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steering collar to the nose wheels.
The steering commands are processed by the SCU and hydraulic system No. 3
pressure is routed through the system when the landing gear is selected down.
In the air mode, the nose wheel is centered electrically by the SCU and mechanically
by centering cams as the shock strut extends under its own weight.
When the steering system is disarmed or with no WOW signal, the nosewheel steering
will revert to free caster and shimmy damping to ensure stability within the nosewheel
circuit. In the free caster mode of operation, steering is accomplished only through
differential braking and differential thrust.
The SCU continuously monitors the nosewheel steering system and any fault within
the system will be displayed as a caution message. Fault detection will result in
nosewheel steering system shutdown and the system will then operate in free caster
mode of operation.
P I LOT T R A I N I N G GU I D E
LANDING GEAR AND BRAKES
14-26 For Training Purposes Only
Sept 04
NOSEWHEEL STEERING SCHEMATIC
HYDRAULIC
SYSTEM NO. 3
GX_14_025
STEERING MANIFOLD
STEERING
CONTROL
UNIT
WOW
NOSE LANDING
GEAR DOWN
AND LOCKED
TO EICAS
NOSEWHEEL
STEERING SWITCH
STEERING TILLER
PILOT'S SIDE PANEL
RUDDER PEDALS
P
O
S
I
T
I
O
N
S
E
N
S
O
R
S
NOSEWHEEL
STEERING
ACTUATORS
LGECU
NOSE STEER
ARMED
OFF
RMED
For Training Purposes Only
Sept 04
14-27
P I LOT T R A I N I N G GU I D E
LANDING GEAR AND BRAKES
BRAKE CONTROL SYSTEM
The main wheels have carbon disk brakes. They are controlled by the Brake Control
Unit (BCU) using an antiskid system with autobraking capability.
Refer to BRAKE CONTROL SCHEMATIC of this chapter to support the information
that follows.
The brake control system provides hydraulic services to the outboard (hydraulic
system 2) and the inboard braking systems (hydraulic system 3). Hydraulic system 3
will also provide pressure to all brakes via shuttle valves for parking/ emergency brake
application. Brake systems 2 and 3 have an accumulator (backup) for emergency
operation in case of hydraulic system failure. Hydraulic volumetric fuses are installed
at each brake supply line to provide system protection against line rupture.
Two brake shutoff valves control the inlet pressure to the brake control valves and are
used to:
• Turn off hydraulic power to the brake control valves during flight
• Shut off pressure in the event of uncommanded braking
The hydraulic inlet porting of the brakes has a shuttle valve. Brake shuttle valves are
used to isolate brake control function from park/emergency brake function. Four
shuttle valves are used, one for each brake. The shuttle valve provides the higher of the
two input pressures (either brake control/antiskid or parking/emergency brake) to the
brake.
Pilot and/or co-pilot brake input is provided by four brake pedal transducers. The
signal is processed by the electronic Brake Control Unit (BCU) and commands
appropriate brake pressure. If a failure or unreliable signal is present, a pilot/co-pilot
brake fault message will display on EICAS. Wheel speed is monitored by four
transducers installed in each main wheel axle and these signals are used for antiskid
control valve operation. The brake and antiskid control system interfaces with the
BCU for autobrake capability.
The BCU provides the brake control system with built-in-test equipment (BITE) and
the following protections:
• Gear retract braking (stops wheel rotation, after takeoff)
• Inhibiting normal gear braking in flight
• Independent wheel antiskid
• Locked wheel, and touchdown protection
In the air mode (no WOW signal) the BCU configures the airplane to a full brake
dump to prevent wheel lock-up on touchdown. The touchdown protection system is
canceled to allow modulated braking once a predetermined wheel spin-up signal is
present or airplane WOW is detected. The BCU will cancel hydraulic power to any
wheel which is sensed to be at an abnormal deceleration rate (i.e. locked wheel).
P I LOT T R A I N I N G GU I D E
LANDING GEAR AND BRAKES
14-28 For Training Purposes Only
Sept 04
The dual brake control system independently controls the braking of each main wheel
by modulating the pressure outputs of the appropriate brake control valve. This
modulation is controlled by the BCU which monitors individual wheel speed and
deceleration through the wheel speed transducers.
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