Wheel Brakes
Braking force is proportional to the force of the tires on the runway and the coefficient of friction between the tires 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 tire condition and runway surface, (e.g. 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 overemphasized. This minimizes brake temperatures and tire 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.
During the landing roll, use manual braking if the deceleration is not suitable for the desired stopping distance.
The autobrakes should be released by smoothly applying brake pedal force, as in a normal stop, until the autobrake system disarms. Following disarming of the autobrakes, smoothly release brake pedal pressure.
Disarming the autobrakes before coming out of reverse thrust provides a smooth transition to manual braking.
All crewmembers should be alert for autobrake disarm annunciations during the landing roll so that manual braking can be initiated if required.
The airplane speed at which the transition from autobrakes to manual braking is made varies with airplane deceleration and stopping requirements. For runway conditions that produce good deceleration, the transition from autobrakes to manual brakes should be made at about 60 knots. The transition speed should be closer to a safe taxi speed on very slippery runways or when runway length is limited.
A table in the Performance Inflight section of the QRH shows the relative stopping capabilities of the available autobrake selections.
Manual Braking
The following technique for manual braking provides optimum braking for all runway conditions:
The pilot’s seat and rudder pedals should be adjusted so that it is possible to apply maximum braking with full rudder deflection.
Immediately after main gear touchdown, smoothly apply a constant brake pedal pressure for the desired braking. For short or slippery runways, use full brake pedal pressure.
.
do not attempt to modulate, pump or improve the braking by any other
special techniques
.
do not release the brake pedal pressure until the airplane speed has been
reduced to a safe taxi speed
.
the antiskid system stops the airplane for all runway conditions in a
shorter distance than is possible with either antiskid off or brake pedal
modulation.
The antiskid system adapts pilot applied brake pressure to runway conditions by sensing an impending skid condition and adjusting the brake pressure to each individual wheel for maximum braking effort. When brakes are applied on a slippery runway, several skid cycles occur before the antiskid system establishes the right amount of brake pressure for the most effective braking.
If the pilot modulates the brake pedals, the antiskid system is forced to readjust the brake pressure to establish optimum braking. During this readjustment time, braking efficiency is lost.
Copyright . The Boeing Company. See title page for details.
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本文链接地址：757 Flight Crew Training Manual 机组训练手册(72)
