How Safe is the Boeing 737 Max's MCAS System?

In summary, the MCAS system was not the cause of the crash and it is possible for the plane to fly without the system if the angle of attack sensor is not working correctly. However, the plane is more likely to stall if the angle of attack sensor is not working correctly and the pilots need to manually fly the plane back to correct pitch attitude.
  • #631
anorlunda said:
No. No. No. When the plane stalls you must put the nose down to increase airspeed, not get level. As a glider pilot, I'm used to flying at the edge of stall speed for prolonged periods. Adding an engine changes the parameters, but it does not change the basic physics of flight.

It is counter-intuitive at first. If you stall close to the ground, you must immediately push the stick forward to put the nose down. But after training, the counter-intuitive becomes intuitive.
Pushing the nose down, is increasing velocity, which is what thrust or power results in. When I said "get level", I didn't mean to imply pull up on the yoke. Pushing down to increase airspeed could be part of the maneuvers to become "level". But your point is well taken, and worth pointing out as I'm not a pilot. And it is indeed extremely counter intuitive pitching down when stalling.

*Why am I revisiting this old thread? Is this a faux pas? Please tell me if so.
 
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  • #632
cyboman said:
I recall early on you were quite defensive of any culpability of Boeing and their MCAS system

You recall incorrectly. I never made any such claims.

cyboman said:
I thought your arguments initially seemed to allude that it was more pilot error

I did say that pilot error might also have been a contributing factor. I never said that pilot error being a contributing factor meant that Boeing was not culpable as well.
 
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  • #633
cyboman said:
increasing velocity, which is what thrust or power results in

Not necessarily. There are many cases where increasing power will result in increasing altitude, not airspeed.

Also, the response of the plane to an increase in power is significantly slower than its response to a change in pitch. So if you're in a stall, pushing the nose down is something you have to do regardless of whether you're going to increase power or not, because increasing power by itself won't get you out of trouble fast enough.
 
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  • #634
PeterDonis said:
if you're in a stall

And it's worth noting as a reminder that, as I pointed out at the time the referenced discussion earlier in this thread was taking place, the 737 MAX airplanes that crashed were not actually in a stall. The automated system thought they were because of faulty sensor data, but they actually weren't. So what to do in an actual stall is irrelevant in assessing what needed to be done to avoid those crashes.
 
  • #635
PeterDonis said:
Not necessarily. There are many cases where increasing power will result in increasing altitude, not airspeed.

Also, the response of the plane to an increase in power is significantly slower than its response to a change in pitch. So if you're in a stall, pushing the nose down is something you have to do regardless of whether you're going to increase power or not, because increasing power by itself won't get you out of trouble fast enough.
Agreed, I'm going to guess on an aircraft that size they both increase power and pitch down.
 
  • #636
cyboman said:
I'm going to guess on an aircraft that size they both increase power and pitch down.

Pitch down, yes. Whether to also increase power will depend, as I said, on the specific situation. For example, consider these two different stall situations:

(1) You're on approach for landing and your angle of attack gets too high. Your airspeed drops and you fall below your glide slope.

Here you have to pitch down to increase airspeed, and you also need to add power because your energy is too low--you're below glide slope.

(2) You're in a dive and your angle of attack gets too high and you're close to a stall.

Here you have to pitch down, but the last thing you want to do is add power. You're in a dive, so your airspeed will probably increase pretty rapidly as you pitch down, and you might have to decrease power to avoid too much stress on the airframe (as well as for the obvious reason of decreasing your dive rate).
 
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  • #637
Oops...

1634310833138.png

https://abc7news.com/ex-boeing-test-pilot-indicted-for-fraud-in-737-max-probe/11127038/
 
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  • #638
Boeing itself is apparently "too big to charge" in that they would be precluded from a host of federal contracts if convicted.

Not that this guy isn't culpable as hell IMHO...

.
 
  • #639
Well it seems almost logical that he wasn't acting alone but more like "taking one for the team" so probably got paid to do so , otherwise why would he ?
 
