Definition
A small pressurized oil reservoir, fitted to some feathering propeller systems, that stores engine oil under pressure while the engine is running. When the pilot moves the propeller control out of the feather position to restart a feathered engine in flight, the accumulator releases its stored oil to the propeller hub, driving the blades out of the feathered position so the propeller can begin to windmill and the engine can be restarted.
Plain English
It is a small tank of pressurized oil that helps move a feathered propeller back to a normal blade angle so the engine can be restarted in flight.
Context Anchor
Encountered in multiengine airplane propeller systems, especially when discussing how to restart an engine after its propeller has been feathered.
Derivation
Unfeather is the reverse of feather, which in propeller use means turning the blades edge-on to the wind to stop them spinning. Accumulator comes from the Latin accumulare, meaning to pile up or gather. Here it gathers and stores oil pressure for later use.
Why Pilots Care
Provides the initial oil pressure needed to unfeather a propeller and restart an engine in flight when normal oil pressure is unavailable.
Intuition Check
Do not think of an accumulator as an electrical battery. In this use, it stores oil pressure, not electrical power.
Example Sentence 1
After feathering the right propeller during the air-start demonstration, the instructor explained that the unfeathering accumulator would release stored oil pressure once the prop control was moved forward.
Example Sentence 2
After securing the failed engine, the crew relied on the unfeathering accumulator to supply oil pressure for the restart attempt.