On Airbus family airliners why is it necessary for the pilots to have the facility to trim when there is no aerodynamic feedback through the flight controls?
Airbus FBW aircraft have an Autotrim system, so the trim wheel is only there as a backup. In normal law, the trim system is invisible to the pilot. The sidestick pitch inputs control load factor, unlike in a yoke aircraft where it controls elevator deflection. This means that that the farther you move the stick forward and back the faster it pitches, but at center it is a zero load factor, so it stays at its current pitch angle.
As an example of the difference, if you want to pitch up 10° in a yoke aircraft you pull back on the yoke until it reaches 10° and hold it there. If you let the pressure off it will return to 0° unless you retrim for 10°. Whereas in Airbus normal law, to pitch to 10° you pull on the sidestick until it reaches 10° then let go. The FBW system automatically maintains the new pitch angle. If you don’t do anything else the FBW system will automatically trim the stabilizer to the new pitch angle. There is no feedback as to elevator deflection because it’s not directly controlling the elevator.
Pilot misunderstanding of the Autotrim function was a factor in the crash of XL Airways flight 888T. When the AoA vanes froze, the computer switched to Alternate 2 law, which does not have the Autotrim function. Due to the AoA fault, the autopilot had trimmed the aircraft full nose up. When the AP disconnected and control law degraded it remained there and notified the pilot that he needed to trim manually. The PF did not realize this, so never moved the trim wheel. With the stabilizer in full nose up trim he didn’t have enough elevator authority to prevent a stall and crashed into the ocean.
It's because on FBW aircraft the software is designed to mimic the behaviour of a mechanical system so that the instinctive pilot inputs will have a similar result.
Note that on any aircraft the trim control system is fundamentally a speed selector. Also, airliners with hydraulic controls and moving stabilizers have no aerodynamic feedback (and the neutral position of the column doesn't change with trim changes) and you are just pulling and pushing against springs when you move the stick.
Trimming these airplanes is quite different to trimming an airplane with an elevator trim tab. Trimming to a new speed requires that the column be displaced to pitch the airplane up or down and held there, then a trim input to move the stabilizer, then release some or all of the elevator input to see the result, then another trim input, release etc until the airplane is stabilized at the new trim speed with the control column at neutral. It's a rapid tug/blip-the-trim/release/tug/blip/release/tug/blip/release process that becomes second nature once you figure it out (watch any cockpit video when the pilot is hand flying and slowing down during an approach).
A FBW system will mimic the same responses using software moving the elevator and stab so that the same instinctive pilot inputs will work and trim inputs change trim speed the same way. Not sure about Airbus, but on the FBW C Series there is actually a trim speed bug on the primary flight display that tells you what your new trim speed will be when you make a trim input, taking away the guesswork zeroing in process when slowing down or speeding up.
The trimmed position of the flight controls has different meanings for different control systems:
- For manually controlled flight control surfaces, the flight controls trimmed position is the stick equivalent position that the control surface returns to when it is released, no hand or feet on the controls. The surface position where aerodynamic hinge moments are zero.
- For hydraulically operated irreversible control surfaces, the stick trim position is an offset to the artificial feel spring - the position that the stick feel spring returns to when released. Notice that some irreversible systems have Q-feel: feel springs that change their spring stiffness as a function of dynamic pressure. The B737 has such a system.
In both cases, whether the aeroplane remains in a trimmed state of flight (no heading or altitude change) once the stick is released depends on where the zero force position is located. This location needs to be set by the pilot, via the trim switches or trim wheels, which set the position of the trim tabs or stabilisers.
In the A320, there is an artificial feel spring loading the stick as well. The only difference with the artificial feel spring system of an irreversible hydraulic actuation system, is that the zero force position never changes, and that in normal mode this is also the aeroplane trimmed position.
In degraded modes there is no automatic trimming of the flight state, and the pilot needs to operate the trim wheel in order to set the stabiliser angle. This is also the backup pitch control mechanism for when all hydraulics are off.