  • #640
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  • #641
When I was teaching freshman engineers (this was before being personally exposed to the insides of corporate decisions ), I took some solace that the less technically adept thirds of the class would be likely ensconced somewhere in middle management from whence they could do no real harm. How naive I was.
The saga of Boeing is a tragedy. The maintenance of excellence is never easy, and without recognition of quality by those in charge, impossible. This is an engineering morality play.
 
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  • #642
berkeman said:
Just a comment on the legal side of this. This charge raises an interesting legal issue about the definition of "fraud". Normally, fraud is defined as a dishonest act that causes some kind of deprivation relating to the property interests of another. If the pilot in question provided false information unknowingly as the email indicates, the dishonesty component is lacking. If he did it knowingly, it may still a bit of a challenge to see that the dishonesty relates to deprivation of a property interest.

AM
 
  • #643
This guy crashed the plane in the simulator because of the difficulty of overcoming the actions of MCAS. He then forcefully petitions the FAA that the existence of the system need not be divulged to pilots. A bad idea, stupidly implemented and then hidden.
I guess the law does not recognize the term "reprehensible"
 
  • #644
hutchphd said:
This guy crashed the plane in the simulator because of the difficulty of overcoming the actions of MCAS. He then forcefully petitions the FAA that the existence of the system need not be divulged to pilots. A bad idea, stupidly implemented and then hidden.
I guess the law does not recognize the term "reprehensible"
According to the indictment, in late 2016, Forkner discovered information about an important change to MCAS and withheld it. In an instant-message chain between Forkner and then technical pilot Patrik Gustavsson, the chief pilot described a scenario in which he witnessed the MCAS “running rampant” during a November 2016 simulator session, some four months before the Max received its certification from the FAA.

What exactly does MCAS “running rampant” mean? Was the simulator session flying the normal flight envelope of the plane and simulating failures or were they doing acrobatics. Some context is needed.
 
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  • #645
There were design decisions that must have raised red flags throughout the design team. I believe that upper management relies on the test pilots to give them a realistic evaluation of risks, whereas the engineers are often overly optimistic about their design. This is not such a case. IMO, the design mistakes were severe enough that everyone knew there were problems.
 
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  • #646
Boeing was in competition with Airbus and they had a plane that performed better. Boeing added MCAS to make their new plane competitive.

One overriding feature was the desire to skip pilot training on the new plane saying it handles the same as a prior model and so a test pilots input would have been critical here.

https://en.wikipedia.org/wiki/Boeing_737_MAX
 
  • #647
Auto-trim runaway is what MCAS was causing. Is this what MCAS “running rampant” means?

If that's true then we're back to the standard Auto-trim runaway check-list issue (no manual recovery if started too late) again instead of MCAS specific countermeasures in the crashes.


MCAS put the plane into a "dive" when the AOA sensor malfunctioned by over-trimming the plane.

Some pilots recovered the plane using the auto-trim runaway procedure early and for the rest of the flight but sadly some didn't.

The system should not go into runaway so easily, and it should not have been permitted to run the trim as far as it did.
 
  • #648
nsaspook said:
Some pilots recovered the plane using the auto-trim runaway procedure early and for the rest of the flight but sadly some didn't.
It's not clear that other pilots faced the same degree of runaway that the pilots of Lion Air and Ethiopian Air faced. At 34:34 in the Frontline video, a discussion of Boeing's position is mentioned, followed by a review of the Ethiopian Air (ET-302) crash with an American Airlines 737 pilot, who mentions the crew got it right (37:34) when they shutoff power to MCAS. Unknown to the pilots, an AOA was giving faulty data and MCAS kicked into correct the situation. The first officer tried to use manual trim wheel, but it proved too difficult to move, and by then the plane was traveling too fast (ostensibly, the pilot/co-pilot should have reduced engine power). Then they reactivated MCAS.

