2. Air/Ground Relays_________________
A. The air/ground relays control airplane systems when the airplane is in the air (air mode) or on the ground (ground mode).
B. In ground mode, the wing and body gear trucks are not tilted and both body gear are down and locked. In air mode, the wing and body gear trucks are tilted. Tilt is a condition when the aft wheels of a truck are lower than the forward wheels. The body gear tilt angle is approximately 8 degrees from the horizontal. The wing gear tilt angle is approximately 53 degrees from the horizontal.
C. Proximity sensors, located on the landing gear tell the position of the landing gear. Each sensor supplies a signal to the PSEU. The PSEU changes the sensor input signals to discrete output signals for the air/ground relays.
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MAINTENANCE MANUAL
PSEU EI-3
Landing Gear Multiple Use System Component Location
Figure 1
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D. The relays stop air mode operation on the ground. They also stop ground mode operation in the air. Two air/ground relay systems, the primary and alternate, operate independently. Each relay system has master relays and control relays. Master relays allow connections to other relays. Control relays supply the connections to turn on airplane systems. One of the two relay systems, the primary or the alternate, control most airplane operations. Some critical airplane systems use both relay systems.
(1)
In the air/ground relay systems, there are six master relays. There are four master relays in the primary system and two in the alternate system. Two primary master relays (R910 and R952) and one alternate (R1128) energize in the air (air mode). They do not energize on the ground (ground mode). The other three master relays (R778, R7073 primary, and R988 alternate) energize on the ground. They do not energize in the air. Each master relay can energize one or more control relays. They can also supply a connection to allow an airplane operation.
(2)
There are many control relays in the primary and alternate air/ground systems. The control relays supply connections for airplane operations.
E.
The air/ground relays used in the primary and alternate air/ground relay systems are four pole double throw. They are hermetically sealed with pin-type terminals for electrical plug connections. Some relays are two amp rated and some relays are ten amp rated.
3. Proximity Switch Electronics Unit (PSEU)
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A. The PSEU functions are:
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To supply position data from landing gear, thrust reverser, and auto restow subsystems to the EICAS display units and CMC
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To supply control data to relays in other systems
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To monitor inputs, outputs, and internal circuits with built-in-test-equipment (BITE).
B.
PSEU input signals come from proximity sensors and external switches. The landing gear lever switch, hydraulic pressure, and body gear steering actuators are monitored by discrete switch input devices. These are the input signals from external switches.
C.
The PSEU uses digital logic and output drivers to convert the input signals into digital or discrete outputs. Digital outputs supply messages on the EICAS display unit. These outputs also supply data to the Central Maintenance Computer System (CMCS), Aircraft Condition Monitoring System (ACMS), and Flight Recorder. The discrete outputs supply signals to operate indicating systems.
D.
The ARINC 429 data link is the interface between the PSEU and the other systems it supplies data to.
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