Definition
A hydraulic pump mounted on and mechanically driven by an aircraft engine, which pressurizes hydraulic fluid to operate systems such as the landing gear, flaps, brakes, and flight controls. Because it draws its power directly from the engine's accessory drive, it produces hydraulic pressure only while that engine is running.
Plain English
A pump that is turned by the engine to push hydraulic fluid through the airplane's systems under pressure. If the engine stops turning, this pump stops producing pressure.
Context Anchor
Seen in landing gear and engine-failure discussions, especially when gear movement depends on hydraulic pressure from an engine-mounted pump.
Derivation
"Hydraulic" comes from the Greek hydor (water), referring to systems that move power through fluid under pressure. "Engine-driven" simply means the engine turns it -- as opposed to an electrically driven or hand-operated pump. Naming the power source matters because it tells the pilot what happens to hydraulic pressure when that engine quits.
Why Pilots Care
Loss of the engine stops the pump, so gear must be raised with the remaining engine's pump or alternate means to restore climb performance.
Intuition Check
Do not assume the pump works just because the landing gear handle was moved. If the pump depends on a failed engine, the system may not have enough pressure to move the gear normally.
Example Sentence 1
After securing the failed engine, the pilot remembered that its engine-driven hydraulic pump was no longer producing pressure and selected the electric backup pump for gear retraction.
Example Sentence 2
During single-engine climb practice the instructor verified that the operating engine-driven hydraulic pump maintained normal system pressure.