Anti-skid, auto-brake, brake temperature indicators and brake fans are all systems which enhance the performance of the aircraft brakes.
The anti-skid system, through various mechanisms, compares the speed of the aircraft with the rotational speed of each main wheel. If the speed of any wheel is too slow for the existing aircraft speed, the brake on that wheel (or wheels) is released momentarily to allow the wheel speed to increase and prevent the wheel from skidding. The system is fully automatic and is active from immediately after initial wheel spin up on landing (during which time brake activation is inhibited) down to a design limited minimum speed; usually about 15 knots. Anti-skid systems are designed to minimise aquaplaning and the potential tyre damage which can occur when a wheel is locked or rotating at a speed which does not correspond to the speed of the aircraft. Anti-skid removes the possibility of reverted rubber skids caused by locked wheels. An anti-skid system also greatly improves stopping distance on substandard surfaces such as gravel or grass and is particularly effective on surfaces contaminated with frozen contaminants such as ice or slush by ensuring maximum effective breaking.
Auto-brake systems can be used on takeoff where they will provide maximum braking in the event of a rejected takeoff and on landing where they will provide a scheduled rate of deceleration (dependent upon the auto-brake level selected) using only a single brake application. These features combine to optimize the brake usage against the requirement and to minimize brake wear.
Brake Temperature Indicators
Brake temperature indicators are intended to give the pilots an indication of the temperature in each wheel assembly. While each aircraft type will have its own specific limitations for items such as maximum indicated temperature for initiating takeoff, comparison of the brake temperature indications can give an overall indication of the “health” of the braking system. For example, inappropriately high or low temperatures on a given wheel can indicate the potential of a dragging or an inoperative brake respectively. Similarly, increasing brake temperatures after takeoff could be indicative of a tyre failure which has resulted in a wheelwell fire.
Brake fans reduce brake cooling times by using wheel mounted electric fans to blow ambient air across the brake and wheel assemblies. Note that the maximum recommended temperature for takeoff as indicated on the instrument panel may have a different value dependent on if the brake fans have been used or not.
The parking brake is usually applied by hand lever selection. Hydraulic accumulators are generally required if hydraulic pressure is to remain sufficient to hold parking brake settings for long periods once engines have been shut down and the primary source of hydraulic pressure is no longer available. On some types, the parking brake pressure will bleed off over time and the brakes will eventually release.
All aircraft should be chocked once parked to prevent unplanned movement.
- Overheated brakes
- Loss of braking performance
- Tyre deflation
- Brake failure
- Runway excursions (although this is a very infrequent cause)
- Unwanted aircraft ground movement