Indonesian aircraft missing off Jakarta
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MCAS descriptive wording may not be written in the clearest manner. That aside, I know the design intent and implementation with enough detail to confidently say that MCAS authority is limited to 2.5 degrees of stabilizer (less at higher Mach numbers) and will not add more unless the function is reset. Pilot activation is manual electric switch commanded stabilizer motion is one way to reset MCAS. This appears from the data shown here in PPRUNE to be what was repeatedly occurring during the Lion Air event.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
Last edited by FCeng84; 5th Dec 2018 at 13:39. Reason: Fix typo
FCeng84,
Why did Boeing not fit a stick nudger as was done for the 747 on the British register? This would seem a simple fix that has been used before.
Why did Boeing not fit a stick nudger as was done for the 747 on the British register? This would seem a simple fix that has been used before.
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MCAS descriptive wording may not be written in the clearest manner. That aside, I know the design intent and implementation with enough detail to confidently say that MCAS authority is limited to 2.5 degrees of stabilizer (less at higher Mach numbers) and will not add more unless the function is reset. Pilot activation is manual electric switch commanded stabilizer motion is one way to reset MCAS. This appears from the data shown here in PPRUNE to be what was repeatedly occurring during the Lion Air event.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
So MCAS gets reset when there is a pilot 'electric' trim - so the first series of trim events we see from the pilot is to return the trim to the wanted nose up to level flight. There is a pause and MCAS trims 2.5ND. This is repeated.
The run up to the dive the electric trim is just blipped and trim not returned to level flight. From the description the effect of this on MCAS is that it resets in the current 2.5 ND and then adds another 2.5ND and so on with every blip. So by blipping the trim the PF was actually making MCAS ratchet nose down after each blip reset???
From that description the best approach with an errant MCAS is CUT OFF the stab trim, the next best approach is live with the pull force MCAS has given or trim right back to level. The absolute worst thing to do is just 'blip' the electric trim as each blip resets MCAS which then winds in another 2.5ND.
Which is what happened in the last few seconds of the flight.
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...I know the design intent and implementation with enough detail to confidently say that MCAS authority is limited to 2.5 degrees of stabilizer (less at higher Mach numbers) and will not add more unless the function is reset. Pilot activation is manual electric switch commanded stabilizer motion is one way to reset MCAS.
FCeng84. With the above in mind; my supposition for comment;- perhaps a scenario of 1.1Vs stick-shake (trim point 1.3/1.4Vs), has insufficient stability margin to manage a ‘powering out - approach to stall recovery’, such that the pitching moment from thrust is greater than that could be negated by acceleration. i.e. the aircraft pitches faster than it accelerates such that it reaches stall AoA during recovery (~ even more forward stick during a go around.).
Bergerie. Further supposition following the above; a nudger would apply a nose down force - relating to stick force / speed stability, which in the 737 appears to addressed by STS.
Whereas in the MAX, the problem might be the rate of pitch up relating to thrust and AoA, which feature as inputs to the trim computation.
“… electric trim is just blipped…”.
Or worse still if the pilot holds the finger on the electric trim ???
Bergerie. Further supposition following the above; a nudger would apply a nose down force - relating to stick force / speed stability, which in the 737 appears to addressed by STS.
Whereas in the MAX, the problem might be the rate of pitch up relating to thrust and AoA, which feature as inputs to the trim computation.
“… electric trim is just blipped…”.
Or worse still if the pilot holds the finger on the electric trim ???
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Faulty AoA information
Hmm. So, my understanding of what FCeng84 said is that MCAS is essentially a 'one-shot' pitch ND command when AoA is determined to be 'high'; pulling back on the yoke does not re-arm the MCAS trigger, but activating the trim switches or manually spinning the trim wheels does re-arm the trigger; and if the AoA drops below the threshold MCAS is re-armed.
One other thing occurs to me: if the MCAS system is receiving faulty AoA information, it is not beyond the bounds of possibility that the AoA information received varies, so it is possible that the reported, erroneous, AoA could rise to a value high-enough to trigger MCAS, drop enough to re-arm the MCAS trigger, then rise again to trigger the MCAS action, and do this repeatedly. Of course, there may be some logic to detect this, but in normal flight, nobody expects AoA to see-saw through the trigger threshold repeatedly.
