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

[Alistair_Heaton]

This thread is a continuation of:

thread815-445840


****************************
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.

 

[waross]

Excerpts from[/b] Pete K's second link]Last week, Boeing outlined a software fix that addresses all the details criticized in the story. But the company denied that the original design was inadequate and characterized the changes as merely making the MCAS “more robust.”
.....
The pilots of the Ethiopian Airlines 737 MAX that crashed last month appear to have followed the emergency procedure laid out by both Boeing and the Federal Aviation Authority — cutting off the suspect flight-control system — but could not regain control and avert the plunge that killed all 157 on board.
.....
But Lemme said the Ethiopian pilots most likely were unable to carry out that last instruction in the Boeing emergency procedure — because they simply couldn’t physically move that wheel against the heavy forces acting on the tail.
“The forces on the tail could have been too great,” Lemme said. “They couldn’t turn the manual trim wheel.”
.....
More detailed instructions that conceivably could have saved the plane in this situation are provided in the 1982 pilot manual for the old 737. As described in the extract posted by the Australian pilot, they require the pilot to do something counterintuitive: to let go of the control column for a brief moment.

The entire article in the link is well worth reading.


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

 

[WKTaylor]

pre-engineer... hmmm...


YOU need to explain each element... (a), (b), (c) and (d)... of this 'simplified' equation in relation to what is KNOWN and VERIFIABLE, relative to the certification, manufacture and operation of a new very complex aircraft system.

Over the years I've heard theme/variation of these 'remarks' for various aircraft types/models... IE: just to name a few...

Lockheed Electra
CH-47
CH-53E
C-17
F-22
F-35
V-22
787
A400M
Cessna Conquest
Piper Tomahawk
Piper Aerostar
Beechcraft Bonanza [V-tail]
Etc...



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

Does speed trim include MCAS?

Yes, that's my understanding at this time. To a pilot, "Speed trim" is a good meaningful name for the system in this context. Pilots all have to master continuously managing the aircraft speed, and at times re-trimming this speed, no matter what aircraft they fly. It is one of the most universal aviation skills. A name such as "MCAS" would not be easily recognizable. Remember that Boeing promised minimum re-training of pilots, so they'd cut down on the alphabet soup, especially on stuff like this.

Another thing, as I'm seeing it now, is that I think the pilots and the MCAS system are making adjustments to the exact same trim mechanism, even though they us different controls to do it. The pilot has a big wheel on the center console, while the MCAS has an electric actuator. Adjusting the trim of the elevator means that pilot has to exert more or less force on the control column to maintain the attitude of the aircraft. Usually the pilot adjusts trim until the control column force needed to maintain a steady airspeed is zero or very light.

A lot of folks who aren't pilots know that the pitch of the nose should be "level" when flying normally, but there's more to that. The pitch you set for the nose actually determines the speed you fly. The engines play a role, too, of course, but every time you change the power and re-set the pitch of the nose, you settle out at a new airspeed, and you also have to reset the trim. Again back to this elementary piloting duty to keep the aircraft set properly.

Here we have the 737 Max8 trying to do some of that on behalf of the pilot. It's not supposed to be active during cruise, but during climb the angle of attack is high and the MCAS kicks on at times to compensate for oddball pitching moments which, we are learning, do counter-intuitive things on this aircraft. But the pilots don't know much about the automatic MCAS system, and they're trying to do this job the way they've been drilled to do it for their entire careers. Something the MCAS does is contrary to their expectation, or it wages war with the crew under some conditions.

This is becoming more clear, as to how the trim mechanism can find itself run down to the end of its travel, leaving the pilots with control column forces they cannot overpower.

Releasing the stick for a second

Yeah, not just a second. More like minutes. If the MCAS has the trim bottomed out, then it will take a long time for it to slowly drive the lead-screw back up to the neutral. Oh, and the MCAS needs time to recognize that the airspeed is stable and decide that it CAN begin to reduce the trim adjustment. With all the talk about how the MCAS activates the trim to compensate for high angle of attack, there's been less discussion given to when and how it relinquishes its control, after the aircraft is leveled off.

