APU avail inflight?

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HQ_BRU_Lover
Posts: 326
Joined: 22 May 2013, 20:44

APU avail inflight?

Post by HQ_BRU_Lover »

Hi all

While watching [youtube]https://www.youtube.com/watch?v=fTnKdGtjrIY[/youtube], I saw on the ECAM "APU avail" during approach.

Could someone please explain which technical reason there might be behind this? Curious to see as I thought this was never (well, never) done inflight?

Thanks guys!

EDIT: to be seen at 5:00

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HQ_BRU_Lover
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Joined: 22 May 2013, 20:44

Re: APU avail inflight?

Post by HQ_BRU_Lover »

Thx 737MAX. I know it can be used technically but why should you do that? Bleed air is provided by the engines, no?

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HQ_BRU_Lover
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Joined: 22 May 2013, 20:44

Re: APU avail inflight?

Post by HQ_BRU_Lover »

You rock, 737MAX! At least gives me some more insight. And used to fly the 738 as well back in the days in FS2004, lovely machine!

Homo Aeroportus
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Re: APU avail inflight?

Post by Homo Aeroportus »

If your company SOP is to do SETI, Single Engine Taxi In, you activate the APU (*).
SETI is aimed at reducing fuel burn and CO2 emissions on the ground. A Eurocontrol study shows though that this has no effect if your taxi time is lower than 5 minutes, and I've seen much too often that the APU is kept running for longer than necessary especially if the Gnd Pwr is not made available quickly. I love Hamburg : they have an "APU Sheriff" :lol:

Even though you need about 2-minute cool-down before you may switch off one of your engines after landing, getting the APU able to provide the electrical as 737MAX wrote above, also requires some time (**) and pilots have their hands full during landing. Therefore the APU is often switched On before landing although the A/C is perfectly capable of providing elec and pressurization/airco from its main engines.

(*) There are two (2) fire bottles on board that you can activate in case of an engine fire, each capable of acting on either engine bottle 1 first and you bang the second if the first bottle was not sufficient. Each bottle is on a different elec circuit, one supplied fro the Eng 1, the other by Eng 2. In SETI, you lose elec power from the engine (N°2) that you had switched off, hence limiting your capacity to one fire bottle only.
A couple of years ago Airbus has proposed a mod in the wiring harness in the pedestal that enables taxiing on a single engine whithout requiring the APU for fire protection reason. It is called SETWA, Single Engine Taxi Without APU. I don't thing it is much used though.


(**) First press APU Master push button on the OVHD Panel; it shows APU ON in blue. Wait 2-3 sec and press APU START pb that also turns ON in blue. When ready (fuel, opening of APU hatch, ...) it will show as AVAIL in green as well on the upper DU.

H.A.

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