Boeing 737 Max8 Aircraft Crashes and Investigations [Part 3] 36

[Sparweb]

This is the continuation from:

thread815-445840
thread815-450258

This topic is broken into multiple threads due to the long length to be scrolled, and many images to load, creating long load times for some users and devices. If you are NEW to this discussion, please read the above threads prior to posting, to avoid rehashing old discussions.


Some key references:

Ethiopian CAA preliminary report

Indonesian National Transportation Safety Committee preliminary report

The Boeing 737 Technical Site

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

 

[VEBill]

Satcom Guru has some info.

 

[VEBill]

It's worth noting that Flight Simulators are apparently used as evidence in inquiries, so one would hope that they are representative and accurate in the corners of the flight envelope where incidents occur. If not, hopefully the limitations are explicitly documented.

Perhaps we can agree that the simulator was not "flawed", but they certainly are making "corrections " (<- their own exact word choice).

I'm not sure how one could describe what sort of thing it is that is being 'corrected'...

(This is so far down into the semantic weeds that it's not very interesting from a technical point of view.)

 

[waross]

As for the simulator.
We have seen video's of older simulators set up to demonstrate the extreme forces caused by trim runaway.
We have seen a report from I think the New York Times that Boeing test pilots reported that their Max 8 simulator was developing forces far below what it should have produced.
Putting together the published reports it seems as if only the Max 8 simulator does not reasonable reproduce the forces produced by runaway trim.
I understand that Boeing has recognized this.
So why is it only the Max 8 simulator that has a problem?
Back to the question; What is the source of the problem. Did the simulator make a mistake or did Boeing supply the wrong data?
I don't expect the simulator to be perfect but I do expect it to be pretty good within the flight envelope.
I see the flight envelope as between stalling and crashing.
Once an aircraft is in a non recoverable situation, there is no need for the simulator to simulate. It's too late.
How can pilots be trained to recover from a trim runaway in a simulator that does not reasonably reproduce the extreme control forces?
Apparently they can be trained on a 737 NG simulator, just not on a 737 Max simulator.
It can be done, it has been done. Why wasn't it done this time.
This is not Boeing bashing, this is a serious question. The answer may uncover yet another problem that should be rectified or at least checked.

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

 

[Eufalconimorph]

About the checklist, I'm wondering why it isn't something like the following (change in italics):

Required by AD 2018-23-51
Runaway Stabilizer
Disengage autopilot and control airplane pitch attitude with control column and main electric trim as required.
Check for erroneous AOA input.
If erroneous AOA input is indicated, ensure that control column pitch forces are neutralized using main electric trim.
If relaxing the column causes the trim to move, set stabilizer trim switches to CUTOUT.
If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually.

Note: The 737-8/-9 uses a Flight Control Computer command of pitch trim to improve longitudinal handling characteristics. In the event of erroneous Angle of Attack (AOA) input, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds.
In the event an uncommanded nose down stabilizer trim is experienced on the 737-8/-9, in conjunction with one or more of the indications or effects listed below, do the existing AFM Runaway Stabilizer procedure above, ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.

An erroneous AOA input can cause some or all of the following indications and effects:
• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.
Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT.

The existing AD indicates to do one procedure (the existing AFM Runaway Stabilizer procedure) and only mentions the higher control forces and the need to mitigate them at the end (IE, not part of the procedure). Why wasn't the procedure section modified if the procedure could change? If the pilots are trained to follow checklists to resolve problems then the resolutions to known problems should be in the checklists!

 

[SwinnyGG]

How can pilots be trained to recover from a trim runaway in a simulator that does not reasonably reproduce the extreme control forces?

There's a very important distinction to be made here- trim runaway produces extremely high control forces when allowed to proceed uninhibited for a long time.

The trim system acting strangely does not in and of itself produce high forces- the high forces are the result of the aerodynamic forces generated by the control surfaces being moved into certain positions, and those movements take time. There are also other factors at play that have an effect on the control forces, such as the current pitch attitude and its rate of change, as well as vehicle speed etc.

The Ethiopia flight in particular is a case where the pilots experienced extremely high control forces, because they (for whatever reason) seem to have lost track of airspeed, AND failed to identify the runaway trim condition until it was well developed. If they identified the runaway trim condition earlier, managed airspeed earlier, or both, they very possibly would not have crashed.

Point is, they were operating in a part of the flight envelope where:

1) Had they correctly followed all their procedures, they would not be

2) Had they made other mistakes but had identified trim runaway more quickly they would not be

In either case, they were operating in an area of the flight envelope that those developing simulators and curriculum for said simulators could reasonably have not anticipated the need to simulate.

People developing simulators don't (can't!) assume that in every possible scenario, the pilot in control of the aircraft will disregard every safety or operating procedure in order to reach dangerous areas of the flight envelope and then use those resultant limits to define the envelope for the simulator to operate in.

