B737 Ground Aircond + Apu Aircond

[LEM]

On our 737 Classics the APU is usually quite weak (:mad: ), AND often the ground airconditioning they plug in is weak as well.

Now that it's quite hot, why wouldn't we use the two together?
I can't find any limitation...

Only the Supp procedure says Pack switch OFF to allow cart to operate at maximum efficiency, but actually I find the cooling much better using the two together.

Somebody said it's not allowed, but I think differently....

[Flight Detent]

Hi LEM,

On our -700s, It's quite acceptable to hook up the external air together with operating the APU.
From memory, the crossship valve is closed, and the APU supplies the LH pack ,with the external supplying the RH pack.
The two supply pressures can be monitored on the pneumatic duct pressure indicator.

Cheers, FD

[alexban]

Hy
Lem,did you try using the high flow on the pack (or eng bleed off) while using the Apu? If not working,then maybe it's time for the tech to do their work,clean some filters....
The ground air conditioning cart is connected directly to the mix manifold.So ,I wouldn't use both Apu and the cart on the same time.The cart has,usually ,a higher air pressure than the Apu,so,maybe a chance for a back press on the pack.This is the reason for the recomm to switch of the packs while using the cart.
The Ng has a more powerfull Apu,so you can use both packs with the Apu.No need for the cart on this case.
Brgds...
Alex

[theWings]

Re: back press on packs, thought that this was avoided by

1) mix manifold outlets being held at relative low press and
2) non-return valves b/w pack and mix manifold

after all even the packs, let alone the pre-cond cart, don't supply at exactly the same pressure...

[LEM]

Good afternoon Gents,
There is usually a confusion between the two different ground air they can plug in the aircraft: one is the more powerful, with a smaller hole, the one we use to start the engines if APU not available, which pressurizes the duct and thus can be used to operate the packs.

I'm not talking about this one here.

I'm talking about the other one, much weaker, with a bigger hole, which doesn't pressurize the duct and goes directly to the mix manifold.

I've got the feeling none of the two is powerful enough to backpressure the other one if operated together.
The supp procedure says to turn the pack off to help the ground unit, not he APU.
But actually, who cares of helping the air coming from the finger!

Above all, is there a written reference somewhere? is it forbidden because of some possible problem?

Sometimes the APU is so weak, and the ground air also is weak, that I don't see why we shouldn't use the two together to avoid melting!

Btw, I don't even think about using high flow, as every time I tried, I immediately had a pack trip off.
Always!

[Gary Lager]

Used APU + Ground pre-conditioned air together on Sunday night (tech delay in PMI) - it worked a treat. No direction to the contrary that we could find in the manuals, our inspection of the Tech Manual produced the same conclusion as LEM and Wings have described.

I too have had problems with pack tripping off in HIGH if the cooling demand is large, probably because all our jets are fairly old now and their APUs quite tired.