bahadir
Topic Author
Posts: 1296
Joined: Wed Oct 03, 2001 4:57 pm

Engine Start At The Gate?

Tue Feb 16, 2010 12:08 pm

I was on the TK 1979 IST-LHR flight on this great airplane
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Photo © Spencer Wilmot

the other day and we started the engines at the gate. Usually this is a practice that we used to do when the APU was inop in the airplane, but on this occasion the APU was working. (I could see the heat trail from the tail of the aircraft before boarding) .
To add to the mystery, our normal operating procedure was to gate start the airplane with one engine, push back and then do a cross bleed start once the area behind us was clear.

Any Airbus pilots care to share why they would start both engines at the gate with the APU operational?

Thanks..
Earthbound misfit I
 
musang
Posts: 788
Joined: Sun Apr 08, 2001 4:11 am

RE: Engine Start At The Gate?

Tue Feb 16, 2010 1:43 pm

I'm on 737 classics but I'm sure the philosophy is similar....

Air and Electricity are required for start.

The APU might be running but unable to supply air for some reason. In that case one engine is started at the gate using the external air supply. With one engine running, there is an air supply to start the other, via the Cross-bleed procedure. Therefore there's no reason why the sircraft cannot push back and start the other engine afterward.

A crossbleed start involves using more air pressure than is available at idle rpm, so the running engine is revved up to an intermediate setting before the cross-bleed start is done. We need 30 psi of air pressure, which means setting about 35% N1 (as opposed to the idle rpm of about 21%). For this reason, it is not done while pushing back, only after brakes are set.

Pushing back with asymmetric thrust above idle could cause problems for the tug crew, especially if using a tow-bar, especially on slippery surfaces.

Or the other possibly is that the APU is running, but the generator is unserviceable. So the APU can supply the air for the start, but not the AC power. So the first start is done at the gate using the ground power supply. Once the first engine is started, its generator supplies the electricity. The second start therefore is done with the APU air and the power from the running engine generator.

(On the 737 its possible to start using DC power, i.e. only the battery and any air supply, but its long and complicated so would never be done by choice. I had to do it once. It takes ages.)

Why your flight started both engines at the gate I don't know. Maybe they started one, and were held on stand for other traffic, so decided to start the other anyway. Or if they were dependent on ground supplied air or electricity, they possibly thought - "what the heck - might as well start both here". Then the cross-bleed start procedure (not particularly complicated or time-consuming) is unnecessary.

Regards - musang
 
CanadianNorth
Posts: 3145
Joined: Sat Aug 24, 2002 11:41 am

RE: Engine Start At The Gate?

Tue Feb 16, 2010 8:53 pm

On a side note at YXY the planes are pretty much always started at the gate, but due to lack of push backs. We've got the room to space the planes out and a long enough bridge that we simply park the aircraft farther back than usual and at an angle, then come departure time swing the bridge out of the way, fire up the engines, steer hard right and taxi away.

Back to the topic, it's probably correct above. Power and air are required to start the engines, and normally the APU supplies both. However, if either (or both) systems on the APU are not working you can still depart the aircraft, but need to use either the GPU (power) and/or the air cart (air) to start one engine, and then use that engine to start the other.


CanadianNorth
Way to go, nice and slow, never late, 748!
 
KingFriday013
Posts: 879
Joined: Fri May 18, 2007 7:56 am

RE: Engine Start At The Gate?

Tue Feb 16, 2010 9:02 pm

Here at ROC, the US Airways Express flights operated by Air Wisconsin out of gate 12 (iirc) are started at the gate. Rather than pushing the CRJ back, it just turns to the right and taxis out of the gate area.

-J.
Tho' I've belted you an' flayed you, By the livin' Gawd that made you, You're a better man than I am, Gunga Din!
 
pilotpip
Posts: 2825
Joined: Fri Sep 19, 2003 3:26 pm

RE: Engine Start At The Gate?

Tue Feb 16, 2010 11:38 pm

Maybe the bleed was good but the APU generator was inop. Or perhaps vice versa.
DMI

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