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

 

[VEBill]

BBC: Ethiopian Airlines Boeing 737 pilots 'could not stop nosedive'

... Pilots "repeatedly" followed procedures recommended by Boeing before the crash, according to the first official report into the disaster. ... "The crew performed all the procedures repeatedly [that were] provided by the manufacturer but were not able to control the aircraft," Ms Dagmawit said in a news conference in Addis Ababa. ... [Boeing] has said the system can be disabled - allowing pilots to regain control if there appears to be a problem. But the latest comments from Ethiopian officials suggest that pilots could not regain control, despite following procedures recommended by Boeing. ...

 

[LionelHutz]

SparWeb - From what I have read, MCAS will never return the trim to the original position. It also doesn't take trim control away the control from the pilot. It's operation can be stopped at any time if the pilot activates their yoke mounted trim switch which puts the pilot in control of the trim.

The first problem is that the misbehaving MCAS will re-activate something like 10 seconds after the pilot has released their trim switch and then it begin to move towards nose down trim again. The second problem is that the MCAS can move the trim at least as fast as the pilot can (and possibly slightly faster) which means that if MCAS does operate for 10 seconds and 2.5 degrees of adjustment before the pilot catches it, the pilot then has to spend at least 10 seconds holding the switch to move it back. With the system being "hidden" the pilot might not fully understand why the trim moved and why they have to spend so long moving it back.

What the above means is that the pilot could hold their trim switch until the trim was correct or at least close to correct and then thrown the cut-out switches to stop any further electric trim movement. The true issue and how to resolve it eluded the pilot and co-pilot of both planes as they tried to understand what the plane was doing and sadly the Ethiopian pilot threw the switches with high nose down trim and then couldn't recover the plane from that point.

The operating procedure change as per this FAA document on page 7 supports that they should have tried to moved the trim back to being correct before throwing the cutout switches.

FAA Emergency AD 2018-23-51

So, sadly, it's clear the failure issue was well understood shortly after the first crash and the way to save the airplane was published well before the second crash. I hope the investigations also look into how much effort is put into ensuring pilots quickly learn about changes to operating procedures such as this.

 

[IRstuff]

The article cited above implies that MCAS continued to re-activate itself, even though the pilots turned it off, ostensibly because MCAS continued to get erroneous readings from the AoA sensor.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[LionelHutz]

It barely implies that, if at all. It certainly doesn't say the MCAS kept functioning. Once the switches are thrown, the stabilizer won't be moved by the electrical system.

From what the article says, it can easily be construed that they threw the cutout switches with the stabilizer nose down and couldn't recover from that point.

 

[BridgeSmith]

Sounds like an automatic steering system for a car that the only way to override it is to shut down the entire power steering system, but if you do it with the wheels already turned, you probably won't be able straighten it out. That seems pretty dang stupid. They should get somebody from Tesla to reprogram it for them - at least if the car tries to drive you into a barrier, if you're paying attention, you can override the automatic steering just by turning the wheel, without losing the power steering.

 

[saikee119]

The manufacturer spokesman, who said the fix is to make "a safe plane" even safer, can always take a short flight in a 737 Max with the original MCAS and a faulty AoA sensor to demonstrate how safe is the plane.

 

[LittleInch]

Can anyone describe the relative speeds of the two trim systems?

We appear to know that the MCAS drives the trim at about 0.3 degrees per second for about 9 seconds to get its max 2.5 degree angle.

How fast can the manual trim toggle switch drive the elevator back up?

Does the MCAS cut in even if the manual trim up is being pressed?

It seems from the Lion air data that this results in fairly violent pitching and climbing of the aircraft with no doubt the pilots heaving back on the stick at the same time. It does appear that at max nose down of the trim the control elevators can't provide enough variance to maintain a level flight at 5 degrees pitch down of the main elevator. That doesn't sound great to me, but maybe that's normal?

