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On the a320, can someone explain why this is:

AMM 27-22-00 PB 001
NOTE:The maximum admissible rudder trim limits indicated on the control panel when the aircraft is stabilized in heading with the AP engaged are RH 1 DEG and LH 2.3 DEG.

Why is there a 1.3 difference in deg between LH & RH?

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Both engines turn the same way, and the bypass fans exert torque in order to accelerate the air passing through them. The torque needs to be trimmed out using aileron & rudder trim.

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  • $\begingroup$ This is really interesting. It seems difficult for the pilot. Why don't the engines turn in opposite directions? $\endgroup$ – Brian Gordon Oct 7 '17 at 6:08
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    $\begingroup$ @BrianGordon: that would require two mostly different engines. Twice as much spare engines, higher production costs, and so on. Though, many propeller driven aircraft, where the torque plays a bigger role, have counter-rotating engines/propellers. $\endgroup$ – sweber Oct 7 '17 at 6:41
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    $\begingroup$ Also, it's easier to reverse the direction of a turboprop where you've already got a gearbox before the propeller. Turbofans generally have the fan on the same shaft as the low-pressure compressor and turbine, so you would need to rebuild half the core to change direction. $\endgroup$ – SomeoneSomewhereSupportsMonica Oct 7 '17 at 8:15
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    $\begingroup$ @BrianGordon it's not really a problem for the pilot when the designers have compensated for it, for example, by allowing more left rudder trim than right. $\endgroup$ – Ben Oct 8 '17 at 1:10

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