Boeing was facing stiff competition from Airbus. They promised US airlines that no additional training (that would require a simulator 15:40) would be necessary with the 737 Max based on a commitment to Southwest Airlines (15:52). MCAS was adapted from a military aircraft (17:20). At 18:02 begins the discussion of the simulator test that revealed a potential problem with MCAS, that being even a Boeing test pilot might have trouble overcoming faulty operation of MCAS.

At 22:08 begins a discussion of FAA's delegation to Boeing and statements by Michael Huerta. I disagree with his statements. Quality Assurance is not necessarily fully independent from corporate management and pressure to accept the unacceptable.

At 22:42, discussion of the maiden flight of 737 Max reveals a less than smooth flight particularly at low speed (just after take off). MCAS is further adapted given greater effect (it evolved from minor to major, and perhaps critical in some situations).

At 26:10, begins discussion of Mark Forkner, who became chief technical pilot for the 737. Forkner requested permission from FAA to remove MCAS from pilot manual (27:14-27:21). At 28:49, the video indicates that Forkner became aware that the had misinformed the FAA regarding MCAS. However, Forkner did not alert the FAA and did not correct the record.

Due to the delegation practice, the FAA was unaware of how significant MCAS has become. Clearly the FAA was negligent in this matter, and Boeing was negligent in not informing the FAA, or ensuring that correct information was provided to the FAA.

At 30:20 begins discussion of faulty AOA sensors and impact of MCAS. Boeing engineers raised the issue internally.
 
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  • #649
Astronuc said:
an AOA was giving faulty data and MCAS kicked into correct the situation.
It's my understanding that the AOA faulty data fooled the MCAS into causing the situation. MCAS did not correct anything.
 
  • #650
Astronuc said:
Unknown to the pilots, an AOA was giving faulty data and MCAS kicked into correct the situation.

FactChecker said:
It's my understanding that the AOA faulty data fooled the MCAS into causing the situation. MCAS did not correct anything.
Those two descriptions are not contradictory. It sounds to me like a difference in characterization.
 
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  • #651
FactChecker said:
It's my understanding that the AOA faulty data fooled the MCAS into causing the situation. MCAS did not correct anything
I think what @Astronuc meant is "AOA faulty data fooled MCAS into thinking a situation existed that it had to correct, so it did, when in fact no such situation existed".
 
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  • #652
anorlunda said:
Those two descriptions are not contradictory. It sounds to me like a difference in characterization.
Yes. Sorry. I interpreted the statement another way. I guess "erroneously corrected" would have helped me.
 
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  • #653
FactChecker said:
It's my understanding that the AOA faulty data fooled the MCAS into causing the situation. MCAS did not correct anything.
FactChecker said:
Yes. Sorry. I interpreted the statement another way. I guess "erroneously corrected" would have helped me.
MCAS was attempting to correct the attitude of the aircraft. The faulty input from the faulty AOA (a problem Boeing engineers knew about because it had happened previously on other aircraft) caused MCAS to react to push the nose down, when the nose was already down (the pilot and copilot knew the nose was down, but it's not clear if they knew one of the AOAs was faulty). Apparently there was no cross checking on the pilot-side (left) and copilot-side (right) AOAs, nor an independent redundancy. By the time the Ethiopian pilots tried to manually correct (and they couldn't correct fast enough), it was too late.

The question came up in Congressional hearings (46:33 in video), that the AOA system was modified to compare the two sets of AOAs. Rep. Peter DeFazio asked why wasn't that the case from Day 1. Boeing CEO Muilenburg responds that they (Boeing) asked themselves the same question, and if they knew then what the know now, they "would have made a different decision". But Boeing did know about the AOA faults and concerns about MCAS as internal documents revealed. Apparently, Muilenburg was ill-informed, or in denial, or both.

MCAS was a new feature in commercial aircraft, and it was adapted/evolved to deal with parts of the flight domain (low speeds during ascension following takeoff) for which it had not been designed. Boeing minimized the impact of MCAS, basically indicating to the FAA that pilots did not need to know about MCAS, which would have required training, including simulator training.

