Originally Posted by
deadheader
Avoiding the closed threads, I'd like to enquire about the 737NG HS trim system. Evidence tends to suggest that in a specific and perhaps very niche corner of the flight envelope, due to powerful aerodynamic forces, the manual HS trim on at least one version of the 737 doesn't function as intended and, in all probability based on known facts at this time, the electric trim system can also becone overwhelmed in extreme circumstances. My question is: regardless of how unlikely it may be, how certain is it that other variants of the 737 can not end up in a flight regime, through some chain of undesirable events, whereby the trim system essentially becomes inoperable?
If it is certain this can't occur, how do we know with any certainty?
I'm not quite sure how to address your question, particularly in light the last sentence of your post. Can a B-737 be flown into conditions that may overwhelm its electric trim system? The answer to that is "yes", and there are procedures to deal with that rare circumstance. Can an Airbus A-300 be flown into conditions resulting in catastrophic failure of its vertical stabilizer due to manual rudder inputs? Again, the answer to that is "yes"; however, there are no procedures to deal with that extremely rare circumstance. Anything is possible, and the only thing in aviation approaching real "certainty" is the old saw: "What goes up, must come down". Everything else is risk management.
Since the B-707 days, Boeing has recognized that in a situation combining high airspeeds with a severely out of trim stabilizer, the ability of the either the manual trim wheels or electric drive to move the trim jack screw could be overwhelmed. I quote from the "FAA APPROVED AIRPLANE (B-707) FLIGHT MANUAL, STABILIZER TRIM EMERGENCY OPERATION: "If the stabilizer electric trim becomes inoperative, the manual trim can be used. Due to the flight loads imposed in an out of trim condition, it may be necessary to release the control column to a low force position and retrim while in an "unloaded" condition. If the airplane is much out of trim, this action will have to be repeated until correct trim is obtained." It should be noted that the foregoing is quoted from a November 08, 1960 revision to the manual.
Hopefully following the links in the Posts by PEI 3721 will answer the balance of your questions.
Cheers,
Grog