Remember that a flight simulator is used to train commercial pilots. The goal of a simulator is to provide a way to train people to fly planes in a way that is more cost-effective than actually flying the real planes. Yes there are other uses, but ultimately a flight simulator is not used to exactly replicate every possible operating state of an aircraft- because it's not only not the primary goal of their development, it just isn't possible.

 

[VEBill]

And yet, the flight simulators in this case are being "corrected"; as per Boeing statement.

--

"...exactly replicate every possible operating state..."

Did anyone claim that except you? Usually one doesn't provide one's own strawman arguments. That's something others do, and then you call them out on it.

--

"...failed to identify the runaway trim condition until it was well developed. If they identified the runaway trim condition earlier..."

I understood that the Trim Wheels on each side of the center pedestal plainly rotate when the trim is in movement, from any cause. If there was anything wrong with the crew reaction, then it's probably not from missing the two spinning trim wheels.

So this theory would need further work.

 

[SwinnyGG]

Did anyone claim that except you? Usually one doesn't provide one's own strawman arguments. That's something others do, and then you call them out on it.

Well..

I see the flight envelope as between stalling and crashing.

In any event- the point is that it is not a surprise that there are areas of the flight envelope which the simulators are not designed to simulate, or are not capable of simulating at all.

That an area of the flight envelope which requires several successive violations of procedure as well as airframe performance limits to reach would be one of those not within the realm of normal simulation should not be a surprise to any of us.

I understood that the Trim Wheels on each side of the center pedestal plainly rotate when the trim is in movement, from any cause. If there was anything wrong with the crew reaction, then it's probably not from missing the two spinning trim wheels.

So this theory would need further work.

The trim wheels do turn when the automatic or electric trim adjustments are happening. They spin rather slowly but are marked with a white stripe to make the movement easier to see. Their location, however, isn't really in the direct line of sight all the time.

With a big 'ol adrenaline hit, confusion in the cockpit, stick shaker whacking away, cautions/alarms/vocalizations sounding, and the rapidly approaching ground quickly filling the view out of the windows, yeah I think it's feasible that movement of the trim wheels would be easy to miss.

 

[LionelHutz]

All this back and forth about the simulator is a rather moot point unless the pilots actually train on runaway stabilizer and using the manual wheel. In other words, training that includes the roller-coaster maneuvers to get the trim back to where it needs to be. From every source I have read, this training was dropped many years ago (by the 80's).

The timeline in the Ethiopian crash report says that the pilots couldn't move the manual trim wheel yet the electric trim motors moved the stabilizer just fine in the trim up direction right before MCAS moved it down again. So, that pretty much kills off any conjecturing about the trim motor not being able to put as much force into the jackscrew as the manual wheel.

The Ethiopian plane had the first MCAS misapplication at 5:40:00. The cutout switches were thrown right around 5:40:40. The overspeed indicator first started at 5:41:21. The stabilizer started working again right around 5:43:10. This adds up to about 40 seconds from first runaway until cutout thrown. Then, there was 40 seconds from cutout thrown until overspeed warning and 2.5 minutes from cutout thrown until they turned the cutout switches back on dooming everyone on board.

From the above, the following can be determined,

The pilots only had 40 seconds to react claim in the Times article seems like nothing more than sensational BS reporting.

The pilots did successfully identify the runaway trim rather quickly.

The pilots didn't successfully identify that they could and should have re-trimmed the plane before using the cutout switches, even though the electric trim switch was working just fine.

The pilots had about 2.5 minutes after throwing the cutout switches to try and figure out what the plane was really doing. 2.5 minutes to do the roller-coaster. 2.5 minutes to realize the plane was climbing and accelerating which meant the stall warnings were in error. 2.5 minutes to realize they needed to get the plane speed back under control so the control forces would be controllable.

All the above would indicate the pilots didn't do any difficult runaway trim training. If Bill's claims are correct and the NG simulator does properly simulate the forces, then NG simulator training for runaway trim would have transitioned just fine to the MAX. Remember, there was no simulator training for the MAX, it relied on the previous NG training.

 

[waross]

That's a great link, VE1BLL.
If you follow down to the section ELEVATOR FEEL CONTROL it gives some answers.
Not WAG speculation.
The elevator is no longer directly connected to the controls.

The elevator feel computer changes the control column forces as the airspeed changes and the horizontal stabilizer moves.

All the statements about a simulator not replicating the forces exactly also apply to the actual plane in flight.
The elevator forces on the control column are not the real feel. The elevator control column forces are generated with hydraulic pressure controlled by the elevator feel computer.
When the elevator feel computer in a simulator is fed the same airspeed information, the same elevator position information and the same stabilizer position information as the elevator feel computer in a plane in flight the control column forces should be exactly identical to the aircraft in flight under the same conditions.