Edit/update: Interesting discussion on AOAs by Boeing.
https://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html
AOA can be used for many indications on the flight deck to improve flight crew awareness of airplane state relative to performance limits. Dedicated AOA indicators have been used on military aircraft for many years, but this form of display has not been used often on commercial airplanes. On Boeing models currently in production, AOA is used to drive stall warning (stick shaker), stall margin information on airspeed indicators, and the pitch limit indicator (PLI) on the primary attitude displays. AOA information is combined with other data and displayed as an integral part of flight deck displays.

Discussion on erroneous flight data and accidents
https://www.boeing.com/commercial/aeromagazine/aero_08/erroneous_story.html
Unfortunately, safety data show that not all flight crews have satisfactorily handled situations caused by erroneous flight instrument information. During the past 10 years, more than 300 accidents and incidents have been reported as a result of erroneous flight deck information, including problems with pitot-static probes and air data computers. Several fatal accidents that involved erroneous flight instrument information and six incidents resulting from lost or erratic air data occurred in 1996 alone. Investigations of these events indicate that, with proper preparation, the flight crews involved in these events probably could have prevented them.
 
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  • #654
Again I wish to look at the larger picture. The entire idea of using the trim system to essentially "fool" the pilot into thinking this was the earlier version of the 737 is such bad engineering. I have not designed aircraft, but have worked on life critical medical devices. This bright idea would have set off every alarm in my being instantly. From article https://en.wikipedia.org/wiki/Maneuvering_Characteristics_Augmentation_System#cite_note-Obs-34 quoting
  1. Hart (2019). Boeing 737 MAX Flight Control System : Observations, Findings, and Recommendations(PDF). FAA.

"MCAS used the stabilizer to change the column force feel, not trim the aircraft. This is a case of using the control surface in a new way that the regulations never accounted for and should have required an issue paper for further analysis by the FAA. If the FAA technical staff had been fully aware of the details of the MCAS function, the JATR team believes the agency likely would have required an issue paper for using the stabilizer in a way that it had not previously been used; this [might have] identified the potential for the stabilizer to overpower the elevator."

This is simply reprehensible.. I feel very sad for those good people left at Boeing.
 
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  • #655
IMO the bad engineering wasn't using the trim to change the column force feel of the aircraft because that's what trim is for.

"Hold the required attitude using the yoke, then trim the aircraft until no force is required to hold the yoke"
FBW flight control systems with a yoke emulate column force feel changes during trim adjustments.

Bad engineering is making a dangerously powerful but operationally fragile auto-trim system that could overpower the human capacity to correct in seconds.
 
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  • #656
I agree that the bad engineering derives from making it trim capriciously. The reprehensibility comes from actively shielding the pilot from this knowledge for public relations purposes.
 
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  • #657
nsaspook said:
that's what trim is for
Not really. The quote you give is not describing how trim is used to adjust control force feel when executing any maneuver. It is describing how you correctly trim an airplane for a given angle of attack, i.e., how to execute a particular maneuver. Execute a different maneuver, such as banking the airplane, and you'll feel a control force, and you'd better not try to adjust trim to make it go away or you'll take away a critical piece of feedback for the pilot.

Mechanical/hydraulic control systems typically add weights or something like that to adjust control force feel; they don't depend on the trim system to do that. FBW control systems, as you note, emulate control force feel changes, but they certainly don't use the trim system to do that since there's no mechanical connection between the control force and the aerodynamic force, which removes any possible justification for using trim adjustments to adjust control force.

Many airplanes with mechanical/hydraulic controls, including the 737, do have a "speed trim adjustment" that adjusts the trim depending on airspeed in order to adjust control force. (MCAS was actually piggybacked on this system in the 737.) However, this is done to keep the control force within manageable limits for a human pilot over the full range of allowed airspeed, not to adjust the feel to match some previous profile. To me, the bad engineering with MCAS was using the capability to adjust trim to affect control force for a different purpose than its original intent--namely, to match the control force profile of previous 737s--and the consequent adding of new inputs (AoA sensors, which AFAIK aren't used by the speed trim system) and new failure modes that weren't properly assessed.
 
