Aircraft Brake Linings: A Comprehensive Guide

Understanding Aircraft Brake Linings and Their Maintenance

The braking system is a critical component of any aircraft, ensuring safe taxiing, landing, and ground operations. The brake linings, often referred to as brake pads, are the friction material that, when pressed against the brake discs, slows and stops the aircraft. Over time and with use, these linings wear down and require replacement. Proper maintenance, inspection, and replacement of brake linings are paramount for flight safety. This comprehensive guide will delve into the various aspects of aircraft brake lining maintenance, from initial inspection to complete servicing.

Inspecting Brake Lining Wear

Regularly checking the wear on brake linings is essential. Different brake assemblies employ various methods to indicate when replacement is necessary. Understanding these indicators is key to proactive maintenance.

Wear Indicator Pins

Many brake assemblies are equipped with a built-in wear indicator pin. As the brake linings wear, the exposed length of this pin typically decreases. A minimum specified length indicates that the linings have reached their end of service life and must be replaced. It is crucial to note that the measurement of these pins can vary between different brake manufacturers and models. For instance, Goodyear brake assemblies measure the pin where it protrudes through the nut of the automatic adjuster on the back of the piston cylinder. In contrast, the Boeing brake assembly illustrated in Figure 2 measures the pin's length from the back of the pressure plate when the brakes are applied (dimension L). Always consult the manufacturer’s maintenance information to ensure you are correctly interpreting the wear pin indicators for your specific aircraft.

Alternative Wear Measurement Methods

Not all brake assemblies utilise a wear pin. Some systems rely on measuring the distance between the brake disc and a portion of the brake housing when the brakes are applied. As the linings wear, this distance increases. The manufacturer will specify a particular distance at which the linings should be changed. For example, Cleveland brakes allow for direct measurement of lining wear, as a portion of the lining is typically exposed. The diameter of a #40 twist drill is often used as a gauge, approximating the minimum allowed lining thickness. Multiple disc brakes typically have their lining wear checked by applying the brakes and measuring the distance between the back of the pressure plate and the brake housing. Regardless of the specific method, consistent monitoring and measurement of brake wear are vital to ensure that linings are replaced before they become unserviceable. Linings worn beyond their specified limits often necessitate the removal of the entire brake assembly for replacement.

Brake System Bleeding

The presence of air in the brake system fluid can lead to a spongy brake pedal feel, compromising braking effectiveness. Bleeding the system removes this trapped air, restoring a firm pedal feel. Brake systems must be bled according to the manufacturer’s instructions, as the method employed is specific to the type of brake system.

Bleeding Methods

There are two primary methods for bleeding aircraft brakes: top-down gravity bleeding and bottom-up pressure bleeding. Brakes should be bled whenever the pedals feel spongy or if the brake system has been opened for maintenance.

Master Cylinder Brake Systems

Brake systems incorporating master cylinders can be bled using either gravity or pressure methods. For pressure bleeding from the bottom up, a pressure pot, a portable tank containing brake fluid under pressure, is used. This unit forces pure, air-free fluid from near the bottom of the tank into the brake system. The bleed port on the brake assembly is connected to the pressure pot via a hose with a shut-off valve. Pure, pressurized fluid can also be supplied by other means, such as a hand-pump type unit. A typical pressure bleed involves attaching the pressure tank hose to the bleed port. A clear hose is attached to the vent port of the aircraft brake fluid reservoir or master cylinder, with its end submerged in clean brake fluid in a collection container. Opening the brake assembly bleed port and then the pressure tank valve allows clean fluid to enter the system, expelling air-fluid mixture through the vent port hose. The clear hose is monitored for air bubbles; once they cease, the bleed port and pressure tank valve are closed, and the hoses removed. The fluid level in the reservoir should be checked and adjusted as necessary. It is paramount to use the correct brake fluid for the system.

