Figure 1. Aircraft landing gear can easily top in at more than one ton, requiring the extensive use of hydraulics.Figure 1. Aircraft landing gear can easily top in at more than one ton, requiring the extensive use of hydraulics.In a bygone era of aviation, many aircraft systems relied on the physical strength of the pilot. This limited system weights to a couple hundred kilos, as pilots simultaneously pulled levers or knobs while also flying the plane.

The innovation and integration of hydraulics gradually created bigger systems and bigger aircraft. Landing gear alone can weigh more than a ton on many airplanes.

Common design and operation

The aircraft’s hydraulic system design varies depending on the aircraft type. However, they all share similarities based on basic design principles.

The hydraulic fluid stored inside a reservoir is carefully developed to ensure the proper viscosity. Viscosity is the characteristic of any fluid to resist flow. Hydraulic fluids are designed to be viscous to ensure that it is not easily compressible when pressure is applied by actuators.

Figure 2. Diagram of aircraft hydraulic system. Source: Irene Christian Silvia Manu/CC BY-SA 4.0Figure 2. Diagram of aircraft hydraulic system. Source: Irene Christian Silvia Manu/CC BY-SA 4.0Actuators are installed to actuate movement on either the flight controls, cargo doors or landing gear. Inside the actuators are pistons that displace when a pressurized fluid enters the cylinder.

Piston displacement inside the actuators is made possible by the electrical pump providing a pressurized supply of hydraulic fluid. Hydraulic fluid pressure varies from 3,000 psi to 5,000 psi. The constant speed motor generator (CSMG), which is connected to the accessory gearbox (AGB) of each engine, produces electricity that supplies the hydraulic system electric pump.

Within the organized chaos of valves, tubes, fittings and sensors is a component called the hydraulic system’s “crossfeed valve.” What it basically does is that it allows flow of pressurized hydraulic fluid from one subsystem to another. This usually occurs whenever there is low output pressure from the electrical pump.

The crossfeed valve is controlled by the flight crew through a pushbutton in the cockpit overhead panel as shown in the image above.

Significant difference

The components and operations mentioned above are very similar on many different types of aircraft however one significant difference the system design has is the principle of “hydraulic fluid crossfeed.”

Aircraft hydraulic systems are typically composed of two to three subsystems. For the European made Airbus aircraft, the subsystems are called the Green, Blue and Yellow Subsystems. On the American made Boeing aircraft, the subsystems are called System A, System B and the Standby System.

These subsystems support each other through the employ of sensors that detect if a subsystem is below its acceptable pressure and most importantly the transfer of pressure by passing through the crossfeed valve from the “healthy” subsystem to the “faulty” subsystem. This principle of subsystem-to-subsystem support is vital. Without this principle, the failure of one subsystem may cause permanent loss of the movement of the flight controls, which could be fatal especially when the aircraft is maneuvering in flight. It could also result in the permanent loss of thrust reverser door actuator or landing gear brakes, which can cause runway overshoot or worse, passenger casualty.

The application

Imagine yourself flying from JFK airport in New York City to Heathrow in London aboard an Airbus A320. After take-off, the Green Subsystem that controls the landing gear extension and retraction suffered a failure due to a probable electric pump failure. Due to this failure, the flight crew cannot initially retract the landing gear, which is a requirement for the aircraft to fly faster and at a higher altitude.

This is where the design of the aircraft for hydraulic fluid transfer becomes handy. Once the sensors installed in the Green Subsystem detected a decreased pressure of 1,500 psi from the normal 3,000 psi, the Yellow Subsystem would transfer pressure to the Green Subsystem to enable the landing gear retraction.

Only pressure will be transferred. Unlike other hydraulic system designs wherein pressurized fluid will be transferred through a crossfeed valve, the Airbus design employs a component called the power transfer unit (PTU) to transfer pressure from the healthy subsystem to the faulty subsystem.

Figure 3. Hydraulic fluid draining from a landing gear strut. Source: Airman 1st Class Justin Veazie/U.S. Air ForceFigure 3. Hydraulic fluid draining from a landing gear strut. Source: Airman 1st Class Justin Veazie/U.S. Air ForceThe advantage of this design ensures that it provides a second line of defense especially while the aircraft is operating on such sensitive phases of flight that a hydraulic failure can prove catastrophic. It also allows the preservation of the healthy subsystem when fluid leak is present and the hazard of hydraulic fluid starvation of two subsystems due to crossfeed seems likely.

Pressure only versus pressurized hydraulic fluid

There are two ways the hydraulic system’s crossfeed design operates. The first is transfer through pressurized hydraulic fluid. In this design, the crossfeed valve, which is situated between the subsystems, is kept close as long as there is sufficient pressure in each subsystem. However, once the pressure sensors are located downstream and the electric pumps detect a decrease in output pressure, the crossfeed valve, depending on the system logic design, can open automatically or manually and allow fluid that is pressurized at 3,000 psi to flow toward the side of the subsystem that has significantly less pressure.

Within the design of that type of system are check valves, or one way valves, that prevent pressurized fluid from flowing back to the outlet of the electric pump and causing damage to it. With this, it is ensured that the now sufficient pressurized fluid supplied by the opposite subsystem will only be delivered to the different aircraft systems that it is needed for.

The downside of this design is that many times when there is a hydraulic fluid leak on a specific subsystem that would surely result in low pressure since no fluid will be pumped, the opening of the crossfeed valve will cause the healthy subsystem to lose its stored fluid as it would flow through the crossfeed valve and to the component where the leak is present. If there is a positive takeaway from this design, it is that since it uses fewer components, the cost of implementation in the manufacturing and operation for the airline is lower compared to the next design that will be discussed.

The second design employs the principle of transfer through pressure only design. This means that when a specific subsystem reaches the minimum allowable pressure, instead of transferring pressurized hydraulic fluid by passing through a crossfeed valve, this design employs a PTU.

The idea behind this component is very similar to the transmission in land vehicles. Whenever a gear is engaged, the clutch released and the gas pedal pressed, the power produced by the engine is transmitted through the clutch and drives the drivetrain, thus producing motion for the vehicle's wheels. In the context of aircraft design, the PTU receives pressure from the healthy subsystem, which drives the motor on the PTU. Afterward, the motor powers the pump located on the side of the subsystem with low pressure, which will then start to produce an increasing output pressure.

This design is ideal as it removes the possibility of two subsystems consecutively being drained due to fluid leak. This is ideally fitted on larger aircraft whose purpose is to fly intercontinental routes. The weight and cost penalty does not prove to be a concern as the increase in aircraft reliability and flight safety is extraordinary.

Learning from the past

Surely, this almost perfect design did not come into application overnight. Many lives that were lost in accidents from hydraulic system failure were the seeds and motivation of all the engineers, technicians and pilots that worked tirelessly to ensure a safer and more reliable aircraft hydraulic system.

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