Icarus2001, #76, following on
Water pilot, #72,
The description of trim operation is based on normal operation; relating it to basic flying training. However, the 737 trim system differs from basic training aircraft . The 737 design can inadvertently create bad habits tending to fly the aircraft with trim (Boeing cautions against this).
In normal operation the pilot-aircraft-trim interaction is similar to any other system, the effects of the different design are not readily apparent. However, with large changes in stick position / force, or if the trim malfunctions then the pilot-trim interaction changes (
https://www.skybrary.aero/bookshelf/books/2627.pdf).
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The 737 Max pitch stability is like slice of stale bread, curled up at the edges. MCAS logic applies trim to flatten out the edge bits; small trim changes have a large ‘restorative’ effect, which works.
If large, unwarranted MCAS trim changes occur anywhere in the flight envelope then both aircraft control and restorative trim application is difficult - accidents.
After modification, comparison of sensor input, errors inhibit further MCAS trim movement after one small application; this value will place the aircraft ‘out of trim’. If this occurs in the ‘flat’, centre area of the bread slice, applying trim to flatten a non existent curl might dent or hole the bread causing further problems.
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Before the Max can return to service the latter case must be assessed as satisfactory - within an agreed range of piloting competence. However, if the small amount of trim from MCAS ‘failure’ creates a false impression of pilot-aircraft-trim, via residual stick force, then the piloting task is increased. How this might apply to manual trim wheel operation is not clear, but evidence suggest its not beneficial.
Similarly application of corrective electric trim to reduce any offset could be masked by the higher stick forces. This situation could be further complicated if the aircraft is flown with trim, small trim blips inadvertently amalgamate to a longer application, applied instinctively in surprising situations - several alerts and distracting stick vibration (see trim system design link above, NB accidents - and recent FlyDubai).
Previous posts
https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10642164 and
https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10643566 assume that all electrical trim operations are inhibited with MCAS inhibit. The rationale is that if the dual FGC is used as the monitor for MCAS, and electric trim and AP trim are within the same box, then the most effective safeguard is to switch everything off. If so, could subsequent pilot evaluation via checklists determine which part of the system has failed and then some parts reinstated; particularly advantageous if electric trim is effective in restoring and continuing safe flight.
The above is influenced by not understanding why the Max trim-inhibit switches differ from the NG, separating elect trim and AP trim (extensive Tech Log discussion); did Boeing expect some aspects to be restorable - is the Max more dependent on electric trim than the NG, thus it is desirable to restore it to reduce workload in abnormal conditions; particularly where trim runaway still has to be considered.
It is in these areas which checklist drills and actions could be generating more certification discussion.
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