APU - Time running

[7triple]

Hi all
I would like to know for how long an aircraft can be with the APU running with the passengers inside, for example, before TO.
Normally, the times I travel, it is only working for a short time. Then the engines start running. I noticed that air conditioning only with the APU is weaker, then with the engines running. Why does this happen?
Happy new year

[allthatglitters]

The APU can run as long as there is fuel in the tanks. Also may be required as an alternative if an engine generator is down.

[Mithrasiain]

The APU is obviously much smaller than a wing mounted engine - it cannot supply as much air compared to a main engine, hence reduced flow rate.

[Anotherflapoperator]

Ah, but often, as the compressor is more suited to bleeding air than an engine, the APU can provide higher pressure air, so in some regional types, the noise and flow rates are a lot better than putting engine air bleeds on whilst idling on the ground.

APUs are serviced based on cycles, hours and condition, plus a few older types suffer from oil weeping past seals and causing air quality issues. As Ground handling agencies charge like the Light Brigade for Ground power, airlines often prefer APU use to save costs, particularly with short turnaround times, but SOPs are increasingly turning towards turning APUs off immediately after main engine start and either sitting with airs off, or on engine air.

Also, some aircraft use electric starting engines, (turboprops 146/RJ) and others air starts. In either case, you'll hear the air conditioning, then it goes quiet, as you start to push back, and likely once the start is complete, the noise will change as you taxi out to take off.

Hope that helps.

[allthatglitters]

plus a few older types suffer from oil weeping past seals and causing air quality issues.

Do not have to be older types, even some of the newer types, like the APS3200 still have regular failures with total oil loss through the compressor and out the turbine and exhaust. Smoke plume over several bays (Bleed OFF) or in the cabin (Bleed ON), the oil tank emptied in a few minutes, low oil pressure, then silence.

[Kiwiguy]

On ETOPS flights APUs must be run continuously.

At my local Air New Zealand turn on 733 APUs around 5.00am and they don't switch them off until tie down.

[NSEU]

The APU is limited by altitude. It will not provide enough air for pressurization and airconditioning at medium to high altitudes.

Rgds.
NSEU

[Mr @ Spotty M]

Kiwiguy
You are making a bold statement that Quote,"On ETOPS flights APUs must be run continuously", it might have to be on your fleet type, but as a whole you are incorrect.
I do not know of any aircraft that was designed for ETOPS operations that has to have an APU running continuously while in the ETOPS stages of flight.
The only time that you do have to keep it running is if you have a generator inop.

[Kiwiguy]

Obviously I stand to be corrected, but as I understand it that is the local requirement of NZ CAA.

NZ was one of the earliest countries to embrace ETOPS and for the 767-200 half the experience operating that type on ETOPS was gained on trans Tasman ETOPS in the first decade or so.

Another thing is that APU's depending on the type will not relight above a given altitude (usually early 20Ks) so as to avoid cold soak problems the local requirement is to keep them running.

[call100]

Most airports restrict the time an APU can be run on stand. Usually 30 mins after arrival or 30 mins before departure.

[captain87]

re from captain87

Hello everyone ! ...

7triple
The APU flow rate is not always LOW ... not for all the aircrafts ...
According Airbus system philosophy , the APU has been set to deliver always HI flow to the packs. Besides HI flow is automatically selected by the remaining BMC (Engine 2) in case of one engine failure. At this purpose I've to say that what you stated according the APU Flow is strictly relative. Obviously if we select the Pack Flow manually to HI, we've a flow greater than APU accordingly. I'd like to remind that "APU is a generator and the engine has a generator (IDG's)" ... the purpose of the APU is to reduce the engine workload and acting as a "backup" in such normal and abnormal situations. Said that, I suppose that there shouldn't be any substancial differencies in terms of efficiency.

The APU has some operating limitations for bleed and electrical power supply. The first one (bleed) is often more restrictive according altitude.

Best Regards

by captain87

[Shiny side down]

WRT-The issue of APU running for ETOPS.
The B737 when operated on an ETOPS sector needs the redundancy provided by the APU to cover generators. Because the APU will not (reliably) light above 25000', the APU is left on after start, and remains on throughout the flight until the end of the last ETOPS segment.

So I would imagine it is aircraft/fleet specific.

I cannot obviously comment on other types.

[Swedish Steve]

On the B777 you can dispatch for an ETOPS flight with the APU Inoperative.

But it does have four engine generators.

[7triple]

Hello Captain87- Thanks for your explanation!
Dear Call100 - Where can I find such information about that airports? Can you please give an example?
Concerning the procedure during ETOPS, I didn't know there was a company using it in flight! It is in fact another engine spending fuel for nothing...
Cheers and happy new year!
7triple

[bvcu]

Early ETOPS in UK with 757 required apu on for ETOPS . Once hyd generator and other ETOPS specific mods were incorporated then it just had to be serviceable for dispatch. Not sure about current status as that was late eighties ! Ref 777 at the moment apu reqd to be serviceable for 207 mins ETOPS, if INOP then you're restricted to 180 mins. Not meaning to be picky but to all intents and purposes you have 2 generators on 777, the back up generators arent treated as normal generators for dispatch, just to clarify for non 777 people as it could be misleading !!

[pjvr99]

The origional question was 'how long can the APU be run, and why does APU air delivery seem weaker than engine delivery'?


Without splitting hairs about aircraft types and operator/airport requirements, the answer(s) should be:

indefinitely, as long as there is fuel to feed it, and TBO's on turbine, compressor, and other critical components are not exceeded;

air delivery will seem weaker in most cases because the APU compressor is much smaller than the engine compressor, the amount of air available for bleeding from the APU is less than engine, the delivery pressure from the APU is lower than the engine

My $0.02 worth .....

[NSEU]

"air delivery will seem weaker in most cases because the APU compressor is much smaller than the engine compressor, the amount of air available for bleeding from the APU is less than engine, the delivery pressure from the APU is lower than the engine"

We should be careful not to simplify here. Bleed pressure can depend on rpm and demand. Engines have variable output depending on rpms and whether the high stage or intermediate/low stage is providing air. In contrast, the output of APUs these days is variable, depending on demand, despite rpms running at 100%. If there is only one pack running, the APU will output less bleed air than when, say 2 or 3 packs are running or during main engine start.

Are we talking about a specific aircraft?

[7triple]

Hi Mike Jenvey, I didn't know I couldn't post different posts about different subjects and questions!
Actually, is obvious after reading the 2 posts they are not the same. One has more questions than the other.
Perhaps I asked again the same question, because on the first post I only got an answer that didn't satisfied me at all... and with more questions perhaps people could understand better what I wanted. And that you can understand if you read the 2 posts!
I am a MSc student doing my thesis in Airport Infrastructures - the main subjects is gas emissions and noise inside an airport. I am studying in Instituto Superior Técnico at Universidade Técnica de Lisboa.
Some help about this kind of subjects... is highly appreciated!
And who are you Mike?
Best Regards and happy new year
7triple