Accumulator

A component in a hydraulic system that allows a noncompressible fluid, such as oil, to be stored under pressure. An accumulator has two compartments separated by a flexible or movable partition such as a diaphragm, bladder, or piston. One compartment contains compressed air or nitrogen, and the other is connected into the source of hydraulic pressure. When oil is pumped into the accumulator, the partition moves over and increases the pressure of the air. This air pushing against the partition holds pressure on the oil. An accumulator in an aircraft hydraulic system acts as a shock absorber and provides a source of additional hydraulic power when heavy demands are placed on the system.

Technical Notes

Types of Accumulators (by internal design): Piston type, Bladder type, Diaphragm type.

Gas used: Nitrogen — because it is stable, non-flammable, and does not react with hydraulic fluid or seals under high pressure

Typical operating pressure: Approximately 3,000 PSI (pounds per square inch) in most aircraft hydraulic systems

Main applications in aviation:

1- Brake accumulator — stores hydraulic pressure for emergency braking or parking brake when the main hydraulic system pressure is lost

2- Unfeathering accumulator — supplies pressurized oil to the propeller system to unfeather the propeller blades (bring them back to a positive angle) in flight or on the ground

Other important functions:

Shock absorption — dampens pressure surges (water hammer effect) inside the hydraulic lines

Emergency power source — provides immediate hydraulic flow during sudden high-demand situations (e.g., rapid landing gear extension or heavy braking)

Practical Example :

Example on an Airbus A320 (or similar transport category aircraft):

The brake accumulator in an Airbus A320 stores enough hydraulic pressure to apply the wheel brakes approximately 6 to 7 times — even when both engines are shut down and the main hydraulic pumps are no longer running.

Why is this important?

After landing, if the aircraft experiences a complete loss of hydraulic pressure, the pilot can still stop the aircraft safely using only the stored pressure in the accumulator.

The same accumulator also allows the pilot to set the parking brake after shutdown without needing any external power or hydraulic pressure from the engines.