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-   -   Boeing admits flaw in 737 Max flight simulator (https://www.pprune.org/rumours-news/621681-boeing-admits-flaw-737-max-flight-simulator.html)

Kal Niranjan 19th May 2019 02:45

Boeing admits flaw in 737 Max flight simulator
 


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https://www.ft.com/content/494354da-...2-f785092ab560

Boeing has been forced to correct a flaw in the software of flight training simulators that are meant to reproduce the flying conditions of the 737 Max aircraft involved in two deadly crashes in the past six months. The disclosure of a problem with the simulator software is a further blow to the credibility of the Boeing brand, which has been seriously damaged in recent months by the two crashes, in which 346 people died. Subsequent disclosures of serious design flaws both in the anti-stall system of the Max, the manoeuvring characteristics augmentation system (MCAS), as well as errors involving other safety systems have further undermined the reputation of the world’s largest commercial aircraft maker. Boeing revealed at the weekend that the software used on the Max training simulator was unable to reproduce some flight conditions, including the conditions which led to the crash of Ethiopian Airlines flight ET302 on March 10. The preliminary crash report from the Ethiopian authorities revealed that the pilots of that flight were flying at high speed and were unable to overcome the power of the MCAS system as it drove the plane’s nose down, by doing what Boeing had told them to do in such circumstances: use a manual wheel in the cockpit to bring the nose back up.

Read further below:

https://www.ft.com/content/494354da-...2-f785092ab560

Dee Vee 19th May 2019 03:04

same thing was reported in the Seattle Times a couple of days ago

https://www.seattletimes.com/busines...-airlines-had/


Boeing recently discovered that the simulators could not accurately replicate the difficult conditions created by a malfunctioning anti-stall system, which played a role in both disasters. The simulators did not reflect the immense force that it would take for pilots to regain control of the aircraft once the system activated on a plane traveling at a high speed.

On Ethiopian Airlines Flight 302, the pilots tried to follow Boeing’s instructions and manually adjust or trim the horizontal tail — called the stabilizer — using a large mechanical wheel beside the pilot seat. However the heavy forces on the tail made it impossible to move the wheel. It’s this condition the flight simulators failed to reproduce.

In a statement, an Federal Aviation Administration (FAA) spokesman said the safety agency “is aware that Boeing Co. is working with the manufacturers of Boeing 737 MAX flight simulators to adjust the fidelity of the stabilizer trim wheel.”

The FAA statement added that it “will review any proposed adjustments as part of its ongoing oversight of the company’s efforts to address safety concerns.”


ironbutt57 19th May 2019 03:10

the media will NEVER be corrected on "anti stall" will they? good grief

Gove N.T. 19th May 2019 07:14


Originally Posted by ironbutt57 (Post 10474661)
the media will NEVER be corrected on "anti stall" will they? good grief

Please remember that the media are describing what the system is designed to correct in certain circumstances in simple language for the poor, ill-educated masses, including some politicians it seems, who don’t have the benefits of your inside knowledge.


A and C 19th May 2019 07:18

If the MCAS system failures resulted from an unknown and unintended flaw in the software it is hardly surprising that the simulators had not been programmed with this unknown and unintended flaw.



Bend alot 19th May 2019 07:29


Originally Posted by A and C (Post 10474728)
If the MCAS system failures resulted from an unknown and unintended flaw in the software it is hardly surprising that the simulators had not been programmed with this unknown and unintended flaw.



I think a whistle blower has stated the single AoA reference was a known flaw - but using both sensors, would then lead to "extra training" not a iPad conversion training platform.

If that turns out to be fact it is an intended flaw in the software. MAX simulators are low in numbers for a reason and that reason is there is nothing new here. Certainly not 3 things that play with flight controls.

It should be checked as to the other new items, if they also are accurate on the MAX simulators.

Given the events in Russia recently,that nose wheel one could play a factor.

MemberBerry 19th May 2019 08:11


Originally Posted by A and C (Post 10474728)
If the MCAS system failures resulted from an unknown and unintended flaw in the software it is hardly surprising that the simulators had not been programmed with this unknown and unintended flaw.


Yes, but that's not the issue they found with the simulators. The issue they found was that it was too easy to move the trim wheels manually in the simulator.

That is a problem regardless of the reason causing you to use the manual trim wheels, and it's not just MCAS that can get you in this situation.

safetypee 19th May 2019 09:34

MemberBerry, # correct, to the point :ok:

Of greater concern is the origin of the error.

