Top Detailed Facts About Aircraft Hydraulic Systems

Aircraft Hydraulic Systems: Working Principles, Components, and Operation

There is a need to study and understand Aircraft hydraulic systems because the use of hydraulics in aircraft has immensely contributed to the advancement of aviation. This is because, without hydraulics, there would be much more reliance on human strength to fly and control planes. Hydraulics step in where human limitations cannot.

In this post, I will explain the principles of hydraulics, as well as the composition and operation of a typical aircraft hydraulic system.

NB: I will be using a typical passenger aircraft such as the Airbus A320 as a reference for most of my explanations. The workings, components, and arrangement of other aircraft hydraulic systems may slightly differ.

Basic Hydraulic Principles

Hydraulic systems work based on the concept of Pascal’s Law which states that a pressure change at any point of a confined incompressible fluid is felt throughout the entire fluid – a liquid or gas. A fluid is considered incompressible when if a large force is applied to it, only a very small change in its volume or density occurs.

The incompressible fluid involved here is the hydraulic fluid. Since pressure is the force applied divided by the area and the pressure is the same everywhere, it follows that a small force applied over a small area on one side of a piston would produce a larger force acting on a larger area on another side of the piston.

This larger force is what would be required to move systems with large forces operating on them, such as flight control surfaces and landing gear. Hydraulics ensures that a small input force from the pilot’s end results in a much larger force at the end of the component that needs to be moved.

Heavier aircraft are affected by much larger aerodynamic forces. The heavier structure of the aircraft is needed for the larger load they carry as well as the much higher speeds and higher altitudes they fly at. This heavier structure must therefore come with a more powerful flight control system. Also, the landing gear assembly is very heavy. This calls for the use of hydraulics to provide the needed power which cannot possibly be supplied by the pilot.

Hydraulics are also used in the braking systems of cars.

Components of Aircraft Hydraulic Systems

The major components essential to the operation of a hydraulic system are:

• Pumps: these are components that provide the pressure that enables the fluid to move through the system. They can be powered by different sources such as engines, electric motors, pneumatics, hydraulics, and the hand pump also known as the manual pump. Pumps can also be classified based on their delivery. One of the common classes of pumps used in aircraft is the constant-delivery/ constant-displacement pump whose use requires the presence of a pressure regulator to maintain the normal pressure at the constant volume of fluid that is supplied by the pump. The other class is the variable displacement pump which is generally more complicated than the former.

• Accumulators: for constant delivery, and fixed volume systems, an accumulator is needed. When pressure builds up in the system, the pressurized gas on one side of the accumulator is compressed until the gas pressure and hydraulic fluid pressure on the other side of the piston equalize at normal system pressure. The pump then starts idling and the system pressure is maintained by the accumulator. There are two types of accumulators used in aircraft, namely the piston accumulator and the diaphragm accumulator. In a piston accumulator, the gas and fluid chamber are separated by a piston which does the compression while in a diaphragm accumulator, the chambers are separated by a diaphragm.

The accumulator process keeps happening whenever system pressure rises above normal, absorbing the shock that can occur due to frequent pressure changes within the system. This means the accumulators are useful when there is thermal expansion or contraction of fluid due to changes in temperature that may occur.

Accumulators can provide little emergency supply of fluid for the system if the pump fails. In this event, the fluid will be sufficient for any single service, except for the brakes. Accumulators provide initial fluid to kickstart the system when the pump is cut out.

• Valves:
• Non-return valves (NRV): they are also called check valves. They ensure that fluid flows in only one direction. It does this with the use of a spring which pushes back against what is called the ‘seat’ of the valve when fluid tries to flow in the other direction. This is useful for various components within the hydraulic system. For example, it is beside the accumulator to prevent fluid from flowing from the accumulator back to the reservoir.

