I've been watching YouTube videos and when they're going through the checklist they say "packs off" can anyone elaborate?

  • $\begingroup$ Searching here for "packs" would have answered this question. It's a useful resource. $\endgroup$ Jul 1, 2018 at 11:22

1 Answer 1


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.

  • 1
    $\begingroup$ I suggest adding a paragraph on why the packs are turned off for takeoff. $\endgroup$
    – Terry
    Jul 1, 2018 at 17:35
  • 3
    $\begingroup$ Packs are normally on for takeoff unless you are doing an unpressurized takeoff for some specific reason. If you have an APU running they can be on APU bleed instead of engine bleed so you don't have to take the performance hit. $\endgroup$
    – John K
    Jul 1, 2018 at 18:15
  • $\begingroup$ This was very useful! Thank you $\endgroup$
    – user27910
    Jul 2, 2018 at 16:31
  • $\begingroup$ Good illustration +1, note there is no consistent definition for the (supposed) acronym PACK. Passenger Air Conditioning kit -- Pneumatic air conditioning kit -- Pressurization and air-conditioning kit. Other variations can be found, leading to pleonasms like "pneumatic air cycle kit". Airbus seems to use "AC pack", with pack being a shortcut for package, like in "A pack with a seized ACM must be switched off on ground". $\endgroup$
    – mins
    Sep 19, 2021 at 11:31

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