Ethiopian airliner down in Africa

[PaxBritannica]

Humble SLF here, wondering if there's a general rule about speed/autothrottle in a runaway stabiliser situation?

The emergency AD issued by the FAA doesn't appear to say anything about speed. Is this just a matter of common sense among pilots, or would the PF have felt that it was sensible to keep the speed high to counter nose-down and hopefully gain altitude? Did he disobey the AD?

[MemberBerry]

Originally Posted by Albino

Just to confirm, I'm talking about 'Control airplane pitch manually with control column and main electric trim as needed'

This is pretty basic stuff.

Yes, and I agree. But then you could then argue it shouldn't even be included in the memory items, and all the discussion about step 2 is irrelevant here.

It's not exactly the same situation but, I looked again at Mentour's video that demonstrates the runaway stabilizer memory items. They already have the A/P disconnected when the runaway starts, and the thumb switches are not working at all during the runaway. He does try to use the thumb switches to control the trim, but then so did the Ethiopian crew.

You Tube

This is how it went (some small details omitted):

17:18 - runaway starts
17:28 - Mentour (PF) - "I'm not doing that"
17:40 - F/O (PM) identifies the issue as a runway stabilizer
17:50 - Mentour asks the F/O to perform the runway stabilizer memory items
17:55 - F/O - "control column: hold firmly"
17:57 - Mentour - "holding firmly"
17:59 - F/O - "auto pilot - disengage"
18:01 - Mentour - "autopilot is disengaged"
18:04 - F/O - "auto throttle - disengage"
18:07 - Mentour - "if you can do that for me please"
18:09 - F/O - "disengaged"
18:11 - F/O - "if it continues, I will go with the stab trim cutout switches"
18:13 - Mentour - "ok, let's evaluate this to see if it's moving"
18:20 - F/O - "memory items for the runaway stabilizer complete"
18:21 - Mentour - "yeah, let's evaluate"
18:25 - Mentour - "I have no authority of the trim at the moment"
18:29 - F/O - "roger"
18:32 - Mentour - "I can't control it manually, so it doesn't really work, so if you can continue with the memory items please"
18:37 - F/O - "memory items - stab trim cutout switches to cutout"
18:40 - Mentour - "Continue"
18:41 - F/O moves the switches to the cutout position
18:42 - F/O: "Cutout"

So, between the start of the runaway and using the cutout switches it takes them 83 seconds to perform the runaway stabilizer memory items. With no additional failures and alarms that can distract them and increase their workload. With that in mind, do you think the stabilizer runaway memory items are adequate for dealing with an MCAS failure induced by an AOA sensor failure?

My guess is the Ethiopian crew was aware they could be dealing with MCAS, so they actually skipped some memory items and other procedures to stop MCAS as soon as possible:

05:40:00 - first MCAS AND trim
05:40:12 - pilot ANU trim
05:40:20 - second MCAS AND trim
05:40:27 - the Captain advised the First-Officer to trim up with him
05:40:28 - pilot ANU trim
05:40:35 - the First-Officer called out “stab trim cut-out” two times. Captain agreed and First-Officer confirmed stab trim cut-out.
05:40:41 - third MCAS attempt to apply AND trim, but it doesn't affect trim anymore

They did all that in under 40 seconds.

It's funny. Until the preliminary report was released many people were arguing that the cut-out switches should be used as soon as possible when you are dealing with an MCAS failure. Now people say that you should first bring the aircraft is in perfect trim before doing it. So basically that you should use the cutout switches in a 5 second interval, not too soon, and not too late, or it's pilot error. I disagree. This is not a fighting video game when you have to mash the right sequence of buttons with the right timing to produce a "combo". That's not OK when the lives of hundreds of people are at stake.

[Capn Bloggs]

Originally Posted by Albino

Just to confirm, I'm talking about 'Control airplane pitch manually with control column and main electric trim as needed'

So what are we doing here, flying by memo? Boeing should have revised the Stabiliser Runway memory procedure!

It knew it had a deathtrap on it's hands, why didn't it insert a line into the memory procedure about trimming to neutral and then switch off the stab trim?

