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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.

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

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.

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.

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,
‘… 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.

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.

There are several things a simulator won’t replicate. In some cases fidelity is no way comparable to the aircraft - for example, a manual reversion. I find it pretty unimaginable that the way a simulator characterises MCAS would mirror that of the aircraft. Certainly not in it’s present form.

Didn’t Boeing put several pilots through the sim to show them how good the “fix” is? Perhaps I have that wrong. But if so, how could the pilots make a reasonable judgement?

From #91 , https://www.pprune.org/10475116-post91.html (RVF750)

‘I can confirm that Mechatronix NG simulator we have does not have loading to the trim wheels. Very disconcerting when you try the scenario and the F/O can wind in full Nose down on you.
And no, it's not recoverable in that state. Very sobering.’

Huummm. Mix that in with some earlier speculative assessment at #8, #11, #16.

Re MCAS purpose; from Boeing original statement, definitions.
“Maneuvering Characteristics Augmentation System (MCAS) – flight controlaw implemented on the 737 MAX to improve aircraft handling characteristics and decrease pitch-up tendency at elevated angles of attack.”
https://www.boeing.com/commercial/73...e-updates.page

Also; “The Maneuvering Characteristics Augmentation System (MCAS) flight control law was designed and certified for the 737 MAX to enhance the pitch stability of the airplane – so that it feels and flies like other 737s.”

Gums I always read your posts with great interest because you are far more knowledgeable than I, and many others here, about airplanes. I’m just a lowly Commercial SEL/IR with a thousand hours who quit flying myself around in 1993.

On the the other hand, I know a lot about marketing, risk assessment, and the lengths to which management will go to sell a buyer on the reliability of a system, while at the same time the engineers are telling them their expectations are impossible to meet. The world is full of examples of this phenomenon. Challenger, Columbia, and the FIU bridge collapse being just a few.

The MBAs are rarely able assess risk and come up with all sorts of clever vocabulary to hide it. I’m a Boeing shareholder and have faith in the long term future of the company. Short-term they have blown it big time. Their reluctance to ground the fleet even after it had essentially self-grounded worldwide is clear evidence of management in denial. I suspect that internally the engineers were waving red flags for a long time.

None of us know what was discussed at Boeing, but we do know that control forces became increasingly light as AoA increased, possibly to the point of needing an augmentation system to help the pilot push the nose down.

To me, MCAS was designed to prevent situations which might lead to a stall. As such public perception is going to stay “stall prevention”. A plane requiring a stall prevention system will scare the public. A system augmenting pilot control is an easy sell, like power steering.

This sounds good! Any recipe with the instructions “dump” in it has to be on par with a Clemenza Spaghetti.

Which proves that aeroplanes are more important than navigators.
Mind you, if that didn’t work the old man got out too...

Gums et al, https://www.pprune.org/showpost.php?p=10475209&postcount=36
Re ‘like other 737’, stability ‘enhancement’, or anti stall.

First point, this discussion is probably in the wrong thread. Second, there is very little definitive information on these aspects (see Boeing refs), thus supposition reigns. Third, using what is known about an aircraft to build a simulator is not the same as taking a simulator and inferring what the actual aircraft characteristics are, but there ominous indications in this instance.

‘Like other 737’, is more of a marketing or commercial argument; as such it could be managed by other means (even if not saving $1M per a/c).
‘Stability vs stall’; the critical points are when and where. Any problems with stall ident - the point of stall, or characteristics at that point, or recovery capability, requires quick and forceful action, loud and positive, thus stick push or similar.
‘Anti stall’, not a good term, is more associated with stability, usually prior to stall warning - stick shake; awareness, but not always with action.

I conclude that MCAS is a requirement of stability certification, quiet and subtle background enhancement, ‘dressed-up’ to placate customers. Note the use of AoA vs speed input, flaps up only, and Mach input to manage what appears to be a complex problem in a small part of the flight envelope - wt, cg.

The trim input was intended to be small, incremental, and unobtrusive. The design theory appears to be OK, the engineering implementation, checking, and certification a disaster.
MCAS should not be compared with STS, or even Mach trim; nor its implementation as FBY.
I agree with gums # 36 …

Stability involves ‘feel’, measured as stick force in certification.
As an explanatory description only: consider trimmed flight at constant speed, constant thrust. Enter a turn without changing trim position or thrust (aircraft will descend). The ‘feel’ increases as a pull force; this can be assessed at various bank angles - different AoA.
Repeating these tests for a range of speeds (different trim speed) there should be a consistent relationship between bank angle and pull force. Inconsistent force - reduced pull force with bank angle, or with speed decrease at constant bank angle, could add workload in manual flight.
In extreme, slowing down in a turn or level flight at constant trim should always require more pull force, if not the aircraft feel is an unnatural ‘push to turn’. In some cases reducing forces could tend to oscillation or pitchup. There could be similar interactions with thrust - altitude.

MCAS “will not be a significant issue after modification”, nor require hands-on training except if the handling in the failed state is significantly degraded - cruise flight only. [opinion]

The important issues arising from the simulator mistake are in the processes of checking and certification of training simulators, and the possibly of a shortfall in knowledge about the aircraft, particularly the effect of trim failure.

Gums, [engineering or aerodynamic fix] Instead of lengthening the undercarriage, Boeing should have put the engines on top of the wing [/ fix] ;)