Aircraft Wheels, Tyres, and Brakes

30.1 Wheels

• Aircraft wheels are typically made of aluminum or magnesium alloy and are designed in two
• pieces (split-wheel design) to facilitate tyre changes.
•  Creep: The tendency of the tyre to rotate slightly around the wheel rim on landing. Creep
• marks (a white line on the tyre and wheel) are used to monitor this.
•  Fusible Plugs: These plugs are fitted in the wheels of transport aircraft. If the brakes
• verheat, the core of the plugs melts at a set temperature, allowing the tyre to deflate in a
• controlled manner, which prevents a dangerous tyre explosion.

30.2 Tyres

• Aircraft tyres are designed to withstand very high loads and speeds. They are typically
• inflated with nitrogen because it is inert and does not support combustion.
•  Construction: Built with multiple layers of fabric (plies) embedded in rubber. The ply
• rating is an index of the tyre's strength, not the actual number of plies.
•  Tread: The tread is often ribbed with circumferential grooves to channel water away and
• prevent aquaplaning (when a wedge of water lifts the tyre off the runway surface, causing a
• loss of braking and directional control).
•  Aquaplaning Speed: The minimum speed at which aquaplaning can occur can be estimated
• by the formula: Speed (knots) = 9 × √Tyre Pressure (PSI).

30.3 Brakes

•  Multi-Disc Brakes: Large aircraft use brakes consisting of a stack of alternating rotating
• discs (rotors, keyed to the wheel) and stationary discs (stators, keyed to the axle). Hydraulic
• pressure from pistons squeezes this stack together, creating immense friction.
•  Brake Materials: The discs are made of steel or, more commonly on modern aircraft,
• carbon-carbon composites. Carbon brakes are lighter, can absorb much more heat, and are
• more effective at high temperatures.
•  Anti-Skid System: Similar to ABS in a car, this system prevents the wheels from locking up
• during heavy braking.
• Operation: A wheel speed sensor on each wheel detects if a wheel is decelerating too rapidly
• (indicating an impending skid). The anti-skid control unit then commands a hydraulic valve

• to momentarily release and reapply brake pressure to that wheel, maintaining maximum
• braking effectiveness without skidding.
•  Autobrake System: This system automatically applies the brakes upon landing or during a
• rejected takeoff (RTO). The pilot can select a desired deceleration rate (e.g., LOW, MED,
• MAX), and the system modulates the brake pressure to achieve it.