MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

[Tomaski]

Originally Posted by Lake1952

Can the MAX be recertified without solving the issues related to the manual trim system? And how is the manual trim system handled on older versions?

Since the days of the 707, one of the issues with this trim system is that it is very difficult to operate the manual trim wheel from an excessive out-of-trim condition. The answer then is to not let this aircraft get grossly out of trim which means that flight crews need to keep the stab trim state, and any motions of the trim wheel, very much in their regular cross check, and respond promptly to any undesired trim inputs.

Ironically, before the MAX the 737 stab trim system had become very reliable and significant problems were very rare. This level of reliability generated a great deal of complacency among both crews and airlines which then led to a lack of regular training with and awareness of potential runaway stab scenarios. This training deficiency can potentially contribute to problems with recognition and delayed crew response.

[Tomaski]

Originally Posted by megapete

Not sure that answers the question - if the clutch operates at a load manageable by the pilot via the trim wheel would it not operate when trying to use electric trim at the same load - and hence not be effective?

As I understand the system, if the stab was truly jammed in place - either by aerodynamic loads or some physical malfunction - the electric trim motor would turn, the clutch would slip, and the stab would not move. That being said, I know of no actual case of this happening on a 737.

[Europa01]

Agreed -this generic schematic shows a clutch.
(sorry couldn't make the jpeg work - will try harder)

[Checkboard]

Ironically, before the MAX the 737 stab trim system had become very reliable and significant problems were very rare. This level of reliability generated a great deal of complacency among both crews and airlines which then led to a lack of regular training with and awareness of potential runaway stab scenarios. This training deficiency can potentially contribute to problems with recognition and delayed crew response.

A runaway stab, and MCAS are fundamentally different events.

For a runaway stab, usually it's a failure of the trim switches. That is - the pilot trims, and releases the switch but the trim keeps running. As the pilot initiated the trimming, a problem is almost automatically assumed to be a trim problem in the pilot's mind and added to the noise of the trim in the otherwise quiet cockpit, the runaway trim is obvious and the correction is instinctive (trim against the runaway - firstly with the switches, secondly manually.)

An MCAS event, however, happens after a stick shaker on the Captain's side, AND unreliable airspeed on the Captain's side - and with all of that going on pulling the pilots eyes away from the centre pedastal to the flying instruments the trim spins (with the noise of that covered by the stick shaker). The fact that this event took two experienced pilots to their deaths shows how different, difficult and dangerous the event is to diagnose in the time available.

[Tomaski]

Originally Posted by Checkboard

An MCAS event, however, happens after a stick shaker on the Captain's side, AND unreliable airspeed on the Captain's side - and with all of that going on pulling the pilots eyes away from the centre pedastal to the flying instruments the trim spins (with the noise of that covered by the stick shaker). The fact that this event took two experienced pilots to their deaths shows how different, difficult and dangerous the event is to diagnose in the time available.

Distraction was certainly an issue, just as it would have been an issue if there had been an engine failure amidst a cacophony of alarms. But part of the problem was also recognition of an ongoing stab trim issue. It should be noted that when hand-flying, the primary indication of the aircraft trim state is the feel of the controls and not reference to the trim wheel or stab position indicator. In all three of the MCAS malfunction incidents, the flying pilot was holding significant aft column control pressures and inputting a greater than normal amount of nose up trim, and yet there was either a delayed or lack of recognition that this might be a form of runaway stab trim. I am suggesting that if these flight crews had been exposed to runaway stab trim scenarios on a more regular basis in the sim (it is a "memory item" after all), just as they routinely practice engine-out scenarios, then their response to the MCAS malfunction might have been more prompt.

[Ian W]

Originally Posted by DaveReidUK

"The accident was truly caused by the crew in both cases"

"It amounted to just a runaway trim"

"There was never a reason to ground [the MAX]"

"[Boeing's] largest mistake was to overestimate the quality of the pilots it was selling its airplane to"

Video: CNBC interview with William Langewiesche

During the similarly long AF447 thread where more hostility was allowed toward the cockpit crew than on this thread, the repeated refrain was 'fly the aircraft' - 'pitch and power' - especially when the automatics have lost it and dropped into Alternate Law. I lost count of the number of times that was repeated (sometimes in different words attitude and thrust etc.). The same in this thread a proportion of the posters have repeated 'fly the aircraft' - 'pitch and power' and use trim when required and that, had that been done, like the first Lion Air aircraft the two subsequent aircraft would not have crashed.

