Definition
Hinged inlet and exhaust doors on the airplane's exterior that open to allow outside air to flow into and out of the Auxiliary Power Unit compartment for cooling and combustion when the APU is operating, and close when the APU is shut down.
Plain English
Small movable panels on the outside of the airplane that open to let air reach the small back-up engine that powers the aircraft on the ground, and close when that engine is off.
Context Anchor
Seen in Configuration Deviation List discussions when checking whether an airplane may be flown with certain exterior panels missing, damaged, or not working normally.
Derivation
Ram' refers to air that is forced (or 'rammed') into an opening by the airplane's forward motion or by suction from the running APU. The doors regulate this ram air flow.
Why Pilots Care
Inoperative doors may restrict APU use or require specific procedures listed in the CDL.
Intuition Check
Ram air does not mean the doors are hit or damaged. It means outside air is pushed into the opening by airflow around the moving airplane. Doors here are not passenger doors; they are small aircraft skin panels that control an air opening.
Example Sentence 1
During the walkaround, the captain noted that the APU ram air doors were closed and flush with the fuselage, as expected with the APU shut down.
Example Sentence 2
With the APU ram air doors listed as inoperative on the CDL, the crew monitored APU temperature more closely during cruise.