B744/748 Running 3 PACKs off the APU
 

Hello, I am wondering if it is a bad idea to run more than 2 packs simultaneously off the APU in the 744 or 748. If it is a bad idea, could you explain why. I can find no reference for this but it is widely accepted practice where I work to only run 2 of the 3 packs at once using APU bleed air. Thanks.

Basically comes down to how hot you want the cabin to get.......

It can run 3 at once, or 2 or 1. :ok:

That was my opinion as well, however my more senior colleagues insist that running 3 at once for more than 10 minutes or so puts undue strain on the APU and can cause it to fail.

Part of the issue I'm sure is that we operate predominately freighters and pack 2 is usually kept off since 1 & 3 provide the airflow to the upper deck anyway.

2 packs run more efficiently than 3 with the apu, there is enough volume of air to run 3, but they will not run or cool as well as 2. Have a look at the duct pressure when operating 2 then 3 packs you can also (in the flt deck at least) hear the change in air coming in.

Most times you pick up an a/c from maintenance you will find we only have 2 packs on.

From our FCOM:

At my airline we have 22 B744s and have used three packs on the ground for the last twenty years without any undue difficulties. Of course the ground crews would prefer that we only use two, because it would be a lot quieter for them!

We normally use three packs as well. If on a freighter, then only 1 pack is frequently used to save fuel I suppose.

As stated above, the packs are very noisy outside due to a high pitch whine from the compressors which are annoying even if standing well in front of the aircraft. Even though you are far from the APU(which is in the tail), earplugs are definitely required when doing the walkaround underneath the central part of the aircraft as it is extremely noisy due to the exhaust air and the high pitch whine from the packs. As well, on warm days there seems to usually be a lot of water coming out of the panels on the belly in the area of the louvered pack exhaust outlets(and some jets of high speed steam out of some small circular vents).

On a hot day, depending on the wind direction and strength it can be really uncomfortable checking the wheelwell area due to the very hot exhaust air but it makes up for that with a nice warm breeze in cold locations.

I work on 744's in the middle east. The only way to keep temperatures reasonable on the ground with 400+ pax on board is to run 3 packs.

If one pack is inop for example an external aircon unit needs to be connected to the pack that's not running. There is a big difference in how much better it cools with 3 on instead of 2.

The duct pressure is automatically adjusted to keep everything running properly, and the reason you can hear and feel the difference in airflow as mentioned above between 2 and 3 packs is that with 3 packs running the pack controllers shut down the lower recirculation fans as they are not needed in that configuration.

Running all 3 packs is fine.

However, on hot days, sooner or later pack #2 may trip, as it is getting hot from waste heat from pack #1, which is below it. So most of the maint ground staff in the Mid-East will not run pack #2 on the ground for long periods of time, to prevent it from tripping due to overheat.

Does running more packs actually stress the APU more? It's not obvious the APU would know or care where the bleed air goes downstream.