Pressurisation and Air Cooling Systems - or atleast something like that. Basically, they take compressed air from the engine, cool it, and feed it into the cabin. Keep them on unless you like everyone passing out - Or want extra engine thrust.
http://en.wikipedia.org/wiki/Helios_Airways_Flight_522Renember.
Packs on - less pnuematic air pressure.
Packs off - more pnuematic air pressure.
So have them OFF for engine start.
And off for high performance departures. Typically packs are ON from engine start to aid in passenger comfort.... climbing at 4000ft a min cant be good on the ears.
For the md80 - left engine supplies air the cockpit, right engine supplies air to the cabin. Anti-Ice pneumatic pressure comes from the left or right engines when the pneu X-feed levers are opened or when APU is running. APU is typically in the middle so you will need both Pneu xfeed levers opened if you want air conditioning to cabin or cockpit.
I do not know other aircraft.... but I suspect it will be similar.
Last edited by Slotback on Wed Feb 27, 2008 5:12 am, edited 1 time in total.
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