PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 24th Sep 2019, 14:17
  #2583 (permalink)  
Join Date: Mar 2006
Location: England
Posts: 868
Fly Aiprt, #2572, and comments / diagrams thereafter, tomaski et al.

‘AoA Disagree’ was an option in those aircraft with ADCs which use AoA for internal corrections, e.g. 737 NG. Thus the AoA alert can provide some supporting understanding of an ADC sensor disagree, Speed, Alt, but where all of these systems have an abnormal procedure to compare indications with the appropriate stand-by indicator - use best 2 out of 3, the AoA alert is superfluous in those instances.

The stick shake is similar, but it is very poor practice (not suitable for certification ?) to use an ‘associated’ AoA Disagree alert to deduce the reason for the stick shake. An AoA malfunction is equally likely to be a low value opposed to a high one (MCAS excepted?), thus providing opportunity for a true stick shake to be falsely deduced as an AoA problem - similarly with the display of AoA.

The optional display of AoA value may have be an operational requirement, thus AoA Disagree required. AoA information is already within the EFIS speed display of low speed awareness.

A major weakness of a stand alone AoA display is that with dual system sensors, if one malfunctions it is impossible to identify which of the two is correct (the MCAS saga), additionally many other systems can be ’confused’; irrespective of the disagree alert.
Both EFIS displays should be removed with a disagreement to prevent incorrect interpretation.

Re ‘MCAS active’ - red indication. This display would be unacceptable for certification as it violates the requirement to only use ‘red’ level alerting for failures requiring urgent attention or action (see trim below). The alert also violates recommended principle of not to annunciate normal operation of background systems, e.g. STS.
Alerting for MCAS unavailability could be required if it’s absence affects flight handling.

We can assume that MCAS failure, continuous trim operation as in the accidents, must not happen again (within certification probability) - OnG #2580, but the unrelated trim runaway could occur as the system is unchanged from previous aircraft.
Thus the lesson is that any unwarranted trim operation - ‘PITCH TRIM’ should be alerted in Red for urgent action to isolate the trim system. Alternatively the trim system integrity should be improved (ongoing delay?). This could be a challenging task due to the many inputs and modes of trim operation.
This is aside from the problems of moving the trim wheel to restore a trimmed condition.

The central issue involves awareness, perception, and comprehension, thus display improvements must aid these. Be careful what you assume, what you wish for, or think that can be trained.

Last edited by PEI_3721; 24th Sep 2019 at 15:53.
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