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A320 Rudder Trim & AP.

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A320 Rudder Trim & AP.

Old 5th Apr 2018, 22:39
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A320 Rudder Trim & AP.

Hi guys,

A quick question on the A320’s Flight Controls FCOM:
By looking at the diagrams, I understood that while hand flying the bus, side-stick signals directly go to ELAC, which then computes a yaw rate and sends it to FAC, so that we get a coordinated turn. However, can FAC decide to apply rudder trim during manual flight (let’s say, to keep a very long turn coordinated)?

Second question regarding AP and Rudder Trim: FCOM says that FMGC directly computes Rudder Trim value and sends it to FACs. So what sequence do we get here? AP sends signals to ELAC, which as usual sends a yaw rate to FACs. However, FMGC also sends a direct signal to FACs regarding rudder trim?

Any help will be highly appreciated.

Thanks!

Last edited by AF330; 5th Apr 2018 at 23:02.
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Old 6th Apr 2018, 16:52
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Nobody?
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Old 7th Apr 2018, 09:12
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Originally Posted by AF330 View Post
Nobody?
Well someone should reply, after all you have waited almost 19 hours.
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Old 7th Apr 2018, 13:04
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Oh...one member had replied but deleted his post..

Ok so rudder trim is never applied during manual flight by the computers, unless we turn the rudder trim selector manually. This pretty much answers my first question.
Furthermore, I received a pm telling me that I was getting confused between Turn Coordination and Rudder Trim..

So if rudder trim is never used during long turns (AP engaged scenario), when would AP decide to apply rudder trim (of course, except engine failure..)? Is there any flight phase during normal operations where AP asks for Rudder Trim? Maybe cruise? Or on short final in windy conditions?

Thanks.
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Old 7th Apr 2018, 13:50
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Originally Posted by AF330 View Post
So if rudder trim is never used during long turns (AP engaged scenario), when would AP decide to apply rudder trim (of course, except engine failure..)? Is there any flight phase during normal operations where AP asks for Rudder Trim? Maybe cruise? Or on short final in windy conditions?

Thanks.
AF330, I think there is a bit of confusion about terminology; perhaps what you have in mind is the yaw damping function, that is always active:


Yaw damping stabilizes the aircraft in yaw and coordinates its turns.
In automatic flight (AP engaged) during takeoff and go around, it assists rudder application after an engine failure (short-term yaw compensation).
Note: When the AP is engaged, the FMGS sends orders to the FAC to give :
- Yaw damping during approach
- Yaw control for runway alignment in ROLL OUT mode


How?
through the rudder trim:

The rudder trim function :
- Executes trim orders, entered by the pilot by using the manual trim knob.
- When AP is engaged
 executes trim orders from the FMGS.
 Assists the system in recovering from engine failure (long-term yaw compensation) in all flight guidance modes.
 If the pilot pushes the rudder more than 10 out of trim, it disengages the AP.


Note: When the AP is engaged, the rudder trim knob is inoperative : the master FMGC sends rudder trim orders to the FAC.

Ref: FCOM DSC 22_40-20 yaw function

I hope it helps.

Last edited by EI-PAUL; 7th Apr 2018 at 21:58. Reason: typing correction
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Old 7th Apr 2018, 15:07
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If you watch the rudder trim value while accelerating after v1 cut (still manually flying) you will see the rudder trim value reduce as you accelerate which I deduce is the Rudder Travel Limiter doing its job as the IAS increases. I believe that is the only time the rudder trim will move uncommanded while manually flying - unless someone knows different!!
Cheers,
mcdhu
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