For take-off on heavy jets as B747, 2 of 3 air packs should be turned off. Why?

Question

For take-off on heavy jets as B747, 2 of 3 air packs should be turned off. Why?

Answer 1

Because they run off engine bleed air.  Bleeding off the engine compressor makes the engine less effective.

Answer 2

my pilots do that while taking off at Katmandu's Airport, airport elevation is at around 5300 feet+ but the mountains around is 8000' we wanted to be sure in getting away higher as much as possible. The guy (2nd answerer) i think best said it all. There is also the danger of decompression which pilots are aware and prevent the two packs off like in our 737.

Answer 3

"PACKs"--not "air packs"

"depressurization"--not "decompression"

Answer 4

Air conditioning packs use bleed air from the engines which reduces the available power. Having the packs off improves take off performance but more importantly, will improve climb performance in the event of loss of an engine after V1.You want as much power from the remaining engines as you can get.  Depending on individual airline procedures, the packs are brought back on one at a time when the plane has climbed past the most critical climb segment. Typically 1000 feet or so. Some other aircraft have similar requirements, such as the DC-10 and others.