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Packs
Also from: Air Conditioning Packs
Hot bleed air enters the pack via a pack flow control valve, and passes through a heat exchanger, air cycle machine, and another heat exchanger. A temperature control valve adds hot air that has bypassed the turbine and second heat exchanger if necessary. A bypass valve bypasses the turbine. Conditioned air from the packs is fed to a mixing unit.
Pack flow is always HI(125 or 120% of Norm, depending on version) for single pack operations or with APU bleed supply. The pack flow control valves will not open if any doors are opened.
Pack flow control valves automatically close if the pack overheats, during engine start, operation of the fire or ditching pushbuttons, any door not closed, or insufficient upstream pressure.
Heat exchangers
Cool bleed air using ambient air which enters via a pack air intake and exists via a pack air outlet.
Pack controller
Regulates the pack flow control valve and the temperature control valve, and controls the pack air intake and pack air outlet. The intake and outlet are closed during takeoff and landing. The pack valves are closed for engine start, and reopen automatically. If engine start is delayed more than 30s, the packs come back on automatically.
During takeoff the ram air inlet and outlet flaps close when thrust lever is at or above CL and wheel speed (from the BSCU) is at or above 70 kts. During landing they close as soon as the main landing gear is compressed until 15s after the speed drops below 70 kts.
External conditioning unit
Can be connected via a low pressure point. The air is fed directly into the mixing unit. There are no cockpit indications for an external conditioning unit.
Controls
If bleed air is not available and the PACK 1(2) pb sw is selected on, the FAULT light illuminates. The PACK FLOW selector is set to NORM for a normal passenger load (AUTO on later versions), or can be set to LO for a low number of passengers to reduce bleed demand and save fuel. With the zone temperature selectors in 12 o'clock, a zone temperature of approximately 24 degrees C is demanded. In the COLD position, a temperature of 18 degrees C is demanded (each increment is 2 degrees C).
Failures
| Message | Notes |
| AIR PACK 1(2) OVHT | - HI flow is automatically selected on remaining pack - pack FAULT light illuminates - pack control valve closes automatically - when the overheat condition no longer exists, the pack can be switched back on |
| AIR PACK 1(2) REGUL FAULT | - the corresponding air cycle machine has failed - the bypass valve opens automatically |
| PACK LO FLOW | - not enough flow from the packs to reach the selected temperature - displayed at the top of the COND page |
If both pack controller channels fail, the pack outlet temperature is controlled at 1 and 9 degree C by anti-ice valves (so-called because they prevent the pack air and water particles from freezing), and flow is regulated to 120% of NORM.