Engine start packs position

[Dave23456]

Hey guys,

Just wanted to make sure something. I know the 737 NG doesn't let you to start engines with packs on auto, it needs to be in the off position. what about the classic? I managed to start the engines with packs on auto. is that a true behavior? 

 

cheers!

[Litjan]

It would be possible, depending on the circumstances. But it is not allowed, it places too much strain on the Apu and also risks a hotstart.

Jan

[Dave23456]

Hey Jan!

So practically the ng is capable of starting engines with packs on the auto position? Because PMDG's 737 ng doesn't start the engines with packs auto. Is it a new feature for safety reasons?

 

Cheers!

 

[Litjan]

Hi Dave,

I have no idea how the NG does it, but in the Airbuses the automatic starting just closes the pack valves, so the pilot has no influence over that... Maybe the NG has a similiar feature?

On the classic (and also the 747-400 I flew, we´d start 2 engines at once with one pack still running) this was up to the pilot to do. You need a start-pressure of at least 30 PSI. If you have a cold day on a low airport, you might get away with the APU also powering the pack(s) - but it sure puts a strain on the APU (high EGT) and if the PSI´s drop too much, you can have a bad startup overtemping the engines. Note that these effects are not really modelled to a realistic extent on our 737, so it might be more forgiving than the real system.

Jan

 

[stefans85]

On 20.5.2016 at 6:20 PM, Litjan said:

Hi Dave,

I have no idea how the NG does it, but in the Airbuses the automatic starting just closes the pack valves, so the pilot has no influence over that... Maybe the NG has a similiar feature?

On the classic (and also the 747-400 I flew, we´d start 2 engines at once with one pack still running) this was up to the pilot to do. You need a start-pressure of at least 30 PSI. If you have a cold day on a low airport, you might get away with the APU also powering the pack(s) - but it sure puts a strain on the APU (high EGT) and if the PSI´s drop too much, you can have a bad startup overtemping the engines. Note that these effects are not really modelled to a realistic extent on our 737, so it might be more forgiving than the real system.

Jan

 

That's the prcedure done with GE engines on 747. The RR enginges are started outboard-inboard 2 at same time while the P&W engines needed to be started one by one normalle 3-4-2-1.

The APU of the classic might be able to start the engines but you'll barely come to 25+% N2 so you risk a hot start as Jan said. Normally you'd inject fuel at 26 or more N2 or when the N2 doesn't rise anymore.