Definition
A tank that holds the supply of hydraulic fluid for an aircraft's hydraulic system. It stores fluid for the pump to draw from, accommodates volume changes as actuators extend and retract, and allows air bubbles and contaminants to settle out before fluid recirculates through the system.
Plain English
The tank that holds the fluid used to power things like the landing gear, brakes, and flaps. The pump pulls fluid from this tank, sends it through the system to do work, and the fluid returns here to be used again.
Context Anchor
Seen during landing gear system discussions and preflight checks of airplanes that use hydraulic power.
Derivation
Hydraulic comes from the Greek hydor (water), referring to systems that use fluid under pressure to do work. Reservoir comes from the French réservoir, meaning a place where something is stored. Together: the storage tank for the system's working fluid.
Why Pilots Care
Ensures reliable operation of hydraulically actuated systems such as landing gear extension and retraction; insufficient fluid can cause failure to deploy gear.
Analogy
It is like the fluid tank for a car’s brake system: the tank does not create the pressure by itself, but the system needs that stored fluid to work.
Intuition Check
Do not think of the hydraulic reservoir as the part that creates pressure. It mainly stores and supplies the fluid the hydraulic system needs.
Example Sentence 1
During preflight, the pilot checked the hydraulic reservoir sight gauge to confirm the fluid level was within the normal range.
Example Sentence 2
During troubleshooting, the mechanic found that the hydraulic reservoir had developed a small crack, leading to low system pressure.