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THS in the Airbus A320/21/19...

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THS in the Airbus A320/21/19...

Old 4th Apr 2018, 07:12
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THS in the Airbus A320/21/19...

Sorry for the question that in as far as operating an Airbus goes comes from a plain simmer mindset here :-)

I have 3 different behaviour of the THS as operated manually using the trim wheels...

1) It requires at least the Hyd Elec Pump to be on in order to be able to set it, even being able to move the wheels, for setting the proper trim for takeoff;

2) With the APU running, it is also possible to use the trim wheels;

3) It's only possible to use the trim wheels if either the Hyd Elc Pump OR the engines are running;

What is the typical flow ? Will the crew set the trim while still at the gate with engines stopped, and even before APU start, or is this unusual ? And in this case, for instance for when there are problems with the APU and they don't want to start it, they have to switch on the Elec Hyd Pump in order to be able to set takeoff trim ?

Or, will it be possible and actually done only after APU is running ?

Thx for any enlightenment :-)
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Old 4th Apr 2018, 07:54
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You have to have hydraulics to set the THS. When I flew it, it was part of the after start flow.
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Old 5th Apr 2018, 03:31
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Originally Posted by Cough
You have to have hydraulics to set the THS. When I flew it, it was part of the after start flow.
Still is, which means the trim will be set once the engines, or one engine if using one engine taxi out, are started.
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Old 5th Apr 2018, 07:33
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The actual physical turning of the screw jack that moves the THS is performed by two hydraulic motors, one powered by the green system, the other by the yellow. The trim wheel is a control only - it cannot turn the screwjack itself. So you need green or yellow hydraulics, (usually both). So you need at least one engine running.

I suppose that in theory you could move the THS with the yellow electric pump pressurising the yellow system, but I never tried it and donít know if there is a system restriction against this.
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Old 5th Apr 2018, 16:38
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For the manual movement of the THS you only need green or yellow hydraulic.
Yellow system can be pressurized by an elec pump so you need no engine running for "physical turning of the screw jack".
Without G or Y hydraulic the THS is locked in the actual position.
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Old 5th Apr 2018, 18:35
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APU supplies electric and bleed. It has nothing to do with hydraulics, so eventually with manual trim.
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Old 6th Apr 2018, 07:03
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Thank you very much for all your replies!

Precious info, and very useful for my understanding.

It was also confirmed yesterday by a fellow glider pilot who is also an A320 captain at Air Portugal and confirmed also that the procedures mandate the trim to be set only after at least one engine running.

Yet, as some have pointed out it is also possible through the activation of the Hyd Elec Pump.
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