But from the posts here this is explicitly NOT being taught in training. The teaching seems to be to find the correct NNC(s) to use, then run them by rote. If there is a cacophony of alarms and cavalry charges, flashing lights, EICAS messages, and horns and verbal alerts, the crew are spoilt for choice - which NNC to run? This is the cognitive overload that NTSB is warning about. And as the repetitious posters above would tell them disregard the noise, 'fly the aircraft' - 'pitch and power' and only then while in control of the situation work out what the automatics are complaining about. This approach would have the advantage of reducing the cognitive overload as the human is doing something meaningful and effective.

Do any airlines train crews to 'fly the aircraft' - 'pitch and power' as a an essential underlying task while running NNCs?

[Fly Aiprt]

Originally Posted by Ian W

This is the cognitive overload that NTSB is warning about. And as the repetitious posters above would tell them disregard the noise, 'fly the aircraft' - 'pitch and power' and only then while in control of the situation work out what the automatics are complaining about. This approach would have the advantage of reducing the cognitive overload as the human is doing something meaningful and effective.

Do any airlines train crews to 'fly the aircraft' - 'pitch and power' as a an essential underlying task while running NNCs?

Here is what the NTSB report insists on, and where Boeing and the FAA were wrong : in theory it should have appeared as a trim runaway, but in reality it was totally different.

Emphasis should be place on the fact that the accident aircrews were experiencing control difficulties beside all the cognitive overload, and ultimately failed to maintain pitch attitude.
The aircraft was nosing down, the elevator couldn't counter it, and the trim wasn't runaway, it was behaving in a mysterious manner.

Every pilot that experienced or witnessed the situation in the engineering cab (fixed, no g's, no movement), stated that the situation was extremely difficult to manage, even with no stress and knowing in advance what was to take place, and what the solution was.

[misd-agin]

Originally Posted by Tomaski

Since the days of the 707, one of the issues with this trim system is that it is very difficult to operate the manual trim wheel from an excessive out-of-trim condition. The answer then is to not let this aircraft get grossly out of trim which means that flight crews need to keep the stab trim state, and any motions of the trim wheel, very much in their regular cross check, and respond promptly to any undesired trim inputs.

Ironically, before the MAX the 737 stab trim system had become very reliable and significant problems were very rare. This level of reliability generated a great deal of complacency among both crews and airlines which then led to a lack of regular training with and awareness of potential runaway stab scenarios. This training deficiency can potentially contribute to problems with recognition and delayed crew response.

Exactly. Which is why the stab trim cutout switches are guarded and right next to the Captain's thigh/hip area. Experienced guys know where they are and you'd hope they wouldn't let the stab trim run uncommanded, multiple times for a long period of time, without turning the stab trim cutoff switches to off.

[misd-agin]

Originally Posted by Fly Aiprt

Here is what the NTSB report insists on, and where Boeing and the FAA were wrong : in theory it should have appeared as a trim runaway, but in reality it was totally different.

Emphasis should be place on the fact that the accident aircrews were experiencing control difficulties beside all the cognitive overload, and ultimately failed to maintain pitch attitude.
The aircraft was nosing down, the elevator couldn't counter it, and the trim wasn't runaway, it was behaving in a mysterious manner.

Every pilot that experienced or witnessed the situation in the engineering cab (fixed, no g's, no movement), stated that the situation was extremely difficult to manage, even with no stress and knowing in advance what was to take place, and what the solution was.

Reacting as if it was a trim runaway would have stopped the problem. The plane could have been flown, almost 100% normally, to a safe landing. That's exactly what happened on the flight that the jump seater identified the problem as a runaway trim - the crew turned the trim off using the stab trim cutout switches and continued to their destination. I'm not advocating continuing to your destination after a stab trim problem but their actions show that it's fully controllable once the appropriate steps are taken.

[misd-agin]

Originally Posted by Fly Aiprt

Every pilot that experienced or witnessed the situation in the engineering cab (fixed, no g's, no movement), stated that the situation was extremely difficult to manage, even with no stress and knowing in advance what was to take place, and what the solution was.

Every pilot? Where's that report? And what are the circumstances and conditions? I saw one report that gave the pilots the plane at full runway trim and 300+ (350+?) knots. What were their impressions and reactions if they were flying the entire scenario and took off and had the trim issue at less than 200 kts and didn't keep takeoff power the entire time which ultimately lead to excessive speed which just compounded the problem?

[Fly Aiprt]

Originally Posted by misd-agin

Every pilot? Where's that report? And what are the circumstances and conditions?

