Aircraft Brake Systems

Brake block Torque bar Frame Torque bar bolt Brake frame bolt Brake spider Adjuster 1. An independent system not part of the aircraft main hydraulic system; 2. A booster system that uses the aircraft hydraulic system intermittently when needed; and 3. A power brake system that only uses the aircraft main. hydraulic system(s) as a source of pressure. Systems on different aircraft vary, but the general operation is similar to those described. Independent Master Cylinders In general, small, light aircraft and aircraft without hydraulic systems use independent braking systems. An independent brake system is not connected in any way to the aircraft hydraulic system. Master cylinders are used to develop the necessary hydraulic pressure to operate the brakes. This is similar to the brake system of an automobile.

In most brake actuating systems, the pilot pushes on the tops of the rudder pedals to apply the brakes. A master cylinder for each brake is mechanically connected to the corresponding rudder pedal (i.e., right main brake to the right rudder pedal, left main brake to the left rudder pedal). [Figure 13-93] When the pedal is depressed, a piston inside a sealed fluid- filled chamber in the master cylinder forces hydraulic fluid through a line to the piston(s) in the brake assembly. The brake piston(s) push the brake linings against the brake rotor to create the friction that slows the wheel rotation. Pressure is increased throughout the entire brake systems and against the rotor as the pedal is pushed harder.

Many master cylinders have built-in reservoirs for the brake hydraulic fluid. Others have a single remote reservoir that services both of the aircraft’s two master cylinders. [Figure 13-94] A few light aircraft with nose wheel steering have only one master cylinder that actuates both main wheel brakes. This is possible because steering the aircraft during taxi does not require differential braking. Regardless of the set-up, it is the master cylinder that builds up the pressure required for braking. A master cylinder used with a remote reservoir is illustrated in Figure 13-95. This particular model is a Goodyear master cylinder. The cylinder is always filled with air-free, contaminant-free hydraulic fluid as is the reservoir and the line that connects the two together. When the top of the rudder pedal is depressed, the piston arm is mechanically moved forward into the master cylinder. It pushes the piston against the fluid, which is forced through the line to the brake.

When pedal pressure is released, the return springs in the brake assembly retract the brake pistons back into the brake housing. The hydraulic fluid behind the pistons is displaced and must return to the master cylinder. As it does, a return spring in the master cylinder move the piston, piston rod and rudder pedal back to the original position (brake off, pedal not depressed). The fluid behind the master cylinder piston flows back into the reservoir. The brake is ready to be applied again. Hydraulic fluid expands as temperature increases. Trapped 13-51 Torque bar Return spring Brake block Return spring shield Expander tube Torque bar bolt Tube shield Torque flange Frame Nozzle Frame Adjuster Adjuster bolt Brake frame bolt Rudder pedals Brake master cylinders (2) mechanical linkage.

fluid can cause a brake to drag against the rotor(s). Leaks has the capacity to accept the extra fluid volume. The typical may also result. When the brakes are not applied, fluid must reservoir is also vented to the atmosphere to provide positive be allowed to expand safely without causing these issues. A pressure on the fluid. compensating port is included in most master cylinders to facilitate this. In the master cylinder in Figure 13-95, this The forward side of the piston head contains a seal that closes port is opened when the piston is fully retracted. Fluid in the off the compensating port when the brakes are applied so that brake system is allowed to expand into the reservoir, which pressure can build. The seal is only effective in the forward 13-52 some independent braking systems.

Left brake assembly Reservoir Right brake assembly Master cylinders Parking brake valves (operated by a single control handle Brake pedals direction. When the piston is returning, or is fully retracted to the off position, fluid behind the piston is free to flow through piston head ports to replenish any fluid that may be lost downstream of the master cylinder. The aft end of the master cylinder contains a seal that prevents leakage at all times. A rubber boot fits over the piston rod and the aft end of the master cylinder to keep out dust. A parking brake for this remote reservoir master cylinder brake system is a ratcheting mechanical device between the master cylinder and the rudder pedals. With the brakes applied, the ratchet is engaged by pulling the parking brake handle. To release the brakes, the rudder pedals are depressed further allowing the ratchet to disengage. With the parking brake set, any expansion of hydraulic fluid due to temperature is relieved by a spring in the mechanical linkage.