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  • #658
PeterDonis said:
Not really. The quote you give is not describing how trim is used to adjust control force feel when executing any maneuver. It is describing how you correctly trim an airplane for a given angle of attack, i.e., how to execute a particular maneuver. Execute a different maneuver, such as banking the airplane, and you'll feel a control force, and you'd better not try to adjust trim to make it go away or you'll take away a critical piece of feedback for the pilot.

Mechanical/hydraulic control systems typically add weights or something like that to adjust control force feel; they don't depend on the trim system to do that. FBW control systems, as you note, emulate control force feel changes, but they certainly don't use the trim system to do that since there's no mechanical connection between the control force and the aerodynamic force, which removes any possible justification for using trim adjustments to adjust control force.

Many airplanes with mechanical/hydraulic controls, including the 737, do have a "speed trim adjustment" that adjusts the trim depending on airspeed in order to adjust control force. (MCAS was actually piggybacked on this system in the 737.) However, this is done to keep the control force within manageable limits for a human pilot over the full range of allowed airspeed, not to adjust the feel to match some previous profile. To me, the bad engineering with MCAS was using the capability to adjust trim to affect control force for a different purpose than its original intent--namely, to match the control force profile of previous 737s--and the consequent adding of new inputs (AoA sensors, which AFAIK aren't used by the speed trim system) and new failure modes that weren't properly assessed.

https://www.boeing.com/commercial/737max/737-max-software-updates.page

The approved software fix is still using using trim/stabilizer adjustments to adjust enhance the pitch stability of the airplane, right?
 
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  • #659
nsaspook said:
The approved software fix is still using using trim/stabilizer adjustments to adjust enhance the pitch stability of the airplane, right?
Boeing's comment there about "adjusting the pitch stability" is rather disingenuous. The real purpose, as the very next clause states, is to make the 737 MAX feel similar to previous 737s. And the reason for that has nothing to do with "pitch stability"--it has to do with avoiding time consuming and expensive pilot retraining in a new aircraft type that Boeing's customers do not want to have to pay for.
 
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  • #660
PeterDonis said:
Boeing's comment there about "adjusting the pitch stability" is rather disingenuous. The real purpose, as the very next clause states, is to make the 737 MAX feel similar to previous 737s. And the reason for that has nothing to do with "pitch stability"--it has to do with avoiding time consuming and expensive pilot retraining in a new aircraft type that Boeing's customers do not want to have to pay for.
Exactly. So IMO after a long period of intense investigation it was decided the basic concept was OK but the execution of that concept was the root engineering error by the certification agencies.
 
  • #661
nsaspook said:
https://www.boeing.com/commercial/737max/737-max-software-updates.page

The approved software fix is still using using trim/stabilizer adjustments to adjust enhance the pitch stability of the airplane, right?
They fixed three glaring sins of CLAW design:
1) MCAS only looked at one AOA sensor. That was a sin.
2) MCAS took control repeatedly and for longer times than it allowed the pilots to correct it. That was another sin.
3) MCAS had more control authority than the pilots. That was a third sin.
I am sure that experienced CLAW designers were horrified.
It is not clear to me that any amount of pilot knowledge and training would have made it safe.
 
  • #662
nsaspook said:
So IMO after a long period of intense investigation it was decided the basic concept was OK but the execution of that concept was the root engineering error by the certification agencies.
While that will be the effect of the current ruling (AFAIK the software update described is the only fix, the 737 MAX is still considered the same aircraft type), I'm not sure how much of that decision was driven by actual engineering judgment as opposed to the political implications if such a fix were not approved.
 
