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.
You can be forgiven your unfamiliarity with the 737 cockpit layout, so let me help here. There is a direct reading of the stab position right next to the trim wheel:
This is the view from the First Officer side, but there is an identical trim wheel and index on the Captain's side. You can also see the stab trim cutout switches just aft of the trim index.
The stab trim setting is not in the pilot's forward scan, but it is fairly easy to check at a quick glance. The green band is the takeoff trim zone. Normal takeoff trim is usually between 4.5 to 6.0 units. From personal experience, having the trim lower than 4.0 units or greater than 7.0 units in flight is highly unusual. If you told me I had to operate a flight at one constant trim setting (assuming a normal C.G.), I would say somewhere around 5.0 units would work. We do have procedures for a jammed stabilizer, and any 737 pilot should have been exposed to this non-normal as part of their initial training.
That being said, except for your initial takeoff setting, you don't operate the trim by reference to the index. You operate by feel. If you are holding in any sustained control pressures around any axis (pitch, roll, yaw), you would normally trim away those pressures until the controls were neutral. I say "normally" because there are some flying techniques that involve leaving in some control pressures in certain situations, but that is beyond the scope of this discussion. So in reality, you could tape over the index, and as long as you trimmed out the control pressures, then maintaining aircraft control should pose no problem. Again, this is assuming that you do not have an aircraft that is loaded outside its C.G. envelope.
In terms of increased awareness of a runaway trim situation, I will say that I liked the approach used on the MD-80. There was a aural tone that sounded for every degree of stab movement. A continuous movement of the stab, would produce a steady series of tones. This tone was active with the autopilot on or off, and actually was most useful in a situation where the autopilot was engaged, but the speed was bleeding off (i.e. autothrottles inop or disengaged while autopilot was attempting to hold altitude). Now, before anyone thinks this is a surefire fix, I'll just make the observation that flight crews have, on multiple occasions, demonstrated the ability to not hear aural warnings as well (most notably, gear up landings).
If there is one point I would like to hammer home, it is this. A pilot's primary reference to the aircraft's trim state is through the feel of the controls. If the control pressures are heavy, then they are not trimmed for whatever maneuver is being performed. For an experienced pilot, trimming properly should reside somewhere near the unconscious level - I believe procedural memory is the proper term. Learning to trim correctly is one of those psychomotor skills that can only be developed by constant practice - like riding a bike, throwing a baseball, or learning to dance. If you are already under a heavy cognitive load, and you have to
think about trimming to actually do it, then you are miles behind the aircraft.