Q400 apu

[Peter JB]

If the Q400 has an APU then why are the aircraft plugged in to ground power? I see the ground power being plugged in at Devonport every time I fly there - is it an economy reason (electricity costs less than the fuel the APU burns)? Or is the APU only used when ground power is not available?

[Nigel C]

I presume it's the latter.

But it's not just Q400's that do it this way. At Sydney, many of the international apron aerobridges have been upgraded to accomodate ground power and air conditioning. This would also be a real plus for the apron workers for hearing protection.

[Adam W]

I've seen ground power being used more and more as it is a lot cheaper than running an APU.

[John C]

The Dash 8 APU provides bleed air for air conditioning and electrical power. In the Dash it uses electrics to start the engines, where in the jets it uses compressed air, which is why the apu's are used differently.

In the jets plugging ground power in doesn't help you with engine starts, so unless they have a ground air cart you still need the apu running anyway.

In many jets you can't use ground air to run the air conditioning packs, for those aircraft you need conditioned air, which plugs directly into the air conditioning plenums and distributes already cooled air along the ducts and outlets in the aircraft.

So in many cases the just isn't an alternative to running the apu. The Dash isn't one of those, you can actually buy a Dash without an apu (admittedly I don't know if the -400 can be purchased without an apu, but the "classics" certainly can be - VH-JSJ (the old VH-NID and VH-NJD) doesn't have an apu.

[Brenden S]

Out of all the Dash 8 operators here in WA, Skippers, Maroomba, Cobham, they all use ground power where available.
A) is cheaper to run a GPU on Diesel than an APU on Kero
B) it's cheaper maintenance wise to run the GPU
C) you have less problems with the APU

Yes JSJ still does not have a APU, yet it could be fitted with one. I think there is also a couple of Customs Dash's that don't have APU's either

[Daniel G]

The Q400 is interesting in its electrical set up in whereby external power if plugged in, will power the aircraft, even if the APU is running with the GEN selected on. If the DC external power was to drop out, the APU does not take on this load (like the older models do), it will drain the batteries. In this case the GEN would need to be re-selected to on to begin powering the aircraft.

It gets very warm in the aircraft quite quickly, and 90% of the time the APU will be on to keep the cabin at a comfortable temperature, even while the electrical power is supplied by the power cart.

[Daniel G]

The Q400 is interesting in its electrical set up in whereby external power if plugged in, will power the aircraft, even if the APU is running with the GEN selected on. If the DC external power was to drop out, the APU does not take on this load (like the older models do), it will drain the batteries. In this case the GEN would need to be re-selected to on to begin powering the aircraft.

It gets very warm in the aircraft quite quickly, and 90% of the time the APU will be on to keep the cabin at a comfortable temperature, even while the electrical power is supplied by the power cart.

[John C]

Interesting! The Ejets are the opposite, when the apu or engine gens come on line the ground power goes from online to available.

The EJet particularly is very picky about the quality of the ground power, and will happy drop the ground power if the power is too "dirty"

[Hugh Jarse]

We're all sort of heading in the right direction.

You'll find that the APU cannot simultaneously provide bleed air for the air conditioning and DC power for engine start. (Flight manual limitation) You are permitted one, or the other.

Therefore it's common practice to see them on the gate with both a GPU and the APU running, which is done for passenger comfort reasons.

(I thought I would have forgotten all this by now. Enjoying that lovely Brazilian lady of the skies, si)