A common requirement of all braking systems is for there to be no air mixed in with the hydraulic fluid. Since air is compressible and hydraulic fluid essentially is not, any air under pressure when the brakes are applied causes spongy brakes. The pedals do not feel firm when pushed down due to the air compressing. Brake systems must be bled to remove all air from the system. Instructions for bleeding the brakes are in the manufacturer’s maintenance information. Brake systems equipped with Goodyear master cylinders must be bled from the top down to ensure any air trapped behind the master cylinder piston is removed.

An alternative common arrangement of independent braking systems incorporates two master cylinders, each with its own integral fluid reservoir. Except for the reservoir location, the brake system is basically the same as just described. The master cylinders are mechanically linked to the rudder pedals as before. Depressing the top of a pedal causes the piston rod to push the piston into the cylinder forcing the fluid out to the brake assembly. The piston rod rides in a compensator sleeve and contains an O-ring that seals the rod to the piston when the rod is moved forward. This blocks the compensating ports. When released, a spring returns the piston to its original position which refills the reservoir as it returns. The rod end seal retracts away from the piston head allowing a free flow of fluid from the cylinder through the compensating ports in the piston to the reservoir. [Figure 13-96] The parking brake mechanism is a ratcheting type that operates as described. A servicing port is supplied at the top of the master cylinder reservoir. Typically, a vented plug is installed in the port to provide positive pressure on the fluid.

Boot Seal SealPiston Piston rod To brakeCompensating port Return spring Piston head ports Inlet from reservoir 13-53 Compensator sleeve Piston rod O-ring and washer Compensator port B A shown. Illustration A depicts the master cylinder when the brakes are off. The compensating port is open to allow fluid to expand into the reservoir should temperature increase. In B, the brakes are applied. The piston rod-end seal covers the compensating port as it contacts the piston head. Boosted Brakes In an independent braking system, the pressure applied to the brakes is only as great as the foot pressure applied to the top of the rudder pedal. Boosted brake actuating systems augment the force developed by the pilot with hydraulic system pressure when needed. The boost is only during heavy braking. It results in greater pressure applied to the brakes than the pilot alone can provide. Boosted brakes are used on medium and larger aircraft that do not require a full power brake actuating system.

A boosted brake master cylinder for each brake is mechanically Poppet Piston Toggle Piston rod to brake pedal Spool valve Compensator port Brakes on To system return manifold To system pressure manifold To brake wheel cylinder augments foot pedal pressure with aircraft system hydraulic pressure during heavy braking. attached to the rudder pedals. However, the boosted brake master cylinder operates differently. [Figure 13-97] When the brakes are applied, the pressure from the pilot’s foot through the mechanical linkage moves the master cylinder piston in the direction to force fluid to the brakes. The initial movement closes the compensator poppet used to provide thermal expansion relief when the brakes are not applied.

As the pilot pushes harder on the pedal, a spring-loaded toggle moves a spool valve in the cylinder. Aircraft hydraulic system pressure flows through the valve to the back side of the piston. Pressure is increased, as is the force developed to apply the brakes. When the pedal is released, the piston rod travels in the opposite direction, and the piston returns to the piston stop. The compensating poppet reopens. The toggle is withdrawn from the spool via linkages, and fluid pushes the spool back to expose the system return manifold port. System hydraulic fluid used to boost brake pressure returns through the port.

13-54 Accumulator air pressure Return Hydraulic pressure Emergency brake valve Hydraulic system pressure Pilot’s pedals Shuttle valve Normal anti-skid valves Fuse Autobrake pressure control Emergency IR cylinder Normal brake metering valve Alternate brake metering valve Alternate antiskid valve Air/oil transfer tube Brake accumulator Brake check valve Power brake control valves Antiskid control valves Shuttle valves Brake assemblies B A system is shown in B. Power Brakes on the top of the rudder pedal for braking as with the other Large and high-performance aircraft are equipped with actuating systems. The volume and pressure of hydraulic fluid power brakes to slow, stop, and hold the aircraft. Power required cannot be produced by a master cylinder. Instead, a brake actuating systems use the aircraft hydraulic system as power brake control valve or brake metering valve receives the source of power to apply the brakes. The pilot presses the brake pedal input either directly or through linkages.

