Generic Airliner APU Question

[Ewan Whosearmy]

Can anyone indicate whether it is standard to kill the APU once one or more engines are started on the ground?

I had always understood that the APU is turned off in the climb when a certain altitude is reached, and turned on again during the descent. However, I am being told that I am wrong.

Thanks in advance.

[Love_joy]

Depends entirely on the aircraft type, and the operator.

On my type, we use the APU to start both engines. Provided that we dont need it for pressurisation on departure we shut it down as part of the after start checks. If we use it during the departure, we switch to ENG BLEED and shut it down as part of the after take-off checks.

We use it for departure in situations where we need all the engine performance we can get, and it will happilly run the pressuriastion but not anti ice.

[stue]

In my last company the APU was turned off when we had finished with it. (Once engines started and generators on bus etc.) Then turned on for when we needed it, i.e. while taxiing in to provide power to the A/C once the engines are shut down. If we were doing a Low Vis App it would generally be turned on passing FL100 in the decent because of needing another generator supply.

Just what we did.....

[zlin77]

The Bae 146 requires the APU to be left on after start as it is the source of bleed air for A/C & pressurisation until approx. 1,500 feet, at which point the engine bleeds are turned on and the APU shutdown, it is also started on descent and carries the bleed air load on final approach.
Most other jets have sufficient bleed air available so the APU is generally shut down after the last engine is started.

[Ewan Whosearmy]

Thanks for the answers - very informative.

Cheers

[captjns]

Wait... isn't the 146 powered by 5 APUs?:E

[Checkboard]

On the 737 in Australia, we had a requirement for an engineering look if the APU failed to start on the ground, but not in the air (XS fuel vapour being removed by the airflow, I suppose.). It was therefore standard procedure to start the APU on the descent, if the destination didn't have any engineering support.

[MarkerInbound]

Hard to define a generic airplane. DC-8, no APU so not an issue (but I've heard a few did have APUs.) Seven two seven, not approved for use in flight. DC-9, can be used in flight as a back generator. Seven four seven four hundred, can be used for bleed air up to 15,000 feet, has to be off by 20,000.

[757jetjockey]

B757/767

APU is used to start both engines under normal conditions, then switched off during after start panel scan. Switched on during taxi in to either have it up and running prior to shutting down engines on stand, or at a suitable point for Single Engine taxi.

Of course if req'd by the MEL then it remains on throughout...

[Old Fella]

Gentlemen, APU's are "Started" or "Shut down". Lights, pumps etc are
"switched" on or off.

[411A]

L1011.
APU used for engine starting (first engine only, generally) then shut down.
If one engine IDG inop, APU left operating until FL100, then shut down (MEL requirement).

The APU can be started whilst airbourne up to FL250, and can be used up to FL310.

[vapilot2004]

I have heard more than a few times complaints and concerns about the racket a DC-9 APU fills the rear cabin with. Any nominations for the most vociferous units? Or most likely to drop the A and simply be a PU?

Gentlemen, APU's are "Started" or "Shut down". Lights, pumps etc are
"switched" on or off.

On the phrase "shut down" I am in agreement kind sir, however for starting, I prefer to light my turbines. When running, I consider them lit.

Is my vernacular old fashioned?

[Rick777]

For something a little newer, B777 APU shutdown as part of before taxi procedure and started as part of after landing procedure. It can be used in flight as needed for electrics up to service ceiling and for bleed up to 22.000 ft. It also automatically starts with the loss of both electrical transfer busses in flight.

[Old Fella]

vapilot2004. I guess your vernacular is unlikely to be as old fashioned as mine, so if I had said you could get the APU and/or engines "Turning and burning" it would be OK. I guess then I would have to say the APU and or engines are "Stopped and extinguished" when shutting down. Enjoy your corporate flying.

[Ewan Whosearmy]

More interesting and informative answers. Thank you.

[Eff-Ohhh]

B744 - APU cannot be started in flight.

[ChristiaanJ]

Another brief 'blast from the past'....

Concorde had no APU.... engines were started with compressed air from an external starter trolley.

The two preproduction aircraft and the first two production aircraft (used for certification) had something called an MEPU - Monofuel Emergency Power Unit - that would provide emergency power for about ten minutes after a four-engine flame-out.

Since it ran on hydrazine (which is more like a rocket fuel than something you would like to have around on an airport) the airlines that finally operated Concorde (BA and AF) decided they preferred doing without := .

CJ

[mwaugh]

We used to leave the APU running through the after takeoff flow and start it up descending through 10,000ft.
Then fuel got expensive.
Now it gets turned off at the first opportunity and started again at the last possible minute. To the point where we're cross-bleed starting second engines on the ground and coming to the gate and leave a single engine running until ground power can be hooked up.
I'm not sure anybody is doing the math on the fuel thrown through engines for cross-bleed starts and the cost of running an engine at the gate as opposed to the APU. I think it's based on simple airline management thinking of "APU bad" so we don't run it.

[kijangnim]

Greetings,
Bean counters :8 have decided that it is expensive to leave the APU runing :eek:, so we have to shut it down as soon as no longer required.
Not so long ago we use to do PAcks on APU takeoff, even that is scrapped.
Although nowadays APU have a cool down cycle, big thermal variations in a short cycle is not metal friendly, so in order to save the cost, we might have one day to pay the price.:}

[Cyclone733]

Can anyone give an approximate APU fuel use example for their aircraft type. I'd be interested to see the difference between the regional and long haul aircraft APUs

[kijangnim]

Greetings,
B777 is approx 340 Kg/H

[mwaugh]

Re: APU cycle cost.

Four words "Power by the Hour".

The bean counter mentality is that if shutting down and starting the APU isn't costing them any more money why worry about it. Now, when the contract comes up for renewal they'll be crying all the way to their bonus, but for now it has "no cost".

[TopSwiss 737]

Greetings Cyclone733;

For us on the B747-400 the flight planning section in our OM-B states:
"Average APU fuel flow rate under normal operation on the ground is 300 kg/hr." :ok:
No specifics on the influence of load on fuel flow though.
For example 1 or 2 generators on line (not much difference on the freighter I guess as normally gen 1 is the only one on line. Gen 2 is then used for main deck cargo handling) or 1/2/3 airconditioning packs running... I reckon bleed demand has the biggest impact on fuel flow. :hmm:

Other aircraft:
MD-11: 200 kg/hr on the ground
150 kg/hr at altitude
B767-300: 115 kg/hr

TS737

[Cyclone733]

kijangnim and TopSwiss 737,

thanks for the replies. Higher fuel burns than I'd expected. I wonder if the A380's APU is a more modern design than the 747's with an increase in fuel efficiency or if APU design is a reasonably static element

[ATP_Al]

Dash 8 300 - APU cannot be used in flight. Burns 100lbs/hour. Shut down once both engines have been started, but company policy is to use ground power instead if our handling agents provide it at no extra cost