7: Hydraulic Systems

Hydraulic System Operation

Hydraulic Accumulators

Hydraulic Actuators and Valves

Flight Controls

Roll Control

Tabs

Spoilers

T-Tail

Vortex Generators

High-Lift Devices

Landing Gear and Tires

Ground Safety Switch

Brake System

Antiskid System

Alternate Brake Systems

Hydraulic System Operation

Aircraft hydraulic systems can be used to operate the flight controls, high lift devices, landing gear, brakes and nose wheel steering. See Figure 7-1. The typical hydraulic system has a reservoir, lines, pumps (both electrical and mechanical), actuators and cooling systems. Most aircraft have two or more independent hydraulic systems. This is more efficient and provides redundancy for critical flight controls and other components.

Figure 7-1. Typical hydraulic systems schematic

It is critical that the correct type of hydraulic fluid be used in an aircraft system. The fluids most commonly used in jet aircraft systems are Skydrol 7000, 500, 500A or 500B. There are also the mineral-based oils MIL-H-5606, and Braco 882. These hydraulic oils are color-coded to prevent confusion. MIL-H-5606 and Braco 882 are red, Skydrol 7000 is light green, Skydrol 500 is blue and Skydrol 500A and 500B are purple. None of these fluids are compatible with the others and great care must be taken to avoid mixing them. MIL-H-5606 is used mainly in small aircraft. One of its main limitations is that it is highly flammable.

Most large aircraft hydraulic systems use Skydrol. It is fire resistant and has a wide temperature operating range. It has the disadvantage of being highly corrosive and can soften or dissolve many types of paints, lacquers, enamels and wire insulation. It also can cause painful skin or eye irritation. If it is spilled on the skin, wash completely with soap and water. If Skydrol contacts the eye, flush with water and consult a doctor. Hydraulic fluid can be contaminated with water very easily. It should be stored in a tightly sealed container to prevent such contamination from the moisture in the air.

The hydraulic reservoirs on most jet aircraft are pressurized by engine bleed air. This helps to prevent foaming of the fluid at high altitudes.

The tolerances in hydraulic systems are so small that even very small contaminates can cause damage to seals and cylinder walls. For this reason almost all systems have one or more filters. Some of these filters have a pop-out indicator to show that contaminates may be present in the fluid. Many filters also have a bypass valve that allows fluid to flow to the system if an element of the filter becomes clogged.

Hydraulic pumps can be driven from one of several power sources. The two most common types are engine-driven and electrically-driven pumps. Most aircraft have both types. Regardless of the power source, there are two types of pumps—constant displacement and variable displacement. The constant displacement pump moves a set volume of fluid with each rotation, regardless of system demand. A system using this type of pump must have some sort of pressure relief valve or system bypass valve to regulate pressure when the pump moves more fluid than the system requires. A variable displacement pump only delivers the amount of fluid that the system will accept at that moment. There is no need for relief valves since the pump itself maintains pressure within the desired range.

It is very important that all visible hydraulic lines and fittings be inspected for leaks or signs of damage. If there is any leak from a stationary connection, notify maintenance to repair it before flight. Some parts of the aircraft move enough that flexible hose must be used for hydraulic line. This hose should have support about every 24 inches or less along its length. The hose should be 5% to 8% longer than the distance between fittings. This slack protects the hose from damage during movement of the airframe. Most hoses have a lay line painted or woven on. Any spiral in this lay line suggests that the hose has been incorrectly installed with a twist in it. While conducting the inspection, check that all self-locking nuts have at least one full thread protruding beyond the nut. Also check for any signs of corrosion. Corrosion beneath an aluminum clad surface is indicated by small, dark gray lumps.

If there is a break in a hydraulic line, fluid can be exhausted from the system very quickly. One protection against such a serious leak is the hydraulic fuse. There are two types of hydraulic fuses; one works on the principle of a pressure differential, and the other works on the quantity of flow. With the pressure-differential-type fuse, a leak in the line will drop the pressure downstream of the fuse and the normal system pressure will close the fuse and shut off all further flow. With the quantity-type fuse (as is used with some lockout deboosters), if a line breaks and a specific quantity of fluid flows through the fuse, it will shut off the line and prevent further loss.

