Apu use
 

Good evening everybody. When there are not airport restrictions for APU use, when do you usually turn on? Many times I see it On during boarding or just later boarding start. Is it a waste of fuel? Do you board a small amount of extra fuel when you plan to use during boarding?

Apu use
 

In the middle of winter, as soon as we enter the aircraft (half an hour before we are scheduled to take off) to warm the aircraft up. It takes some time, and our APU burns 90kg/hr. We are not permitted to start the APU when the fuel tanker is connected. We may upload an extra 50kg or so for the APU.

Most jet airliners (the Avro/BAE146 is the only currently flying exception I know of) need air pressure for starting the engines, usually provided by the APU. So using the APU is not a waste of fuel, just a mere necessity. Running it a few minutes before startup is often sensible in order to get the cabin temperature to a comfortable level and on occasion, the electricity provided by the APU may also be required or helpful.

And so, it depends. Extreme temperatures, expected large passenger loads (do not underestimate the heat output of a group of people!) or the lack of an external electric source will see the APU started rather early or run all through the turnaround while moderate temperatures together with low passenger figures will require the APU only for engine start and consequently started only a few minutes before startup. Formally, APU fuel burn is included in taxi fuel; expected heavy APU usage might cause a bit extra fuel to be taken though.

Prior to departure, generally 10 min or so prior to passenger boarding. After landing, the APU is started once on the taxiway.

What about the 787?

Whenever board to aircraft. Just after fuel is loaded and truck disconnected, of course. Ground power which is 28 volts usually don't provide air conditioning. APU is more reliable for AC most of the time. Considering it a waste of fuel is not reasonable, as Performance Initialization calculates all the APU fuel burns for the reserve fuel, but of course you can change it via cross-feed from a tank which required. After landing, APU is started in a time between exiting the runway and shutting the engines down, mostly on taxiway.

We start the apu after exiting the runway and leave it on till after pushback. We never use GPU, too costly apparently. (738)

I normally start the APU 5 minutes before pushback unless I need the packs for cabin temperature control.