4
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ You can find the answer in an answer to this question, it sounds like the computer takes care of it normally but its there in case of failure. I wont mark as dupe since you are asking a different root question but others may feel it is a dupe. $\endgroup$ – Dave Jun 6 '18 at 14:46
  • 1
    $\begingroup$ Note that most non-FBW big jets have artificial feedback as the controls are irreversible. $\endgroup$ – ymb1 Jun 6 '18 at 14:51
  • $\begingroup$ Yes I’m aware but I’m referring specifically to the Airbus sidestick which, forgive me if I’m wrong, has no bearing on the control surface deflection relative to its position $\endgroup$ – J.FLLT Jun 6 '18 at 15:36
  • 1
    $\begingroup$ I don’t think it is strictly speaking necessary. It used to be the mechanical backup for FBW failures on early Airbusses, then it was kept a while for commonality. I am not sure whether the A350 still has it? $\endgroup$ – Cpt Reynolds Jun 6 '18 at 19:05
  • $\begingroup$ @CptReynolds - Good question, just asked it. $\endgroup$ – ymb1 Jun 6 '18 at 19:45
5
$\begingroup$

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.

$\endgroup$
1
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Minor nit, but nothing about humans flying airplanes is "instinctive". It may be included in early basic training, and it may be something that pilots will fall back onto even under large amounts of stress, but it's not instinctive. If anything about flying airplanes was instinctive, then by definition no training would be required for that part of flying; no explanation would be required other than for illustrative purposes; and doing it any other way would require significant extra training. $\endgroup$ – a CVn Jun 6 '18 at 19:14
  • $\begingroup$ You might be applying a bit too literal an interpretation. It's common in English to use "instinctive" as a short hand to describe actions that are ingrained sufficiently by training/repetition/experience to be done without having to think about it. $\endgroup$ – John K Jun 6 '18 at 20:02
  • $\begingroup$ @MichaelKjörling I completely disagree. There is a huge amount of human factors work that underlies flight control and dynamics. The aircraft is engineered so that a human can handle it. This includes things like control forces, damping and pilot-induced oscillations. The science of flight is inherently about control and stability, and the human is right in the middle of the loop. If you take a undergrad class on control theory, you find things like the number of poles in a control axis, the bandwidth of the system, and then how difficult it is for a human to stabilize the system. $\endgroup$ – user71659 Jun 7 '18 at 3:08
  • $\begingroup$ @user you're thinking way too hard. It was just a comment about the interpretation of a word, i.e., literal vs vernacular usage. $\endgroup$ – John K Jun 7 '18 at 13:11
0
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.