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System rundown: Hydraulics 101

Big forces from a small system

By Neil Singer

As aircraft get bigger, more and heavier parts need to be moved. Heavy landing gear raised against the force of gravity, flap panels that extend into the resistance of 200-knot slipstreams, or brake systems that must generate tremendous force to resist the rotation of tires—these all need some serious power behind them.

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The Cessna Citation Mustang has one of the simplest hydraulic systems. A single electrical pump produces pressure only when needed, and serves only two systems—the landing gear and the brakes.

Illustration by Kevin Hand

Traditionally these jobs have been tasked to hydraulic systems. As airplanes become heavier, it’s a general rule that more systems become “clients” of the hydraulic system. So although Cessna Citation Mustangs and early Embraer Phenom 100s only need hydraulic pressure for the landing gear and brake systems (getting by with electric motor actuation of other components, such as flaps), the heavier CJ series also uses hydraulic force for flap and speed brake movement, and the Phenom 300 puts multifunction spoilers, stick pusher, and rudder boost on the list.

All hydraulic systems operate on the same general principle: utilizing incompressible hydraulic fluid as a medium of transmitting force from a pump to the piece of equipment that needs to be moved. Some systems store the hydraulic fluid under high pressure until needed, while others only pressurize the fluid when a client is requesting it. How much pressure? Quite a bit, thank you—commonly in the neighborhood of 3,000 pounds per square inch (psi), or about the ambient pressure of the ocean a mile and a quarter below the surface.

Smaller aircraft with fewer demands on the hydraulic system typically pressurize the fluid through an electrically driven pump. Depending on the system, the pump may either cycle on and off to maintain pressure in a desired range, or only turn on when one of the clients demands pressure. In either case, it’s common to have an accumulator of some sort on the pressurized side of the system to assist the pump. A hydraulic accumulator can be thought of as a hydraulic “battery” that can be gradually topped off by the pump so it can quickly discharge when the pump can’t keep up with the pressure demand.

Accumulators typically consist of a heavy-duty canister with a sliding divider. On one side of the divider sits a volume of trapped gas, often nitrogen. As hydraulic pressure is applied to the other side of the divider, the gas becomes more and more compressed, and tries to push back against the divider with thousands of pounds of force. Should the pressure in the hydraulic system drop below the pressure inside the accumulator, the accumulator will push against the fluid, effectively boosting the hydraulic system pressure temporarily.

Larger airplanes with more and heavier demands on the hydraulic system need more pressure than an electric pump can generate. These aircraft utilize engine-driven hydraulic pumps instead. Directly connected to the high-pressure spool of the engine, these pumps turn any time the engine is turning. Again, differences in system design exist, with some manufacturers choosing to have high pressure always present in the system, and other designs feature fluid circulating under low pressure until a client is used. The 525 series of Citation jets mixes the two types of pumps, with the majority of hydraulic clients receiving power from two engine-driven pumps, while the brakes are served by a completely separate hydraulic system with an electric pump for pressurization.

However the pressure is generated, with the exception of fluid sent to the brake system, it is destined to end up in a hydraulic actuator—a means of converting pressure into movement. High-pressure fluid enters the actuator and pushes against a sliding component that is ultimately connected to the piece of the aircraft to be moved.

Actuators are designed to be two-way streets: Apply pressure to one side, for example, to raise the flaps—or against the other side to lower them. Valves controlled by the client system are moved, typically by electrical power, so as to route pressure to the appropriate side of the actuator.

During normal operations, pilot involvement with the hydraulic system is minimal to nonexistent. It’s common for a pilot to have no direct control over the hydraulic system at all, other than the ability to pull a circuit breaker to disable an electric pump. Abnormal operations are most often a result of the system not producing enough pressure, either because of a pump failure, in the case of a single pump system, or a fluid leak, in the case of a multiple-pump system. As the most critical hydraulic clients—such as landing gear and brakes—feature a backup means of actuation, a hydraulic failure is considered an abnormal event, rather than a full-blown emergency.

However, this doesn’t mean it’s not to be taken seriously. Braking systems, in particular, are known to be touchy and subject to overcontrol on the part of the pilot, easily leading to blown tires. That’s because they’re manually operated and don’t often provide antiskid or locked-wheel protections.

What’s worse is that a hydraulic failure will often involve systems that compound the braking difficulty: a loss of speedbrakes and ground spoilers, for example, to help create drag and maintain friction between the tires and the runway. Or if the braking system isn’t compromised, the flaps may be, and as backup flap actuation isn’t a feature of typical light jets a no-flap landing with higher approach speeds will be required. For these reasons, hydraulic failures usually necessitate a look for a much longer runway than would normally be acceptable. AOPA

Neil Singer is a Master CFI with more than 9,500 hours.

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