Definition
A secondary hydraulic pump, usually electrically driven, that supplies pressure to the aircraft hydraulic system independently of the engine-driven main pump. It is used for ground operations, system servicing, and as a backup if the primary pump fails in flight.
Plain English
A backup pump that pressurizes the hydraulic system when the main engine-driven pump isn't running or isn't working. It lets you operate hydraulic equipment on the ground or keeps things working if the main pump quits.
Context Anchor
Seen in aircraft systems descriptions, preflight checks, and emergency or abnormal procedures involving landing gear, flaps, brakes, or other parts moved by fluid pressure.
Derivation
Auxiliary comes from the Latin auxilium, meaning 'help' or 'aid.' An auxiliary pump is literally a helping pump — it isn't the main one, but it steps in when needed.
Why Pilots Care
Provides essential backup pressure for flight controls, brakes, and landing gear when the primary hydraulic source is lost, allowing continued safe operation and preventing loss of control.
Intuition Check
Auxiliary does not mean unimportant here. It means an additional pump that can help or take over when the normal hydraulic pressure source is not enough or is not available.
Example Sentence 1
Before starting the engine, the mechanic switched on the auxiliary hydraulic pump to pressurize the system and check the brakes.
Example Sentence 2
During the preflight inspection the auxiliary hydraulic pump was tested to confirm it could extend the landing gear if needed.