All the background is lovely stuff, but Boeing, you're the experts, give me a couple of lines to hang my hat on and I will not crash the aeroplane. That's what memory items are for, are they not? Certainly, the current Stab Runaway is not appropriate in these cases where the stab had already lost the plot.

[DaveReidUK]

Originally Posted by PaxBritannica

Humble SLF here, wondering if there's a general rule about speed/autothrottle in a runaway stabiliser situation?

The emergency AD issued by the FAA doesn't appear to say anything about speed. Is this just a matter of common sense among pilots, or would the PF have felt that it was sensible to keep the speed high to counter nose-down and hopefully gain altitude? Did he disobey the AD?

The 737 FCOM Runaway Stabilizer checklist reproduced in the Preliminary Report (Appendix 3) has, as Item 3: "Disengage (and do not re-engage) the autothrottle".

[Just the fax maam]

The overall trim system now appears to be under scrutiny by several XAA's.

Also, what is more likely in the final dive of both crashes:

Pilots pulling with as much physical strength as they can muster to avert certain death did not think to also apply more than 2 brief blips of ANU trim to help...

Or

For the same reason manual trim is impossible in specific conditions, so is electrically operated ANU trim...

?

Seems to me that once the stab due to MCAS or runaway trim reaches a certain point it is unrecoverable without the old roller coaster manouvre few have been trained on or practiced. And possibly not just the Max.

I hope I am wrong of course, but this story may yet have further twists and turns to come.

[meleagertoo]

Let's not forget that the manual trim may only become immovable at or near max stabiliser travel at normal airspeeds.
It is more than a bit harsh to criticise Boeing when the pilots so mishandle (overspeed) the aircraft it is completely out of all parameters for the trim to work, and neither do they take any recovery action - ie slowing down.
Additionally, Boeing would not expect or anticipate a crew allowing the trim to get so far out of position, the rollercoaster method was a confidence demo to show that even if you did let the trim go to extremes it could never normally achieve in reality it was still recoverable. These pilots did let it get there.
If the pilots don't fly the aeroplane/procedures properly how can Boeing's procedures be expected to cope with that?
There's no certification need or requirement to have manual trim operable at full deflection above VMO - how could there be?

[FlightDetent]

Like a tiny little go-pro, recording a 10 minutes loop onto a titanium-encased SD card with a radio-isotope pinger. Good question why no such thing is already commonplace for at least 15 years. Asking out loud will not win you friends, but the answers are a great research material.

[Capn Bloggs]

It is more than a bit harsh to criticise Boeing when the pilots so mishandle (overspeed) the aircraft it is completely out of all parameters for the trim to work, and neither do they take any recovery action - ie slowing down.

So we're test pilots too now? No mention anywhere by Boeing of keeping the speed down, these guys were probably totally focussed on the MCAS issue, not your bog-standard UAS/80%. Ask yourself: when did you last pull off a handful of thrust at 1000ft above ground, straight after takeoff with the stick shaker going? I would suggest that some pilots don't even touch the thrust levers after takeoff normally, let alone have it in their muscle memory to do it when all hell has just broken loose.

I suspect watching the crew deal with the multitude of alarms and contradictory instruments readings, plus proximity to the ground, on the two Lionair and one ET flight would focus professionals minds a lot more on the human factors issues of these emergencies...

Any professional pilot would already have a very good visual concept of what happened here. Absolutely no need to watch someone dying, thanks.

[meleagertoo]

When all's said and done there was one thing and one thing only missing that on it's own would have totally prevented all this, from LionAir onwards.

I hope I'm not banned for using foul and profane language but the followiing contains adult concepts that some may find deeply offensive and disturbing.

Spoiler

 

 

AIRMANSHIP

[Albino]

Originally Posted by Capn Bloggs

So what are we doing here, flying by memo? Boeing should have revised the Stabiliser Runway memory procedure

It's in my memory items, right under disconnect autothrottle. Do you fly the 737? Next time you're in the sim put the stab forward to 2 units and leave it there. See how you get on, I doubt you'll consider it to be controlling the aircraft.

