Definition
A pilot-operated valve in a pressurized aircraft that, when opened, allows cabin air to escape to the outside atmosphere, removing cabin pressurization and equalizing cabin pressure with the ambient pressure outside the aircraft.
Plain English
A switch or control the pilot can use to manually let the pressurized air out of the cabin, so the inside and outside air pressures match.
Context Anchor
Seen in pressurized aircraft systems, abnormal or emergency procedures, shutdown checks, and maintenance checks involving cabin pressure.
Derivation
Manual means operated by hand rather than automatically. Depressurization is built from the prefix de- (removing) plus pressurization (the act of adding pressure). Together it describes a hand-operated control for removing cabin pressure.
Why Pilots Care
Provides a backup way to equalize cabin pressure when the automatic system fails, allowing safe descent or door operation.
Analogy
It is like pressing the valve on a tire to let air out, except this valve is built into the aircraft system and is used according to the aircraft procedure.
Intuition Check
Manual does not mean improvised or outside the system; it means the valve is intentionally operated by hand. Depressurization does not always mean a sudden explosive loss of pressure; here it can mean a controlled release of cabin pressure.
Example Sentence 1
Before opening the cabin door after landing, the pilot opened the manual depressurization valve to equalize cabin pressure with the outside air.
Example Sentence 2
During the preflight, the crew verified that the manual depressurization valve moved freely and sealed properly when closed.