Originally Posted by
Smythe
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What concerns me is that the initial CFD modelling showed at 0.6 degrees would be enough to mitigate the issues. On flight test, that was found not to be sufficient, and somewhere, the 0.6 turned in 2.5 degrees.
That is an exponential difference, and I would really like to see the modelling and assumptions that supported either numbers...
There is also the NYT report where the test pilot stated MCAS orginally used G force, and that was dropped somehwere along the line...now what was that used for? It seems that the high G manuevers the pilot was talking about such as wake turbulence or avoidance (that caused stall?)
Again, the CFD assumptions that required MCAS or where mitigated by MCAS would be very, very interesting to look at.
Vortex tabs to prove laminar flow over the wings should not required at this point in design evolution. I was really surprised when they showed up on the 787....will see on the 777-W
Hi. The NYT article gives the background that really answers those questions.
Originally the MCAS is conceived and accepted as a high g & high AOA triggered SAS device, which doesn't need a great range of motion to achieve it's outcome, the 0.6 value is born. Along the way, the device gets repurposed to being a low speed "anti-stall" device, and as the issue is a low speed event, the g trigger is removed, and the motion rage gets increased markedly. Ooops. In doing so, the two trigger AND logic that protected from single point failures was removed, and the response rate had increased by more than 4 times. doing this set up JT and ET accidents. The requirement in here was likely a need to comply with §25.203(a), "
No abnormal nose-up pitching may occur. The
longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls." § 25.145 Longitudinal control may also have triggered the repurpose, § 25.145(a) It must be possible, at any point between the trim speed prescribed in
§ 25.103(b)(6) and stall identification (as defined in
§ 25.201(d)), to pitch the nose downward
so that the acceleration to this selected trim speed is prompt with(1)
The airplane trimmed at the trim speed prescribed in § 25.103(b)(6);
(2) The landing gear extended; (3) The wing flaps (i) retracted and (ii) extended; and (4) Power (i) off and (ii) at maximum continuous power on the engines.
§ 25.103 Stall speed (b)(6) The
airplane trimmed for straight flight at a speed selected by the applicant, but not less than 1.13VSR and not greater than 1.3VSR.
So, it is possible that the early flight tests showed less than desirable nose down pitch authority near or in the stall with the stab trimmed at 1.13VSR to 1.3VSR. That this was found in early flight test wouldn't have needed a large error in CFD modelling to occur. It is why testing is done.
A simple fix would have been to add trim at that point, which it appears is what the repurposed design would now do. But doing so, the removal of the g trigger of the original accepted high speed, high g design now opened up a single failure point design which could trigger a very powerful trim change authority at any time.
Underneath this the testing issue would range from, an unacceptable under any case pitch up tendency, to a "less than prompt" response near or in the stall when set up under the rules for demonstration. I would guess to the pitch up tendency, but both are possible. In any event, while focusing on one problem, a simple fix probably set up the latent conditions for lack of fault tolerance. The design at this stage would probably be a no go item for an AOA probe failure of any form, I understand it alternates use between the single sensors, but is not user selectable to the other side in a failure case. As an SAS system that was deemed necessary to meet the § 25 subpart B requirements, it would have needed a warning system to the crew of a failure, and that puts the design as built into likely conflict with the rules. Sad day all round. The repurpose suggests that the low speed model was slightly different to expectation, the high speed case may have been found not to occur, or may still have existed, conclusion can't be drawn i\on the high speed case from the info provided. The decision makers probably didn't think that there was any consequential risk in removing a trigger requirement, it's not like adding more stuff, they removed a criteria that could have faults as well, but in so doing, made an intolerant design without failure alerting, alerting that had already been missed in the first set of design acceptance review. The process to end up at this point doesn't need overt action by any players, it just needs assumptions and lack of imagination or curiosity, and in a project under commercially derived time constraints and a policy driven by the customer through commercial to avoid changes necessitating differences training, well, the engineers are human, and the system has room for improvement.
Organisationally, the QA whistleblower & KC scandals pointed towards a broken high level institution in need of some navel contemplation, and such activity if still occurring in this case would be pretty ugly in the impending proceedings. Whether the ethics issues have been fixed will be on display in the wash up.