Definition
A tube that routes air, fluid, or gas from inside a system or instrument to the outside of the aircraft, discharging it into the slipstream rather than venting it into the cabin or another compartment.
Plain English
A pipe that carries something — usually used air — out of the aircraft and dumps it overboard into the airflow outside.
Context Anchor
Seen in diagrams of vacuum systems that power gyroscopic flight instruments, such as attitude and heading instruments.
Derivation
‘Overboard’ comes from old nautical use, meaning ‘over the side of the ship.’ A ‘vent’ is an opening that lets something escape. So an overboard vent line is simply a line that lets something escape over the side — out of the aircraft.
Why Pilots Care
Maintains proper vacuum pressure by allowing continuous airflow through the system, ensuring reliable instrument operation.
Intuition Check
Do not read “overboard” as something falling out by accident. Here it means intentionally routed outside the aircraft.
Example Sentence 1
After spinning the gyros, the air in the vacuum system is routed through an overboard vent line and discharged into the slipstream.
Example Sentence 2
A blockage in the overboard vent line can cause reduced suction and erratic gyro instrument readings.