Would you ever see 5 degrees up or down on a trim? Is the real issue here that max trim is excessive?

So how easy it actually is for the pilots to hit the cut out switch at the right point seems to me to be very difficult.

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

 

[SwinnyGG]

which means that if MCAS does operate for 10 seconds and 2.5 degrees of adjustment before the pilot catches it, the pilot then has to spend at least 10 seconds holding the switch to move it back.

Is it clear that this is how the system functions?

My understanding (which someone will hopefully correct if I'm wrong here) was that when the switch on the column which disables MCAS is depressed, it only stops MCAS from making further trim adjustments, but it DOES NOT return to whatever the previous trim setting may have been.

I.e. if MCAS decides it needs to add 2 degrees of nose-down rear stab trim, and halfway through that process the pilot depresses the column switch (not the panel-mounted cutoff switch) then the rear stab is held at 1 degree, and does not return back to zero.

This makes the situation even worse. If MCAS returned to zero every time the momentary switch was depressed, all the pilot(s) would have to do is hold that switch down, wait for trim to stabilize at the original value while maintaining safe pitch attitude (or as close as possible), then flip the cutoff switches on the panel. This would be much easier than the alternative, which is what I thought was actually required- flip the cutoff switches to disable automatic trim, and then run the rear stab back to the required trim setting manually using the trim wheel.

 

[IRstuff]

The preliminary report is here:

http://www.ecaa.gov.et/documents/20...AVJ).pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[LionelHutz]

LittleInch - I read 0.2* per second with flaps up for pilot initiated movement. The MCAS was 0.27* per second I believe. Probably anything within 0.5* would be OK maybe even more, so hitting the switches within a couple of seconds of reaching the correct trim would work.

jgKRI - Information available says the pilot still have over-riding control via the yoke switch. As I described and the AD I linked directs, it should be possible to run the stabilizer back, by holding the switch on the yoke control, to the correct trim before turning off the switches. Nothing I've read so far says the MCAS will ever reverse any nose down trim adjustments it has made, but that really is irrelevant when talking about the pilots ability to change the trim via their control switch.

 

[saikee119]

IRstuff,

Thanks for the Preliminary report link.

So in the report the pilots at 05:40:35, presumably from the voice recorder, did apply the Stab Trim Cut Out switches as advised by Boeing via an Emergency Airworthiness Directive issued by FAA only after the Indonesian 737 Max Crash.

Thus the first 737 Max crashed in the Java sea because the Indonesian pilots were kept in the dark about MCAS.

The second 737 Max Crashed because the Boeing/FAA Emergency Airworthiness Directive didn't work for the Ethiopian pilots.

Is this the reason Boeing went back to do more work on the MCAS countermeasures which claimed to be ready last week and were previewed at Renton recently?

Is this another example of the accountants/lawyers taking over the engineering?

 

[Pete_K]

To the point about it being difficult to manually trim the plane (with the wheel) if the stabilizer is at full nose down, the excerpt from the flight manual has a statement about reducing control column force by running the electric trim (with the yoke switches) to a neutral condition before hitting the trim cutout switches.

It seems to me that unless the recovery procedure is well trained to the point of being rote, the pace of the events and the "fog of war" during this problem make it difficult to ascertain what is going on and correctly identify the proper way to fix it.

 

[SwinnyGG]

but that really is irrelevant when talking about the pilots ability to change the trim via their control switch.

So the column switches actually provide trim adjustment?

Maybe that's the point of confusion for me- what I know about this system comes pretty much from reading this thread, so this is all new to me. I was thinking (apparently incorrectly) that the column switch only disabled automatic trim, but was not actually a control switch (i.e. the pilots can adjust trim electronically).

I was under the impression that pilots adjusted trim using the wheel only.