Originally Posted by MemberBerry

He does try to use the thumb switches to control the trim, but then so did the Ethiopian crew.

From the trace they blipped it a couple of times with one slightly larger correction before the cutouts were used. If i've got this wrong then I apologise but as I see it they retained full control of the stabilizer with electric trim at all times (apart from when they cut it out) and it would have overridden any MCAS AND command. After the initial AND command the aircraft is never put back into trim.

[meleagertoo]

Originally Posted by Capn Bloggs

So we're test pilots too now? No mention anywhere by Boeing of keeping the speed down, these guys were probably totally focussed on the MCAS issue, not your bog-standard UAS/80%. Ask yourself: when did you last pull off a handful of thrust at 1000ft above ground, straight after takeoff with the stick shaker going?

The last time I pulled a handful of power 1000ft-ish from the ground with stickshaker going was probably the last time I got one in the sim - as I accelerated towards flap limiting speed having already determined on which side the error lay, with correct pilot flying and confidence in my airspeed. How 'bout you?

You must know by now that MCAS didn't (couldn't) start it's tricks until flap was retracted...er...don't you?
Question is, would Test Pilot Bloggs actually retract flap in that situation? Even if you were climbing to 4-5000agl to complete the troubleshooting what would you be doing with the power at that point? Leaving it at climb thrust like these guys? Climbing at 200Kts, Flap 5 at 6000fpm and a pitch angle of 15degrees, with a stickshalker going???? But they were flying level-ish, not climbing!
Does Boeing actually need to tell you that?

See my post #3515 above...

[MemberBerry]

Originally Posted by Albino

From the trace they blipped it a couple of times with one slightly larger correction before the cutouts were used. If i've got this wrong then I apologise but as I see it they retained full control of the stabilizer with electric trim at all times (apart from when they cut it out) and it would have overridden any MCAS AND command. After the initial AND command the aircraft is never put back into trim.

No, I agree, they didn't fully reverse the MCAS AND trim before using the cutout switches. I was just saying that they did use the switches in the ANU direction, to reverse part of the trim applied by MCAS, not that they fully reversed it.

I can only guess why they didn't fully reverse the AND trim applied by MCAS. Maybe it was the fear of MCAS doing it again, so they hurried to use the cutout switches. Maybe it was the fear of being close to a stall, and didn't want to make any large adjustments in the ANU direction.

Also, the trim adjustment speed available to MCAS is higher than the speed available to the pilots, 0.27 units/s for MCAS vs 0.18 units/s for the thumb switches. So, if MCAS trims for 9 seconds AND, you would then need to trim with the thumb switches for 14 seconds in the ANU direction to fully reverse the 2.5 units of AND trim introduced by MCAS. Anything less than that, and you will lose the fight at some point.

[Derfred]

Best as I can piece together from the report, the successful 11-second (approx) electric ANU trim from 0.4 to 2.3 units took place at 325-330 kts (05:40:28 to 39). Capt control column aft displacement during this time varied from 12 to 5 degrees.

The two “blips” of electric trim (suggested by some as failure of electric trim) from 2.1 to 2.3 units took place at 360-365 kts (05:43:11). Capt control column aft displacement during this time varied from 9 to 6 degrees.

Airspeeds deduced from the narrative based on the F/O’s airspeed (the presumed correct one). VMO for is aircraft is 340 kts. Control column inputs based on the FDR chart.

A question then becomes, is the difference between 325-330 kts and 360-365 kts enough to overwhelm the electric stab trim with these kind of control column displacements?

Obviously the second case is outside of design parameters, and therefore may not even be able to be tested in a simulator (unless the simulator is certified to 365 kts).

On another note: some have suggested that the “blips” would be due to severe G-forces resulting from each blip. I don’t buy it. G-forces are mapped in the FDR plot (vertical acceleration), and they are insignificant - certainly much less than the pilot-induced G-forces of the previous couple of minutes. And let’s get serious for a moment - is a bit of trim-G going to bother you if you’ve had two pilots pulling on the column for dear life for the past few minutes?