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

 

[waross]

Lionel,in regards to your statement:

The timeline in the Ethiopian crash report says that the pilots couldn't move the manual trim wheel yet the electric trim motors moved the stabilizer just fine in the trim up direction right before MCAS moved it down again. So, that pretty much kills off any conjecturing about the trim motor not being able to put as much force into the jackscrew as the manual wheel.

I'm not arguing, rather asking for clarification.
Did the flight recorder actually record stabilizer movement or did it record a command for movement?
Thank you in advance.


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

 

[VEBill]

_{Edit to add: This is in response to jgKRI (Mechanical) 22 May 19 03:25. Since the thread got busy while composing reply.}

----

And yet, Boeing *is* correcting the simulators.

(Because they need to be.)

QED.

--

"...movement of the trim wheels would be easy to miss."

You might wish to review the Aircraft Accident Investigation Preliminary Report (PDF) No. AI-01/19, dated March 10, 2019.

The sequence of events on pages 10 to 11 clearly indicate your speculation is not what happened. The crew were working the trim issue from the outset.

--

On some of these points, we're now looping without making progress. So I may abandon further discussion on those points.

 

[SwinnyGG]

The elevator is no longer directly connected to the controls.

That's not correct. The feel computer is a filter for the forces transmitted from the hydraulic circuit that drives the elevators back to the control columns- but the control columns are still connected to the elevators directly with cables.

If they weren't, a failure of A and B hydraulics would mean complete loss of elevator control.

The crew were working the trim issue from the outset.

I've read the report... did you actually read to the end? One of the last actions the pilots took was to reset the trim cutout switches. After their last manual electric ANU trim input, the automatic system applied the final, fatal AND input over a period of, per the report, 5 seconds. This was discussed days/weeks/pages ago in this thread, and it's not conjecture. It's very clear.

They didn't recognize they were in a trim runaway situation, and either missed the manual trim wheels moving during the last automatic AND trim movement, or did not understand the implications thereof... Had they known what we know, and turned the system off using the cutout switches after their last manual electric trim command, it's possible this story ends with a not-so-fun emergency landing as opposed to hitting the ground at 40 degrees nose down and 450+ kts.

 

[VEBill]

The timeline in the Ethiopian crash report says that the pilots couldn't move the manual trim wheel yet the electric trim motors moved the stabilizer just fine in the trim up direction right before MCAS moved it down again. So, that pretty much kills off any conjecturing about the trim motor not being able to put as much force into the jackscrew as the manual wheel.

Agreed.

But then it leaves the other question. How do the pilots stop the powerful trim motor with their hand on the trim wheels?

Ref. As mentioned in procedures.

It has the appearance of nontransitive strength, like Rock > Scissors > Paper > Rock > etc.

 

[SwinnyGG]

How do the pilots stop the powerful trim motor with their hand on the trim wheels?

There's a gearbox which mixes inputs from the hand wheels and motor, and outputs to the jack screw. The torsion coupling inside decouples the drive motor from the jack screw if a certain torque input is applied by the manual trim wheel. It's like a mechanical relay.

 

[Alistair_Heaton]

by all accounts using your foot is quite successful for stopping the trim wheel on the 737. If you use your hand your going to loose bio mass and add blood into the equation.

When the trim wheel goes its very noisy and gives huge vibration right next to your knee. And if your knee is against it when it goes you will be limping for a few days afterwards.

 

[VEBill]

"...torsion coupling inside decouples the drive motor..."

Thank you for the explanation on that point.

 

[waross]

According to SatCom Guru:

The Boeing 737 uses Power Control Units (PCU), a hydraulic actuator, to drive the elevator. The indirect control introduces the need for an elevator feel computer, to artificially provide "representative aero loads" onto the pitch control yoke with a variable "neutral shift".

The elevator control cables move the ELEVATOR PCU INPUT POGO.
The PCU input POGO moves a control valve on the PCU. The only feel on the cables is the small constant pressure needed to operate the valve.
Please read the links before commenting on the information from them.

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

 

[VEBill]

_{edit to add: this is in reference to Bill's discussion of the Elevator.}

But the Trim operates on the Horizontal Stabilizer via the Jack Screw (in the middle of the diagram posted above).

The Elevator is kinda/sort-of separate (emphasis on kinda/sort-of).

Am I confused?

 

[Alistair_Heaton]

No the elevator is only the rear portion of the tail plane some 25% of the area.

The trim effects the whole angle of attack of the tail plane.

There is also a balance tab on one side.

It's easier to see if you look at a picture of it.

 

[SwinnyGG]

The PCU input POGO moves a control valve on the PCU. The only feel on the cables is the small constant pressure needed to operate the valve.

The elevator PCU acts as a booster to forces on the cables- it does not terminate them. It's exactly like power steering in your car. You can't draw conclusions from the simplified schematic.

Think about it- if the elevator activation was purely hydraulic and the ONLY control column cable forces (and thus the only forces in the control column) were the pressure required to move the valve, there would be no huge control force increases due to aerodynamic loads, which there are.