums sends...
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FCeng84
Thanks for your multiple clarifications! I appreciate the fact that my 'dumb' questions elecit enlightening responses.
Something that still puzzles me (I refetred to previously) is the contradiction between making MCAS safe under faulty AOA scenario, and effective under actual high AOA conditions.
1. Under the faulty AOA scenario MCAS activates but is only allowed one trim down increment, after which it is inhibited, with a relatively harmless outcome.
2. Under an actual high AOA condition MCAS will activate, and start to trim nose down. If (for whatever reason) the pilot decided to trim the nose fractionally up, this would inhibit MCAS. Following the new rules MCAS is only allowed to make one nose down operation. If the pilot does nothing further for 5 seconds and the AOA remains high, does the remainder of the nose down cycle complete? Or does it remain where the pilot left it, leaving the aircraft in a degraded condition, potentially falling outside regulatory requirements?
3. If an aircraft encounters two high AOA events in sequence, is MCAS inhibited for the second event? What time interval or event, would allow MCAS to be enabled for a second valid high AOA event activation?
Does any of this make sense, or have the details been worked out, and I am creating imaginary difficulties?
Edit: gums is asking the same kind of question.
Edit: Disabling MCAS if AOA disagree would remove gums fault scenario.
Edit: I see similar questions (and answers) being covered in the Stabiliser Trim thread. Potentially the moderators could move this entire discussion, though it follows closely on from earlier posts.
Thanks for your multiple clarifications! I appreciate the fact that my 'dumb' questions elecit enlightening responses.
Something that still puzzles me (I refetred to previously) is the contradiction between making MCAS safe under faulty AOA scenario, and effective under actual high AOA conditions.
1. Under the faulty AOA scenario MCAS activates but is only allowed one trim down increment, after which it is inhibited, with a relatively harmless outcome.
2. Under an actual high AOA condition MCAS will activate, and start to trim nose down. If (for whatever reason) the pilot decided to trim the nose fractionally up, this would inhibit MCAS. Following the new rules MCAS is only allowed to make one nose down operation. If the pilot does nothing further for 5 seconds and the AOA remains high, does the remainder of the nose down cycle complete? Or does it remain where the pilot left it, leaving the aircraft in a degraded condition, potentially falling outside regulatory requirements?
3. If an aircraft encounters two high AOA events in sequence, is MCAS inhibited for the second event? What time interval or event, would allow MCAS to be enabled for a second valid high AOA event activation?
Does any of this make sense, or have the details been worked out, and I am creating imaginary difficulties?
Edit: gums is asking the same kind of question.
Edit: Disabling MCAS if AOA disagree would remove gums fault scenario.
Edit: I see similar questions (and answers) being covered in the Stabiliser Trim thread. Potentially the moderators could move this entire discussion, though it follows closely on from earlier posts.
Last edited by GordonR_Cape; 26th Mar 2019 at 19:48. Reason: Other thread questions.
Saute!
Good questions Gordon.
The way FCeng described it on another thread was that the MCAS returns trim to previous/ past position if the trigger event/condition no longer exists after the initial nose down trim command. So you can understand the logic, as the MCAS was only to reduce the nose up pitch moment as AoA approached stall value.
It was obvious with Lion 610 that the abnormal AoA was still present every time the 5 seconds elapsed. And the fact that every time the pilot beeped the trim switch that the uncommanded nose down disappeared.
[added] The sad part of the story is that the same AoA sensor MCAS was using was the one making the stick shake. And it may be that the left/right logic for the flight director and other stuff has a human factor deficiency as well
Gums sends...
Good questions Gordon.
The way FCeng described it on another thread was that the MCAS returns trim to previous/ past position if the trigger event/condition no longer exists after the initial nose down trim command. So you can understand the logic, as the MCAS was only to reduce the nose up pitch moment as AoA approached stall value.
It was obvious with Lion 610 that the abnormal AoA was still present every time the 5 seconds elapsed. And the fact that every time the pilot beeped the trim switch that the uncommanded nose down disappeared.
[added] The sad part of the story is that the same AoA sensor MCAS was using was the one making the stick shake. And it may be that the left/right logic for the flight director and other stuff has a human factor deficiency as well
Gums sends...