I’ve never hand-flown a B737 at 365 kts before, but I have hand-flown one at 320 kts plenty of times - and trimming is not an G-force inducing event at that speed.

[Derfred]

Anything less than that, and you will lose the fight at some point.

That’s not entirely true, as soon as MCAS starts trimming you can stop it with a reverse blip. You don’t need to wait for it to punish you for 9 seconds.

[MemberBerry]

To stop it yes. But your "blip" still needs to be 50% longer than the "blip" from MCAS, to fully reverse it.

For example if you let MCAS run for 2 seconds, your "blip" needs to be 3 seconds long to not just stop MCAS, but fully reverse what it did.

[PEI_3721]

yo gums, F16 Guy,
Slightly off topic, but an an analogy of the 737 situation.
The only F16 experience I have, apart from sitting in the prototype at Edwards, was to fly the T33 simulation with side stick, and fixed centre stick (equally effective).
In that time frame the problem of restricting flight envelopes was being evaluated (F16?); thus with increasing AoA, at some point the nose up control was restricted and commanded nose-down preventing limit excursion. However, a high g turning departure / flick with associated roll-coupling put the aircraft above the AoA limit - now what.
The aerodynamics place the aircraft beyond a limit, the software prevents recover (situation not known).

The converse, analogy with the 737 Max is that the software puts the aircraft beyond the limit, and the aerodynamics / mechanics prevent recovery (situation not known).

[Derfred]

Originally Posted by MemberBerry

To stop it yes. But your "blip" still needs to be 50% longer than the "blip" from MCAS, to fully reverse it.

For example if you let MCAS run for 2 seconds, your "blip" needs to be 3 seconds long to not just stop MCAS, but fully reverse what it did.

Correct, or you could do 3x 1-second "blips", each 4 seconds apart. Or 6x half-second "blips". Or instead of counting, just trim repeatedly and/or continually until the elevator force is neutral, like a pilot. Which he never did.

He did say one strange thing though: if it is correct in translation, he asked the F/O "to trim up with him". This was after the second MCAS. He had only responded to the first MCAS with a relatively short "blip" (2.1 to 2.4 units). The response to the second MCAS (presumably with the F/O "helping") was around 11 seconds and took it from 0.4 to 2.3 units (if only they'd kept going another 11 seconds).

So why did he need the F/O's help to activate main electric trim? It's a thumb switch. Was the stick-shaker that annoying he couldn't activate the thumb switch? Could this mystery have anything to do with the two fatally short "blips" near the end of the flight?

[Chronus]

Here is an interesting piece on the STM :


"• Boeing 737NG • Improve field reliability of the Horizontal Stabilizer Trim Motor (STM) • Solder joint fatigue of the memory chip • 6355C0001-01 to 6355C0001-02
""Joint investigation into the STM’s field reliability with Boeing, along with lessons learned from 737MAX qualification, have yielded design improvements to the Control PWA.".

The link is at:

file:///C:/Users/Tony/Downloads/Jay%...A302_FINAL.pdf

[hans brinker]

Originally Posted by Majorbyte

This constant assumption by aircraft manufacturers that the pilots don't know f*** all about flying and insist on installing complicated systems to act as a safety pilot is crazy, how often do pilots stall airliners?? very very rarely is the answer, and if they were stupid enough to stall the aircraft, recovery is relatively simple, assuming some altitude is available. A message to Boeing and Airbus, please let the pilots fly the aicraft you build and stop confusing them with unnecessary and untimately dangerous technology.

Not that simple. As stated already hundreds of times, MCAS is required because of aerodynamic differences with the older model (bigger nacelle, further forward), making the required pull to increase AOA at high AOA too light (best way I can phrase it). You could blame Boeing for not designing a new aircraft, but based on all their other new designs i am sure it would have more, not less envelope protection.

[hans brinker]

Originally Posted by astonmartin

But they stayed in ‘level change’ mode all the time. That means that the autothrottle kept the engines at climb thrust all the time.

Both altitude and speed were uncontrolled.

Agree, I meant they had the intent to level of and reduce speed. Because the never got to ALT hold and Speed mode (or whatever Boeing calls it) they remained at full thrust.