So, I am very familiar with the 737NG, and there is a trim switch on the yoke for vertical trim. However, I can't seem to locate this trim switch in the A320. Is there a trim switch on the joystick in an A320? If not, how do they trim?
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$\begingroup$ You can see on videos that when the pilot activate the switch, the trim wheel turn. On both B737 and A320 the trim wheel is located between pilot's and turn and can be turned manually by pilots as a backup way to trim the aircraft (i.e. to move the THS) $\endgroup$– Manu HCommented Dec 11, 2019 at 9:24
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$\begingroup$ @ManuH The point is that there is no such switch on Airbus aircraft. Turning the wheel by hand is the only manual way to change pitch trim in an A320. $\endgroup$– BianfableCommented Dec 11, 2019 at 10:03
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$\begingroup$ @Bianfable the comment was to highlight that both type have a common way to (manually) trim the aircraft. The OP seems to ignore this way exists on the B737. $\endgroup$– Manu HCommented Dec 11, 2019 at 10:26
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2 Answers
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The pilots do not manually trim an Airbus in pitch. The flight control computers provide automatic trim:
The normal-law flight mode is a load-factor-demand mode with automatic trim and protection throughout the flight envelope.
Following normal law, the sidestick controllers set the elevators and THS to maintain load factor proportional to stick deflection and independent of speed.
With the sidestick at neutral, wings level, the system maintains 1 g in pitch (corrected for pitch attitude), and there is no need for the pilot to trim by changing speed or configuration. Pitch trim is automatic both in manual mode and when the autopilot is engaged.
(A320 FCOM - Flight Controls - Description - Normal Law - Flight Mode)
There is however a mechanical backup:
The pilots use mechanically interconnected handwheels on each side of the center pedestal to control the trimmable horizontal stabilizer.
(A320 FCOM - Flight Controls - Description - Cockpit controls)
You can see the full flight control logic in the following image. The pitch trim is indicated by THS (Trimmable Horizontal Stabilizer):
(image source: A320 FCOM - Flight Controls)
answered Dec 11, 2019 at 7:58
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2$\begingroup$ Related: What is the trim wheel behavior on the A320 family? $\endgroup$– user14897Commented Dec 11, 2019 at 10:47
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$\begingroup$ I don't understand half of the diagram, but my guess is that essentially, the computer system on the A320 calculates where the pilot wants the nose at, and trims it for them $\endgroup$– lpydawaCommented Dec 11, 2019 at 14:23
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$\begingroup$ @lpydawa Something like that: the pilot controls the target load factor with the sidestick, the computer then calculates the required pitch angle to reach that load factor and then actuates elevators and THS accordingly. $\endgroup$ Commented Dec 11, 2019 at 14:25
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2$\begingroup$ A key to the other abbreviations in this diagram would be useful: e.g. FAC=Flight Augmentation Computers; ELAC=Elevator Aileron Computers; SEC=Spoiler Elevator Computers. FCDC=Flight Control Data Concentrators; ... $\endgroup$ Commented Dec 11, 2019 at 15:44
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1$\begingroup$ @lpydawa You could say the same about the entire flight control logic of Airbus planes, but how often do you hear about an Airbus just randomly crashing? There is more than one of each of these computers and they are constantly cross-checking each other. Have a look at How dissimilar are redundant flight control computers? $\endgroup$ Commented Dec 12, 2019 at 7:52
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Autotrim :D or Manual trim by moving the trim wheel. Trim wheel has priority over Autotrim. But if you move the wheel with AP on, it will disengage. Also AP doesn't need to be on for Autotrim to work. It's always working in normal and alternate law, until bank angle of 33 Deg, then you are on your own.
