For several decades, the common challenge for aircraft designers was to produce aircraft with engines that can fly at supersonic speed and return them as at when necessary to normal speed using an efficient braking system. This braking system requires diligent attention, inspection, and servicing because nothing must go wrong with it.
In general, airplane brakes can last between 1000-2000 landings before they are changed. An active airplane makes an estimate of two landings per day; this means the airplane needs to be changed after 18-36 months.
However, the longevity of a braking system also depends on the material it is made from. There are basically two main materials from which aircraft brakes can be made namely steel and carbon.
A steel brake can last about 1000 landings while a carbon brake which costs more can last about 2000 landings.
Modern airplanes use carbon brakes because they are more durable, lighter, and require lower maintenance costs. Additionally, carbon brakes can also resist higher temperatures (about 3000°C/5000°F), making them more fuel economical than the steel brakes.
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Factors Affecting How Frequently Aircraft Brakes Are Changed
The design considerations of aircraft brakes will ultimately determine how durable the brakes would be before they are changed. Asides durability, an optimal braking system must be able to perform the following basic functions:
- It must be able to hold the aircraft at full static run up;
- It must provide adequate control during ground taxi operations;
- It must be able to smoothly stop the aircraft within the length of the runway during landing and roll out and;
- It must be able to stop the aircraft from moving from where it is parked.
To achieve all of these impressive functions, there are some design considerations that must be taken.
The basic considerations for the design of aircraft brakes include the number of discs, the diameter of the discs, the material of the discs and the worst case rejected takeoff scenario (RTO) at the maximum rolling speed (V1), known as the decision speed.
Above V1, takeoff cannot be aborted without the risk of the airplane not stopping before the end of the runway. In this case, the brake must be able to absorb energy more than any other scenario.
Brake Materials
Steel was the most common brake disc material until 1963. Afterwards, beryllium was introduced because of its improved thermal properties. Unfortunately, beryllium created a new difficulty owing to the toxicity of the beryllium oxide. About twenty years later, carbon brakes became widely available for commercial airplanes.
Compared to steel, carbon’s higher specific heat (the amount of heat absorbed by a unit mass of carbon to raise its temperature by one 1°F/1°C) enables reduced brake weight and ultimately the overall weight of the aircraft.
Carbon’s specific strength is relatively constant over a very wide range of temperatures, unlike steel and beryllium, which both decline sharply in specific strength at higher temperatures. Carbon’s higher thermal conductivity allows faster heat transfer and uniform distribution of heat through the disc.
Furthermore, carbon has lower thermal expansion, higher thermal shock resistance, and a higher temperature limit than steel.
Other brake materials are also available such as Cerametalix, a sintered combination of powdered metals and ceramics.
Brake Pedal Profile
The brake cylinder must deliver the proper pressure and fluid volume to the caliper for optimum braking. A general rule is to design the brake pedal profile to have a 2:1 ratio of pedal travel to brake cylinder travel.
Size of Brake Discs
The brake system absorbs heat energy developed during braking. The larger the mass of the disc, the more heat energy it can absorb. The designer’s task is to deliver a brake system with an adequate disc mass without excessive weight.
Aircraft Brake Inspection and Change
Brakes by default operate under extreme stress and generate heat due to the effect of friction. This makes them vulnerable to malfunction and damage. A few common brake problems are hereby discussed below that may necessitate the change of aircraft brakes.
Overheating
Frictional heat is important in braking to slow down and stop a vehicle in motion but this necessary element becomes a problem when the heat becomes excessive. Excessive heat can weaken the brake parts even to the point of failure.
The inspection for brake overheating involves checking the brake housing for cracks, buckling, and the replacement of all the seals. This is important because any form of brittleness caused by overheating could cause the brake to fail under high pressure.
Squealing
This is caused when brake linings do not align smoothly along the disc. Overheating of discs may cause damage to the surface layer of the disc which may be transferred to the adjoining disc, resulting in uneven surfaces leading to chatter or squeal. Squealing is chattering at a higher frequency.
The brake technician must probe suspicions and reports of brake chattering and squealing to avoid further damage to the brake and landing gear system.
Dragging
A brake may drag when there is a malfunctioning of the return mechanism when the brakes are no longer applied. This drag can cause excessive lining wear and overheating, leading to the damage of the disc.
Dragging can be caused by the expansion of air in the brake fluid line by heat. Bleed the brakes to remove air in the system to eliminate drag. Dragging could also occur when the return pin slips into the auto adjuster pin grip. ]This is corrected by returning the units to the appropriate positions.
Aircraft brakes are also often inspected according to the following characteristics:
● Power
● Pedal feel
● Durability
● Smoothness
● Continuous power dissipation
● Fade
● Peak force
Which Brake System Parts Can Be Replaced?
Common items that must be inspected when brakes are installed and removed on the aircraft include
- fluid quantity level
- brake lining wear
- bolt and threaded connections
- cracks and glazing in discs
- brake housing and piston condition
- leaks (of brake seals, etc)
- bolting torque.
The following brake system parts should be inspected and replaced:
- The brake assembly when it has experienced undue loads/stress, for example, a ground loop.
- The brake caliper when there is corrosion, leaking hydraulic fluid, cracks, or damage.
- The torque plate when there is corrosion, cracks, and loose bolts that attach it to the axle.
- The brake line and brake line fittings when there are signs of damage/leakage and when linings have been contaminated with fluid.
- Brake lining material when it is worn beyond limits.
Many brake assemblies have a built-in wear indicator pin, the exposed pin length decreases as the linings wear and a minimum length is used to indicate the linings must be replaced.
Some other brake assemblies use the distance between the disc and a portion of the brake housing when the
Cost of Brake Replacement
The cost of brake system parts vary from one manufacturer to the other. Below are the ranges of some of the costs for aircraft brake systems.
- A complete dozen-piece brake set of a Boeing 777 – about $100 000
- Overhauled main brake assembly for Pilactus PC-12 – $3249 – $4595
- Alaskan Bushwell 1.5 inch brake caliper – $663
- Cleveland brake cylinder assembly only – $1226
- An Alaskan torque plate – about $150 Speedbrakes – $5508 – $8000
- BX-1000 Black Max Hydraulic Brake System – $500 – $660
Honeywell Aerospace, Messier-Bugatti-Dowty, Meggit Aircraft Braking Systems, and Goodrich remain large brake manufacturers in the aircraft brake market.
Safety Precautions
Common precautions maintenance experts observe include:
- Never approach facing the tread edge-on and do not spray hot brakes with water or extinguishing agent unless fire or any imminent danger is present.
- Always follow the manufacturer’s maintenance manual and always use the right tool to bleed brakes.
- Always use the proper parts in the brake assembly. Avoid improper parts in the retraction assembly.
- Avoid unnecessary cutting of costs by patching or repairing when a complete replacement is needed.
Final Note
The horrendous Atlantic Airways Flight 670 crash that occurred on 10 October 2006 at Stord Airport, Sorstokken, Norway was because the aircraft’s spoilers failed to deploy, causing inefficient braking.
Similarly, at Vnukovo Airport, December 30, 2012, a Russian airliner crashed onto a highway outside Moscow killing five of the eight crew members due to faulty brakes.
These sad events amongst others show that the proper adjustment, inspection, and maintenance of the braking system is very essential for the overall safety of passengers and effective operation of any aircraft.
A stitch in time saves nine! (but in this case, it saves hundreds of lives and millions of dollars).
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