• Automatic Cut Out Valves (ACOV): it provides an idling circuit for the system when no services have been selected. It contains a piston that can sense the pressure of the system and controls the poppet valve in accordance. In the cut-out mode of the ACOV, the pump output pressure of the hydraulic system is greater than the pressure of the spring of the ACOV so the poppet valve opens enabling some of the pump output to return to the reservoir in the return line.
• Fire Shutoff Valves: this is present in some aircraft systems that are engine-driven. In the event of fluid leaks at very high temperatures, the fluid is flammable and can cause the engines to set fire. To prevent this from happening, the fire shutoff valves which is located between the reservoirs, and their respective engine-driven pumps are closed.

• Leak Measurement Valve: each system has one close to the primary flight controls. The valves measure the leakage in each system and close once commanded in the cockpit.
• Selector valves: it is used to control the direction of the fluid flow in an actuator. It provides a path for the fluid to flow in and out of an actuator at the same time. The fluid can either flow into the actuator for the operation of a service or out of the actuator and back to the hydraulic system (reservoir). The selector valve can also change the direction of fluid flow within the actuator.

• Sequence valves: they close and open in a manner that allows the sequential operation of a branch that feeds two hydraulic units. Once the first hydraulic unit is actuated, the valve then allows the fluid to follow the subsequent one. Sequence valves can either be pressure-controlled or mechanically operated.

• Priority valves: they cut off the hydraulic power supply to non-critical heavy load users in the event that the hydraulic pressure in the system is lower than the normal system pressure. The primary system in the diagram below comprises the critical systems while the second comprises the non-critical.

• Pressure-relief valves: to safeguard the system or hydraulic lines from failure or rupture, it is imperative that the pressure is regulated within the system. An accumulator, in a way, performs this function, however, pressure-relief valves can also be incorporated into the system as safety valves. Spring-loaded valves within the relief valve open when the spring tension is overcome and allow fluid to go through the return port to the return line of the reservoir when the system pressure surpasses the set pressure of the valve.

They can be used as pressure regulators for electrically driven pumps or in small lower pressure hydraulic systems. Pressure-relief valves are also useful as thermal-relief valves which are necessary to maintain the pressure in the system when the hydraulic fluid expands due to an increase in temperature of the aircraft’s surroundings.

• Reservoirs: this is the component in which the fluid is kept. In order to prevent cavitation from happening within the pump, the reservoirs are usually pressurized for aircraft which fly at high altitudes. The reservoirs are also kept a reasonable distance above their respective pumps to prevent this from happening. Cavitation is a process that happens when bubbles form in the hydraulic fluid when the fluid moves from a region of low pressure to high pressure. When the bubbles pop within the pumps, they cause serious damage. The reservoir of a hydraulic system may also be non-pressurized if the reservoir is located in a pressurized part of the aircraft.

• Filters: these are usually found in both suction and pressure lines of the hydraulic systems. This means they are on both sides of the pumps: the return line and the supply line. The function of the filters is to sift out particles bigger than (usually) 10 microns, thereby protecting the seals and component surfaces of the hydraulic systems. Case drain filters allow for maintenance as engine wear can be tracked by checking if there are metallic scraps in the filters.

• Actuators: these are cylinders with pistons that convert the hydraulic energy into mechanical energy for the operation of components outside the hydraulic systems. Actuators are connected to these components. When the use of any of these components is demanded, the hydraulic system works to move the actuator so that the service can be provided. In that case, the fluid pressure on one end of the piston would overcome the spring tension causing the piston to move in the direction of the control surface. Once hydraulic energy has been used to do work on the component (by causing it to move), the spring tension will cause the piston to move back, allowing the fluid to pass through the return line.  Actuators can either be double-action or single-action. The former can be used to actuate two components at a time.

• Ram Air Turbine (RAT): it operates like a windmill and is typically located in the belly fairing of the aircraft. The Airbus A320 allows the blue hydraulic system – one of 3 systems – to function if electrical power fails and kicks in automatically if the AC electrical buses are lost. However, the automatic kick-in is only for the electrical system, so it has to be put on manually from the overhead panel in the cockpit for hydraulic usage. It is used to power the blue system in the case of dual engine failure. It can only allow the blue system to operate at a lower pressure output, so priority services alone must be selected. It can only be stowed when the aircraft is on the ground.

