APU start after pushback?

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I rode in an ExpressJet ERJ a few days ago on business, and noticed that the APU was turned on during pushback.  I thought it was turned on at the gate.


The engines are usually started during the pushback, and thus the APU would have to be on for that to happen. Before pushback ground power could be used, but I would bet they turned on the APU at least as soon as the ground power was disconnected.

Real ERJ jocks can jump in anytime on this.


We had pushed back from the gate, and I heard the sound of the engines starting and running.  Then I heard the sound end, and thought that the engines had quit, and we weren't going anywhere.   A few moments later, I noticed us moving.  

I see this has been here awhile, but it's an interesting question with a fairly straight forward answer.  The Rolls Royce Allison 3007A1 engines require compressed air to start, hence there has to be either an APU running or an air cart attached externally.  Since you were being pushed back, it's safe to assume that the APU was running and there was no air cart attached.  What you likely heard were to Air Conditioning Packs or ACM's (Air Cycle Machines) automatically cycle off during the main engine start.  The reason for this is that the APU must now use the compressed air that it is generating to spin the main engines in order to start them.  While the APU also powers the ACM's when the pilots select it to do so, it has to stop using all that compressed air to run them in order to have enough to start the main engines.  The ACM's are quite loud in the cabin.  First, you'll hear them as a loud high pitch "jet engine" type of sound, then they'll go away and you'll hear an engine start.  You may hear this sequence occur again if they start the other engine right away, or maybe later if they decide to taxi on one engine (we must all save fuel!).  It's not likely you'll even hear the APU over the sound of the ACM's.  The APU is all the way at the tail of the aircraft and the ACM's are located near the wing roots, one on each side.  Hope this helps!


Nice discussion! If you ever happen to sound record that sequence where packs cutout for engine start, I'd sure love to have it to include in the panel.



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