I don't know if the traces for AoA and Automatic Trim from the FDR are capable of showing whether or not such a behaviour occurred. You'd probably need to know the fine details of how the MCAS threshold varied according to reported AoA and Mach number, and I'm not in a position to determine that.
Semreh
One other thing occurs to me: if the MCAS system is receiving faulty AoA information, it is not beyond the bounds of possibility that the AoA information received varies, so it is possible that the reported, erroneous, AoA could rise to a value high-enough to trigger MCAS, drop enough to re-arm the MCAS trigger, then rise again to trigger the MCAS action, and do this repeatedly. Of course, there may be some logic to detect this, but in normal flight, nobody expects AoA to see-saw through the trigger threshold repeatedly.
I don't know if the traces for AoA and Automatic Trim from the FDR are capable of showing whether or not such a behaviour occurred. You'd probably need to know the fine details of how the MCAS threshold varied according to reported AoA and Mach number, and I'm not in a position to determine that.
Semreh
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MCAS descriptive wording may not be written in the clearest manner. That aside, I know the design intent and implementation with enough detail to confidently say that MCAS authority is limited to 2.5 degrees of stabilizer (less at higher Mach numbers) and will not add more unless the function is reset. Pilot activation is manual electric switch commanded stabilizer motion is one way to reset MCAS. This appears from the data shown here in PPRUNE to be what was repeatedly occurring during the Lion Air event.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
One question that was posed a few days ago is what would have been the MCAS response if the crew were to use manual mechanical nose up stabilizer trim via the cockpit trim wheel following an errant MCAS nose down stabilizer increment. In that case MCAS would not have followed with more stabilizer motion as it’s authority limit would already have been met. I am not suggesting that this would be proper pilot approach to ending the MCAS down, pilot up cycling seen with an erroneously high AOA indication, but mention this scenario by way of providing better system understanding.
On the the question of stall prevention there are a couple of things that clearly reveal that this is not the objective of MCAS. First is it’s limited authority. Clearly generous aft column without pilot pitch trim input would provide much more nose up command that the limited increment of nose down provided by MCAS. The crew can definitely command the airplane into stall in the presence of MCAS. Second is the slow rate of MCAS stabilizer motion. Even running the stabilizer at the relatively quick 0.27 degrees per second it takes almost 10 seconds to insert MCAS’s full increment of stabilizer motion. A sharp column pull would be able to get to a stall before MCAS has a chance to complete its stabilizer motion.
jimtx, you mistakenly associate the successful outcome as an indication that the crew took the correct action, i.e. that they understood the situation.
It is just as likely, or more so, that they formed ‘a strong but wrong’ belief that the AoA and IAS alerts - the airdata was interfering with STS, so trim was disabled. This was after they had followed the drills for UAS, and ALT Disagree. See report p 21.
The accident crew could similarly have concentrated on the air-data problems and were managing the conflicting stick-shake and low speed awareness, with the different airspeed indications - as did the previous crew.
However, thereafter with flap retraction and trim problems, the latter crew held an equally ‘strong but wrong’ belief that UAS was the dominant issue. Again refer to the report re ATC and speed / height checks p 23.
Outcome knowledge does not reflect the quality of the decision - hindsight bias.
It is just as likely, or more so, that they formed ‘a strong but wrong’ belief that the AoA and IAS alerts - the airdata was interfering with STS, so trim was disabled. This was after they had followed the drills for UAS, and ALT Disagree. See report p 21.
The accident crew could similarly have concentrated on the air-data problems and were managing the conflicting stick-shake and low speed awareness, with the different airspeed indications - as did the previous crew.
However, thereafter with flap retraction and trim problems, the latter crew held an equally ‘strong but wrong’ belief that UAS was the dominant issue. Again refer to the report re ATC and speed / height checks p 23.
Outcome knowledge does not reflect the quality of the decision - hindsight bias.
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WSJ https://www.wsj.com/articles/behind-...d=hp_lead_pos1
Dec. 5, 2018 11:04 a.m. ET An automated flight-control system on Boeing Co.’s BA -4.85% 737 MAX aircraft, which investigators suspect played a central role in the fatal Oct. 29 jetliner crash in Indonesia, was largely omitted from the plane’s operations manual and was the subject of debate inside Boeing, government and industry officials say.Pilots of Lion Air Flight 610 battled systems on the Boeing 737 MAX for 11 minutes after the plane took off from Jakarta until it crashed into the Java Sea, killing all 189 people on board. Boeing now is devising a software fix and trying to assure carriers, aviators and safety experts that the 737 MAX’s computerized system is safe.
