Reducing the speed (or not letting it grow) is an obvious remedy but easier to say afterwards. If they have the stick shaker since the take-off, with a normal airspeed, they may think the fault is on airspeed and apply the procedure to keep N1 at a relatively high value that will accelerate the aircraft beyond controlability limits...

Also, 787 do not need any MCAS :{

Instead of a 737 MAX Boeing should have built a 787 MIN :)

Keep trimming and if the system continuously trims you nose down as you are accelerating know that is the opposite direction of STS and thus incorrect => shut the stabilizer down. First and foremost keep trimming to keep the column forces from building up. The problem of getting progressively more out of time takes 10s of seconds to build up and will stop if you keep trimming. If MCAS decides it is time to put in 10 seconds of nose down trim and you determine after a couple of seconds that it is doing the wrong thing and add your own nose up trim, MCAS will stop until you have been off of the trim for 5 continuous seconds. If all you do is blip the trim once every 5 seconds MCAS won't do anything even if the AOA it is getting is out of whack high! Eventually it is hoped that the crew will decide that the automatic stabilizer is doing far more harm than good and turn if off via the cutout switches.

Oh - did I mention enough times that you should keep trimming?!

I'm not trying to argue that MCAS as currently design is satisfactory, but with the system knowledge that we had before the end of 2018 I was confident that we had an acceptable workaround until improvements could be brought to the fleet.

If it turns out the the Ethiopian accident was the result of the same issues that led to the Lion Air accident our industry has some major soul searching to do.

MCAS appears to have been designed based on three assumptions that I have suggested before:
1. If a pilot trims column force will be trimmed to or near zero.
2. Having the automatic stabilizer control continuously take the airplane away from trim when otherwise flying a relatively steady condition, pilots will recognize that it is not healthy and would shut it down via the stabilizer cutout switches.
3. An errant AOA signal feeding MCAS would be acceptable because of (1) and (2) above.
Lion Air showed us that for the crew the day before the accident assumptions (1) and (2) above were reasonable so when (3) happened they were able to fly home.
- In fact they elected to go on to their destination with the stick shaker rattling the whole way!
Lion Air further showed us that for the pilot on the accident flight assumption (1) above held. He made more than 20 corrections and was able to maintain his apparent target altitude of 5000 feet reasonably well. He did not get to (2) above, but might have had his partner been able to keep up with (1) after transferring control.
The Lion Air accident clearly showed that neither assumption (1) nor (2) held true for the second pilot and thus control was lost as a result of not meeting assumption (3).

737MAX was permitted to continue flying after the data from the Lion Air accident revealed the facts outlined above, through so many PPRUNE pages, and across so many other information outlets including FAA and Boeing communications because an assumption was made that with increased awareness pilots would be able to live up to assumptions (1), (2), and (3) above. I would like to think that most of the pilots flying 737MAX airplanes in February were prepared to live up to these assumptions. If the recently decoded FDR from the Ethiopian accident shows that to be a repeat of the Lion Air event this latest assumption will clearly not have been valid for at least one 737MAX crew.

While the MCAS software update developed after the Lion Air accident that is almost ready to go to the fleet will likely remove reliance on the three MCAS design assumptions listed above and thus would have greatly improved the likelihood of a safe outcome for the Ethiopian event we are left with a huge elephant in the room. After making the planned update we still must address the following:
A. How many other key points in the 737MAX safety story are based on pilot response assumptions that may not be valid?
B. How about other airplane models? Are they deemed safe based on faulty assumptions regarding pilot action?
- For instance, how may current 737 crews (all models) would not respond quickly enough to a classic stabilizer runaway that was not arrested by column cutout (i.e., pulling the column far enough)? I know this is covered in simulator sessions for 737 pilots, but is that enough?
C. Moving forward with the current status and future of commercial aviation have we gotten to the point where basic flying skills and system awareness are so low that we are at risk throughout the whole industry?
D. Can current and future pilot reaction short falls be addressed through training? If so, what kind, how much, and how often?
E. How will we know that we have achieved a sufficient industry wide level of safety?

Hoping to see FDR data from the Ethiopian accident soon. I sure hope someone from the PPRUNE community will find a way to get ahold of it and share it here.

FCeng84,

Given your familiarity with 737 systems I wanted to address a question to you:

As far as I understand, FCC-enabled MCAS receives single channel sensory data from the ADIRU, mainly AoA (so as to trigger MCAS in the first place) and speed (so as to regulate just how much trim input is required based on Mach number schedule).