Based on what is now known, it’s is unlikely that Boeing had flight tested the extreme condition, an offset stab (unable to control the aircraft). Thus the simulation may have extrapolated steady-state data from operation in the normal flight envelope, which was alluded to in the EASA query.
If so, this might clarify the choice of wording of the emergency AD drill; the belief that the trim wheel could be moved to recover the aircraft. A caveat on this is circumstance where the abnormal ‘yo-yo’ manoeuvre is required - when the trim could not be moved without relief. Furthermore, whether this circumstance is the same as previous variants (assumption in that what worked in the -200, would also work in the -700, NG, etc).

This point is also at the crux of the training / pilot error debate; how much time would the crew have before encountering excessive stick forces, and/or the trim not being available. (MCAS ‘failure’ was a pulsed input spread over a longer time period than might be assumed for a trim runaway - depending on trim rate).
The simulator demos in the media might correctly replicate the difficulty of control and trim, even with lower forces, but would be inaccurate because the difficulties are encountered earlier, a lower stab displacement, and at the stab limit the forces would be much higher.

An associated issue is if the simulated contribution of elevator stick forces were similarly too low; tail trim load is a combination of stab trim and elevator. This can be identified in flight test by mis-trimming the stab and counteracting with elevator, but only up to the point of excessive control force. Of interest this type of test is similar to that which identifies the longitudinal stability compliance - too little at low speed (need MCAS), too much at higher speed (need to reduce the stick force).

A conclusion from the simulator error could be that the 737 Max differs considerably from previous 737 variants, and that these differences are more than Boeing knew of, or had anticipated, - what was the extent of their assumptions.



Fly Aiprt 19th May 2019 10:08


Originally Posted by MemberBerry (Post 10474763)

Yes, but that's not the issue they found with the simulators. The issue they found was that it was too easy to move the trim wheels manually in the simulator.

This raises a question : were the sim demonstrations last month conducted on updated simulators, or not ?

1_of_600 19th May 2019 12:18

Not a flaw....
 
It's not a flaw, it's an omission!
You need to look into the source of the programming of the MAX simulators. Shouldn't take very long to track it down.
Delivered as a "binary" to the sim manufacturers, who are essentially just box integrators.
There was a very definite decision on the part of the software provider to include ONLY what is referenced in the FCOM and QRH.

Just sayin.....

safetypee 19th May 2019 12:27

Fly Aiprt,
‘… were the sim demonstrations last month conducted on updated simulators, or not ?’

See my comments #8, “The simulator demos…”
It might not be possible to establish the standard of simulator used, but the timescales between demo and public notification of the problem suggest that the demo simulator was not updated. However, the standard of simulation might be irrelevant, particularly as the principle of the problem was identified during the demo.

The future relevance of a modified simulator is in the realism of a trim runaway. Particularly the crew’s ability to recognise the failure amongst potential confusion of ‘recently’ added trimming systems (STS, MCAS) and lack of ‘failure’ annunciation, and then physically recover the aircraft to a trimmed condition. The latter even more important if the point of ‘inability’ is significantly before the limit of the stab screw jack, different to that previously thought - less time for recognition and action.

A more speculative problem would be if the Boeing engineering simulator had a similar weakness, only identifiable when compared with the accident FDRs.

1_of_600, an ‘omission’ if the aircraft characteristic was previously known. If unknown then its a ‘flaw’, a big flaw in aircraft certification.





DaveReidUK 19th May 2019 12:46


Originally Posted by safetypee (Post 10474808)
A conclusion from the simulator error could be that the 737 Max differs considerably from previous 737 variants, and that these differences are more than Boeing knew of, or had anticipated, - what was the extent of their assumptions.

I'd be surprised if the real-life trim wheel resistance varies significantly between the Max and the NG in any configuration, given the similar aerodynamics.

Of course on the NG, in the absence of MCAS, having to model accurately the effect of the combination of full AND stab trim and Up elevator in the sim probably sounded a crazy idea that would never happen in real life.

1_of_600 19th May 2019 13:00


Originally Posted by safetypee (Post 10474904)

1_of_600, an ‘omission’ if the aircraft characteristic was previously known. If unknown then its a ‘flaw’, a big flaw in aircraft certification.

You're right, semantics do matter sometimes...