Has some graphs
Lion Air Flight 610 pilots battled systems on the Boeing 737 MAX for 11 minutes before the plane crashed into the Java Sea.
By
Andrew Tangel andAndy Pasztor
Dec. 5, 2018 11:04 a.m. ET An automated flight-control system on Boeing Co.’s BA -4.85% 737 MAX aircraft, which investigators suspect played a central role in the fatal Oct. 29 jetliner crash in Indonesia, was largely omitted from the plane’s operations manual and was the subject of debate inside Boeing, government and industry officials say.Pilots of Lion Air Flight 610 battled systems on the Boeing 737 MAX for 11 minutes after the plane took off from Jakarta until it crashed into the Java Sea, killing all 189 people on board. Boeing now is devising a software fix and trying to assure carriers, aviators and safety experts that the 737 MAX’s computerized system is safe.
Lion Air Flight 610 pilots battled systems on the Boeing 737 MAX for 11 minutes before the plane crashed into the Java Sea.
----
Probably best to read rest of article
graph is credited to flight radar 24 but does not reproduce well here
Probably best to read rest of article
graph is credited to flight radar 24 but does not reproduce well here
Last edited by CONSO; 5th Dec 2018 at 15:42. Reason: graphs dont work here
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FCeng84. With the above in mind; my supposition for comment;- perhaps a scenario of 1.1Vs stick-shake (trim point 1.3/1.4Vs), has insufficient stability margin to manage a ‘powering out - approach to stall recovery’, such that the pitching moment from thrust is greater than that could be negated by acceleration. i.e. the aircraft pitches faster than it accelerates such that it reaches stall AoA during recovery (~ even more forward stick during a go around.).
Some information from the WSJ article CONSO linked to:
"Boeing concluded pilots weren’t ever likely to encounter situations where the new anti-stall system kicked in, according to a Southwest memo reviewed by The Wall Street Journal. “They would never see the system in action,” a person familiar with Boeing’s development of the system said."
"..... Boeing’s position has some support. A top executive at a 737 MAX customer agreed pilots didn’t need to know the system’s details. “They’re not engineers and their job is to fly the aircraft,” this executive said."
"The FAA confirmed it is reviewing its decision to accept Boeing’s initial risk analyses of the automated system and other approved systems on the new plane. The FAA and Boeing also are developing a test of the entire MCAS system, which wasn’t previously required."
"Boeing concluded pilots weren’t ever likely to encounter situations where the new anti-stall system kicked in, according to a Southwest memo reviewed by The Wall Street Journal. “They would never see the system in action,” a person familiar with Boeing’s development of the system said."
"..... Boeing’s position has some support. A top executive at a 737 MAX customer agreed pilots didn’t need to know the system’s details. “They’re not engineers and their job is to fly the aircraft,” this executive said."
"The FAA confirmed it is reviewing its decision to accept Boeing’s initial risk analyses of the automated system and other approved systems on the new plane. The FAA and Boeing also are developing a test of the entire MCAS system, which wasn’t previously required."
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jimtx, you mistakenly associate the successful outcome as an indication that the crew took the correct action, i.e. that they understood the situation.
It is just as likely, or more so, that they formed ‘a strong but wrong’ belief that the AoA and IAS alerts - the airdata was interfering with STS, so trim was disabled. This was after they had followed the drills for UAS, and ALT Disagree. See report p 21.
The accident crew could similarly have concentrated on the air-data problems and were managing the conflicting stick-shake and low speed awareness, with the different airspeed indications - as did the previous crew.
However, thereafter with flap retraction and trim problems, the latter crew held an equally ‘strong but wrong’ belief that UAS was the dominant issue. Again refer to the report re ATC and speed / height checks p 23.
Outcome knowledge does not reflect the quality of the decision - hindsight bias.
It is just as likely, or more so, that they formed ‘a strong but wrong’ belief that the AoA and IAS alerts - the airdata was interfering with STS, so trim was disabled. This was after they had followed the drills for UAS, and ALT Disagree. See report p 21.
The accident crew could similarly have concentrated on the air-data problems and were managing the conflicting stick-shake and low speed awareness, with the different airspeed indications - as did the previous crew.