My question is about the total control column forces with both SMYD stall protection features ANDhaving MCAS in its maximum setting. Allow me to explain further:

1. MCAS: assuming an AoA sensor failure where the data is relayed as equal or higher than the activation threshold of MCAS, then MCAS kicks in. Assuming a low speed readout by MCAS, it is then set to its maximum throughput (10 seconds of trim and 2.5 degrees nose down, since it works on a speed schedule based on Mach).
2. SMYD/EFS: As I understand, the SMYD computer enables the EFS (Elevator Feel Shift) and STS (Speed Trim System) to do the following during low airspeed conditions - EFS on one hand doubles Hyd A pressure to control column in its AFT axis, and STS trims the stab Nose Down (though respecting the column cutout switches, something MCAS ignores altogether).

The conclusion is the STS trimming you down (so as long as you don't pull aft), the EFS doubling aft forces, and MCAS also trimming at up to 2.5 degs nose down within 10 seconds (if uninterrupted). Essentially, you can end up in full Nose Down stab trim with doubled elevator control forces, thus severely affecting your authority.

Do you agree with my reasoning that SMYD/EFS interaction with MCAS (when low speed is relayed) can be additive in terms of struggling to keep the nose up and having limited authority?
Makes you wonder just how much attention Boeing engineering gave to both these systems' interaction at the design stage.

Non of which is particularly relevant to good basic aerodynamics and safe logical system design and some points of which even contradict themselves !

nose down trim by a stick pusher a very different thing than motoring the whole stab 'in the background' without so much as a 'by your leave' or whisper to the pilot !!!

As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.

But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)

My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.

Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"

(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.

(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.

(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.

Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.

We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.

An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.

A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.

deltafox44
I love logic!!!!

How many turns of the wheel are required to change trim by one unit?

Why not allow MCAS ‘if activated’, to operate one cycle with aural alert, no repeat for 60 seconds?

This, to me, a once-upon-a time mech and long-time engineer, is the "truthiest" post of this long thread. From the moment, in the Lion Air thread that we began to understand the existence and potential reality of MCAS, my image of being on the flight deck of one of these accident aircraft has been eerily like Aloha_KSA's depiction.

I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.

737 power structure in descending order: SWA-Boeing-FAA.

10 rotations for one unit of trim.
(Source: Boeing FCOM Bulletin FLR-6(FR) dated August 10, 2012. Subject: Runaway Stabilizer Procedure. From Background '.. the stabilizer trim wheel coasted up to 40 turns (four units of trim).')

I am a human factors researcher working in aviation safety, a long time lurker on pprune (5+ years), and this is finally the post which I feel the need to commend so much that I have created an account. This post, which takes into account human psychology and environmental factors, should be required reading for anyone following any of the recent developments regarding the MAX's safety record.

I am personally shocked at how many posters here truly believe that the pilots should be clairvoyant when presented with a cacophony or warnings and little time to react. It has been covered in other posts but both the Lion Air and Ethiopian crashes presented the pilots with a series of stimuli that did not match the stimuli presented in (most carrier's) training for stabilizer runaway; it is thus not expected for the pilots to immediately reach for the cutout switches. Many of the other proposed actions by the crew(s), which some posters seem to consider common sense, again fail to consider human performance limitations in cognitively demanding situations.

Boeing may have a lot to answer for in terms of design philosophy, flight manual omissions, training deficiencies, etc.. However, I think another critical realization the professional aviation community should draw from these recent tragedies is that basic human-automation interaction principles are still not being given adequate consideration by manufacturers.

Sums it up beautifully from my perspective. Very easy to look at the issue out of context and say things like "they should have simply flicked the stab trim cutout switches", but when you have two seemingly very similar instances of that not happening, you need to look a bit deeper at the why, rather than reverting to the 1970s and citing 'pilot error' as alarmingly, some are still quick to jump to.

Thank you.

As a professional pilot we all hope it isn't our day. A wise aviator once remarked "no pilot assumes at the start of the day it is their last day"
It is part of the human condition to assume we are not that guy.

A cacophony of noise, spurious alarms, warnings, surges in power, vestibular disturbance. A growing awareness that this is getting really bad and eventually a windshield full of earth approaching at a breakneck speed with a windshield and radar dome between you and it is not something many want to think about. That four pilots found themselves in that situation is something we reluctantly accept. There but for His grace goes I.

However, that a regulatory process is compromised by a cost focused regulator and manufacturer that knew potentially of the problem (at least after Lion Air) is inexcusable.
That the CEO of Boeing calls the President begging for the aircraft to continue flying when they knew there was a probability it was the same problem that occurred last year is inexcusable.

Trim wheel
 

For context here is the trim wheel on the throttle quadrant. The "Green Band" is the normal takeoff range. On the NG it is usually around 5 units for takeoff Don't know on the Max. You can see the handle folded in to the wheel across from the white stripe, which is there to indicate rotation visually. If, for example, one were to go from 0 back to 5 units, that would apparently take 50 rotations. Lower your hand next to your knee in a sitting position and make 50 X 25cm (10inch) rotations to simulate full deflection.

The cutout switches are on the bottom right front.

https://cimg9.ibsrv.net/gimg/pprune....9d49ec0e43.png