13-55 Valve body Springseat Compensating chamber Return spring Brake metering valve slideBrake metering valve sleeve Autobrake piston Link assembly Brake input shaft Crank Crank stop To outboard brake To return Autobrake To inboard brake To return “A” system pressure “B” system pressure hydraulic system fluid to the brakes. The pressure developed is in proportion to the amount the rudder/brake pedal is depressed and the amount the slide is displaced. The slide/spool also simultaneously controls the return of fluid to the hydraulic system return manifold when brake pressure is released. The valve meters hydraulic fluid to the corresponding brake assembly in direct relation to the pressure applied to the pedal.

Many power brake system designs are in use. Most are similar to the simplified system illustrated in Figure 13-98A. Power brake systems are constructed to facilitate graduated brake pressure control, brake pedal feel, and the necessary redundancy required in case of hydraulic system failure. Large aircraft brake systems integrate anti-skid detection and correction devices. These are necessary because wheel skid is difficult to detect on the flight deck without sensors. However, a skid can be quickly controlled automatically through pressure control of the hydraulic fluid to the brakes. Hydraulic fuses are also commonly found in power brake systems. The hostile environment around the landing gear increases the potential for a line to break or sever, a fitting to fail, or other hydraulic system malfunctions to occur where hydraulic fluid is lost en route to the brake assemblies. A fuse stops any excessive flow of fluid when detected by closing to retain the remaining fluid in the hydraulic system.

Shuttle valves are used to direct flow from optional sources of fluid, such as in redundant systems or during the use of an emergency brake power source. An airliner power brake system is illustrated in Figure 13-98B. Brake Control Valve/Brake Metering Valve The key element in a power brake system is the brake control valve, sometimes called a brake metering valve. It responds to brake pedal input by directing aircraft system hydraulic fluid to the brakes. As pressure is increased on the brake pedal, more fluid is directed to the brake causing a higher pressure and greater braking action. A brake metering valve from a Boeing 737 is illustrated in in Figure 13-100. Two sources of hydraulic pressure provide redundancy in this brake system. A brake input shaft, connected to the rudder/brake pedal through mechanical linkages, provides the position input to the metering valve.

As in most brake control valves, the brake input shaft moves a tapered spool or slide in the valve so that it allows hydraulic system pressure to flow to the brakes. At the same time, the slide covers and uncovers access to the hydraulic system return port as required. 13-56

Pressure

comparator

Pressure

comparator Left gear Right gear M 1 2 3 1 3 2 1 23 M “A” system pressure Charging valve System “A” solenoid valve System “B” solenoid valve To system return Accumulator Transmitter Sealing disconnect fitting (type) System return System pressure Brake pressure Outboard wheel Inboard wheel Swivel (type) Gauge Main landing gear “UP” line Automatic wheel snubbing Brake valve feel augmentation actuator Brakes ON Parking brake shutoff valve “B” system pressure Ground interconnect valve Brake metering valve Antiskid valve Shuttle valve When the rudder/brake pedal is depressed, the slide in the metering valve moves to the left. [Figure 13-99] It covers the return port so pressure can build in the brake system. The hydraulic supply pressure chamber is connected to the brake system pressure chamber by the movement of the slide, which due to its taper, unblocks the passage between these two. As the pedal is depressed further, the valve slide moves farther to the left. This enables more fluid to flow to the brakes due to the narrowing shape of the slide. Brake pressure increases with the additional fluid. A passage in the slide directs brake pressure fluid into a compensating chamber at the end of the slide. This acts on the end of the slide creating a return force that counters the initial slide movement and gives feel to the brake pedal. As a result, the pressure and return ports are closed and pressure proportional to the foot pressure on the pedal is held on the brakes. When the pedal is released, a return spring and compensating chamber pressure drive the slide to the right into its original position (return port open, supply pressure chamber and brake pressure chambers blocked from each other).

