Boeing 737 Max8 Aircraft Crashes and Investigations [Part 2] 44

[Alistair_Heaton]

This thread is a continuation of:

thread815-445840


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Another 737 max has crashed during departure in Ethiopia.

To note the data in the picture is intally ground 0 then when airborne is GPS altitude above MSL. The airport is extremely high.

The debris is extremely compact and the fuel burned, they reckon it was 400knts plus when it hit the ground.

Here is the radar24 data pulled from there local site.

It's already being discussed if was another AoA issue with the MCAS system for stall protection.

I will let you make your own conclusions.

 

[Lou Scannon]

Boeing now changes the approach and take responsibility.

That is a good approach. If one admits one's fault one is likely to go out all one's way to fix it. No one doubts Boeing's ability to fix it. The financial burden isn't one that can break Boeing. An early real fix will enable Boeing rebuild its reputation, return 737 Max back to service, win back customers and emerge as a leader in aviation again.

That goes a long way to repair my respect for Boeing's integrity.
I fear that having admitted responsibility, they have exposed themselves to debilitating lawsuits. Not that it wasn't the right thing to do.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[LionelHutz]

It seems odd they would disable both and then turn one back on since flipping both points to them understanding that both had to be off.

Is that earlier switch combination from a NG or an even earlier version of the 737?

 

[Spartan5]

Regarding the root cause of the major AOA discrepancy, I found this curious:

At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1° then increased to 35.7° while value of right AOA indicated 14.94°. Then after, the left AOA value reached 74.5° in ¾ seconds while the right AOA reached a maximum value of 15.3°. At this time, the left stick shaker activated and remained active until near the end of the recording. Also, the airspeed, altitude and flight director pitch bar values from the left side noted deviating from the corresponding right side values. The left side values were lower than the right side values until near the end of the recording.

At 05:38:43 and about 50 ft radio altitude, the flight director roll mode changed to LNAV.

At 05:38:46 and about 200 ft radio altitude, the Master Caution parameter changed state. The First Officer called out Master Caution Anti-Ice on CVR. Four seconds later, the recorded Left AOA Heat parameter changed state.

What is the "heat parameter" that changed state?

 

[VEBill]

They should get somebody from Tesla to reprogram it for them...

Perhaps not... You could make Tesla's Autopilot swerve into traffic with a few stickers on the road

 

[Sparweb]

When reading accident reports, I admit that I get very wrapped up with the sections that tell the sequence of events. I try to understand what it was like to be in the cockpit as the trouble unfolded. Concealed in the technical jargon, the culmination of events in the cockpit of this aircraft is dramatic.

Seconds after takeoff, the crew were immediately confronted with the Master Caution and stick shakers. Realizing they had an aircraft in trouble, they remained calm, stuck to procedures, and worked the problem. They did what the AFM told them to do, and came very close to bringing the problem under control. With both men pulling back on the yoke to keep the nose just barely level, they got it turned around back to AA airport, enough to start thinking about the descent to land. They even got their hands free enough to work the manual trim a little, but it must have been a battle to do it, when trying to pull back on the yoke at the same time. They were so close to bringing everyone home.

Then, for no apparent reason, this happened:

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.

Something commanded the electric trim to nose-down drastically, even while the electric trim switches were set to cut-out. With yoke forces too great for two adult men to overcome, the aircraft thundered into the earth seconds later.

I read it, but I still can't believe it.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[waross]

The root problem:
Am I completely misunderstanding this?
The regs require that the force on the control column be linear.
That is, as the pilot increases the angle of attack, the pressure on the control column must increase proportionally.
Problem; The placement of the oversized engines resulted in the pressure on the control column to no longer be linear as required by the regulations.
It may have been a lot simpler, cheaper and safer to just add a force to the control column so that the force felt by the pilot would continue to increase as the angle of attack increased.
This could be done electrically (a torque motor type arrangement) or pneumatically (an electrically controlled pressure regulator).
Either way, the system could be overcome by the pilot by either exerting more or less force on the control column depending on the failure mode.
The maximum force added would be the maximum force needed to maintain the "feel" of the controls when the aircraft was flying in the problem angle of attack attitude.
Turn it off, the added force goes away.
It turns itself on, the same force is again applied, but no windup until disaster.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MDEAus]

I'm noting a few things here, my main concern is that this isn't entirely a repeat of the Lion air accident. I have a few questions before I get into any speculation. at 05:35:45 we see that the aircraft leaves the ground based on altitude readings (pressure and radar), yet approximately the same time the pitch is 18 degrees nose up. Is 18 degrees considered high for a 737 max8, could this have caused a tail scrape? At 05:40:27 we see the TRIM AND is almost immediately countered with TRIM ANU (Control column inputs less then during final moments), This trimmed deeper than the final TRIM AND event which was unrecoverable, immediately after the final TRIM AND event there's a drop in control column position (significant?) followed by large input which fails to arrest the nose down pitch of the aircraft. Almost immediately after the final TRIM AND event there is a sudden change in the left AOA value (05:43:26) at the same time the pilots appear to be attempting to roll the aircraft from 30 degrees one direction to 18 degrees the opposite direction at time off impact.

the left AOA Trace also looks out of place, after the jump to 75 degrees it maintains a near flat trace while the right AOA trace has noise which seems fairly consistent with other noise profiles or fluctuations with the aircraft (Lion air was out by +20 degrees in regards to the right AOA, this one "jammed" at 75 degrees). would that seem to indicate a bird strike? however that doesn't explain the change in the LEFT AOA value at 05:43:26.