One of the key elements to the baseline MCAS logic is that it will only put in a single increment of stabilizer motion as long as no pilot trim command is given. This goes in over no more than 10 seconds (less if operating at Mach number greater than 0.4 where MCAS single increment authority is less than 2.5 degrees). The amount of elevator needed to balance one MCAS increment of stabilizer motion will be approximately 5 degrees due to the 2:1 ratio of stabilizer to elevator pitch control power. That amount of elevator can readily be commanded via the column with plenty of additional pitch control authority available to perform any maneuvers needed to maintain desired flight path and speed (if, for instance, climbing with a fixed throttle while varying path to maintain speed). Even in the presence of errant AOA data causing MCAS to activate when actually at AOA well below the intended MCAS activation point, MCAS will not move the stabilizer more than one increment without the crew making a pitch trim input. The only path to compromised pitch control authority via the column is for the crew to make pitch trim inputs but not make sufficient pitch trim inputs to return the stabilizer to the proper trimmed position.
Where this notion of "40 seconds" comes from is a total mystery - particularly when coupled with the statement "... without the crew using the trim switches". The scenario that led to problems after 40 seconds must have included short, ineffective periods of crew pitch trim switch commands that did not establish column force free pitch trim but did enable MCAS to insert another increment of airplane nose down stabilizer.
FCeng84 provides clarity ...
Where this notion of "40 seconds" comes from is a total mystery - particularly when coupled with the statement "... without the crew using the trim switches". The scenario that led to problems after 40 seconds must have included short, ineffective periods of crew pitch trim switch commands that did not establish column force free pitch trim but did enable MCAS to insert another increment of airplane nose down stabilizer.
FCeng84 provides clarity ...
Do I understand this correctly?
If MCAS detects an AOA that satisfies whatever criteria is needed, it activates 10 seconds (or less, depending on Mach number) of nose down trim for a maximum of 2.5 degrees nose down trim and stops.
If NO pilot trim input is commanded, MCAS remains disabled?
Thanks, GY
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Thank goodness. But there still appears to be confusion.
FCeng84 and other sources I've read over the course of this and the Lion Air thread seem to say this is not true. In the existing MCAS, there is only a single 10s (or less) nose-down trim provided, even if the trigger event/value is still present, unless the pilot makes a pitch trim input. This resets the MCAS state, resulting in a 5 second delay, followed by more nose-down trim if the condition is still met. If no pitch trim inputs are made, MCAS will not trim further down (but may restore trim). I can understand the logic to reset the MCAS state, because it can no longer make a trim/AOA association once the pilot changes the trim.
Is there a reference for what gums says?
I can understand cirticism of an MCAS that should be more fault-tolerant and pilot-tolerant, and apparently that is what Boeing is designing and documenting. This has been said before, but it appears the engineers didn't consider that the pilot may make pitch trim changes that do not put the h stab into a proper position, and that the resulting state reset then could have bad consequences if the pilot hadn't already turned stab trim off.
I was happy to see data early and often from Lion 610. Having an open, informed discussion of what happened is the best way to improve safety. I hope to see this soon for ET302.
Is there a reference for what gums says?
Those Lion troops did a good job but for treating the problem as an MCAS quirk or STS working backwards as previous crew asserted, or a "runaway trim" that was not continuous. And besides, if they had heard about MCAS, then it was suposed to work making steep turns or slowing way down at cruise altitude or whatever and not be a stall prevention gizmo, and not activating just after flap retraction on takeoff.
Hoping new computer folks at Boeing do a better fault tree analysis this time.
Gums sends...
Hoping new computer folks at Boeing do a better fault tree analysis this time.
Gums sends...
I was happy to see data early and often from Lion 610. Having an open, informed discussion of what happened is the best way to improve safety. I hope to see this soon for ET302.
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Sometimes it is very good to go back to basics. => I have read the lot, and the focus is on the wrong point
IF, and I say ; "IF" , no AOA sensor had failed, not a soul would have heard about what the MCAS was / is / is going to do.
All these events start 'or are triggered by" failing AOA sensors. => And naturally the single probe problem MCAS surfaces, and then what MCAS is trying surfaces, but it all starts with the failing AOA SENSOR/system.