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  • #663
PeterDonis said:
While that will be the effect of the current ruling (AFAIK the software update described is the only fix, the 737 MAX is still considered the same aircraft type), I'm not sure how much of that decision was driven by actual engineering judgment as opposed to the political implications if such a fix were not approved.
+1
I would certainly hope political implications were at the bottom of the heap for that decision.

https://www.faa.gov/foia/electronic_reading_room/boeing_reading_room/media/737_RTS_Summary.pdf
Boeing proposed multiple updates to the MCAS function to address Safety Item #1: USE
OF SINGLE ANGLE OF ATTACK (AOA) SENSOR, Safety Item #2: MCAS RESET GENERATES
REPETITIVE MCAS COMMAND and Safety item #3: MCAS TRIM AUTHORITY in the
previous chart. The MCAS activation software now includes a maximum limit of one nosedown stabilizer activation during a single elevated AOA event and cannot be reset by pilot
activation of the electric trim switches. An AOA sensor monitor was added to prevent
MCAS from using an AOA input if it differs from the other AOA input by more than 5.5
degrees. Boeing incorporated a maximum command limit to disable the MCAS and speed
trim operations if the stabilizer position exceeds a reference position. This limit ensures
sufficient elevator control is available to provide maneuvering capability using control
column inputs alone.
13. FAA Conclusion
Following a thorough, transparent and inclusive process, the FAA determined that Boeing’s
changes to the 737 MAX design, flightcrew procedures and maintenance procedures
effectively mitigate the airplane-related safety issues that contributed to the Flight 610
and Flight 302 accidents. The FAA further determined that the design change addressed
additional safety concerns beyond those identified during the accident investigations. This
report does not address other safety issues that might have contributed to the accidents
but are not related to airplane design, including maintenance, aircraft operator and air
traffic control. The FAA believes recommendations related to these other potential
contributing factors should be addressed by the appropriate organizations. Further, the
FAA evaluated Boeing’s proposed flightcrew training through the Flight Standardization
Board process. The FAA issued a final Boeing 737 Flight Standardization Board Report
documenting the results of the operational evaluation.
 
  • #664
nsaspook said:
Was it something like the several second timer that repeats the erroneous trim adjustment? Was it a design control law problem or a problem with some like the software PID implementation of a control law? Typically with a PID control loop the error term has the integral term gain limited (equivalent to a one time adjustment here) to only be able to give X amount of feedback (to combat mechanical system windup to control limits) to adjust the total error signal to balance the control set-point. One of the problems that prevented recovery was the pilot would correct the pitch error but MCAS would just push the nose back down again and again. The pilots were able to counter the nose-down movement multiple times but eventually they ran out of airspace. Obviously the repeated adjustment mode was 'fixed' to one time only now.

The main thing to avoid in airplane stability & control, is an aerodynamic nose up moment that is not commanded by the pilot. The uncommanded nose-up moment would not auto-stabilize, but rapidly get progressively larger with increasing angle of attack, and run away to a stalled airplane.

During certification of a passenger airplane, many tests are carried out to check if the airframe does not start to have a mind of its own.

  • If the pilot does not provide a control input, the airframe must return to the trimmed position.
  • Forces and inputs to move the airplane away from trimmed position must be such that there is an ever increasing force required to achieve an ever increasing nose-up position. The nose-up position must always be commanded by the flight control surfaces, elevator and stabilizer, in a predictable way.
One of the tests to be performed during certification is stick-force-per-g. Bank the airplane and start turning while pulling the stick back in order to maintain altitude. Then bank more and pull back more, in ever tightening turns. It must be progressively harder to pull back on the stick to maintain altitude, never easier.

It was during this wind-up turn that due to the engine configuration of the MAX an aerodynamic nose-up moment appeared, which would cause the pitch stick force to suddenly become less than expected. Not as bad as a runaway pitch, but still an undesirable situation when the pilot is still straining to maintain the maneuver. This is the situation that MCAS was originally designed for, to auto-compensate for this situation only.

