Brake Units

Taxi Speed and Braking

Taxi speed should be closely monitored during taxi out, particularly when the active runway is some distance from the departure gate. Normal taxi speed is approximately 20 knots, adjusted for conditions. On long straight taxi routes, speeds up to 30 knots are acceptable, however at speeds greater than 20 knots use caution when using the nose wheel steering tiller to avoid over controlling the nose wheels. When approaching a turn, speed should be slowed to an appropriate speed for conditions. On a dry surface, use approximately 10 knots for turn angles greater than those typically required for high speed runway turnoffs.

Note: High taxi speed combined with heavy gross weight and along taxi distance can result in tyre sidewall overheating

Heat Generation

As severe as the effects of high centrifugal forces are, heat has a more detrimental effect. Heavy loads and high speeds cause heat generation in aircraft tyres to exceed that of all other tyres.

The vertical dotted line at 30 knots indicates the recommended maximum taxi speed.

Even when an aircraft tyre is properly inflated and operated at moderate taxi speeds, the heat generation will always exceed the heat dissipated. The farther the taxi distance, the hotter the tyres will be at the start of the take-off.

When the airplane is equipped with steel brakes, avoid prolonged brake application to control taxi speed as this causes high brake temperatures and increased wear of brakes. If taxi speed is too high, reduce speed with a steady brake application and then release the brakes to allow them to cool. Braking to approximately 10 knots and subsequent release of the brakes results in less heat build-up in the tyres and brakes than when the brakes are constantly applied.

Under normal conditions, differential braking and braking while turning should be avoided. Allow for decreased braking effectiveness on slippery surfaces. Avoid following other aircraft too closely. Jet blast is a major cause of foreign object damage.

Carbon Brake Life

Brake wear is primarily dependent upon the number of brake applications. For example, one firm brake application causes less wear than several light applications. Continuous light applications of the brakes to keep the airplane from accelerating over a long period of time (riding the brakes) to maintain a constant taxi speed produces more wear than proper brake application. During taxi, proper braking should involve a steady application of the brakes to decelerate the airplane. Release the brakes as lower speed is achieved. After the airplane accelerates, repeat the braking sequence.

Antiskid Inoperative

With antiskid inoperative, tyre damage or blowouts can occur if moderate to heavy braking is used. With this condition, it is recommended that taxi speed be adjusted to allow for very light braking.

Wheel Brakes

Braking force is proportional to the force of the tyres on the runway and the coefficient of friction between the tyres and the runway. The contact area normally changes little during the braking cycle. The perpendicular force comes from airplane weight and any downward aerodynamic force such as speedbrakes.

The coefficient of friction depends on the tyre condition and runway surface, (eg concrete, asphalt, dry, wet or icy).

Automatic Brakes

Boeing recommends that whenever runway limited, using higher than normal approach speeds, landing on slippery runways or landing in a crosswind, the autobrake system be used.

For normal operation of the autobrake system select a deceleration setting.

Settings include:

• MAX AUTO: Used when minimum stopping distance is required. Deceleration rate is less than that produced by full manual braking

• 3 or 4: Should be used for wet or slippery runways or when landing rollout distance is limited

• 1 or 2: These settings provide a moderate deceleration effect suitable for all routine operations.

Note: Autobrakes 2 or greater results in a continuous brake application, which can increase carbon brake life.

Flight crew/airline experience with airplane characteristics relative to the various runway conditions routinely encountered provide initial guidance as to the desirable level of deceleration selected.

Immediate initiation of reverse thrust at main gear touchdown and full reverse thrust allow the autobrake system to reduce brake pressure to the minimum level. Since the autobrake system senses deceleration and modulates brake pressure accordingly, the proper application of reverse thrust results in reduced braking for a large portion of the landing roll.

The importance of establishing the desired reverse thrust level as soon as possible after touchdown cannot be overemphasised. This minimises brake temperatures and tyre and brake wear and reduces stopping distance on very slippery runways.

The use of minimum reverse thrust almost doubles the brake energy requirements and can result in brake temperatures much higher than normal.