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The ECU is the primary interface between system components. Circuit boards in
the ECU control Combined hydraulic system braking and anti-skid functions,
Auxiliary hydraulic system braking and pulsed anti-skid braking. One circuit board
is dedicated to the Built-in Test Equipment (BITE). The ECU receives signals from
the LVDTs, WSSs, and commands from cockpit switch panel selections (Anti-skid
ON / OFF, Brake Test, and Brake Nutcracker Override). In response to received
data, the ECU is programmed to apply braking in response to LVDT commands,
reduce brake application if a wheel skid is detected, or provide a fault detected
signal to the DAUs for annunciation on EICAS displays. Other ECU functions
include wheel speed data for control of automatic ground spoiler deployment,
prevention of a locked wheel on landing, and wheel spindown on landing gear
retraction.
The ECU controls the main wheel brakes with inputs to the Hydraulic Brake
Control Module (HBCM). The HBCM is connected to pressure lines from the
Combined hydraulic system and the Auxiliary hydraulic system. A simplified
diagram of the hydraulic components of the Brake-by-Wire system is shown in
Figure 15. During normal operations, Combined system pressure displaces a
shuttle valve in the HBCM and is routed to right and left brake control valves that
are electronically operated by the ECU. The ECU signals the valves to open
OPERATING MANUAL
2A-32-00 PRODUCTION AIRCRAFT SYSTEMS
Page 36
January 31/02
Revision 6
proportional to LVDT inputs. Combined system pressure is then directed through
brake lines incorporating hydraulic fuses and pressure transducers to the shuttle
valves at the wheel brakes. Indication of hydraulic pressure applied to the brakes
is available on the cockpit EICAS displays using data transmitted from the
pressure transducers to the DAUs. If there is a failure of the Combined hydraulic
system, the Auxiliary hydraulic system will provide wheel braking. A pressure
switch in the HBCM monitors Combined hydraulic system pressure. If Combined
system pressure falls, the HBCM will automatically switch to the Auxiliary system
when all of the following conditions are met: (with AUX Pump armed) Combined
system pressure drops below 1500 psi, either main landing gear nutcracker
switch is in the ground mode, and a brake pedal (LVDT) is depressed to a least
10% of full travel. As Auxiliary system pressure increases, it displaces the HBCM
shuttle valve that closes off Combined system pressure input and routes Auxiliary
system pressure to the brake control valves. All normal brake system functions
are available with Auxiliary hydraulic power.
If Combined or Auxiliary hydraulic pressure is not available, a parking/emergency
brake system with stored accumulator pressure may be used to stop the aircraft.
The brake accumulator is in the nose wheel well, and has a nitrogen gas precharge
of 1200 psi. The accumulator is shown in Figure 10. When normal
hydraulic pressure is initially applied to the aircraft, the accumulator is fully
pressurized with hydraulic fluid at 3000 psi. Accumulator pressure may be read at
the accumulator in the nose wheel well, or on the indicator on the copilot lower
instrument panel. The accumulator, when fully charged contains sufficient
pressurized hydraulic fluid for approximately 5 to 6 brake applications. The PARK
/ EMER BRAKE handle on the cockpit center console is connect by cable to the
modulating valve at the accumulator. Brake modulation can be accomplished by
pulling out the handle between ¼ and ½ inch. Handle travel beyond approximately
½ inch results in the application of full accumulator pressure. No anti-skid is
available with emergency braking. To apply accumulator pressure for parking the
aircraft, the PARK / EMER BRAKE handle is pulled up and then rotated clockwise.
To release the PARK/EMER BRAKE, pull the handle up and rotate
counterclockwise.
The Brake-by-Wire system provides automatic spindown of the wheels during
landing gear retraction. When the landing gear lever is selected to the up position
and at least one nutcracker (squat) switch is in the air (weight off wheels) mode,
the ECU signals an initial partial brake application to slow wheel spin, followed by
a full brake application to stop wheel spin. The initial brake application is for two
(2) seconds, the subsequent application is for eight (8) seconds, with the hydraulic
pressure applied ranging from 400 psi to 1500 psi.
2A-32-44: Electronic (Brake-By-Wire) Anti-skid Protection
1. General Description
NOTE:
This section pertains to aircraft S/N 1000 - 1213,
excluding 1183, without ASC 307.
　
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