Definition
A gas held under pressure inside a sealed container, storing energy that can be released to perform mechanical work such as extending landing gear when normal hydraulic systems fail.
Plain English
Gas squeezed into a bottle at high pressure. When you open a valve, the gas rushes out and that force can push something — like pushing the landing gear down.
Context Anchor
Seen in emergency landing gear extension systems, especially where a backup gas bottle is used to help extend the gear.
Derivation
From Latin comprimere, 'to press together.' The gas has been pressed into a smaller space than it would naturally occupy, which stores energy. When released, it expands back out and that expansion is what does the work.
Why Pilots Care
When hydraulics fail, the stored pressure from the compressed gas supplies the force required to lower and lock the landing gear.
Analogy
Think of a shaken soda bottle with the cap on. Energy is stored in the pressure. Open the cap and that pressure escapes fast — in an aircraft, that escaping pressure is harnessed to push the landing gear into place.
Intuition Check
Compressed gas does not mean engine compression or ordinary cabin air. Here it means gas stored under pressure in a system so it can do work when released.
Example Sentence 1
When the hydraulic pump failed, the pilot used the compressed gas bottle to blow the landing gear down.
Example Sentence 2
Pre-flight checks confirmed the compressed gas bottle pressure was within limits for possible emergency extension.