Question APU vs external power
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Is the APU on the BAE146 always switched on before boarding - i.e. relatively early - or only shortly before engine start? I'm asking because the checklists suggest that the aircraft at the gate is mostly powered by the APU (and not by external power).
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Either the APU or GPU can be used to power the aircraft prior to engine start. The GPU may be preferable as it will not use any of the aircraft's internal fuel, however, the advantage of the APU is that it can provide air to the PACKS for air conditioning.
Mark - Just Flight
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So, lets say, on a hot day the APU is used early for pax comfort and on a cold day the APU is started prior the engines?
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I believe it would typically be up to the airline's standard operating procedures for what power source would be used as well as what equipment is available at the airport. For instance, some of the smaller airports the 146 flies to may not have the correct GPU to power the 146, so the APU would be used throughout the whole turnaround. If the APU was not functional, then engine 1 or 4 would be kept running to power the aircraft on the ground.
Mark - Just Flight
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Ours OPS required the use of a GPU if available. At out-ports, we'd use the APU.
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To add to the discussions above.
During a turnaround at an out station, if no APU or GPU available were available, the preference would be to leave engine #4 running. This is because pax entry/de-plane would normal be conducted through the left front and rear stairs. If fitted, his would be via the aircraft’s own steps. Great care and a full briefing to baggage loaders would be required, otherwise extra paperwork would follow.
In circumstances were no APU/GPU and engine 1 or 4 is kept running, a x-start procedure can be used to start the other engines.
Another reason that crews could find the APU not available on ground is airfield noise restrictions banning their use.
btw, there is also the dreaded DC battery cart ‘dark start’ procedure. But such talk is best left until Halloween…
