Originally Posted by
MemberBerry
Obviously hitting the mechanical limit disables trimming. It's a hard limit. But there are also electric switches preventing you from reaching the full nose down (and up) position using electric trim even before you hit the mechanical limit. Boeing regards this additional limitation as a safety feature. And indeed, in case of electric trim runaway, it can limit the amount of trim the runaway could apply, so it can improve safety in some situations.
Later edit:
You can read in more detail about that in the latest Satcom Guru article:
https://www.satcom.guru/2019/04/stab...and-range.html
In that article there is a table describing the available ranges for the various types of electric trim, compared to manual trim with the trim wheels.
For example, for the 737-800, the full trim range available with the trim wheels is -0.20 to 16.9 units. For manual electric trim with the flaps retracted the range available is 3.95 units to 14.9 units, so far away from the mechanical stops.
That's what the EASA document was talking about. You can't access the full range of mechanical trim positions using manual electric trim. With the flaps retracted on the 737-800 you would only have access to a range of 10.95 units of trim out of the total range of 17.1 trim units available with the trim wheels.
Unfortunately MCAS doesn't use the same limits as manual electric trim, so it can bring the trim closer to the full nose down limit. In the Ethiopian flight it manages to bring it to 0.4 units of trim at some point.
Question, when it says manual electric trim is unavailable outside of the range you mentioned, does that mean in both directions? I understand you can't use electric manual trim to go further ANU than 14.5 and not further than 3.95 AND, but if you find yourself with the flaps up and full AND trim can you use the thumb switch to correct, or do you have to crank the handle till reaching 3.95 AND trim beforhe the thumb switch works? (all this based on me thinking the low number being AND, and the high number ANU)
Asked and answered while I was typing:
Originally Posted by
RatherBeFlying
At initial stick shaker, the aircraft can be expected to be in trim or not far from it. Better to head off MCAS at the pass than let it compound an already difficult situation to one where recovery is uncertain.
Electric Trim Availability?
It's been pointed out that the range allowed for electric trim is less than can be obtained from the trim wheel (airload permitting).
Does that mean electric trim can not be used to return trim outside its range back to within its range?
Originally Posted by
bsieker
No, the manual says explicitly that electric trim can always be used to bring it back into the normal range, but not to move it outside that range:
from 737NG FCOM 9.20.8 Stabilizer TRIM:
Quote:
In the event the stabilizer is trimmed to the end of the electrical trim limits, additional trim is available through the use of the manual trim wheels. If manual trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer trim switches may be used to return the stabilizer to electrical trim limits.
Bernd