What is this?

[plasmarb]

I understand it as a Ground Air Unit, supplying air for aircon, power & startup air?

But my next question goes how do the engines start on push back if it's disconnected from the supply?

[Denti]

That is a unit/duct for preconditioned air, or simply external AC. Not used to start the engines, but to keep the cabin in roughly livable condition without the need to use an APU.

[halfofrho]

If you require an Air Start Unit because you have an inop APU then you would start one engine on stand whilst connected to the ASU, and then disconnect it and usually start the second after the pushback using crossbleed air from the running engine.

[plasmarb]

Both engines started after push back, so this would have been from the APU even though we was at the stand?

[Piltdown Man]

Many airports prohibit the use of APUs on stand except for the first and last five minutes or so. Therefore, air conditioning units (identified by the large diameter flexible pipes) are used to provide fresh, cooled or heated air, as appropriate. So in your case, the APU would have been fired up just before you left and probably turned off after engine start.

Cynics believe restrictions on APU usage are to sell Fixed Electrical Power and air conditioning. Airlines are told it is for environmental reasons. It's normally sold in 15 minute blocks for £50 or so. If you think airlines are greedy, you should see the invoices created by airports! They would charge you for the air you breathe given half a chance.

PM