Gravity bleeding from the top down is also an option for master cylinder systems. This method is similar to that used in automotive systems. Additional fluid is supplied to the brake reservoir to prevent it from running dry during the process, which would introduce more air. A clear hose is connected to the brake assembly bleed port and submerged in a container of clean fluid. With the brake pedal depressed, the bleed port is opened, allowing the master cylinder piston to force air-fluid mixture out through the hose. While the pedal is still depressed, the bleed port is closed. Pumping the brake pedal replenishes fluid from the reservoir. The process is repeated – pedal down, bleed port open – until no more air is expelled. Finally, the bleed port fitting is tightened, and the reservoir fluid level is confirmed.

Power Brake Systems

Top-down brake bleeding is the standard method for power brake systems, as these systems receive fluid from the aircraft’s main hydraulic system. Bottom-up pressure bleeding is not suitable because any trapped air would be forced into the main hydraulic system, which is unacceptable. Many aircraft with power brakes can be bled using an auxiliary hydraulic mule to pressurize the system. Regardless, the aircraft system must be pressurized for bleeding. A clear hose is attached to the brake bleed port, with its end in a container of clean hydraulic fluid. With the bleeder valve open, the brake is carefully applied, allowing aircraft hydraulic fluid to enter the system and expel the air-contaminated fluid. Once no air bubbles are visible, the bleeder valve is closed, and the hydraulic system is returned to normal operation. Due to the variations in power brake systems, always consult the manufacturer’s maintenance information for the correct bleeding procedure. It is also essential to bleed auxiliary and emergency brake systems concurrently with the normal system.

Important Bleeding Considerations

• Ensure reservoirs and bleed tanks remain full throughout the bleeding process.
• Use only clean, specified hydraulic fluid.
• After bleeding, always check the brakes for proper operation, any leaks, and ensure the correct fluid level.

Brake Fluid: Quantity and Type

Using the correct hydraulic fluid is absolutely critical. Brake system seals are designed for specific fluids, and exposure to incompatible fluids can cause deterioration and failure. Mineral-based fluids (e.g., MIL-H-5606, red oil) must never be mixed with phosphate-ester based synthetic hydraulic fluids (e.g., Skydrol®). Contamination of brake or hydraulic systems requires complete fluid evacuation and replacement of all seals before the aircraft can be released for flight. Maintaining the correct fluid quantity is also vital. Technicians are responsible for determining and maintaining the proper fluid level as specified by the manufacturer.

Inspection for Leaks

Aircraft brake systems must remain leak-free. Any evidence of a leak, no matter how minor, must be investigated to identify its cause. Leaks can be a precursor to more significant damage. Many leaks occur at brake system fittings. While tightening a loose connection might seem like a simple fix, over-tightening fittings can cause damage and worsen the leak, especially with MS flareless fittings. It is recommended to relieve hydraulic pressure, disconnect, and inspect fittings. Replace any fittings suspected of damage. After repairing a leak, the system must be re-pressurized and tested for function and to confirm the leak has been resolved. Some brake housings may seep fluid; consult the manufacturer’s manual for acceptable limits and replace any assembly that seeps excessively.

Proper Bolt Torque

The landing gear and brake systems experience significant stress. Therefore, all bolts must be torqued to the manufacturer’s specified values. Bolts attaching brakes to the strut, and any other landing gear or brake bolts, require precise torque. Always use a calibrated torque wrench for these critical applications.

Off-Aircraft Brake Servicing and Maintenance

Certain maintenance tasks are best performed with the brake assembly removed from the aircraft. This allows for a thorough inspection of all components.