The MAIN FOCUS should be on WHY the AOA probe/system fails.
IF, and I say ; "IF" , no AOA sensor had failed, not a soul would have heard about what the MCAS was / is / is going to do.
All these events start 'or are triggered by" failing AOA sensors. => And naturally the single probe problem MCAS surfaces, and then what MCAS is trying surfaces, but it all starts with the failing AOA SENSOR/system.
The MAIN FOCUS should be on WHY the AOA probe/system fails.
Salute!
Good questions, Hawk.
The Lion 610 data clearly shows the MCAS reset and then the 5 second delay. All the while the AoA from the pilot sensor is up at 20 degrees or so from the other side. You can also see the flap position logic in action. So the MCAS worked as designed, but had FUBAR AoA data. I would love to see MCAS act in an approach to stall in a banked turn where the pilot could simply relax back pressure and the AoA would go down. Then see what MCAS does.
Without a data table to check the traces second by second, it is hard to tll if MCAS re-trimmed back to what is was when it first trimmed or that the pilot commanded a whole lotta nose up trim. From the plot, looks like he trimmed up a lo, but all the way back to the original stab angle. Right at the end you can see that the FO did not rettrim as much, so I am questioning the complete reset
Gums sends...
Good questions, Hawk.
The Lion 610 data clearly shows the MCAS reset and then the 5 second delay. All the while the AoA from the pilot sensor is up at 20 degrees or so from the other side. You can also see the flap position logic in action. So the MCAS worked as designed, but had FUBAR AoA data. I would love to see MCAS act in an approach to stall in a banked turn where the pilot could simply relax back pressure and the AoA would go down. Then see what MCAS does.
Without a data table to check the traces second by second, it is hard to tll if MCAS re-trimmed back to what is was when it first trimmed or that the pilot commanded a whole lotta nose up trim. From the plot, looks like he trimmed up a lo, but all the way back to the original stab angle. Right at the end you can see that the FO did not rettrim as much, so I am questioning the complete reset
Gums sends...
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This multiple and varied mix of fault reports (all ADIRU related) suggests the problems are nothing to do with a faulty AoA sensor, but a problem within the ADIRU. Perhaps something like a erratic reference voltage to the A to D converter affecting several channels. The AoA sensor was replaced possibly just as a hopeful guess. The assumption then seems to have been jumped on that as the MCAS uses AoA, the errant behaviour of MCAS must have been due to an AoA fault. Possibly the ADIRU fault is 'confusing' the MCAS; perhaps it cannot handle AoA values that are all over the place and gets trapped in a logic loop. Can I ask a question of the many experts on this forum, are the ADIRUs, the displays and MCAS (FCCs) all on the same ARINC bus?
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A 737 MAX on ferry just had an emergency landing (though has no relation with the crashes, being an engine problem)
https://edition.cnn.com/2019/03/26/u...ing/index.html
https://edition.cnn.com/2019/03/26/u...ing/index.html
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Nobody knew MCAS even existed. It & the need to use trim cutout to correct "STS running Backward" was not ever written up.
Last edited by BobM2; 26th Mar 2019 at 22:39. Reason: spelling
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nice to go into the simulator well prepped on what's happening reminds me of the sims they conducted to see if Sully could have made it back to the airport..startle factor missing, 40 seconds long gone..
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I was thinking Boeing and the FAA had been a bit slack here, but I am starting to change my mind. Clean stall, the back stick is supposed to increase all the way to the break. It doesn't in this airplane apparently because of nacelle lift. So they put a mouse in the system to drive the stab L.E. up at Vs plus a few knots to start to fly the airplane nose down so we have to pull that little bit harder. Fine. All this at low speeds - stick forces light and any runaway a non-event. Also they put it the FTM.
Now we have a situation where a dozy alpha vane sends spurious information to the mouse when hand flying it clean at say 240 knots. The airplane tends nose down in response, the pilot holds attitude in the normal fashion and trims the load out with the pickle switches. Back where we started. Five seconds later it does it again. At some stage the flying pilot or the support pilot or both are going to notice the trim wheel is running while the pilot is not trimming and follow the drill. Oppose the runaway, stop the wheel, and / or operate the stab trim cut-outs. (Is that right? - it is many years since I have been in a 737)
I don't see how it got away from them. There has to be more to it. I gather in both cases the stab trim jack has been found full scale stabilizer nose up. Unfortunately post-crash mechanical evidence is rarely relevant because everything changes during the break-up.
