Overview
Conditioned air for the cabin comes from either the airplane air conditioning system or a
preconditioned ground source. Air from the preconditioned ground source enters the air
conditioning system through the mix manifold. The air conditioning system provides
temperature controlled air by processing bleed air from the engines, APU, or a ground air
source in air conditioning packs. Conditioned air from the left pack, upstream of the mix
manifold, flows directly to the flight deck. Excess air from the left pack, air from the right
pack, and air from the recirculation system is combined in the mix manifold. The mixed air
is then distributed through the left and right sidewall risers to the passenger cabin.
Ram Air System
The ram air system provides cooling air for the heat exchangers. Operation of the system is
automatically controlled by the packs through operation of ram air inlet doors. On the
ground, or during slow flight with the flaps not fully retracted, the ram air inlet doors move to
the full open position for maximum cooling. In normal cruise, the doors modulate between
open and closed. A RAM DOOR FULL OPEN light illuminates whenever a ram door is fully
open. Deflector doors are installed forward of the ram air inlet doors to prevent slush
ingestion prior to liftoff and after touchdown. Deflector doors extend when activated
electrically by the air–ground safety sensor.
Cooling Cycle
The flow through the cooling cycle starts with bleed air passing through a heat exchanger
for cooling. The air then flows to an air cycle machine for refrigeration and to a water
separator which removes moisture. The processed cold air is then combined with hot air
The conditioned air flows into the mix manifold and distribution system. Overheat protection
is provided by temperature sensors located in the cooling cycle. An overheat condition
causes the pack valve to close and the PACK TRIP OFF light to illuminate.
