A320 Apu bleed

[leebrensten]

Hi, quick question, discussion today about putting on bleed as apu was starting,I vaguely remember something about this being controlled by bmc, and therefore it was ok, as the bmc will give you the bleed as soon as the apu is up and pressure is sensed. Fcom ref would be great!!

[OPEN DES]

Totally acceptable on ground by Airbus as per instructor directives.

The APU bleed is pneumatically operated and electrically controlled. So without pressure the valve will simply not open.

Inflight DO NOT do this. The engine bleeds will close instantly and also the use of APU bleed is restricted as you know in a few cases.

[leebrensten]

any reference for this? Cant find it in FCOM

[IFixPlanes]

...being controlled by bmc...No, it is controlled by the ECB.

any reference for this? Cant find it in FCOMDSC-36-10-30 (APU BLEED VALVE OPENING LOGIC) :ok:

[nnc0]

FCOM 06 JUN 12
AIB A318/A319/A320/A321 FLEET
DSC-36-10-30 P 1/2

DESCRIPTION - APU BLEED AIR SUPPLY
GENERAL
Applicable to: ALL

The APU bleed valve operates as a shut-off valve to control APU bleed air. It is electrically-controlled and operated by fuel pressure*.
The APU BLEED pb-sw, on the AIR COND panel, controls the APU bleed valve. When the flight crew selects ON with the pushbutton, APU bleed air supplies the pneumatic system, if the APU speed is above 95 %. This opens the crossbleed valve and closes the engine bleed automatically. A check valve near the crossbleed duct protects the APU, when bleed air comes from another source.

(The text in blue above was introduced in FCOM after we installed Ham Sund APS320 APU. For the Honeywell APU the text would read pneumatically controlled. I think the intent in saying it is operated by fuel pressure is that it implies N>95% before the valve can open )

[leebrensten]

Thanks guys!

[Microburst2002]

There is no reference saying you cannot do it.

Personally I do it like that every time I start the APU and want the bleed, like shoot and forget, so I can continue with the preparation. Except if I suspect that the guy in the left seat could dissaprove it. Then I Prefer not to start another stupid argument where there are two options: I am wrong or I am not wrong but anyhow I have to act as if I were or else destroy the good atmosphere. ( if you flew in my airline you would understand what I mean)...

[MD83FO]

hi all, any recent reference against this practice? i keep getting heat for it.
its just common sense to me, the bleed will come online when its ready, just like the engine bleeds

[vilas]

As op des stated APU bleed is automatic and can be put on and will open only when pressure is available. But it is not done because on ground it has to be ensured that no external air is connected before that and after landing if it comes on early then it will suck engine fumes so again it's held back.

[Amadis of Gaul]

Indeed. In fact, in our case in a limitation that they not both be on.

[safelife]

It can be pre-selected on and will open only when the APU is up and running.
However be aware that many manufacturers recommend 20 seconds to up to 2 minutes running the APU without bleed on other types, and there is a reason for that.
Oil is much more likely to enter the bleed system when the APU was just started. Giving it a short time to settle is likely to reduce the likelihood of aircon smell incidents.

[TURIN]

Why the rush to load up? The APU needs to take on the electrical loads and stabilise. Let it settle down first. Then hit the air bleed button. Just because you can doesn't mean you should.

[vilas]

The bleed doesn't open when it is up and running. It opens only when pressure is available. When the bleed is not going to open how is it going to load up?

[Uplinker]

For the same reason that you don't set Flex or TOGA thrust until your main engines have warmed for at least 3 or 5 mins (depending on type), it is good practice to allow an APU to stabilise thermally before applying a load. (By thermally, consider the APU's oil temperature and viscocity, the oil seals and the evenness of the temperature of the turbine blades.)

Those who start the APU and then as soon as they see 'available' bang on the air and the packs make me wince - especially if they are using APU GEN as well.

In a cold dark aeroplane with no GPU/FEGP, obviously you are going to start the APU, and obviously the APU GEN will come on line as soon as the APU is available. So given that electrical load, don't then bang on the air and packs as well - do something else for a couple of minutes such as continuing your cockpit set-up, or read the OEBs or do your flysmart or cockpit security checks, then put the air on, then the packs after a further few moments.

Give the damn thing a chance: Be gentle. You wouldn't start your own car on a cold morning and immediately drive off at full acceleration* so why do it to an APU? I have seen photos of abused APUs and their internals are a mess. You might need your APU in an emergency one day, and it won't be much good to you if it fails.


*Yes, there are those who do this and their engines sound like a bag of bolts and don't last very long. Have some consideration for your mechanical systems ! :ok:

[vilas]

These are your theories not supported by Airbus. what they say you can but for those practical realities of parallel pneumatic supply and ingesting fumes you may not. Actual words are you can but think before you do.

[Feather44]

However, the FCOM DSC APU is pretty clear about it:
Avail: this green light comes on when N is above 95% or 2 sec after N reaches 95%.
The APU may now supply bleed air & electrical pwr.

It doesn't leave any ambiguity.

Maybe you would prefer to wait 1 extra minute.
But if we follow your reasoning, then another person would say no, let's wait 2.5 minutes and a third one would say 2 minutes in fine.

What's wrong with doing something the standard way??
If Airbus would like you to wait, don't worry they'll tell or advise you.

[Uplinker]

Wow, guys, calm down.

But thank you: you have both illustrated my point beautifully.

Can either of you remember why we use Flex thrust on most take-offs rather than TOGA? If you said 'To reduce engine wear and stress to extend engine life'; go to the top of the class.

The more gently you use them the longer they will last. Same principle applies to APUs, but of course Airbus or Sunstrand don't care how many replacement APUs your company has to buy.

Remind me not to buy a used car from you :ok: :)

[Rocket3837]

Anybody can verify if APU bleed PB is selected ON inflight with APU not running, will that action shut down ENG bleed valves..... In my opinion it will not.

[vilas]

APU bleed does not open unless there is correct pressure. So nothing will happen.