Be that as it may, we have two brand new, on the evidence to date perfectly flyable airplanes planted face first into the planet in broad daylight.
Either there is something the matter with the way Boeing and the FAA are doing things, or there is something the matter out here in the rest of the world. Something serious. MCL for instance raises it's hand immediately. Simulators are great training tools but you can't get hurt in one and those basic piloting defensive thought processes don't grow. The FAA is right to call for 1500 hours pre requisite for getting into an airliner cockpit. The rest of the world must follow that lead. We are going to have to toughen up. It has been made too easy. It is an airplane, not a sago pudding. It is not just a computer game, although many are selling that line. High time, I think for the regulators and operators world-wide to have a hard look at the way they are going about things.
It is them going to get the midnight phone calls.
Now we have a situation where a dozy alpha vane sends spurious information to the mouse when hand flying it clean at say 240 knots. The airplane tends nose down in response, the pilot holds attitude in the normal fashion and trims the load out with the pickle switches. Back where we started. Five seconds later it does it again. At some stage the flying pilot or the support pilot or both are going to notice the trim wheel is running while the pilot is not trimming and follow the drill. Oppose the runaway, stop the wheel, and / or operate the stab trim cut-outs. (Is that right? - it is many years since I have been in a 737)
I don't see how it got away from them. There has to be more to it. I gather in both cases the stab trim jack has been found full scale stabilizer nose up. Unfortunately post-crash mechanical evidence is rarely relevant because everything changes during the break-up.
Be that as it may, we have two brand new, on the evidence to date perfectly flyable airplanes planted face first into the planet in broad daylight.
Either there is something the matter with the way Boeing and the FAA are doing things, or there is something the matter out here in the rest of the world. Something serious. MCL for instance raises it's hand immediately. Simulators are great training tools but you can't get hurt in one and those basic piloting defensive thought processes don't grow. The FAA is right to call for 1500 hours pre requisite for getting into an airliner cockpit. The rest of the world must follow that lead. We are going to have to toughen up. It has been made too easy. It is an airplane, not a sago pudding. It is not just a computer game, although many are selling that line. High time, I think for the regulators and operators world-wide to have a hard look at the way they are going about things.
It is them going to get the midnight phone calls.
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Several people have asked MCAS clarification questions in response to some of my recent posts. Rather than respond to them individually I offer the following as hopeful a fairly comprehensive description of the MCAS system that has been in the 737MAX fleet to date. Note that this does not reflect any of the changes about to be released. Hopefully Boeing will provide clear detail of those soon.
As always, if you still have questions after reading and trying to absorb the following please ask.
1. MCAS was designed to command airplane nose down stabilizer in response to high AOA up to an authority limit of 2.5 degrees for Mach less than 0.4 with lower authority at higher Mach numbers. If the pilot does not make any pitch trim commands, once AOA goes low MCAS will run the stabilizer in the airplane nose up direction back to the location from which it started.
2. MCAS is activated when all of the following are true:
a. Flaps are up
b. Autopilot is not engaged
c. Sensed AOA is above the MCAS activation AOA threshold
3. Once activated, MCAS will not command more than one increment of airplane nose down stabilizer motion until it has been reset. MCAS will be reset by either:
a. Pilot pitch trim command followed by a period of 5 consecutive seconds with no pilot pitch trim command.
- The assumption is made that pilot activation of pitch trim will be closely followed by continued pilot use of pitch trim to return the airplane to a column neutral pitch trim condition. MCAS seeing no further pilot pitch trim for a period of 5 seconds is interpreted as indication that the pilot has achieved column neutral pitch trim.
b. Return (by MCAS) of the stabilizer to its starting position per (1) above.
- Having returned the stabilizer to its pre-MCAS event starting point it is assumed that the airplane is back to a column neutral pitch trim condition.
