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Wed Jul 05, 2017 1:13 am

Can anyone discuss on comparative economic studies between utilising APU versus GPU, Group Air Conditioner and Ground Air Start unit during transits?
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Re: APU vs GPU

Wed Jul 05, 2017 4:39 am

It all depends on the charges levied by the airport authority. I have no idea of dollar numbers, but I estimate at my operator we use ground air/power at about half our outports, and remain on 'ship's power' for the other half – so I guess the costs must be reasonably close if there is such a distribution. We always start our engines using the APU.

Some airports, especially in Europe, mandate minimum APU usage (typically only permitted 5-10 minutes before engine start or after shutdown) which makes the decision for you.

Ground air is an interesting one. At certain airports the ground conditioned air has a reputation of being less than adequate, especially in summer, and we will disconnect it and switch to on-board A/C provided by the APU before passengers start boarding, even if it is "cheaper" to use the ground air.
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Re: APU vs GPU

Wed Jul 05, 2017 5:11 pm

The big factor is the cost of overhauling an APU, so you are instructed to use the GPU as much as possible.
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Re: APU vs GPU

Thu Jul 06, 2017 5:55 am

Each airline has their own operating limitations on APU use. Standard procedure for most airlines is to use ground power and air if available. Fuel costs are the primary factor for this. A 737 will burn approximately 240lbs or 35 gallons of fuel per hour. Move up to something like an A330 and you now are at 450lbs or 68 gallons per hour. Multiply that over a fleet of 600+ aircraft for some of the larger carriers and you are in the millions of dollars just while the planes sit on the ground.
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Re: APU vs GPU

Thu Jul 06, 2017 6:37 am

IqmalJ wrote:
Can anyone discuss on comparative economic studies between utilising APU versus GPU, Group Air Conditioner and Ground Air Start unit during transits?

The biggest difference for me with the Apu is all in one device. Cannot use LP and HP air at the same time.

Using HP air for starts requires the use of additional procedures.

LP air is often ineffective when you have cargo doors open and a couple of cabin doors.

More dangerous for HP air start, have to get people closer to the aircraft while an engine is running to remove equipment and chocks. Start sequenct takes longer, some airports need a safety officer present for an external air start to keep ground personnel clear (mainly baggage,cargo, and catering) which often travel behind aircraft on the side of the taxiway.
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Re: APU vs GPU

Thu Jul 06, 2017 6:16 pm

Another thing to take into account is APU start cycles. I remember reading in some Airbus publication, that on short turnarounds (<40min). It may be beneficial to leave the APU running, rather than shutting it down after taxi-in and restarting before engine start. Apparently the cost of extra APU fuel burn is less than the maintenance cost associated with an additional APU start cycle. Another source I know says that every start of a 737 sized APU is a maintenance cost on order of ~150$
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Re: APU vs GPU

Sun Jul 09, 2017 4:43 am

Hi: A point in this discudion should be GPU quality. So many times, a/c ready for departure, with all instruments systems including navigation IRS, on and ready and GPU shutdown before APU start, due to Overload, EGT High, or similar
So we need to account for delay time and/or cost of troubleshoot and repair of several LRU that became inop with electrical surge.
For this reason, many companies let APU running in several specific airports.
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Re: APU vs GPU

Sun Jul 09, 2017 2:15 pm

I work at an airline that has a specific hub that gives all kinds of problems in terms of certain gates with bad ground power causing all sorts of electrical gremlins. Granted, my specific airplane, we generally always takeoff and land with the APU on as well, for both power and cooling in the cabin, but we run the APU nearly constantly in the summer time, even hours between flights.
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Re: APU vs GPU

Fri Jul 14, 2017 4:06 am

Thank you so much for all your respondss

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