Definition
A descriptive term for a system or device that operates using a combination of a liquid (hydraulic fluid) and a compressed gas (typically nitrogen or air) to perform work, absorb shock, or store energy.
Plain English
Something that works using both a liquid and a gas together. The liquid moves and transmits force, while the trapped gas acts like a spring to cushion or store pressure.
Context Anchor
Seen in landing gear, shock strut, brake, and hydraulic system discussions.
Derivation
From 'hydro-' (Greek hydor, meaning water or liquid) and 'pneumatic' (Greek pneuma, meaning air or breath). Combined, the word literally points to a system that uses both liquid and gas — which is exactly how these components work.
Why Pilots Care
Proper function of hydropneumatic struts is critical for absorbing landing loads and preventing damage to the airframe.
Grounding Statement
Picture the landing gear strut compressing on touchdown as oil controls the movement and compressed gas provides the spring-like cushion.
Intuition Check
Hydropneumatic does not mean the system uses only water and air. In aviation, it usually means oil plus compressed air or nitrogen.
Example Sentence 1
The main landing gear uses a hydropneumatic shock strut, with hydraulic fluid below and compressed nitrogen above.
Example Sentence 2
Hydropneumatic landing gear cushions touchdown by letting the fluid and gas work together inside the cylinder.