4. Pilot pitch trim input at any time during the MCAS sequence as described in (1) above will stop MCAS stabilizer motion and end the current MCAS event while immediately moving the stabilizer in the direction of the pilot command.
a. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose down, that motion will stop and the stabilizer will immediately move in the direction of the pilot command.
b. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose up, that motion will stop and the stabilizer will move in the direction of the pilot command.
c. If pilot pitch trim input is issued after MCAS has completed its airplane nose down motion but prior to MCAS acting to take that motion out (as a result of return to low AOA), the stabilizer will immediately move in the direction of the pilot command.
d. It is assumed that the pilot issuing a pitch trim command is indication that the pilot is taking over the pitch trim task and will return the airplane to a column neutral pitch trim condition.
As a result of 1 through 4 above, given an AOA sensor that is failed so as to give an erroneously high reading (similar to what data appears to indicate occurred during the Lion Air accident flight), the following MCAS related scenarios can occur:
A. MCAS will activate (if flying manually) as soon as the flaps are retracted to up. Note that the stick shaker will have activated as soon as the airplane lifted off the ground regardless of the takeoff flap setting.
B. If the pilot does not make any pitch trim inputs, MCAS will run the stabilizer airplane nose down for one MCAS increment (as much as 2.5 degrees over approximately 10 seconds if Mach is less than 0.4). Without pilot pitch trim input, MCAS will not command further stabilizer motion in either direction. There is plenty of pitch control authority via the elevator using the column to counter the pitch disturbance generated by one MCAS increment of stabilizer motion.
C. If during or after the MCAS stabilizer motion per (A) above the pilot gives a pitch trim command the stabilizer will immediately start moving in the direction of the pilot command.
a. If the pilot pitch trim commands drive that stabilizer back to a column neutral pitch trim condition then the airplane will be back where it started from. In this event, provided sensed AOA is still high, MCAS will activate again once it sees a period of 5 seconds with not pilot pitch trim input. This process will repeat itself with the stabilizer never getting further than one MCAS increment out of trim. It appears that this sequence was repeated 20 or more times by the Lion Air accident flight pilot before he handed control over to his first officer.
b. If the pilot pitch trim commands are activated, but do not drive the stabilizer back to a column neutral pitch trim condition it is possible that successive activation of MCAS triggered by high AOA signal and ineffective pilot pitch trim inputs will lead to the stabilizer moving progressively further in the airplane nose down direction. It appears that this is what took place on the Lion Air accident airplane once the first officer took over control.
As for the reference in media reports this week regarding the crew’s need to intervene within 40 seconds of errant MCAS response to an AOA signal failed high, the following sequence of events would have to occur to so compromise pitch control power in that amount of time:
(a) The flight crew would have to allow a full increment of MCAS airplane nose down stabilizer motion to go in over 10 seconds without interrupting that motion via pilot pitch trim input.
(b) The flight crew would then have to have made a very short pitch trim command that triggered MCAS to reset, but did not re-establish anywhere near a column neutral pitch trim condition.
(c) After a 5 second pause wherein MCAS inferred that no more pilot pitch trim indicated the airplane was back to column neutral pitch trim, the flight crew would have to allow a second full increment of MCAS airplane nose down stabilizer motion to be inserted over another period of 10 seconds. Once again the crew would have to allow this stabilizer motion to go in without interruption via pilot pitch trim command.
(d) Once again, there would have be a repeat of (b) with a short, ineffective pilot pitch trim command.
(e) The final 10 seconds of this proposed 40 second sequence would be allowing another errant MCAS airplane nose down stabilizer motion increment to go in without interruption by pilot pitch trim command.
To summarize, getting in trouble over the course of just 40 seconds requires the crew to not trim when then should and make two very short, ineffective trim inputs following each of the first two MCAS stabilizer motion increments. This represents a worst case scenario.
As always, if you still have questions after reading and trying to absorb the following please ask.
1. MCAS was designed to command airplane nose down stabilizer in response to high AOA up to an authority limit of 2.5 degrees for Mach less than 0.4 with lower authority at higher Mach numbers. If the pilot does not make any pitch trim commands, once AOA goes low MCAS will run the stabilizer in the airplane nose up direction back to the location from which it started.
2. MCAS is activated when all of the following are true:
a. Flaps are up
b. Autopilot is not engaged
c. Sensed AOA is above the MCAS activation AOA threshold
3. Once activated, MCAS will not command more than one increment of airplane nose down stabilizer motion until it has been reset. MCAS will be reset by either:
a. Pilot pitch trim command followed by a period of 5 consecutive seconds with no pilot pitch trim command.
