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the anti-skid system on, hydraulic pressure to the brakes is blocked, since the lack
of wheel rotation signals a locked wheel. On touchdown, compression of the
nutcracker switch to the ground (weight-on-wheels) mode starts a seven second
delay during which hydraulic pressure remains blocked to the wheel brakes
unless the wheels spin up and attain a speed equivalent of 22 knots. The time
delay provides protection for the wheels/brakes in the event of a bounce upon
landing that would compress the nutcracker switch and allow application the
brakes prior to the subsequent touchdown. Normal hydraulic pressure is available
at wheel speeds above 22 knots regardless of nutcracker switch position. Should
a nutcracker switch fail in the flight, or weight-off-wheels mode, normal braking
with full anti-skid protection is available until the aircraft slows to below 22 knots.
Below wheel speeds of 22 knots, the logic of the touchdown protection circuitry
reverts to nutcracker switch position. The brakes on the wheels of the strut with
the normally operating nutcracker switch will operate with full anti-skid (since the
seven second delay has expired), but the brakes on the wheels of the strut with
the inoperable nutcracker switch will be without hydraulic pressure, since the
nutcracker switch did not transition to the ground (weight-on-wheels) mode to
start the timing of the seven second delay. This hazardous condition of full antiskid
braking present on one strut and no braking available on the other strut is
avoided by selecting anti-skid off prior to slowing the aircraft below 22 knots.
Similar protection acts as a backup to normal anti-skid protection to avoid locked
wheels on the ground during roll out. If a rotational speed difference of
approximately thirty-three percent (33%), (as measured between inboard wheels
or between outboard wheels) is detected on one wheel, hydraulic pressure is
completely removed from the slow wheel brake. This feature is useful in
circumstances of severe hydroplaning or icing conditions where wheel rotation is
minimal or nonexistent. Removing all hydraulic brake pressure to the locked or
non-rotating wheel prevents damage / blow outs when the aircraft exits conditions
of very low friction. This feature differs from touchdown wheel protection in that
brake pressure is removed from a single wheel, rather than both symmetrical
wheels.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-32-00
Page 35
January 31/02
Revision 6
In addition to the wheel speeds supplied by the WSSs to the anti-skid system,
wheel rotation data is supplied to the automatic spoiler system to operate ground
spoilers if there is a failure in the nutcracker switch system. When automatic
ground spoilers are armed, they normally deploy when the nutcracker switch on
each main landing gear is in the ground, or weight-on-wheels mode. If a
nutcracker switch fails, wheel speed can initiate spoiler deployment. If at least one
WSS on each strut supplies a signal equivalent to a speed of approximately fortyeight
(48) knots, the ECU will supply a signal to the ground spoiler system to
deploy. However, as the aircraft slows, the strength of the signal supplied by the
WSSs will degrade, and when wheel rotation has slowed to approximately thirtynine
(39) to twenty-three (23) knots, the ground spoilers will retract.
2A-32-43: Electronic Braking System (Brake-by-Wire)
1. General Description
The Electronic or Brake-by-Wire Braking System uses electronic signals in place
of mechanical linkages to transmit cockpit brake pedal commands to the main
wheel brakes. All wiring is dual channel, providing redundancy for system
components. DC power to channel 1 is provided by the forward emergency
battery through circuit breaker BCS CHL #1, located on the CPO at position A-2 ,
and channel 2 is powered from the DC Essential Bus through circuit breaker BCS
CHL #2, located on the CPO at position B-2. Anti-skid protection for braking is fully
integrated into the system using information from wheel speed sensors (WSSs).
The wheel speed sensor circuits are powered by the Left Main DC Bus through
the WHEEL SPEED circuit breaker, located on the CPO at position C-10.
Pilot and copilot left and right brake pedals are equipped with linear variable
differential transducers (LVDTs). As a brake pedal is depressed, an electrical
signal proportionate to pedal displacement is generated. Brake pedal feel is
provided by bungees incorporated into the pedal structure. The electrical signals
from the LVDTs are sent to the Electronic Control Unit (ECU). If both pilots
depress their respective brake pedals, only the signal corresponding to the
greatest pressure is sent (the electric signals are not additive).
　
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