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processes this information to command landing gear and other aircraft systems’
operation.
In the case of the landing gear, the PSEU monitors the position of the landing gear,
nose doors, uplocks, downlocks and Weight On Wheels (WOW). It also electrically
controls the hydraulic selector valves during normal landing gear extension and
retraction. After takeoff, the PSEU disables the anti-skid and nosewheel steering
systems. At touchdown it rearms the systems for use during the landing roll and taxi.
There are two WOW proximity sensors installed on each landing gear assembly, for a
total of six WOW sensors. Various aircraft systems require “ground” or “air” mode
information for operation. The PSEU determines “ground” mode and “air” mode as
follows:
PSEU MODE DETERMINATION REQUIRED CONDITIONS
Ground - Any three of the four main gear proximity sensors indicate
Weight On Wheels
Air - If “ground” mode conditions are not satisfied, the PSEU is in
“air” mode
PSEU Mode Determination Table
Table 15-3
LANDING GEAR / BRAKES
For Training Purposes Only
May 03
15-13
P I LOT T R A I N I N G GU I D E
Main Wheel Bins and Overheat Detection
Description
Each main gear assembly retracts inward into wheel bins that are recessed in the main
landing gear bay.
Components and Operation
Wheel Bins
The wheel bins isolate the wheels from the main landing gear bays and prevent tirethrown
debris from damaging the equipment located within the MLG bay. The bins
can be removed to allow maintenance personnel access to the MLG bay.
Main Landing Gear Overheat Detection
An overheat detection unit and heat-sensing loop continuously monitor each wheel bin
for overheat conditions. If an overheat condition exists, the MLG BAY OVHT
warning EICAS message will be displayed and the aural “GEAR BAY OVERHEAT”
will sound.
If a failure is detected (shorted heat-sensing loop), the MLG BAY OVHT FAIL
caution EICAS message will be displayed. For additional information, see Chapter 9
Fire Protection.
MLG Bay Overheat Test
The MLG overheat detection system can be tested at the LDG GEAR control panel.
The MLG BAY OVHT TEST switches simulate an overheat or failed condition and
generate EICAS messages as explained above.
MLG BAY OVHT TEST Switches
Figure 15-6
MUTED
OFF
TEST
MLG BAY OVHT TEST
OVHT FAIL
UP DN
LDG GEAR
DN LCK
REL
ANTI-SKID
ARMED
NOSE STEER
ARMED
HORN
OFF
P604_15_030
FAIL Test Switch
Tests MLG bay overheat
failure detection system
OVHT Test Switch
Tests MLG bay overheat
detection system
LANDING GEAR / BRAKES
15-14 For Training Purposes Only
May 03
P I LOT T R A I N I N G GU I D E
Nosewheel Steering System
Description
The nosewheel steering system is hydraulically actuated through a steering actuator
mounted on the nose landing gear assembly. There is no mechanical connection
between the flight deck controls and the steering actuator. Nosewheel steering
commands are transmitted electronically using “steer-by-wire” technology.
Components and Operation
The nosewheel steering system is armed by the NOSE STEER switch located on the
LDG GEAR control panel. The system is activated when hydraulic system 3 is
pressurized, the nose landing gear is down and locked and the PSEU “ground” mode is
satisfied.
Steering control is initiated via the steering tiller and/or by the rudder pedals.
Nosewheel deflection up to 55 degrees on either side of center is possible when using
the steering tiller. Rudder pedal movement will deflect the nosewheel up to 7 degrees
either side of center.
With the NOSE STEER switch selected to ARMED, the 55-degree limit may
inadvertently be exceeded if sufficient differential braking is applied in the direction of
the turn. When the 55-degree angle is exceeded, the nosewheel steering system is
automatically disabled and the nosewheel casters freely to a maximum of 99 degrees.
.
The nosewheel steering system cannot be reengaged unless the
steering angle is less than 55 degrees and the NOSE STEER
switch is selected to OFF and then to ARMED again.
Electronic Control Unit (ECU)
The nose steering electronic control unit (ECU) controls the electrohydraulic valve
that directs hydraulic system 3 pressure to the steering actuator to turn the nosewheels.
Nosewheel position feedback is sent to the ECU from transducers mounted on the
steering actuator assembly.
In the air, the nosewheel is centered electronically by the ECU, and mechanically by
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