PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 28th Nov 2019, 12:53
  #4184 (permalink)  
Join Date: Apr 2015
Location: Under the radar, over the rainbow
Posts: 707
Originally Posted by safetypee View Post
Oldn, #4178 "2.2. On the peculiarities of the trim (relief) of forces" beginning on page 159.

Also from the recommendations: - It is recommended that the Boeing Company. Page 175
“5.21. Consider the practicability to implement the design changes of the stabilizer control system to reduce the risk for the pilot to set stabilizer in-flight into out of trim position.”

And the objection and rebuttal in the note:-

In the Comments to the draft Final Report the aircraft manufacturer suggested to remove this recommendation, reasoning that the Boeing Company design philosophy implies the pilot can fully operate with the available deflection of flight controls, including the stabilizer control. This may be required in a variety of non-normal situations, for example at the total loss of hydraulic system/hydraulic circuits’ pressure. At the same time according to the manufacturer, the aircraft design provides for reasonable amount of engineering concepts for the PM to stop the inflight stabilizer setting into the out of trim position by the PF.
The investigation team agrees that the aircraft design allows for it. But, the air accidents investigation practice shows that the PM, who monitors the flight management and aircraft control actions by the PF, is not always able to promptly identify the out-of-trim stabilizer position, as well as to detect the mere fact of the stabilizer prolonged motion. The investigation team notes that at the current level of technological development the combination of the mentioned engineering concepts is a possible solution: the limitation of the stabilizer deflection angles, when this could result in the adverse consequences, and the full travel/deflection when actually necessary.”

If the Max, after MCAS modification, remains similar to the NG, then the above applies - particularly if the operation of the aircraft depends on the assumption of timely crew awareness and action, then this cannot be assured in all circumstances.

As designed (work as imagined) is not the same as work as done, a gap between theory and practice, identified by accident investigation ?
Yes, I cited the part of the report that included the relevant analysis and forgot to cite the recommendation.

I'm just a grounded -- and now retired -- engineer, but that very thorough report on the Rostov-on-Don crash seems to me to contain some very powerful and disturbing insights into the realities of the human-machine interface and the sometimes-scary distance between expected systems behaviors and the way things work when, for instance, the humans get outside their usual envelope.

Because I was primed to do so by your including the link to the report in this thread, the possible relevance to the MAX dawned on me early in the reading. Overall, as someone said in the thread on the report, the reading experience was extremely informative and seriously dreadful.

I hope others here will consider the elements we've highlighted and weigh in with their thoughts.
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