 

[itsmoked]

Really wild AoA numbers and "flat lines with little noise" points at an electrical issue rather than a mechanical one. Perhaps bad connectors?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Something commanded the electric trim to nose-down drastically, even while the electric trim switches were set to cut-out. With yoke forces too great for two adult men to overcome, the aircraft thundered into the earth seconds later.

Instead of sounding like a broken record I'll just post what I previously did again....

"You missed that the pilots did 2 short trim up commands which did move the stabilizer starting at 5:43:11 - about 2.5 minutes after the stab switch call-out in the CVR and the MCAS trim down command not moving the stabilizer. Both the pilot trim switch and MCAS became function again about 2.5 minutes after the stabilizer switches were cutout. This tends to disproves any conspiracy that the MCAS was left functional even in stab cutout."

The above pilot ANU commands occured 5 seconds before the last MCAS AND command.

4 automatic AND commands occurred which were presumably MCAS. The 2nd one drove the stabilizer to 0.4 units and the last/4th one drove it to 1 unit. So, I will also add this again - "Why the pilots could recover from the second automatic stabilizer movement to 0.4 units and not the 4th and last automatic stabilizer movement to 1 unit will be another tough question to answer."

 

[SwinnyGG]

If the Ethiopian pilots, familiar with the non-Max version of 737, had flipped just one of the Cutout switchs thinking it was Main Electric to restore the electric trim then that would have let the MCAS slip through the net.

The two switches are in series- there's a B/U switch so that if the PRI switch fails NC/has a short or other closed contact failure, elec trim can still be disabled.

Unless Switch A has a closed contact failure, flipping switch B only does not re-enable electric trim or vise versa.

A closed contact failure on one of these switches is certainly possible, but highly unlikely. It's a very rare fault.

It may have been a lot simpler, cheaper and safer to just add a force to the control column so that the force felt by the pilot would continue to increase as the angle of attack increased.

This would still require AoA sensors as part of the feedback loop which would be subject to the same system architecture which failed in this case. Also, I'm suspicious that the control gradient requirement is really there so that submitted designs have a self-righting moment AWAY from stalled pitch attitudes. Augmenting control column forces alone doesn't produce that behavior. Aerodynamic changes (which MCAS provides if operating correctly) do provide self-righting pitch moment. I don't know that for sure, it's a suspicion of mine though.

 

[LittleInch]

There's so much data here that looks confusing and difficult to call from afar.

Some points I have include:

1) When the electric trim was active, i.e. before they apparently threw the cutout switch why did the trim only increase from the low point of 0.4 to 2.3 instead of the initial 4.6? Why did they take the finger off the trim up switch?
2) With the trim seemingly stable at 2.3 for a long period, the aircraft was climbing slowly but control position and apparent difficulty in controlling the aircraft would appear to have nothing to do with movement of the trim position - what is going on here?
3) There is a continuous and in the end quite significant roll to the right - what was going on here?
4) The small stab trim inputs just before the final dive and again seemingly after flicking the electric power back on don't seem to have materially affected the trim position so why bother??
5) Why connect the system again?
6) The faults mentioned in Dec 2018 were not exactly fixed as no faults could be found. This doesn't sound like a good fix and if there were "several" (see section 1.6.4) write ups surely this should have resulted in some replacement of components in that event even if no fault can be found there is clearly something wrong.
7) The AOA sensor reacts very strangely right at the end so was not totally broken, but clearly in error.
8) As Lionel says above, they managed to recover from the first nose down and had more height this time, so what happened t 0543 20 ( the aircraft was in a 30 degree roll at that point mind)

I hope someone can figure this out as it doesn't look solely like the mcas issue here, thought that is clearly the primary cause of all the trouble along with the faulty AOA reading.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LionelHutz]

LittleInch - i commented on the possibly being another issue before and I would tend to agree with you now. I'm currently not convinced that changing MCAS is the full solution. As to your point 2, I'm sure it's a fight to hold the stick back when the plane is in a nose down trim configuration.

I had it somewhat wrong in my previous post. There were 7 automatic ANU commands, 4 of which were a significant length. The first 3 were short and attributed to the autopilot. The 5th one (2nd significant one) forced the stabilizer to the furthest down position yet the plane remained controlled. The last one didn't go quite as far but the plane wasn't controlled.

 

[waross]

What amount of down trim would be required to counteract the worst condition of extra lift from the engine housings?
One would think that that would be the reasonable limit of MCAS override.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sparweb]

Bill, it seems capable of exceeding that amount of trim - greatly.
It's possible that when nearly at stall AOA, the MCAS does need that much authority to counteract the lift from the engines.
When it's active at any other time, however, it applies a control force that two crew members cannot overcome.


