Crossbleed start

[aw8565]

I've searched previous threads but did not find the answer I was looking for...

I understand that during a crossbleed start, air is used from one engine to start the other. Certainly at Gatwick (I don't know about other airports) that permission to start the crossbleed is only granted once aligned with the taxiway centerline.

What is the reason for this?

Cheers

AW

[Maz11]

I assume its to do with the fact that you have to increase thrust on the 'live' engine to provide sufficient air bleed pressure to start the second engine. Because of this it is usually conducted on the taxiway centreline to minimise the risk of ingesting debris/people and reduce the chance of the jetwash causing damage. Thats what i think anyway

[dixi188]

On the A300-B4 we start no. 2 from the APU and then start no.1 with cross bleed air. The bleed air pressure is sufficient at idle so we don't have any limitations as to when on pushback we start the other engine.
I used to jumpseat on DC8-73,s and they started no's 2 & 3 from external air and then push back. When pushback was complete the running engines were boosted to give 40 psi. to cross bleed start the other engines, so this had to be done clear of other aircraft.
It all depends on the aircraft or engine type.
Hope this helps.

[Capt Claret]

On the Douglas B717 ground idle is about 65% N2. To achieve the required airflow, 75% to 80% N2 is required.

[aw8565]

Thanks for the answers chaps, sorry about the delay in replying I been in Spain.

Just wanted to rub that in...