Hydraulic Accumulators

Many hydraulic systems have accumulators that protect the system against pressure surges and provide a means of storing hydraulic pressure. When the hydraulic pumps are turned off, the air pressure from the accumulator will maintain pressure in the hydraulic system as long as there are no system leaks. This is commonly done on the brake system where the accumulator is used to maintain pressure for the parking brake after the aircraft is shut down. The most common type is the piston accumulator. See Figure 7-2. This is a cylinder with a free moving piston inside. On one side of the piston is a pre-charge of air (usually around 1,000 PSI). The cylinder on the other side of the piston is connected to the hydraulic system. As the pressure of the hydraulic system rises above the pre-charge pressure, the piston is driven up the cylinder causing the pressure of the trapped air to equal that on the hydraulic fluid side. Nitrogen is often the gas used on the "air" side of the accumulator because its pressure won't vary as much with changes in temperature. See Figure 7-3.

Figure 7-2. Hydraulic accumulator (hydraulic pressure off)

Figure 7-3. Hydraulic accumulator (hydraulic pressure on)

The piston is free to "float" in the cylinder, so any momentary surge in hydraulic pressure causes the air in the cylinder to be compressed. This shock absorber effect prevents damage to the lines and fittings in the hydraulic system.

Another type of accumulator is spherical in shape and uses a bladder-type diaphragm to separate the hydraulic fluid and the air pre-charge. Most modern aircraft use the piston-type accumulator because it takes up less area than the spherical type.

The hydraulic system pressure gauge reading is often taken from the air side of the accumulator. This means that when the hydraulic system is unpressurized, the pressure gauge will still indicate the air pre-charge pressure. When the hydraulic system is pressurized, the gauge pressure will equal system pressure.

If the air pre-charge pressure is lost, the hydraulic system pressure will read zero though system pressure may be normal. See Figure 7-4. Any zero pressure reading indicates a loss of pre-charge pressure and says nothing about the current system pressure. This is true of both piston- and diaphragm (spherical)-type accumulators.

Figure 7-4. Hydraulic accumulator (precharge lost)

Hydraulic Actuators and Valves

Actuators convert hydraulic pressure into mechanical work. The most common kind of actuators are linear type. These consist of a cylinder and a piston. There are three kinds of linear actuators: single acting; double acting, unbalanced; and double acting, balanced.

The single-acting type has a piston that is moved in one direction only by hydraulic pressure. When the pressure is removed, another force, usually a spring, returns the piston to its original position. Brakes use this type of actuator.

The double-acting, unbalanced actuator uses hydraulic pressure to move its piston in both directions, but because of its design, much greater pressure is applied going in one direction than in the other. Landing gear systems use this type of actuator because much more force is required to raise the landing gear than to lower it.

The double-acting, balanced actuator applies the same amount of force moving in either direction. Automatic pilot servos use this type of actuator.

Some aircraft systems are designed so that hydraulic actuators must operate in the correct sequence. For example, when raising the landing gear the gear doors must open, then the gear must raise and retract into the wheel well and finally the doors must close again over the top of the retracted landing gear. This type of operation is accomplished with sequence or priority valves. These two types of valves accomplish the same action in slightly different ways: a sequence valve is opened by mechanical contact; a priority valve opens by hydraulic pressure.

Flight Controls

High airloads make it is very difficult to move the flight control surfaces of jet aircraft with just mechanical and aerodynamic forces. Flight controls are usually moved by hydraulic actuators.

Flight controls are divided into primary, secondary, and auxiliary flight controls. See Figure 7-5. The primary flight controls are those that maneuver the aircraft in roll, pitch and yaw, which includes the ailerons, elevator and rudder. Secondary flight controls include all types of tabs. Auxiliary flight controls include leading- and trailing-edge flaps, speed brakes, spoilers and slats.