The metering valve operates as described simultaneously for the inboard and the outboard brakes. [Figure 13-99] The design of the link assembly is such that a single side of the metering valve can operate even if the other fails. Most brake control valves and metering valves function in a similar manner, although many are single units that supply only one brake assembly. The auto brake, referenced in the metering valve diagram, is connected into the landing gear retraction hydraulic line. Pressurized fluid enters this port and drives the slide slightly to the left to apply the brakes automatically after takeoff. This stops the wheels from rotating when retracted into the wheel wells. Auto brake pressure is withheld from this port when the landing gear is fully stowed since the retraction system is depressurized.

13-57 The majority of the rudder/brake pedal feel is supplied by the brake control or brake metering valve in a power brake system. Many aircraft refine the feel of the pedal with an additional feel unit. The brake valve feel augmentation unit, in the above system, uses a series of internal springs and pistons of various sizes to create a force on the brake input shaft movement. This provides feel back through the mechanical linkages consistent with the amount of rudder/ brake pedal applied. The request for light braking with slight pedal depression results in a light feel to the pedal and a harder resistance feel when the pedals are pushed harder during heavy braking. [Figure 13-101] Emergency Brake Systems As can be seen in Figure 13-100, the brake metering valves not only receive hydraulic pressure from two separate hydraulic systems, they also feed two separate brake assemblies. Each main wheel assembly has two wheels. The inboard wheel brake and the outboard wheel brake, located in their respective wheel rims, are independent from each other. In case of hydraulic system failure or brake failure, each is independently supplied to adequately slow and stop the aircraft without the other. More complicated aircraft may involve another hydraulic system for back-up or use a similar alternation of sources and brake assemblies to maintain braking in case of hydraulic system or brake failure.

Note: In the segmented rotor brake section above, a brake assembly was described that had alternating pistons supplied by independent hydraulic sources. This is another method of redundancy particularly suitable on, but not limited to, single main wheel aircraft. In addition to supply system redundancy, the brake accumulator is also an emergency source of power for the brakes in many power brake systems. The accumulator is pre-charged with air or nitrogen on one side of its internal diaphragm. Enough hydraulic fluid is contained on the other side of the diaphragm to operate the brakes in case of an emergency. It is forced out of the accumulator into the brakes through the system lines under enough stored pressure to slow the aircraft. Typically, the accumulator is located upstream of the brake control/metering valve to capitalize on the control given by the valve. [Figure 13-102] Some simpler power brake systems may use an emergency source of brake power that is delivered directly to the brake assemblies and bypasses the remainder of the brake system completely. A shuttle valve immediately upstream of the brake units shifts to accept this source when pressure is lost from the primary supply sources. Compressed air or nitrogen is sometimes used. A pre-charged fluid source can also be used as an alternate hydraulic source.

Parking Brake The parking brake system function is a combined operation. The brakes are applied with the rudder pedals and a ratcheting system holds them in place when the parking Piston assemblyBleed port Spring Pressure regulator Slide assembly Port No. 1 brake pressurePort No. 3 system “A” pressure Spring-loaded check valveBrake metering valve shaft PistonRoller Crank arm Port No. 2 to return 13-58 FWD UP Brake accumulator Nitrogen charging valve Accumulator pre-charge pressure gauge Brake pressure gauge and accumulator air charging valve System B brake accumulator System A brake accumulator precharged with nitrogen to deliver brake fluid to the brakes in the event normal and alternate hydraulic sources fail.

brake lever on the flight deck is pulled. [Figure 13-103] At the same time, a shut-off valve is closed in the common return line from the brakes to the hydraulic system. This traps the fluid in the brakes holding the rotors stationary. Depressing the pedals further releases the pedal ratchet and opens the return line valve. Brake Deboosters Some aircraft brake assemblies that operate on aircraft hydraulic system pressure are not designed for such high pressure. They provide effective braking through a power brake system but require less than maximum hydraulic system pressure. To supply the lower pressure, a brake debooster cylinder is installed downstream of the control valve and anti-skid valve. [Figure 13-104] The debooster reduces some pressure from the control valve to within the working range of the brake assembly.

Brake deboosters are simple devices that use the application of force over different sized pistons to reduce pressure. [Figure 13-105] Their operation can be understood through the application of the following equation: Pressure = Force/Area Parking brake lever pedestal throttle quadrant. High-pressure hydraulic system input pressure acts on the small end of a piston. This develops a force proportional to the area of the piston head. The other end of the piston is larger and housed in a separate cylinder. The force from the smaller piston head is transferred to the larger area of the other end of the piston. The amount of pressure conveyed by the larger end of the piston is reduced due to the greater area over which the force is spread. The volume of output fluid increases since a larger piston and cylinder are used.