- The assumption is made that pilot activation of pitch trim will be closely followed by continued pilot use of pitch trim to return the airplane to a column neutral pitch trim condition. MCAS seeing no further pilot pitch trim for a period of 5 seconds is interpreted as indication that the pilot has achieved column neutral pitch trim.
b. Return (by MCAS) of the stabilizer to its starting position per (1) above.
- Having returned the stabilizer to its pre-MCAS event starting point it is assumed that the airplane is back to a column neutral pitch trim condition.
4. Pilot pitch trim input at any time during the MCAS sequence as described in (1) above will stop MCAS stabilizer motion and end the current MCAS event while immediately moving the stabilizer in the direction of the pilot command.
a. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose down, that motion will stop and the stabilizer will immediately move in the direction of the pilot command.
b. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose up, that motion will stop and the stabilizer will move in the direction of the pilot command.
c. If pilot pitch trim input is issued after MCAS has completed its airplane nose down motion but prior to MCAS acting to take that motion out (as a result of return to low AOA), the stabilizer will immediately move in the direction of the pilot command.
d. It is assumed that the pilot issuing a pitch trim command is indication that the pilot is taking over the pitch trim task and will return the airplane to a column neutral pitch trim condition.
As a result of 1 through 4 above, given an AOA sensor that is failed so as to give an erroneously high reading (similar to what data appears to indicate occurred during the Lion Air accident flight), the following MCAS related scenarios can occur:
A. MCAS will activate (if flying manually) as soon as the flaps are retracted to up. Note that the stick shaker will have activated as soon as the airplane lifted off the ground regardless of the takeoff flap setting.
B. If the pilot does not make any pitch trim inputs, MCAS will run the stabilizer airplane nose down for one MCAS increment (as much as 2.5 degrees over approximately 10 seconds if Mach is less than 0.4). Without pilot pitch trim input, MCAS will not command further stabilizer motion in either direction. There is plenty of pitch control authority via the elevator using the column to counter the pitch disturbance generated by one MCAS increment of stabilizer motion.
C. If during or after the MCAS stabilizer motion per (A) above the pilot gives a pitch trim command the stabilizer will immediately start moving in the direction of the pilot command.
a. If the pilot pitch trim commands drive that stabilizer back to a column neutral pitch trim condition then the airplane will be back where it started from. In this event, provided sensed AOA is still high, MCAS will activate again once it sees a period of 5 seconds with not pilot pitch trim input. This process will repeat itself with the stabilizer never getting further than one MCAS increment out of trim. It appears that this sequence was repeated 20 or more times by the Lion Air accident flight pilot before he handed control over to his first officer.
b. If the pilot pitch trim commands are activated, but do not drive the stabilizer back to a column neutral pitch trim condition it is possible that successive activation of MCAS triggered by high AOA signal and ineffective pilot pitch trim inputs will lead to the stabilizer moving progressively further in the airplane nose down direction. It appears that this is what took place on the Lion Air accident airplane once the first officer took over control.
As for the reference in media reports this week regarding the crew’s need to intervene within 40 seconds of errant MCAS response to an AOA signal failed high, the following sequence of events would have to occur to so compromise pitch control power in that amount of time:
(a) The flight crew would have to allow a full increment of MCAS airplane nose down stabilizer motion to go in over 10 seconds without interrupting that motion via pilot pitch trim input.
(b) The flight crew would then have to have made a very short pitch trim command that triggered MCAS to reset, but did not re-establish anywhere near a column neutral pitch trim condition.
(c) After a 5 second pause wherein MCAS inferred that no more pilot pitch trim indicated the airplane was back to column neutral pitch trim, the flight crew would have to allow a second full increment of MCAS airplane nose down stabilizer motion to be inserted over another period of 10 seconds. Once again the crew would have to allow this stabilizer motion to go in without interruption via pilot pitch trim command.
(d) Once again, there would have be a repeat of (b) with a short, ineffective pilot pitch trim command.
(e) The final 10 seconds of this proposed 40 second sequence would be allowing another errant MCAS airplane nose down stabilizer motion increment to go in without interruption by pilot pitch trim command.
