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Normal Brake Hydraulic System
The normal brake hydraulic system is powered by hydraulic system 4. The brake
pedals provide independent control of the left and right brakes.
Alternate Brake Hydraulic System
If hydraulic system 4 pressure is low, hydraulic system 1 supplies pressure to the
alternate brake hydraulic system. If hydraulic pressure in systems 4 and 1 are low,
system 2 powers the alternate brake system.
Loss of hydraulic systems 4, 1, and 2 causes the brake source light to illuminate.
Brake Accumulator
The brake accumulator provides for parking brake application.
Antiskid Protection
Antiskid protection is provided in the normal and alternate brake hydraulic
systems.
• normal brake hydraulic system
• alternate/reserve brake
hydraulic system
• brake accumulator
• antiskid protection
• autobrake system
• parking brake.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
14.20.4 D6-30151-425
Landing Gear -
System Description
The normal brake hydraulic system provides each main gear wheel with individual
antiskid protection. When a wheel speed sensor detects a skid, the associated
antiskid valve reduces brake pressure until skidding stops.
The alternate brake hydraulic system provides antiskid protection to tandem wheel
pairs (fwd and/or aft pair on each truck) rather than to individual wheels.
Touchdown and hydroplaning protection is provided using airplane inertial
ground speed. Locked wheel protection is provided using a comparison with other
wheel speeds.
If antiskid power is off on all wheels, or the parking brake valve is not fully open,
or a brake system control unit power loss occurs, the disabled system message
ANTISKID displays on the gear synoptic adjacent to the affected wheels.
Brake Torque Limiter
A brake torque sensor is provided at each wheel. The sensors detect excessive
torque during braking to prevent damage to landing gear. When excessive torque
is detected, a signal is sent to the antiskid valve to release brake pressure to that
wheel. If the alternate brake system is used, brake torque is sensed on an individual
wheel basis, however the signal is sent to the alternate antiskid valve and brake
pressure is released on a laterally paired wheel basis.
If a brake torque limiter failure occurs on more than one wheel per truck, or the
parking brake lever is released and the parking brake valve is not fully open, or a
brake system control unit power loss occurs, the disabled system message
LIMITER displays on the gear synoptic adjacent to the affected wheels.
Autobrake System
The autobrake system provides braking at preselected deceleration rates for
landing and full pressure for rejected takeoff. The system operates only when the
normal brake system is functioning. Antiskid system protection is provided during
autobrake operation.
Rejected Takeoff
Selecting RTO (rejected takeoff) prior to takeoff arms the autobrake system. The
RTO mode can be selected only on the ground. The RTO autobrake setting
commands maximum braking pressure if:
• the airplane is on the ground
• groundspeed is above 85 knots, and
• all thrust levers are closed.
Maximum braking is obtained in this mode. If an RTO is initiated below 85 knots,
the RTO autobrake function does not operate.
April 01, 2001
747 Operations Manual
Landing Gear -
System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 14.20.5
Landing
Five levels of deceleration can be selected for landing. However, on dry runways,
the maximum autobrake deceleration rate in the landing mode is less than that
produced by full pedal braking.
After landing, autobrake application begins when:
• all thrust levers are closed,
• ground mode is sensed, and
• the wheels have spun up.
To maintain the selected airplane deceleration rate, autobrake pressure is reduced
as other controls, such as thrust reversers and spoilers, contribute to total
deceleration. The system provides braking to a complete stop or until it is
disarmed.
Autobrake – Disarm
The system disarms immediately if any of the following occur:
• pedal braking applied
• any Thrust lever advanced after landing
• Speedbrake lever moved to DOWN detent after speedbrakes have
deployed on the ground
• DISARM or OFF position selected on Autobrake selector
• autobrake fault
• normal antiskid system fault
• loss of normal brake hydraulic pressure.
　
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