The reduced pressure is delivered to the brake assembly. The spring in the debooster aids in returning the piston to the ready position. If fluid is lost downstream of the deboost cylinder, the piston travels further down into the cylinder when the brakes are applied. The pin unseats the ball and allows fluid into the lower cylinder to replace what was lost. Once replenished, the piston rises up in the cylinder due to pressure build-up. The ball reseats as the piston travels above the pin and normal braking resumes. This function is not meant to permit leaks in the brake assemblies. Any leak discovered must be repaired by the technician.

A lockout debooster functions as a debooster and a hydraulic fuse. If fluid is not encountered as the piston moves down in the cylinder, the flow of fluid to the brakes is stopped. This prevents the loss of all system hydraulic fluid should a rupture downstream of the debooster occur. Lockout deboosters have a handle to reset the device after it closes as a fuse. If not reset, no braking action is possible. 13-59 Emergency brake valve Hydraulic system pressure Emergency IR cylinder Air/oil transfer tube Brake accumulator Brake debooster Brake check valve Power brake control valves Antiskid control valves Deboosters Shuttle valves Brake assemblies Air pressure Hydraulic pressure Deboost pressure Hydraulic return components of a power brake system.

Anti-Skid Large aircraft with power brakes require anti-skid systems. It is not possible to immediately ascertain in the flight deck when a wheel stops rotating and begins to skid, especially in aircraft with multiple-wheel main landing gear assemblies. A skid not corrected can quickly lead to a tire blowout, possible damage to the aircraft, and control of the aircraft may be lost. System Operation The anti-skid system not only detects wheel skid, it also detects when wheel skid is imminent. It automatically relieves pressure to the brake pistons of the wheel in question by momentarily connecting the pressurized brake fluid area to the hydraulic system return line. This allows the wheel to rotate and avoid a skid. Lower pressure is then maintained to the brake at a level that slows the wheel without causing it to skid.

Maximum braking efficiency exists when the wheels are decelerating at a maximum rate but are not skidding. If a wheel decelerates too fast, it is an indication that the brakes are about to lock and cause a skid. To ensure that this does not happen, each wheel is monitored for a deceleration rate faster than a preset rate. When excessive deceleration is detected, hydraulic pressure is reduced to the brake on that wheel. To operate the anti-skid system, flight deck switches must be placed in the ON position. [Figure 13-106] After the aircraft touches down, the pilot applies and holds full pressure to the rudder brake pedals. The anti-skid system then functions automatically until the speed of the aircraft has dropped to approximately 20 mph. The system returns to manual braking mode for slow taxi and ground maneuvering.

There are various designs of anti-skid systems. Most contain three main types of components: wheel speed sensors, anti- skid control valves, and a control unit. These units work together without human interference. Some anti-skid systems provide complete automatic braking. The pilot needs only to turn on the auto brake system, and the anti-skid components slow the aircraft without pedal input. [Figure 13-106] Ground safety switches are wired into the circuitry for anti-skid and auto brake systems. Wheel speed sensors are located on each wheel equipped with a brake assembly. Each brake also has its own anti-skid control valve. Typically, a single control box contains the anti-skid comparative circuitry for all of the brakes on the aircraft. [Figure 13-107] 13-60 Piston area 1 square inch Vent System pressure Deboost pressure Piston area 5 square inch To brake cylinders From power brake control valve 1,500 psi 300 psi Wheel Speed Sensors Wheel speed sensors are transducers. They may be alternating current (AC) or direct current (DC). The typical AC wheel speed sensor has a stator mounted in the wheel axle. A coil around it is connected to a controlled DC source so that when energized, the stator becomes an electromagnet. A rotor that turns inside the stator is connected to the rotating wheel hub assembly through a drive coupling so that it rotates at the speed of the wheel. Lobes on the rotor and stator cause the distance between the two components to constantly change during rotation. This alters the magnetic coupling or reluctance between the rotor and stator. As the electromagnetic field changes, a variable frequency AC is induced in the stator coil. The frequency is directly proportional to the speed of rotation of the wheel. The AC signal is fed to the control unit for processing. A DC wheel speed sensor is similar, except that a DC is produced the magnitude of which is directly proportional to wheel speed. [Figure 13-108] ANTISKID INBD OUTBD AUTO BRAKE ANTI SKID INOP ANTI SKID INOP ON ON OFF OFF MED MIN MAX OFF Control Units The control unit can be regarded as the brain of the anti- skid system. It receives signals from each of the wheel sensors. Comparative circuits are used to determine if any of the signals indicate a skid is imminent or occurring on a particular wheel. If so, a signal is sent to the control valve of the wheel to relieve hydraulic pressure to that brake which prevents or relieves the skid. The control unit may or may not have external test switches and status indicating lights.