To summarize, getting in trouble over the course of just 40 seconds requires the crew to not trim when then should and make two very short, ineffective trim inputs following each of the first two MCAS stabilizer motion increments. This represents a worst case scenario.
Last edited by FCeng84; 26th Mar 2019 at 23:18. Reason: formatting
Salute!
BobM2
The STS running backwards was from a photo or copy of an entry by the previous Lion crew. It was early in the Lion crash threads. And the lack of entering the shaker on the normal log or squawk book is obvious. And nothing about turning off power to the trim motor? Hmmmm....
What at would YOU do if you knew the previous flight had the shaker going on and manual trim was required because they had turned of trim motor power?
Gums asks...
BobM2
The STS running backwards was from a photo or copy of an entry by the previous Lion crew. It was early in the Lion crash threads. And the lack of entering the shaker on the normal log or squawk book is obvious. And nothing about turning off power to the trim motor? Hmmmm....
What at would YOU do if you knew the previous flight had the shaker going on and manual trim was required because they had turned of trim motor power?
Gums asks...
I was thinking Boeing and the FAA had been a bit slack here, but I am starting to change my mind. Clean stall, the back stick is supposed to increase all the way to the break. It doesn't in this airplane apparently because of nacelle lift. So they put a mouse in the system to drive the stab L.E. up at Vs plus a few knots to start to fly the airplane nose down so we have to pull that little bit harder. Fine. All this at low speeds - stick forces light and any runaway a non-event. Also they put it the FTM.
Now we have a situation where a dozy alpha vane sends spurious information to the mouse when hand flying it clean at say 240 knots. The airplane tends nose down in response, the pilot holds attitude in the normal fashion and trims the load out with the pickle switches. Back where we started. Five seconds later it does it again. At some stage the flying pilot or the support pilot or both are going to notice the trim wheel is running while the pilot is not trimming and follow the drill. Oppose the runaway, stop the wheel, and / or operate the stab trim cut-outs. (Is that right? - it is many years since I have been in a 737)
I don't see how it got away from them. There has to be more to it. I gather in both cases the stab trim jack has been found full scale stabilizer nose up. Unfortunately post-crash mechanical evidence is rarely relevant because everything changes during the break-up.
Be that as it may, we have two brand new, on the evidence to date perfectly flyable airplanes planted face first into the planet in broad daylight.
Either there is something the matter with the way Boeing and the FAA are doing things, or there is something the matter out here in the rest of the world. Something serious. MCL for instance raises it's hand immediately. Simulators are great training tools but you can't get hurt in one and those basic piloting defensive thought processes don't grow. The FAA is right to call for 1500 hours pre requisite for getting into an airliner cockpit. The rest of the world must follow that lead. We are going to have to toughen up. It has been made too easy. It is an airplane, not a sago pudding. It is not just a computer game, although many are selling that line. High time, I think for the regulators and operators world-wide to have a hard look at the way they are going about things.
It is them going to get the midnight phone calls.
Now we have a situation where a dozy alpha vane sends spurious information to the mouse when hand flying it clean at say 240 knots. The airplane tends nose down in response, the pilot holds attitude in the normal fashion and trims the load out with the pickle switches. Back where we started. Five seconds later it does it again. At some stage the flying pilot or the support pilot or both are going to notice the trim wheel is running while the pilot is not trimming and follow the drill. Oppose the runaway, stop the wheel, and / or operate the stab trim cut-outs. (Is that right? - it is many years since I have been in a 737)
I don't see how it got away from them. There has to be more to it. I gather in both cases the stab trim jack has been found full scale stabilizer nose up. Unfortunately post-crash mechanical evidence is rarely relevant because everything changes during the break-up.
Be that as it may, we have two brand new, on the evidence to date perfectly flyable airplanes planted face first into the planet in broad daylight.
Either there is something the matter with the way Boeing and the FAA are doing things, or there is something the matter out here in the rest of the world. Something serious. MCL for instance raises it's hand immediately. Simulators are great training tools but you can't get hurt in one and those basic piloting defensive thought processes don't grow. The FAA is right to call for 1500 hours pre requisite for getting into an airliner cockpit. The rest of the world must follow that lead. We are going to have to toughen up. It has been made too easy. It is an airplane, not a sago pudding. It is not just a computer game, although many are selling that line. High time, I think for the regulators and operators world-wide to have a hard look at the way they are going about things.