So, by omitting data and information about a known characteristic, did Boeing produce a flawed certification, and a flawed training simulation model? If MCAS "runaway" wasn't a known characteristic, it will not be in the simulation.

Regarding the forces necessary to move the stab trim wheel, I think you mentioned extrapolation in an earlier post... You're probably right about that. However the sim manufacturers also have limitations set by the characteristics of the hardware they install in the sim. They probably would not include a simulated trim wheel system capable of producing that level of force without expensive design to limit it's own potential runaway, purely from safety considerations. Probably one of the many things being re-thought now.

Smythe 19th May 2019 14:18


I'd be surprised if the real-life trim wheel resistance varies significantly between the Max and the NG in any configuration, given the similar aerodynamics.
My concern would be that at 250kts, they could not manually trim the ac. (in the sim)

On the NG, you had 2 switches that operated elec trim and AP trim. On the MAX, the 2 switches are redundant, and either shuts stab trim completely off, going to manual wheel input only.
So far, it looks like MCAS has only initiated on AoA faults, so assume MCAS does initialize, you shut down the stab trim, and now have to rely on manual trim with faulty AoA measurements?

safetypee 19th May 2019 15:54

Dave #12,
Agreed, the force characteristic of the trim wheel might not differ that much. However, the differences in the ‘not so similar’ aerodynamics (why was MCAS required) could result in the Max reaching the same value of force with a lower stab displacement, thus the inference is that the Max would have higher forces at the limit of the screw jack. Even a small difference could have a much greater effect if the elevator feel system was active due to AoA error.

Ignoring AoA (and MCAS, it must not fail the same way again), then the assumption in the trim runaway drill that there would be some elevator available to aid recovery from trim runaway - nose up ‘yo-yo’, could be severely challenged. (see FAA investigates drill)

Crazy (and surprising) things happen every day, :ok:
Even crazier - the 737 NG could differ in these characteristics from previous variants.
The more that is established from the investigations and ‘fall-out’ from these accidents, the more we don’t know. So who knew, who did not know, who needs to know, now.

In reality, a simple view could be that the sim might not have triggered the 4x force from the elevator feel shift at the same time as the MCAS induced trim movement, because MCAS was not expected to fail.
Trim runaway and AoA fail might also be an excluded failure in certification due to extreme improbability.


bill fly 19th May 2019 16:09

And on your Aircraft?
 
I just wonder, presuming that all airliners are certified to the same parameters, how other (jet) transport aircraft stabilisers perform at high speed and extreme trim ranges.

Might be worth checking - and whether their sim software accurately reflects the true case in terms of
a. Elevator load and
b. Stab mechanism stall

Of course, we never go there - but if we do...


thcrozier 19th May 2019 16:45


Originally Posted by ironbutt57 (Post 10474661)
the media will NEVER be corrected on "anti stall" will they? good grief

Maybe it really is an anti-stall device, with a clever name to hide the fact.

Byros 19th May 2019 18:27

If the 737 MAX simulator cannot correlate to real aircraft, then no amount of sim training could have prevented the outcome.

Boeing is in deep trouble.

fgrieu 19th May 2019 19:11


Originally Posted by MemberBerry (Post 10474763)

The issue they found was that it was too easy to move the trim wheels manually in the simulator.

What substantiates too easy ? The only source I know is Boeing's statement "changes will improve the simulation of force loads on the manual trim wheel". I have yet to find any authoritative source on if the change makes turning the wheel of the simulator easier or harder, and in which part of the flight envelope / position of the ailerons.

OldnGrounded 19th May 2019 19:32


Originally Posted by fgrieu (Post 10475128)
What substantiates too easy ? The only source I know is Boeing's statement "changes will improve the simulation of force loads on the manual trim wheel". I have yet to find any authoritative source on if the change makes turning the wheel of the simulator easier or harder, and in which part of the flight envelope / position of the ailerons.

Virtually all of the news reports say that the flaw is that the trim wheels are too easy to turn, thus not actually simulating a situation such as the ones the accident flight crews faced, with extreme loading of the H-stab.

The NY Times story says:


Boeing recently discovered that the simulators could not accurately replicate the difficult conditions created by a malfunctioning anti-stall system, which played a role in both disasters. The simulators did not reflect the immense force that it would take for pilots to regain control of the aircraft once the system activated on a plane traveling at a high speed.
Emphasis added.


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