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An actuator is a component that is designed for moving or controlling a mechanism or a system, for example operating flaps on an aircraft. There are two main types of actuators, linear, oscillatory, and rotary. Linear actuators do exactly what they sound like they do: they create motion in a single direction creating a straight line movement on a single axis. Oscillatory actuators are designed to perform a single action most often rotational in movement, for example opening and closing a valve. Rotary actuators are used to create rotational motion like the oscillatory actuator, however the amount of rotation is much greater than that of the oscillatory actuator. Linear, oscillatory and rotary actuators can be operated by various methods, but are most commonly operated by electrical, hydraulic, mechanical and pneumatic principals.

Spherical Accumulator Operation
A spherical accumulator with a bladder showing operation of the accumulator.


Electronic actuators can be classified in three categories: electromechanical, electrohydraulic and linear motor.

  • Electromechanical actuators convert the rotational force of an electric motor into a linear movement with the use of a belt or screw.
  • Electrohydraulic actuators use an electric motor to operate a hydraulic accumulator which is used to transmit the actuation force.
  • Linear motor actuators are different from electromechanical actuators. They are essentially a motor that produces a linear motion instead of a rotational motion.


Hydraulic actuators typically consist of a cylinder that uses hydraulic fluid pressure to perform a mechanical action. The output from the hydraulic pressure can be used to generate linear, rotary or oscillatory motion. Because hydraulic fluid and liquid in general can not be compressed, a hydraulic actuator can be used to generate large forces at the cost of lower actuation speeds.

The hydraulic cylinder consists of a tube along which a piston can slide, much like the design of a cylindrical accumulator. There are two types of hydraulic actuators, single acting and double acting. In a single acting actuator, hydraulic fluid pressure is applied to a single side, meaning the piston can only move in one direction. A spring is typically used to provide the power to return the piston to the resting position.

Double acting hydraulic actuators have hydraulic fluid pressure provided to both sides of the piston. When a pressure differential is created on one side of the piston, it causes the piston to move in the opposite direction.

Cylindrical Accumulator Operation
A cylindrical accumulator with a piston showing operation of the accumulator.


Spherical and cylindrical accumulators operate in essentially the same manner. In operation, the air chamber is charged to a pressure lower than that of the system operating pressure. This initial charge is referred to as the pre-charge. To illustrate the operation of the accumulator, let’s use the cylindrical accumulator as an example.

Let’s assume the accumulator is pre-charged to 1,500 PSI and will operate in a hydraulic system where the operating pressure is 3,500 PSI. When the pre-charge of 1,500 PSI is applied to the accumulator, the system pressure is at zero so the piston will actuate to the fluid end of the cylinder. Because the accumulator pre-charge is 1,500, the hydraulic system will need to generate a pressure greater than 1,500 PSI before the hydraulic fluid can enter the accumulator causing the piston to move backwards. As the fluid fills the cylinder it causes the piston to move backwards compressing the air in the air chamber. At 2,500 PSI, the piston will have backed up several inches, and at 3,500 PSI the piston will be in it’s normal operating position compressing the air until it occupies less than half of the cylinder. It is the natural tendency of the accumulator to maintain equal pressure on both the fluid side and the air side of the cylinder. As the system pressure increases it pushes the accumulator piston back to compress the air which also increases the air pressure, until the air pressure is equal to that of the system pressure. When the use of hydraulic units lower the system pressure, the pressure on the fluid side of the accumulator will decrease and the higher pressure on the air side of the accumulator will force the piston forward expelling hydraulic fluid from the accumulator to provide fluid for the increase in demand on the hydraulic system.

Many aircraft make use of several accumulators in the  hydraulic system. Typically there will be a main system accumulator and an emergency system accumulator. There can also be auxiliary accumulators that operate for other aircraft systems. Regardless of the number, type and location in the system, all accumulators perform the same function: storing extra hydraulic fluid under pressure, and providing that fluid in times of peak demand or emergency.


Accumulator maintenance is very simple and can be accomplished in a few easy steps:

  1. Visually examine accumulator for evidence of fluid leaks. Leaks are present when hydraulic fluid is visible on the outside of the accumulator. Hydraulic fluid can be amber, red, green or purple in color. For an oil accumulator it can be amber, grey or black.
  2. Examine accumulator for evidence of air leaks by brushing soapy water on the accumulator. Bubbles will develop where an air leak is present.
  3. Check for internal fluid leakage by loosening the air valve. If fluid comes out the unit should be removed and sent for repair.
  4. Check the pre-charge pressure after releasing the hydraulic system pressure. Many accumulators on aircraft are equipped with air pressure gauges for this purpose. When a pressure gauge is not equipped, a high-pressure gauge may be installed at the air pre-charge fitting for this purpose. The required pressure will be stated in the aircraft manual for each aircraft.

If any of the checks failed, the accumulator should be immediately removed and sent for overhaul or repair. When removing an accumulator to send it for repair, be sure to completely relieve the system pressure first, then completely relieve the air pressure.

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