Aircraft brakes turn the kinetic energy of a landing airplane into thermal energy so the aircraft can slow safely on the runway. Large jets use carbon multi-disc brakes because they can absorb enormous heat while keeping weight lower than many steel brake designs. Antiskid systems help the tires keep rolling instead of locking, which improves directional control and shortens stopping distance on many runway surfaces.
Understanding these systems is important for pilots, engineers, and technicians because braking performance affects every landing and rejected takeoff.
Understanding Aviation: Aircraft Brakes and Antiskid
A wheel brake is built as a stack of rotating and stationary discs inside each main wheel. The rotating discs turn with the wheel. The stationary discs are held by the brake housing.
Hydraulic pistons squeeze the stack when the pilot presses the pedals or when an automatic brake command is active. Friction between the disc surfaces resists rotation.
Heat first enters the discs, then spreads into the wheel, surrounding air, and nearby parts. This is why a brake assembly needs careful cooling time after a hard landing or a high speed rejected takeoff.
The useful grip comes from the small contact patch between each tire and the runway. A rolling tire can produce strong braking force, but a sliding tire usually loses grip. It can then skid across the surface, wear a flat spot into the tire, and make the airplane harder to steer.
Runway condition changes this grip greatly. Water, slush, ice, rubber deposits, or loose material can reduce it.
Crosswinds add another challenge because the tires must help slow the airplane while keeping it aligned with the runway. Pilots use rudder, nosewheel steering when appropriate, and balanced brake use to maintain direction.
Antiskid works through fast feedback. Sensors measure wheel rotation many times each second. A control unit compares wheel behavior with aircraft motion and looks for a wheel that is slowing more rapidly than it should.
It briefly reduces hydraulic pressure at that brake, then reapplies pressure as the wheel recovers. The cycle is so quick that it may feel like a pulsing action, similar to antilock braking in many cars.
Each wheel can receive a different brake pressure because left and right wheels may have different runway grip. This matters on a runway with patches of standing water or uneven contamination.
Stopping an aircraft is a system task rather than a brake-only task. After touchdown, aerodynamic drag falls as speed decreases, so wheel brakes become more important later in the roll. Spoilers reduce lift and place more aircraft weight on the wheels, which gives the tires more available grip.
Reverse thrust can reduce the demand placed on the brakes, especially at high speed. Pilots select braking levels based on runway length, wind, surface reports, aircraft weight, and the need to preserve brake temperature for a later departure. Students should pay close attention to the difference between brake capability and tire grip.
Powerful brakes cannot create more traction than the runway and tire can provide. They should also remember that landing performance data uses specific assumptions, so real operations require safety margins and careful judgment.
Key Facts
- Braking converts kinetic energy into heat: KE = 1/2 mv^2.
- Stopping distance depends strongly on speed because kinetic energy increases with v^2.
- Braking force at the tire is limited by friction: Fmax = μN.
- Antiskid reduces brake pressure when wheel speed drops too quickly, helping prevent a locked wheel.
- Autobrake systems command a selected deceleration level after touchdown or during a rejected takeoff.
- Brake temperature rises when more energy is absorbed, and overheated brakes can reduce braking effectiveness or damage components.
Vocabulary
- Carbon multi-disc brake
- A brake assembly using alternating rotating and stationary carbon discs to create friction and absorb high amounts of heat.
- Antiskid system
- An aircraft braking control system that monitors wheel speed and adjusts brake pressure to help prevent wheel lockup.
- Autobrake
- A system that automatically applies wheel brakes to achieve a selected deceleration after landing or during a rejected takeoff.
- Torque tube
- A structural part in the brake assembly that supports stationary brake discs and carries braking torque into the landing gear.
- Brake fade
- A reduction in braking effectiveness caused by excessive heat, material limits, or changes in friction behavior.
Common Mistakes to Avoid
- Assuming more pedal force always means shorter stopping, because tire friction has a limit and locked wheels can slide with less control.
- Ignoring the v^2 in kinetic energy, because doubling landing speed gives four times as much energy for the brakes to absorb.
- Thinking antiskid removes the need for pilot judgment, because runway contamination, tire condition, and aircraft weight still affect stopping distance.
- Treating brake heat as only a comfort issue, because excessive temperature can cause wear, fuse plug release, brake fade, or maintenance limits.
Practice Questions
- 1 A 70000 kg aircraft lands at 70 m/s. Estimate its kinetic energy using KE = 1/2 mv^2.
- 2 If the maximum braking friction coefficient is 0.40 and the aircraft weight on the braked wheels is 600000 N, what is the maximum braking force using Fmax = μN?
- 3 Explain why an antiskid system usually improves stopping and steering compared with a locked-wheel skid on the runway.