Thanks for challenging me, sir ! You know the conditions are not public at the moment.
Maybe if you go a long way back in this thread and others, you'll find statements from pilots who "flew" the event in the engineering cab. Also maybe you heard Capt. Sullenberger report what he saw during those "fligts" and what he told the Committee.
You might be tempted to say that's "every pilot who flew and reported said...", and you'd be right.
But then I'd challenge you in return : tell us more about this report you are citing ;-)

[Fly Aiprt]

Originally Posted by misd-agin

Reacting as if it was a trim runaway would have stopped the problem. The plane could have been flown, almost 100% normally, to a safe landing. That's exactly what happened on the flight that the jump seater identified the problem as a runaway trim - the crew turned the trim off using the stab trim cutout switches and continued to their destination.

With due respect, sir.
No certifying agency, pilot union representative, or member of the NTSB share your perspective, so pending more credentials on your part, I'll stick to my opinion considering what whe do know of the accidents, human factors and what my experience as a flight instructor taught me.
I'll add that any video of a "I'd do better" pilot confronted with multiple failures in the sim might help us form a considered opinion on the matter.

[nevillestyke]

Originally Posted by Europa01

(sorry couldn't make the jpeg work - will try harder)

Have you tried the manual wheel to make JPEGs work?

[Speed of Sound]

Originally Posted by nevillestyke

Have you tried the manual wheel to make JPEGs work?

“It’s too stiff Captain, it won’t turn!”

[jdawg]

Originally Posted by misd-agin

Reacting as if it was a trim runaway would have stopped the problem. The plane could have been flown, almost 100% normally, to a safe landing. That's exactly what happened on the flight that the jump seater identified the problem as a runaway trim - the crew turned the trim off using the stab trim cutout switches and continued to their destination. I'm not advocating continuing to your destination after a stab trim problem but their actions show that it's fully controllable once the appropriate steps are taken.

Why would you react to a trim runaway if the problem did not present itself that way? You're confusing us.

[Tomaski]

Originally Posted by jdawg

Why would you react to a trim runaway if the problem did not present itself that way? You're confusing us.

This touches on another training issue that I have previously commented on, that being the overly structured and highly procedural approach that has crept into pilot training. And I will again remind everyone that every manufacturer will clearly state that they cannot possibly devise a checklist for every situation, and sound judgement is always called for when evaluating an aircraft malfunction.

For example, not all engine malfunctions conform to the typical failures seen in the sim. Back in my 727 days, I once had an engine that was producing some odd readings (mainly a mismatch between EPR and RPM's with a very slight amount of adverse yaw). The engine was producing thrust, none of the readings were out of limits, and none of the checklists addressed what we were seeing. Yet something was "off." We decided to secure the engine and land. Upon inspection after landing, maintenance determined that a bleed duct had failed and would have likely resulted in further damage if we had not shut down the engine. In this case, if a crew had been looking for some kind of specific written guidance on how to handle this malfunction, they would have come up empty.

You are absolutely correct that a MCAS malfunction does not look like a "classic" runaway stab trim problem particularly in that it lacks that "continuous" aspect referred to in the QRH. However, from the perspective that this malfunction resulted in uncommanded and undesired stab trim inputs, then the Runaway Stab Trim NNC was a suitable tool by which to address it. That thought process, however, required something of a "big picture" perspective that I believe is not adequately taught at many airlines today.

[Checkboard]

just as it would have been an issue if there had been an engine failure amidst a cacophony of alarms.

An engine failure has ONE alarm - with a big flashing attention getter to stop that alarm. You cannot (easily) turn off a stick shaker.

[babemagnet]

Another week went by. Nothing happend. What the heck are they doing!

[Tomaski]

Originally Posted by Checkboard

An engine failure has ONE alarm - with a big flashing attention getter to stop that alarm. You cannot (easily) turn off a stick shaker.

I can assure you that if you have a major engine malfunction that starts slinging parts and taking out various other systems, you will have more than one alarm, some of which you may not be able to silence right away. Or perhaps that flock of birds the plane flew through took out a few more things than just an engine. There is no immutable law the prohibits multiple things from going wrong at the same time, yet current training regimes will still present scripted problems one at a time with clearly defined symptoms and clearly defined actions.

[pilotnik]

Originally Posted by jdawg

Why would you react to a trim runaway if the problem did not present itself that way? You're confusing us.

Are you for real? So the aircraft is almost uncontrollable, you pull the yoke as hard as you can, you see the aircraft stabilizing itself adversely to what you would expect and then you say to yourself: "oh wait! it's not a continuous runaway! Everything is just fine!". Think for a minute: you in fact argue that these two accidents wouldn't have happened if the word "continuously" had been removed from the checklist. That's ridiculous! The commander of the aircraft is a commander not because he/she can literally read the checklist and execute it like a computer software executes an algorithm but because he/she is competent in airmanship and has demonstrated the ability to counteract a developing dangerous situation. It is exactly for this reason why manufacturer states "In all situations, the captain must assess the situation and use good judgment to determine the safest course of action."