Originally Posted by
GordonR_Cape
737 Driver
A long time ago I asked a critical question, to which I did not get a satisfactory answer. Perhaps you can indulge me.
On the B737, the flight computers (and the FDR post-crash readout) know exactly how much nose-down trim has been applied to the horizontal stabiliser, but the pilots do not.
The only way for them to determine this is to look at the sliding scale between the manual trim wheels, or count the number of rotations, or infer from the elevator feel forces.
Stabiliser trim position is not shown on any of tbe primary flight displays, and
AFAIK is not part of any routine instrument scan.
It seems that pilots have to infer runaway trim, rather than a big flashing display that could easily warn them of that fact. How un-ergonomic is this setup, that the most powerful flight control, is the one they have least information about?
I don't think i've ever looked at the trim setting in flight whilst hand flying. Throw in the stick shaker and IAS disagree, it would probably the the last thing that would get my attention too. Even in a runaway trim, just trying to communicate to the person sitting beside me what I thought was going on and what I was going to do would be tough.
Remember the trim is there to reduce workload. The first lesson of flying is to trim trim trim. Smooth small pitch and thrust adjustments & keeping the aircraft in trim significantly frees up the mind to process other details. An out of trim aircraft consumes ALL your cognitive resources, the instant you take you eyes off the attitude the aircraft will move significantly away from where you want it. You have to start putting in large inputs and things feel very out of control. That is why trainees from the Airbus struggle initially, they have to consciously remember to trim, as the aircraft won't sit where they left it.
It would be an interesting experiment in the sim to do a difficult sequence
without touching the trim at all from takeoff to landing. I suspect even flying a bog standard circuit would be a very challenging exercise with takeoff trim set, particularly if in the forward region for a F25 takeoff. Perhaps someone with a bit of spare time in the sim could try this and report.