PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 3rd Dec 2019, 23:01
  #4241 (permalink)  
PEI_3721
 
Join Date: Mar 2006
Location: England
Posts: 860
… the aircraft design allows for it, but practice shows …

Yo gums,

The investigation team agrees that the aircraft design allows for it. But, the air accidents investigation practice shows that it 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.”

This is directly relevant to the 737 Max, subject to the NG and Max having similar trim (aero) characteristics - unlikely given the Max accidents and investigations.
5.21, if applicable to the Max, could be read as - change the aero issues to match the current trim / control system capability.
- “reasonable amount of engineering concepts” -

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.

Note 38.
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.
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