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Why the APU is kept running and wasting fuel on ground?
Aviation - General
Written by Capt Lim   
Friday, 18 January 2013 14:56

Boeing 727 APU - A Centrifugal Jet which starts the aircraft

Hi Captain Lim

Waiting to board an airliner at the gate area I have always noticed the tail cone exhausting hot air which I assume the APU is running even though the aircraft is plugged to the Ground Power Unit.

My questions:

Is ground power unit alone able to power up the Avionics, Cabin lighting and Air condition during pre-push back? If so, why is the APU kept running and wasting fuel for an hour or more?

Is the APU on throughout the flight or only during certain stages (from ground power disconnect to full stop)?

Thank you for your answers and your site is a pleasure to visit.

Kevin Chung

Hi Kevin,

I don’t know which airport you are referring to. In many International airports in Europe and elsewhere, due to noise pollution, the APU (auxiliary power unit) is required to be switched off once the Ground Power Unit is connected to the plane. It is restarted prior to the next engine restart – normally about 15 minutes before departure.

The electrical Ground Power Unit can power all avionics and cabin lightings but cannot start the engines or provide air conditioning. To start the jet engines, you require the APU or an external Ground Cart that comes with the compressors to supply high pressures for turning the engine turbines during the starting sequence.

Sometimes the APU is left on during transit to provide air conditioning if the external air cart is not available on a hot or very cold day for cooling or heating purposes.

Normally the APU is switched off once the engines are started and left off throughout the flight until landing unless it is required to be used as a back-up in the air in case of a generator failure.

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Boeing 737 APU Starting

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