Component Inspections

• Bolt and Threaded Connections: Inspect all bolts and threaded connections for wear and damage. Ensure self-locking nuts retain their locking feature. Verify that all hardware is per the manufacturer’s parts manual, as aircraft brake bolts are often specialized. Check threads and O-ring seating areas for condition.
• Discs: Inspect both rotating and stationary discs for wear and cracks, particularly around relief slots. In multiple disc brakes, check the key slots for wear and widening. Discs should engage the torque tube without binding. Cracks or excessive key slot wear warrant rejection. Brake wear pads or linings should also be inspected for wear and uneven wear patterns.
• Rotating Discs: Examine rotating discs for glazing (indicating overheating), which can cause brake squeal. Glazed discs may be refinished if permitted by the manufacturer. Inspect drive key slots or tang areas for wear and deformation.
• Pressure Plate and Back Plate: Inspect for freedom of movement, cracks, general condition, and warping. Linings may be riveted to these plates; this process requires specific tools and techniques. Minor warping can sometimes be straightened.
• Automatic Adjuster Pins: Malfunctioning automatic adjusters can cause brake drag. The return pin must be straight and free of surface damage to move freely through the grip. Magnetic inspection may be used to detect cracks. The grip and tube assembly should be cleaned and inspected for proper movement.
• Torque Tube: Visually inspect for wear, burrs, and scratches. Magnetic particle inspection is used to check for cracks. Key areas must be checked for dimensions and wear against manufacturer specifications.
• Brake Housing and Piston Condition: Inspect the brake housing for scratches, gouges, corrosion, or blemishes. These can often be dressed out, but cracks necessitate replacement. Cylinder areas should be dimensionally checked against manufacturer limits. Brake pistons should also be checked for corrosion, scratches, and wear. Insulators on the bottom of some pistons must be inspected for cracks. Minor irregularities can be smoothed with a file.
• Seal Condition: Brake seals are crucial for proper operation. Over time, they can harden and lose resilience, leading to leaks. Always replace all seals with new ones obtained by part number from a reputable supplier. Ensure new seals have not exceeded their shelf life (typically three years from the cure date). Back-up rings, often made of Teflon®, support O-ring seals and prevent extrusion; they are typically installed on the side of the O-ring away from fluid pressure and are often reusable.

Replacement of Brake Linings: Specific Procedures

The replacement of brake linings is a common task performed in hangars. The procedures can vary significantly between brake types.

Goodyear Brakes

For Goodyear single-disc brake assemblies, the aircraft must be jacked and supported. Anti-rattle clips are detached, and the wheel is removed from the axle. The disc, located between the inner and outer lining pucks, is then extracted to allow access to the old pucks. New pucks are installed into the cavities in the housing, ensuring the smooth braking surface faces the disc. The disc is reinserted, the wheel and anti-rattle clips are reinstalled, and the axle nut is tightened per manufacturer instructions. The aircraft is then lowered.

Cleveland Brakes

Cleveland brakes offer the convenience of replacing linings without jacking the aircraft or removing the wheel. The torque plate is bolted to the strut, with the rest of the brake assembly on anchor bolts. Linings are riveted to the pressure plate and back plate. By unbolting the cylinder housing from the backplate, the backplate can be moved away, allowing the assembly to be pulled apart. The pressure plate then slides off the torque bolts. Old rivets are removed using a knockout punch. After inspection, new linings are riveted to the plates using a rivet clinching tool. Kits are available with all necessary components. Reassembly is in reverse order, ensuring any shims are correctly placed. Bolts securing the backplate to the cylinder assembly must be torqued to specification and safetied. A manufacturer’s burn-in procedure, involving controlled brake applications at specific speeds, is then performed to prepare the new linings for service.

Brake Malfunctions and Damage

Aircraft brakes are subjected to extreme stresses and varied conditions, making them susceptible to malfunctions and damage.

Overheating

While aircraft brakes convert kinetic energy into heat, excessive overheating is detrimental. It can damage and distort brake parts, weakening them and potentially leading to failure. Protocols are in place to prevent overheating. If brakes show signs of overheating, they must be removed, disassembled, and inspected. All seals must be replaced, and the brake housing checked for cracks, warping, and hardness. Discs must be inspected for warping and surface damage. After reassembly, the brake should be bench tested for leaks and pressure before reinstallation.

Grove Brake Pads and FAA Approval

Grove brake pads are designed for optimal performance when used with Grove wheel and brake products. They have undergone rigorous testing and are FAA-TSO and FAA-PMA approved for installation on certified aircraft. These pads feature a non-asbestos organic compound, offer a direct replacement for most aircraft, reduce brake disc wear, and provide a long service life, contributing to lower operating costs. Pre-assembled brake pad assemblies are also available for convenience.

In conclusion, maintaining aircraft brake linings is a multifaceted process that requires diligence, adherence to manufacturer specifications, and a thorough understanding of the systems involved. Regular inspections, correct fluid usage, proper bleeding techniques, and timely replacement of worn components are all critical for ensuring the safety and reliability of the aircraft’s braking system.

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