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Pressurization of Hydraulic Reservoirs

How Are The Hydraulic Reservoirs Pressurized?


Pressurization of Hydraulic Reservoirs


As we all know each and every modern day commercial aircraft are contained with hydraulic systems. By depending on the design of the aircraft it may have one hydraulic system, two hydraulic systems or even more. Hydraulic fluid is the lifeblood of modern day airliners and used for most of its light-duty and heavy-duty tasks. All the flight control surfaces, slats, flaps, thrust reverses, landing gears and doors and many more actuators are actuated by hydraulics.


Most of the aircraft flying today are equipped with 3 hydraulic systems(Green, Blue, Yellow) and newer aircraft such as A350 and A380 are designed with only 2 hydraulic systems(Green and Yellow). The most important fact to keep in mind when studying about hydraulic systems is all hydraulic systems are separate and independent, no mixture of fluid between each system due to any circumstance and hydraulic fluids are same in color, Skydrol LD-4  has a purple color.This image shows a Skydrol LD-4 hydraulic can where its quantity is 1 quart.

This image shows a Skydrol LD-4 hydraulic can where its quantity is 1 quart.


Note: Due to the variation of system designs, from here onward all the information are presented for A320 aircraft. Most of the other aircraft is exactly similar with minor variations.

Hydraulic fluid is stored in a reservoir and the aircraft has dedicated reservoirs for each of its hydraulic systems. Below image shows a Green hydraulic reservoir of an A320 aircraft.

Green Hydraulic Reservoir Located in the Wheel Well.

Green Hydraulic Reservoir Located in the Wheel Well.

With the assigned tasks for the system, reservoirs may contain different volumes of hydraulic fluid.

In A320 aircraft,

  • Green hydraulic reservoir contains 14.5 L of fluid.
  • Yellow hydraulic reservoir contains 12.5 L of fluid.
  • Blue hydraulic reservoir contains 6.5 L of fluid.

In the good old day’s hydraulic reservoirs were not sealed and pressurized like modern day reservoirs, they were vented to the atmosphere. But with the introduction of sophisticated technology, aircraft flew higher and higher. With the increase of altitude, those hydraulic systems faced different kinds of problems. Due to the very low temperature at high altitudes, hydraulic fluid tends to make foam and reduced the fluidity creating blockages. Another problem that aroused with the high altitude was pump cavitation and due to very low head up pressure, hydraulic fluid tends to form cavitation. Ultimately this fluid cavitation can end up with pump failure. As a solution for all those drawbacks pressurized reservoirs were introduced.


sketch of a pressurized hydraulic reservoir and by using pressurized air


Above image shows a sketch of a pressurized hydraulic reservoir and by using pressurized air significant head up pressure(about 50 psi) is created above the hydraulic fluid upper surface. This will terminate pump cavitation and fluid foaming.


So How The Hydraulic Reservoirs Are Pressurized?

Bleed air is tapped from the engines and use for various tasks such as cabin pressurization, wing anti-icing, engine anti-icing, water tank pressurization, hydraulic reservoir pressurization and for engine starting. LP(low pressure) and HP(high pressure) bleed is used depending on the requirement and LP bleed is tapped from low-pressure compressor and HP bleed is tapped from high-pressure compressor(In CFM-56-5B engine: it has 9 compressor stages and LP bleed is taken from 5th stage while HP bleed is taken from 9th stage.) That tapped air is passed through PRV(Pressure Regulating Valve) and OPV(Over Pressure Valve) to adjust the pressure while pre-cooler reduces the temperature. Pressure and temperature of the bleed air are sensed by a thermostat solenoid located ahead of pre-cooler and correct pressure and temperature by adjusting the PRV and FAV(Fan Air Valve) respectively.

dedicated duct carrying HP bleed

As shown in the above diagram a dedicated duct carrying HP bleed is used to pressurize the hydraulic reservoirs and if this pressure drops due to any reason bleed air from the cross bleed duct can be utilized to pressurize the reservoirs.

Air conditioning packs and other bleed air consuming units are connected to the cross bleed duct as shown in the diagram below. Cross bleed duct can receive bleed from engines, APU and from a ground air supply cart. There are 3 positions for the cross bleed valve. In the below diagram it is in the auto position(maybe in SHUT!) and it can be switched to shut off(SHUT) position or open position(OPEN).

Air conditioning packs and other bleed air

Controls Related to Air Conditioning System in the Overhead Panel.

Even if the engines are not running we can pressurize the reservoirs by using APU or a ground air supply. If we do not have provisions to start the APU or to connect a ground cart we can pressurize the reservoirs by using Nitrogen with the help of a Nitrogen cart.


During an AIR DUAL BLEED FAULT, we will lose both Green and Yellow hydraulic systems due to LOW AIR PRESSURE on the reservoirs. In this situation no point of opening the cross bleed valve because there is no bleed pressure from both engines. Reservoirs are pressurized back to nominal pressure by starting the APU and selecting the APU bleed. When selecting the APU bleed, engine one bleed valves will be closed automatically and engine two bleed valves will be kept open due to the closed position of the cross bleed valve.

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