However, thereafter with flap retraction and trim problems, the latter crew held an equally ‘strong but wrong’ belief that UAS was the dominant issue. Again refer to the report re ATC and speed / height checks p 23.
Outcome knowledge does not reflect the quality of the decision - hindsight bias.
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"..... Boeing’s position has some support. A top executive at a 737 MAX customer agreed pilots didn’t need to know the system’s details. “They’re not engineers and their job is to fly the aircraft,” this executive said."
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That is an interesting set of statements.
So MCAS gets reset when there is a pilot 'electric' trim - so the first series of trim events we see from the pilot is to return the trim to the wanted nose up to level flight. There is a pause and MCAS trims 2.5ND. This is repeated.
The run up to the dive the electric trim is just blipped and trim not returned to level flight. From the description the effect of this on MCAS is that it resets in the current 2.5 ND and then adds another 2.5ND and so on with every blip. So by blipping the trim the PF was actually making MCAS ratchet nose down after each blip reset???
From that description the best approach with an errant MCAS is CUT OFF the stab trim, the next best approach is live with the pull force MCAS has given or trim right back to level. The absolute worst thing to do is just 'blip' the electric trim as each blip resets MCAS which then winds in another 2.5ND.
Which is what happened in the last few seconds of the flight.
So MCAS gets reset when there is a pilot 'electric' trim - so the first series of trim events we see from the pilot is to return the trim to the wanted nose up to level flight. There is a pause and MCAS trims 2.5ND. This is repeated.
The run up to the dive the electric trim is just blipped and trim not returned to level flight. From the description the effect of this on MCAS is that it resets in the current 2.5 ND and then adds another 2.5ND and so on with every blip. So by blipping the trim the PF was actually making MCAS ratchet nose down after each blip reset???
From that description the best approach with an errant MCAS is CUT OFF the stab trim, the next best approach is live with the pull force MCAS has given or trim right back to level. The absolute worst thing to do is just 'blip' the electric trim as each blip resets MCAS which then winds in another 2.5ND.
Which is what happened in the last few seconds of the flight.
So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
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Some information from the WSJ article CONSO linked to:
"Boeing concluded pilots weren’t ever likely to encounter situations where the new anti-stall system kicked in, according to a Southwest memo reviewed by The Wall Street Journal. “They would never see the system in action,” a person familiar with Boeing’s development of the system said."
"Boeing concluded pilots weren’t ever likely to encounter situations where the new anti-stall system kicked in, according to a Southwest memo reviewed by The Wall Street Journal. “They would never see the system in action,” a person familiar with Boeing’s development of the system said."
If the safety system will never be used, why develop and install in production aircraft in first place?
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I see lots of folks assuming MCAS trims 2.5ND each time - but that's not what I get from the description of MCAS, which provides: 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.
So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
Salute!
The ratchet effect is worthy of discussion and consideration by Big B and those here.
The so-called “reset” does not appear to move the stab back to last manually commanded angle, best I can tell from the limited explanations from B and some here, plus looking at the available data.
So I beep trim and Hal stops for 5 seconds, but does not move stab back to previous angle, right? Hence, we see the ratchet.
This implementation of this flight control feature (MCAS) seems more and more like a kludge to get around/satisfy some certification requirements. And it makes me mad. And scared.
Gums sends....
The ratchet effect is worthy of discussion and consideration by Big B and those here.
The so-called “reset” does not appear to move the stab back to last manually commanded angle, best I can tell from the limited explanations from B and some here, plus looking at the available data.
So I beep trim and Hal stops for 5 seconds, but does not move stab back to previous angle, right? Hence, we see the ratchet.
This implementation of this flight control feature (MCAS) seems more and more like a kludge to get around/satisfy some certification requirements. And it makes me mad. And scared.
Gums sends....
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"The FAA confirmed it is reviewing its decision to accept Boeing’s initial risk analyses of the automated system and other approved systems on the new plane. The FAA and Boeing also are developing a test of the entire MCAS system, which wasn’t previously required."
Heads should roll . . .
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that attributed to Boeing - "pilots weren’t ever likely to encounter" sounds like the kind of wording used in failures assessment guidance. (I suspect if I dug deep enough i could find it verbatim in guidance somewhere) On the other hand, "it'll never happen" is not the kind of statement any certification engineer would ever be capable of or comfortable making, and I'm guessing it's a paraphrasing (with a dollop of simplification) of the former statement.