What are packs on aircraft?
Most jetliners are equipped with "p-a-c-k-s" which stands for
  Pressurization Air Conditioning Kits".


The air conditioning (A/C) packs are usually located at the lower wing/fuselage root fairing beneath the fuselage.



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On some jetliners (Douglas Aircraft DC-9 Series) the A/C packs are located in the tail, while other aircraft such as the McDonnell Douglas DC-10/MD-11 and Lockheed L-1011 are located in the front of the aircraft beneath the flight deck. 

Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747, Lockheed L-1011, and McDonnell-Douglas DC-10/MD-11 have three.