Figure 7-5. Typical flight controls

Roll Control

Jet aircraft roll control is accomplished by ailerons and flight spoilers. The exact mix of controls is determined by the aircraft's flight regime. In low-speed flight, all control surfaces operate to provide the desired roll control. As the aircraft moves into higher-speed operations, control surface movement is reduced to provide approximately the same roll response to a given input through a wide range of speeds.

Many aircraft have two sets of ailerons: inboard and outboard. The inboard ailerons operate in all flight regimes. The outboard ailerons work only when the wing flaps are extended and are automatically locked out when flaps are retracted. This allows good roll response in low-speed flight with the flaps extended and prevents excessive roll and wing bending at high speed when the flaps are retracted.

Ailerons move in opposite directions when the control wheel is moved. When the control wheel is rotated to the left (left wing down), the aileron on the left wing rises and the aileron on the right wing lowers. When the control wheel moves to the right, the movement of the ailerons reverses.

Tabs

Flight control surfaces are sometimes equipped with servo tabs. These tabs are on the trailing-edge of the control surface and are mechanically linked to move opposite the direction of the surface. If the surface moves up, the tab moves down. This servo movement reduces the aerodynamic pressures on the control surface and so assists movement. A variation of this is the balance panel that does the same thing, but uses pressure changes created by the control surface rather than mechanical linkage to move the panel.

One method of modifying the downward tail load through changes in airspeed and configuration is by using trim tabs. Trim tabs are moved by a separate trim control from the cockpit. Movement of the trim tab, like the servo tab, is opposite that of the primary control surface. Adjustable trim tabs do not move relative to the control surface when the primary surface is moved.

Anti-servo tabs move in the same direction as the primary control surface. This means that as the control surface deflects, the aerodynamic load is increased by movement of the anti-servo tab. This helps to prevent the control surface from moving to a full deflection. It also makes a hydraulically-boosted flight control more aerodynamically effective than it would otherwise be.

Some jet aircraft have control tabs for use in the event of loss of all hydraulic pressure. Like a servo tab, movement of the control wheel or rudder pedals moves the tab which causes the aerodynamic movement of the control surface. The control tab is only used during manual reversion; that is, with the loss of hydraulic pressure. When it is under hydraulic pressure, the tab is locked to the control surface.

Some older aircraft have aileron balance panels situated forward of the aileron in the wing. When the aileron deflects, airloads on the balance panel assist in moving the aileron.

Spoilers

Spoilers increase drag and reduce lift on the wing. Raised on only one wing, they aid roll control by causing that wing to drop. If the spoilers raise symmetrically in flight, the aircraft can either be slowed in level flight or can descend rapidly without an increase in airspeed. When the spoilers rise on the ground at high speeds, they destroy the wing's lift which puts more of the aircraft's weight on its wheels, which in turn makes the brakes more effective.

Often aircraft have both flight and ground spoilers. The flight spoilers are available both in flight and on the ground. However, the ground spoilers can only be raised when the weight of the aircraft is on the landing gear. In flight, a ground-sensing squat switch prevents deployment of the ground spoilers.

T-Tail

On some aircraft, the horizontal stabilizer is at the top of the vertical stabilizer. The advantage of this configuration is that the horizontal tail is above the turbulent air flow created by the wing. The main disadvantage to this is the heavier structure required to support the horizontal tail. This adds weight in the tail of the airplane and may require a thicker chord in the vertical stabilizer than is aerodynamically ideal.

Vortex Generators

Vortex generators are small (about an inch high) aerodynamic surfaces located in different places on different airplanes. They prevent undesirable airflow separation from the surface by mixing the boundary airflow with the high energy airflow just above the surface.

When located on the upper surface of a wing, they prevent shock-induced separation from the wing as the aircraft approaches its critical Mach number. This increases aileron effectiveness at high speeds. Vortex generators also can increase rudder or elevator effectiveness at low speed by preventing the flow separation associated with high angles of attack.

A disadvantage of vortex generators is that they slightly increase drag. This is particularly true at low speeds because of the low aspect ratio of the generators.

High-Lift Devices

Swept wing jet aircraft are equipped with a number of high-lift devices. These include leading-edge flaps, slots or slats and trailing-edge flaps. The purpose of all high lift devices is to increase lift at low airspeeds and to delay stall until at a higher angle of attack.

