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To prevent wheels from skidding during braking, the Electronic or Brake-by-Wire
System provides anti-skid protection. Anti-skid is operable between ten (10) and
one hundred seventy (170) knots. (Below 10 knots, the main landing gear wheels
may be locked to accomplish tight-radius turns.) Anti-skid is accomplished by
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-32-00
Page 37
January 31/02
Revision 6
maintaining wheel rotation during braking. Each wheel assembly is equipped with
a Wheel Speed Sensor (WSS). The speed sensors are mounted each wheel hub
and act as generators driven by wheel rotation. As a wheel turns, the WSS
produces alternating current (AC) voltage at a frequency proportional to wheel
speed. The variable frequency voltage of each wheel is monitored by the Brake
Electronic Control Unit (ECU). Circuit boards in the ECU are programmed to
compare the frequencies, and therefore the speeds, of adjacent wheels on each
strut (the speed of the outboard wheel on the left strut is compared with the speed
of the inboard wheel of the left strut). If the ECU detects a difference in rotational
speed between the two wheels, the circuit boards in the ECU determine that the
slow wheel is in a skid, and the ECU signals the brake control valve on the
Hydraulic Brake Control Module (HBCM) to reduce hydraulic pressure to both
wheel brakes on the strut with the skidding wheel. When the rotational speed of
the slow wheel recovers to match that of the faster wheel, hydraulic pressure is
restored to both wheel brakes. Anti-skid protection is also available using Auxiliary
system pressure if the Combined system fails.
If the anti-skid system fails or is disabled, a type of anti-skid braking with degraded
performance is available. This type of braking is termed open-loop pulsed antiskid.
This mode does not compare wheel rotational speeds. Brake pressure is
proportional to brake pedal LVDT displacement, but is applied only three (3) to five
(5) times per second, resulting in pressure fluctuations from zero to commanded
pressure levels. Because full brake pressure is not constantly applied, wheel
skids are less likely. The abrupt fluctuations between zero and full commanded
brake pressure can be felt in the brake pedals, thus this mode of braking is
popularly called “bang-bang” braking.
The anti-skid Wheel Speed Sensors in combination with the nutcracker (squat)
switches are also used to provide data for other braking functions and protective
features. With the anti-skid system selected on, the nutcracker switches block
brake application to the wheels when the aircraft is in flight (weight-off-wheels).
When the landing gear is lowered in preparation for landing, brake pressure
should not be available until the aircraft is on the ground and both the nutcracker
switches compressed (weight-on-wheels). An additional locked wheel circuit in the
ECU protects against a failure of the nutcracker switch system that would allow
application of the wheel brakes prior to landing, causing flat-spotted or blown tires
and/or wheel damage. The circuit incorporates the following logic. When the
landing gear is initially lowered, the wheels do not rotate. The WSSs signal the
lack of rotation to the ECU that equates this condition to locked wheels and blocks
hydraulic pressure to the brakes. The locked wheel signal is preserved in the ECU
until the aircraft wheels spin up on runway contact and reach a speed equivalent
of thirty-five (35) knots as sensed by the WSSs, or until one of the nutcracker
switches compresses and enters the ground (weight-on-wheels) mode. A time
delay in the circuit will also maintain the locked wheel signal for seven seconds
after the nutcracker system transitions from flight to ground mode in order to
provide protection in the event of an aircraft bounce on landing. If a nutcracker
system malfunction prevents a switch from achieving the ground (weight-onwheels)
mode, the locked wheel circuit preserves brake system operation with
anti-skid protection until the aircraft slows to thirty-five (35) knots or below, at
which time anti-skid protection is lost, but full pulsed braking is available if the
anti-skid selected off.
In conditions of severe hydroplanning, a separate locked wheel circuit is effective
at aircraft wheel speeds above twenty-six knots. This circuit releases hydraulic
OPERATING MANUAL
2A-32-00 PRODUCTION AIRCRAFT SYSTEMS
Page 38
January 31/02
Revision 6
pressure to the brake on a wheel that is rotating at a speed thirty percent (30%)
less than the speed of the other paired wheel. This protection decreases the risk
of loss of directional control when the aircraft exits the hydroplanning condition,
　
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