B737-300
Pneumatic Control Panel (overhead panel)
Air Conditioning PACK Switch :
OFF - pack signalled to OFF.
AUTO -
- with both packs operating, each pack regulates to low flow
- with one pack operating, operating pack regulates to high flow in flight with flaps up
- when operating one pack from APU (both engine BLEED air switches OFF), regulates to high flow.
HIGH -
- pack regulates to high flow
- provides maximum flow rate on ground with APU BLEED air switch ON.
and
Ram Air Inlet (left and right wing roots)
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. The RAM DOOR FULL OPEN light illuminates whenever the ram door is fully open.
A turbofan is located in in each ram air exit duct just upstream of the exit louvres. It augments the ram airflow on the ground or during slow flight (flaps not retracted).
How does the pack logic know that the
flaps are up or that the
flaps are not retracted?
The
flapswitch senses flap position and provides a "flaps up" signal to the pack logic.