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Old 5th Apr 2019, 11:58
  #3296 (permalink)  
Join Date: Aug 2007
Location: Brazil
Age: 67
Posts: 131
Originally Posted by PJ2 View Post
Is that what it is?...250 turns of the trim wheel from stop to stop? That info wasn't available anywhere in the AMM, the FCOM or even the FCTM. Like some, I wondered how many turns it was, so thanks.

The full range (from the AMM) of the stab is 17. This range is not available to all controls, (electric, manual, wheel), as noted in this thread, but let us assume the available range is -1 AND to 16 ANU using the big wheel.

Now we can calculate turns and degrees of trim, (remembering the comments on the difficulty of turning the wheel under some circumstances).

250 turns / 17 degrees = 14.7 turns per degree, or,
17 degrees / 250 turns = 0.07 per turn.

If the thinking and the math is correct, we can see that there is a lot of work and time to manually modify the stab trim using the wheel. In fact, those who fly/flew the B727/B737 will recall just how fast the wheel goes around when electrically-trimming in normal flight, flaps-up; the white mark on the wheel is almost a blur...

Do you know how many hand inputs from the pilot will take to complete one turn? Three, four? Supposing the answer is three, the pilot will have to make 44 hand movements to change one degree. If the pilot is able to execute one turn in 4 seconds (fast), a change of only one degree will take 59 seconds, or one degree per minute.
Not enough time to recover from a dive. If my math is correct, the only way to counteract MCAS input is with electric trim. But if you need to cut off trim motors to "kill" MCAS, how the pilot will manually turn the trim wheel to achieve enough amplitude to cancel MCAS inputs?
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