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system provides anti-skid protection during a hydraulic malfunction of the
Combined system, and uses the Auxiliary hydraulic system for braking.
Anti-skid protection for the main landing gear brakes operates by regulating
hydraulic pressure applied to the brakes to maintain wheel rotation. HMAB antiskid
is effective at taxi speeds above ten (10) kts. (Below 10 knots, the main wheel
brakes may be locked by brake pedal commands in order to turn the aircraft within
a minimum radius.) Hydraulic pressure is metered by the anti-skid control valve
modules in response to commands from the skid control box or Electronic Control
Unit (ECU). The ECU, located in the left avionics equipment rack aft of the
cockpit, monitors wheel speeds using data from the Wheel Speed Sensors
(WSSs). Each main landing gear wheel is equipped with a WSS mounted on the
wheel axle that is spun by the rotation of the wheel. The WSSs act as generators,
producing an alternating current (AC) signal at a frequency proportional to wheel
revolution. The ECU compares the frequency of the AC signals produced by the
rotation of the wheels to detect a skidding wheel, indicated when one wheel is not
rotating as fast as others. If the ECU detects a rotational speed difference on one
of the main landing gear wheels during braking, it signals the anti-skid control
valve module to reduce hydraulic pressure to the brake on the slower wheel and
to the corresponding wheel on the opposite strut. The ECU controls the hydraulic
pressure to wheel brakes in pairs - inboard wheels and outboard wheels. By
regulating the same hydraulic pressure to symmetrically paired wheel brakes,
aircraft directional control is easier to maintain.
The ECU has three circuit boards for monitoring wheel speeds and controlling
brake application. Two boards are used during primary anti-skid system operation:
one board for the inboard wheels on each strut, and one board for the outboard
wheels on each strut. The third circuit board is used during alternate anti-skid
operation and controls the application of Auxiliary system hydraulic pressure to
both the inboard and outboard wheel brakes on each strut. Each circuit board is
powered separately for redundancy. The board for the outboard wheels is
powered by the Emergency Battery 2B Bus through circuit breaker OUTBD ANTI
SKID, located at position B-2 on the Copilot Overhead (CPO). The inboard wheel
circuit board is powered by the Emergency Battery 1A Bus through circuit breaker
INBD ANTI SKID located at position B-1 on the CPO. The circuit board for the
alternate anti-skid system is powered by Emergency Battery 2B Bus through the
WHEEL SPEED circuit breaker at location C-10 on the CPO.
The ECU circuit boards communicate with three anti-skid control valve modules.
There are two modules controlled by the primary anti-skid system circuit boards,
one for each main landing gear strut. The third module is controlled by the
alternate anti-skid board, and is linked to all four main landing gear wheels. The
OPERATING MANUAL
2A-32-00 PRODUCTION AIRCRAFT SYSTEMS
Page 34
January 31/02
Revision 6
alternate module and corresponding circuit board are always electrically powered,
even if the primary system components are operating the anti-skid system.
During primary system operation, the circuit board for the outboard wheels
compares and controls outboard wheel speed though the WSSs and the left and
right strut anti-skid control valve modules. The inboard wheels of each strut are
monitored and controlled in the same way. During alternate system operation with
Auxiliary hydraulic system pressure, a single circuit board and anti-skid control
valve module controls all four wheel brakes. The alternate anti-skid system is not
directly linked to the WSSs, it relies upon the inboard and outboard ECU circuit
boards for wheel speed monitoring and comparison. Unlike the primary system
that controls symmetrically paired wheel brakes, the alternate system controls
each strut as a unit. If, for instance, a decrease in wheel speed is detected in the
left inboard wheel, the alternate system reduces hydraulic pressure application to
both the inboard and outboard wheel brakes of the left strut.
Additional protective features are provided by the anti-skid system. To avoid flatspotted
and/or blown tires, touchdown protection prevents hydraulic pressure
from reaching the wheel brakes until the nutcracker (squat) switch is in the ground
mode (weight-on-wheels) or the wheels have spun up to twenty-two (22) knots.
When the wheels are lowered prior to landing, the nutcracker switch is in the flight
(weight-off-wheels) mode and the wheels are not rotating. In this condition, with
　
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