PACKS and APU

[Red Ice]

Hi all

New to this forum but keen on learning about all things aircraft!

Question:

I have a video of a Virgin Atlantic flight to SFO from LGW. They talk about Packs on the one of the pre flight checks and later in little detail during the flight.

Can anyone explain to me what they are/ do and there relevance before take off!

Also in general is the APU used to pass compressed air through jet engines to start the fans turning before fuel injection etc

Cheers

Red Ice

[Capt Pit Bull]

The packs require a source of compressed air which would could be either engine air or apu air. The exact procedures are type dependant, but if you draw air from the engines, then their available thrust for take off is reduced.

Hence, use of engine air for take off will tend to reduce max take off weight. I can't speak for whatever type was in the video you were watching, but on the aircraft I fly we would generally use the APU as the air source for the Packs during take off.

However, sometimes things prevent that. E.G. if the APU air is U/S, (or the whole APU), we can't. If we've just been deiced, we avoid using it as fluid tends to get into the apu air intake and make the conditioned air fairly unpleasant or at worst smoky.

In these cases we would try to use engine air instead. But circumstances may prevent that as well. As mentioned earlier, we would take a performance hit, which might be unacceptable. Or if we are taking of in atmospheric icing conditions we would need Engine Anti Ice protection, which on my type should not be used at take off thrust at the same time as Engine Air. So in these circumstances, we might take off with the packs off, then drive them with engine air as soon as we come out of take off thrust (say 1,000').

So basically, the circumstances of the day determine the air sourcing configuration of the aircraft at take off, and also determine the switchery soon after take off, so for this reason we always include it in our briefing to make sure the necessary procedures have been dragged out of the long term memory and are ready for immediate use.


With regards to turbine engine starting. Bigger engines use compressed air driven through a small turbine 'starter'. That air source could be external, APU, or another engine ('cross start').

Smaller engines tend to use DC Electric motor starting instead.

Hope that is of some use.

CPB

[inbalance]

Those Packs are responsible for the Airconditioning and the Pressurization of the Cabin. They or operated with hot bleed air from the Compressorstage of the Turbine. This hot air normaly is mixed with cold Air to get a normal temperature for the cabin.

[Red Ice]

Thanks guys,

useful stuff. The aircraft was a 747-400!

Red Ice:)