It is them going to get the midnight phone calls.
One has to wonder whether LionAir should have been let off the EU banned list so soon...
Boeing may have made errors in their systems design but the more I read the more I see how appallingly badly LionAir crews (and maintenance?) mishandled the accident flight and those that preceeded it whilst having all the means at hand to prevent the disaster.
The Ethiopan report, should it be open and transparent, will go a long way to corroborate or refute these remarks but I'm not holding my breath on either count.
Last edited by meleagertoo; 27th Mar 2019 at 11:12.
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I rather agree with you TFX, a previous LionAir crew recognised the root-cause of their problem and selected STAB TRIM CUTOFF switches to CUTOFF so the problem is far from dififcult to identify. Indeed they later took several other NNC measures that seem, on the face of it, to be rational and sensible. Thus they identified the threat, if not the cause which in this case is largely irrelevent. But what scares the crap out of me they soon selected the electric trim back on! That isn't the way of a professional crew. They soon selected it back off again, thank God, when the trim runaway re-occurred but then inexplicably continued the flight in manual control(no a/p) and manual trim with the stick shaker running continuously! These are fundamental aberrations of simply unbelieveable proportion and something no normal Professional pilot would consider doing for a moment. Lion Air's crew attitude and operating standards must surely take a massive hit from these appalling errors. It isn't good enough to hide behind thecompany's shameful, disgraceful shoulder-sloping repetition of the wet excuse that "nowhere in these checks did they say to land at the nearest suitable airport." That is an utterlt disgraceful way for a company with sloppy procedures to try to shift blame onto Boeing's manuals and thus demonstrates a company culture of blissful unawareness of normal Professional procedures, airm*****ip (Oh bugger! I nearly swore again!) I mean cheesy excuses for blind unthinking magenta-line following and no apparent command presence at all.
One has to wonder whether LionAir should have been let off the EU banned list so soon...
Boeing may have made errors in their systems design but the more I read the more I see how appallingly badly LionAir crews mishandled the accident flight and those that preceeded it whilst having all the means at hand to prevent the disaster.
One has to wonder whether LionAir should have been let off the EU banned list so soon...
Boeing may have made errors in their systems design but the more I read the more I see how appallingly badly LionAir crews mishandled the accident flight and those that preceeded it whilst having all the means at hand to prevent the disaster.
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Salute!
BobM2
The STS running backwards was from a photo or copy of an entry by the previous Lion crew. It was early in the Lion crash threads. And the lack of entering the shaker on the normal log or squawk book is obvious. And nothing about turning off power to the trim motor? Hmmmm....
What at would YOU do if you knew the previous flight had the shaker going on and manual trim was required because they had turned of trim motor power?
Gums asks...
BobM2
The STS running backwards was from a photo or copy of an entry by the previous Lion crew. It was early in the Lion crash threads. And the lack of entering the shaker on the normal log or squawk book is obvious. And nothing about turning off power to the trim motor? Hmmmm....
What at would YOU do if you knew the previous flight had the shaker going on and manual trim was required because they had turned of trim motor power?
Gums asks...
The first question should be "what would maintenance have done?". Hopefully more than just resetting breakers & cleaning cannon plugs. Of course the airplane had been flying revenue trips for 3 days in an unairworthy condition with capt "unreliable airspeed & altitude". Were they flying that way in RVSM airspace? Maintenance that was done had only made it much more unairworthy with continuous stick shaker & "sts running backwards", but "just press on". Do they not get paid if they cancel a trip or was it a hot date waiting in Jakarta? Indonesia has one of the worst safety records & from the operation of this "airline" I can see why. Do they carry maintenance discrepancies to the next D-check in 20,000 hrs? Of course, it's easy to blame Boeing for not selling them an airplane that can fly safely with mounting unairworthy discrepancies for an indefinite time. Three days is not nearly enough.
What would I have done? On my airline, I would have sat comfortably in the crew room while someone more qualified made a maintenance test flight, since these write-ups involved serious control issues.