It is common for it to be located in the avionics bay of the aircraft. [Figure 13-109] The Boeing anti-skid control valve block diagram in anti-skid control unit. Other aircraft may have different logic Wheel sensor Control valveControl unit a control valve (right) are components of an anti-skid system. A sensor is located on each wheel equipped with a brake assembly. An anti-skid control valve for each brake assembly is controlled from a single central control unit. 13-61 Hubcap fairing Spider Hubcap assembly Lockring Wheel Axle Transducer alignment rivet Electrical connector Transducer Transducer adapter Drive coupling with the wheel.

airliner. RESET VERIFY PRESS/ TEST BIT NORM BREAK TEST INBD A/B 1 2 3 4 OUTBD OFF ENABLE/ VERIFY 1 L R 2 3 4 to achieve similar end results. DC systems do not require an input converter since DC is received from the wheel sensors, and the control unit circuitry operates primarily with DC. Only the functions on one circuit card for one-wheel brake assembly are shown in Figure 13-110. Each wheel has its own identical circuitry card to facilitate simultaneous operation. All cards are housed in a single control unit that Boeing calls a control shield. The converter shown changes the AC frequency received from the wheel sensor into DC voltage that is proportional to wheel speed. The output is used in a velocity reference loop that contains deceleration and velocity reference circuits.

The converter also supplies input for the spoiler system and the locked wheel system, which is discussed at the end of this section. A velocity reference loop output voltage is produced, which represents the instantaneous velocity of the aircraft. This is compared to converter output in the velocity comparator. This comparison of voltages is essentially the comparison of the aircraft speed to wheel speed. The output from the velocity comparator is a positive or negative error voltage corresponding to whether the wheel speed is too fast or too slow for optimum braking efficiency for a given aircraft speed.

The error output voltage from the comparator feeds the pressure bias modulator circuit. This is a memory circuit that establishes a threshold where the pressure to the brakes provides optimum braking. The error voltage causes the modulator to either increase or decrease the pressure to the brakes in attempt to hold the modulator threshold. It produces a voltage output that is sent to the summing amplifier to do this. A lead output from the comparator anticipates when the tire is about to skid with a voltage that decreases the pressure to the brake. It sends this voltage to the summing amplifier as well. A transient control output from the comparator designed for rapid pressure dump when a sudden skid has occurred also 13-62 Transducer Converter Spoiler control Locked wheel comparator Locked wheel Pressure bias modulator Summing amplifier Valve driver Valve Deceleration reference Velocity reference Velocity comparator Lead Transient control Left inboard wheel card To right inboard lock wheel comparator sends voltage to the summing amp. As the name suggests, the each brake assembly. A torque motor uses the input from input voltages to the amplifier are summed, and a composite the valve driver to adjust the position of a flapper between voltage is sent to the valve driver. The driver prepares the two nozzles. By moving the flapper closer to one nozzle or current required to be sent to the control valve to adjust the the other, pressures are developed in the second stage of the position of the valve. Brake pressure increases, decreases, valve. These pressures act on a spool that is positioned to or holds steady depending on this value. build or reduce pressure to the brake by opening and blocking fluid ports. [Figure 13-111] Anti-Skid Control Valves As pressure is adjusted to the brakes, deceleration slows to Anti-skid control valves are fast-acting, electrically within the range that provides the most effective braking controlled hydraulic valves that respond to the input from without skidding. The wheel sensor signal adjusts to the the anti-skid control unit. There is one control valve for 13-63