• Cooling system: during operation, the hydraulic system can begin to overheat. The cooling system acts as heat exchange between the fuel which usually gets cold due to the low temperatures at high altitudes of the aircraft’s operation and the hot hydraulic fluid. Heat moves from the high concentration (fluid) to the lower concentration (fluid).

• Temperature sensing probe:  it senses the temperature of the hydraulic fluid and activates the warning light in the cockpit in the case of overheating.
• Computer and system display: in modern aircraft, computers process data regarding the hydraulic systems. Also, there are displays in the cockpit that show the pilot(s) the status of the hydraulic system and whether they need to take action. For example, the fill level indicator on the ECAM (in Airbus aircraft) goes from green to amber when the hydraulic fluid level gets too low.

Operation of an Aircraft Hydraulic System

The Airbus A320 hydraulic system will be used as a reference for this section.

The normal pressure hydraulic systems in most aircraft operate with is 3,000 PSI (Pounds per square inch). In the A320, there are 3 independent hydraulic systems – blue, green, and yellow – which each power different components. The multiple systems allow for redundancy which is the case in most aircraft too.

At the start of flight in an A320, Engine 2 which powers the yellow system pump is started first because it controls the brakes and the nose wheel gear which is essential for taxiing the runway. When the system is fully pressurized to 3000 psi, it shows on the cockpit display.

While Engine 2 is running, the electricity required for the blue hydraulic pump electric motor is provided automatically. The cockpit display then signals to the pilot to disconnect the nose wheel steering.

The hydraulic systems of the A320 have a Power Transfer Unit (PTU), a computer that allows one system’s pump to pressurize another system in the event that there is a pump failure. Before taking off, the PTU is tested to ensure that both the yellow and green systems can pressurize each other. Once the PTU’s good working condition is confirmed, the plane’s landing gear is retracted using the green hydraulic system.

Each time a component is to be operated on, the pumps pressurize their corresponding systems from 0 to 3,000 psi, and then when the operation is finished, the system pressure goes down. Sometimes, it goes all the way back to 0, as for the cargo doors. Otherwise, it just goes down a bit e.g. landing gear.

Each pump and PTU have their pushbutton switches in the cockpit which activate or deactivate them. The RAT can be extended by activating the push button switch on the overhead panel.

The hydraulic system ensures the safe operation of the A320 through its multiple systems (blue, green, and yellow). The availability of 3 systems ensures that services that need to be operated by hydraulics are not lost if any one of the hydraulic systems fails. This is possible because each service component is connected to more than one hydraulic system.

Also, the yellow and blue hydraulic systems have backup devices which can drive their pumps while the PTU allows the green system to be pressurized by the yellow hydraulic system. This reduces the chance of failure of any single system.

The sensors within the systems enable important parameters of pressure and temperature within the systems so the flight crew can act accordingly to prevent failure.

References

• hydraulic. (2019). 46 A320 HYDRAULIC SYSTEM QUESTIONS. [online] Available at: http://hydraulicsawopitu.blogspot.com/2018/07/46-a320-hydraulic-system-questions.html
• Facebook.com. (2019). Airbus 320 Theory. [online] Available at: https://www.facebook.com/A320Theory/posts/ata-32-landing-gear-part-1a-main-landing-geareach-mlg-includes-these-components-/706699886136743/.
• Nata.aero. (2019). [online] Available at: https://www.nata.aero/agso/astgcache/d0f22090-866b-4adf-970c-5e94a2f2eb00.pdf
• Avia, C., Collier-Wright, M. and Nazir, C. (2019). Airbus A320 Specs – Modern Airliners. [online] Modern Airliners. Available at: http://www.modernairliners.com/airbus-a320-introduction/airbus-a320-specs/
• Hydraulic flight crew training manual. (n.d.). [ebook] pp.1-8. Available at: http://www.smartcockpit.com/docs/A320-Hydraulicl.pdf
• n.d. DC 10 Flight Crew Operating Manual Hydraulic Systems. [ebook] Available at: <http://www.smartcockpit.com/docs/McDonnell_Douglas_DC-10-40-Hydraulic_Systems.pdf>