The pilots of ET302 needed to unload the stabilizer, and you would only need to unload the stabilizer if you've allowed the trim runaway to progress to a point at, or very near, the full nose-down limit AND you failed to reduce thrust and your airspeed is excessive. The Ethiopian flight's indicated airspeed reached 390 knots.

Neither accident crew accomplished the steps on either the runaway stabilizer or unreliable airspeed checklists. Both of which have as either their 2nd (IAS) or 3rd (trim) step to disengage the autothrottles.

They did not.

1. Control Column, hold firmly
2. Autopilot (if engaged), disengage
3. Autothrottle (if engaged), disengage
4. Choose one:
... The runaway stops after the autopilot is disengaged:
... End of procedure

... The runaway continues after the autopilot is disengaged:
... ... STAB TRIM CUTOUT switches (both), CUTOUT
... ... ... If the runaway continues:
... ... ... ... Stabilizer trim wheel, Grasp and hold

The ET Captain engaged the A A/P (the one on the side with the invalid data) while still well below the minimum A/P engagement altitude.

They never disengaged the autothrottles and that allowed the airspeed to build to around 390 knots indicated.

They didn't use the stab trim cutout switches until the stab trim was already near the full nose-down position. Then they took the switches back to normal and allowed MCAS to drive the stab the rest of the way to the full nose-down position.

The backup trim system on all 737s is the same as what was used on the B707, B720, and B727 which all operated with that system for many decades. The system is designed to work in stages depending on the amount of force required in a given situation.

The trim wheels have fold-out handles. When the airplane is close to in-trim, the flying pilot folds out his handle and can operate the trim with his inboard hand while flying the airplane with his outboard hand.

As the out-of-trim condition increases, the forces are higher and the pilot-monitoring will operate the trim wheel on command of the flying-pilot. i.e. "Trim down", "Stop trim", etc.

As the out-of-trim condition increases further, the two pilots work together with their inboard-hands turning the wheels together. The two fold-out handles are located 90 degrees of rotation apart. This is so that one pilot has his handle in a position which provides good leverage at any point in the wheel's rotation.

When you get to a situation where the trim is at the full nose-down stop, and your airspeed is around 150 knots faster than it should be, the stabilizer loads are too high to manually move the stabilizer which require alternate periods of unloading and trimming with periods of regaining altitude.

The key to successfully handling any runaway stabilizer event is to accomplish the runaway stabilizer checklist in a reasonably prompt manner so that the runaway is stopped before the trim reaches the full nose-down limit. If you don't, you have made recovery significantly more difficult.

The additional training that these crews needed was not in MCAS, how MCAS works, or even in handling a runaway stabilizer. The training they needed was in the proper prioritization of tasks in an emergency. 1. Fly the airplane, 2. Silence the warnings, 3. Confirm the emergency. This process is how you move past the distractions to find, confirm, and correctly action the emergency.

None of which would have been necessary if they had continued to fly the airplane by retrimming each time the nose got heavy from the MCAS activations. The Lion Air Captain did exactly that through 21 MCAS activations. The F/O, to whom he transferred control, did not.

As for the Ethiopian accident, the excessive airspeed over the stabilizer in the full nose-down position far outweighed the nose-up moment from the high power setting.

You can't fly the airplane at 390 KIAS (Vmo is 340) and full nose-down stab trim. That would be true in every transport jet.

If they had followed the correct procedure, the trim never would have reached full nose-down, the airspeed never would have reached 390 KIAS, and they would have been able to trim manually just as the crew of the Lion Air incident flight, which landed safely, did.
 
  • #665
EAG711 said:
If they had followed the correct procedure, the trim never would have reached full nose-down, the airspeed never would have reached 390 KIAS, and they would have been able to trim manually just as the crew of the Lion Air incident flight, which landed safely, did.
And if they had made some wise design decisions, the pilots would have never been put to the test at all. Do you think that the design corrections they made now should not have been made? Flight controls should reduce the risks, not introduce new ones.
 
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