 

[saikee119]

There are still some mysteries in the ET302 FDR from the report

(1) A huge 75 degree disagreement between two AoA sensors (In Indonesian plane it was 20 degree). What could cause such huge disagreement and sustained 60 dgree to the end and then with a sudden swing, from +60 to -60 degree?, just before the plane hit the ground ? In both crashes it was the Left AoA faulty but in the Indonesian case the fault stated immediately.
(2) The MCAS kicked in initially with 3 small Automatic Trim Down Commands changing the Pitch Trim from 6 to 5 degree over 12 seconds. It then followed by two 3 and 2 sec aggressive Automatic Trim Down Commands to reduce the pitch Trim from 5 to 0 units. The pilots immediately counteracted with Manual Trim up to pull the Pitch Trim back to about 2 units. At the same time both pilots were heard to apply and confirm the Stab Trim Cut Out switches. Thus the aggressive third long 4-sec trigger by MCAS at 05:40:45 did nothing to the Pitch Trim. However the last Automatic Trim Down Command, at about 05:43:20, pushed the pitch Trim down beyond the point of recovery. Was this a design flaw in the system or the pilots had reversed the Stab Trim Cut Out? If it was an action by the pilots the voice recorder would have a record similar to the one occurred previously and stated in the report "At 05:40:35, the First-Officer called out“ stab trim cut-out” two times. Captain agreed and First-Officer confirmed stab trim cut-out."

 

[saikee119]

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.

 

[LionelHutz]

jgKRI - yes, the yoke switch allows the pilots to electrically change the trim.

saikee119 - 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.

Why the stabilizer started moving electrically again might be one of the toughest questions to answer.

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.

 

[saikee119]

One speculation is the Ethiopian pilots had applied the Cutout switch to the MCAS but might have switched it back again. It is this point I like to see the views from the members here.

The observations are:

(a) The report states “At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First-Officer confirmed stab trim cut-out.” Since both the CVR and DFDR have been recovered and available to other agencies so the information is no longer private. If a voice recording can be heard for applying the Cutout switches once one would expect the same should be available from the pilots for reversing the Cutout switches.

(b) It is somewhat unimaginable that the pilots, knowing well by heart that MCAS can cause the plane to node dive, having killed off MCAS with the Cutout switches to fly the plane if not steady but slowly upward and gaining altitude progressively over a period for nearly 3 minutes would suddenly had a dead wish to activate the MCAS again themselves without recording their action orally.

A possible explanation behind the pilot might have inadvertently switched on the MCAS can possibly explained by Peter Lemme description of the differences of the Stab Trim Cutout switches between a normal 737 and the Max model.

In a normal 737 that most pilots fly one of the Stab Trim Cutout switch is to disable the autopilot and the other is to disable the electrical power to the Electric Trim. Just flip the autopilot Cutout a pilot can disable the MCAS (if fitted) and still be able to use the electric trim to control the plane a lot easier than just use the manual trim.

In the 737 Max the two switches are marked a PRI and B/U. Peter Lemme was asked about what these refer to and he guessed Primary and Back up but was unsure what their functions are.

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. (FAA Emergency Airworthiness Directive mandates MCAS must be disabled by applying both Stab Trim Cutout switches)

It is evident from the black box data after the pilots applied both Cutout switches the plane was steady up to 05:43:10 without any electrically-assist manual trim while the aft columns were worked on continuously. If the pilots needed a relief to their workload and mistakenly thought to reverse the Cutout switch normally for restoring electrical power in a non-Max 737 then we could see some electrically-assist manual trim events in the black boc data. This is indeed the case. The report at 05:43:20 shows two such electrically-assist manual trims did occur, as pointed out by LionelHutz.

About 5 seconds after the last electrically-assist manual trim the restoration of electrical power also allowed MCAS to activate itself from the faulty AoA left sensor. It is that last 5 to 6 second MCAS activation that pushed the nose down beyond the point of recovery and destroyed the plane.

Like everyone I am just speculating but the black box data does support this theory.