The two most common types of leading-edge devices are slats and Krueger flaps. The Krueger flap extends from the leading edge of the wing, increasing its camber. The slat also extends from the wing's leading edge, but it creates a gap or slot. This slot allows high energy air from under the wing to flow over the top of the wing which delays stall until a higher angle of attack than would normally occur. It is common to find Krueger flaps and slats on the same wing.

Landing Gear and Tires

The typical aircraft will have two landing gear indicator lights for each landing gear. A green light indicates that the landing gear is down and locked. A red light indicates either that the gear is in an unsafe condition or that it is not in the position selected by the gear handle. If the landing gear handle has been moved to the "up" position and the gear is up and locked, no light will be illuminated. See Figure 7-6.

All tires should be carefully preflighted because the high stresses imposed during takeoff and landing can cause failure at high speeds. Part of that inspection should be for proper tire inflation. An under-inflated tire shows more tread wear on the shoulders than in the center. An over-inflated tire shows accelerated centerline wear while leaving rubber on the shoulders.

Aircraft with aft-mounted engines have chined nosewheel tires to deflect water or slush away from the engine intakes. Chines are ridges molded into the sidewall of the tire that deflect the spray from the tire. Tires on single-nosewheel aircraft have chines on both sides of the tire. In a dual-nosewheel installation, chines are required only on the outside of the tires.

Tires are protected from high temperatures by fusible melt plugs. When exposed to excessively high temperatures, these plugs will melt, blow out and allow the tire to deflate rather than explode. Maintenance should be called if any fusible plug on a tire shows signs of the core melting, since all the plugs have been exposed to high temperatures and the tire should be replaced.

Figure 7-6. Typical landing gear indicators

Ground Safety Switch

There are many aircraft systems that are designed to be operated only in flight. There are other systems that are limited solely to ground use. The ground safety switch is the device that either arms or deactivates, as appropriate, these systems when the aircraft is on the ground. The switch is operated by compression of one of the main landing gear struts.

Brake System

The main wheel brakes on most large aircraft are powered by one of the hydraulic systems. However, the hydraulic system pressure is often too high for use on the brakes, so a deboost valve is installed in the brake lines. This valve reduces the system pressure to a lower pressure more suitable for the brakes. Also it increases the volume of fluid going to the brakes.

The deboost valve often includes a lockout feature that isolates the brake system from the main hydraulic system. If a brake line is damaged, the lockout will prevent the loss of hydraulic fluid from the entire system. The lockouts usually include a feature that allows replenishing the brake side of the lockout with fluid from the main system when the aircraft is on the ground.

Antiskid System

Most large airplanes are equipped with an antiskid system to aid in effective braking, especially in wet or slippery conditions. It is an electro-hydraulic system that uses skid detectors to control the rate of wheel deceleration. When the brakes are applied, the antiskid monitors each wheel. If one or more slows too quickly (indicating a skid), the system releases pressure to that brake until its rate of deceleration is normal. This allows maximum braking without inducing a locked-wheel skid. Usually the system is automatically deactivated below about 20 MPH so that the aircraft can be brought to a full stop.

The antiskid system is armed with a switch in the cockpit and is active anytime the aircraft is on the ground and rolling at more than about 20 MPH. The system is also deactivated in the air with the landing gear down, to prevent touchdown with the brakes locked. This feature prevents any brake application in the air.

Alternate Brake Systems

The brakes are usually protected by double or triple redundant systems. If the primary hydraulic system fails, a second and even a third can be selected by the flight engineer. If all hydraulic power is lost, the brake accumulator provides limited braking capability. Many aircraft are also equipped with a pneumatic brake system. This works by directing compressed air or nitrogen into a separate brake system and allows pressure to be applied equally to all the brakes.

When pneumatic brakes are used, there is no differential braking and no effective antiskid since these features of the normal brake system are bypassed. Pressure to the brakes can be regulated through a pneumatic brake control.

Moisture in the emergency pneumatic brake system may cause corrosion.