Chapter 30: 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.