No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[waross]

It's possible that when nearly at stall AOA, the MCAS does need that much authority to counteract the lift from the engines.

If you are correct, and it takes that much trim to counteract the engine lift, the corollary may be that if MCAS fails to engage when it is needed, it may require more control force than two crew members can apply to avoid a stall.
Suppose that a new system is deployed that will detect AoA sensor failures and automatically disconnect the MCAS. The next crash may be as a result of a high angle of attack leading to a stall and both crew members not being able to apply enough force to bring the nose down without MCAS.
Another thought; with the trim full down, will the elevators be able to counteract the trim with any amount of control column force?

Maybe it's time to set aside the MCAS and look at the engine placement.
I sincerely hope that the FAA takes a hard look at whether this configuration should be accepted under the old type approval.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[saikee119]

It appears on record Boeing did not want the pilots to know MCAS so they don't have to be trained to deal with it. FAA was party to this arrangement.

It took a plane with 189 lives to show the world MCAS exists and a malfunction of it during take off could be fatal.

Boeing's solution was: treat it as a standard runaway stabilizer scenario, stop it by throwing two cutout switches and use the crank wheel to trim the plane manually. Boeing did indicated "higher control force may be needed to overcome any stabilizer nose down trim already applied". FAA dispatched this solution as an Emergency Airworthiness Directive.

It took another plane with 157 lives, having a malfunction of MCAS during a take off, to reveal that Boeing first solution may not work because by a loss of electrical power, consequential to throwing the two Cutout switches, the pilots were unable to cope with the manual trim using the slow mechanical crank wheels in an emergency situation to overcome quickly enough the heightened aerodynamic resistance in the nose dive.

That looks something fundamentally flaw in the plane design. I can't see how revising software can get Boeing out of this hole. Seem to me at least the wiring has to changed to give the pilots back the necessary electrical power to trim plane out of the nose dive situation.

It would be interesting to see what Boeing has to offer in the next round.

waross's corollary suggests even the MCAS is fixed this time we could still in future see 737 Max dropping off from the Sky (when MCAS has to be disabled on account of a faulty AoA vane and the plane has an excessive nose up that can only be cured quickly by MCAS).

 

[LionelHutz]

It took another plane with 157 lives, having a malfunction of MCAS during a take off, to reveal that Boeing first solution may not work because by a loss of electrical power, consequential to throwing the two Cutout switches, the pilots were unable to cope with the manual trim using the slow mechanical crank wheels in an emergency situation to overcome quickly enough the heightened aerodynamic resistance in the nose dive.

The preliminary report doesn't support this claim, at all.

#1. There is no evidence they attempted to operate the manual wheel after throwing the cutout switches.
#2. The cutout switches appear to have been on right before the plane crashed.

The next crash may be as a result of a high angle of attack leading to a stall and both crew members not being able to apply enough force to bring the nose down without MCAS.

There is no evidence I've seen that this is true. Please share it if you have it, otherwise it serves no purpose to speculate about it.

Another thought; with the trim full down, will the elevators be able to counteract the trim with any amount of control column force?

The initial accident report shows that the pilots held the plane fairly level with the stabilizer almost full down, at 0.4 units (degeees?) long enough to operate it back to 2.3 units. One could conclude from this that there is an amount of force that can counteract it.

 

[Alistair_Heaton]

"It may have been a lot simpler, cheaper and safer to just add a force to the control column so that the force felt by the pilot would continue to increase as the angle of attack increased."

There is already a thing called a feel unit on powered controls. Which increases the control forces with airspeed.

Now trimming. Basically you are zeroing the control forces at a particular airspeed. Think of it as a U and you trimming the control forces to be at the bottom of the U. If you speed up the elevator produces more down thrust due to L=1/2 x rho Vsqt x coefficient of lift. So to get it back to producing the same amount of down force you have to change the coefficient of lift by changing the elevator position. We call this trimming. Trimming is fundamental in flying in all three axis. When you get it right you can fly a 747 with your finger tips. Take you hands off the controls and the plane is in equilibrium and will continue straight and level. Get it wrong and any lapse in concentration and the aircraft will depart the required flight path. Its taught in the 3 lesson of pilot training after effects of controls. Pitch trimming is the basic one, trimming yaw is also important in commercial aircraft especially in an engine failure situation. All 4 controls effect each other so the order you trim is quiet important. Power, then yaw, then roll and lastly pitch. Once you have been flying multi engine for a bit you don't think about it you just do it like changing gear in a car. Jet aircraft there isn't much yaw change with power change unless you are flying on one engine.

 

[LittleInch]

Alistair, happy to see you back. What's your take on the initial report which has a few of us quoite confused as to what the pilot actions and thinking was.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[charliealphabravo]

One could conclude from this that there is an amount of force that can counteract it.

This conclusions premature in the face of the preliminary report. This thread, indeed this entire forum, is an exercise in forensic problem solving based on the available information, fully acknowledging that more information will become available and that more informed experts exist. Personally, I can only justify my participation in this forum if ALL my statements are taken as speculation and NONE of them are taken as public expressions of my professional opinion.

Peace.