first officer's ADC instrument source select switch is selected to, respectively.
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RIGHT IRU
IRS - Power Distribution and Control
Figure 8
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28V DC BUS 4
F/O IRS INST SOURCE SEL SWITCH
IRS Select Relays
Figure 9
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(c)
(d)
(e)
(f)
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2) The center IRU uses whichever ADC the left IRU is using if the first officer's IRS instrument source select switch is in the R position and the center IRU uses whichever ADC the right IRU is using if the first officer's IRS instrument source select switch is not in the R position.
Each IRU has two low speed ARINC ports dedicated to FMC input. When initial position is entered on the CDU, the data is transmitted to both FMCs. First, the onside FMC transmits initial position to all three IRUs in a burst, then, the offside FMC transmits initial position to all three IRUs. This allows either forward CDU to be used to initialize the IRUs even if one FMC is not present. The IRUs receive set heading through these ports in a similar manner during ATT mode.The yaw damper system requires IRS data for its self-test. If either yaw damper is placed in self-test, all three IRUs will be given an interface test discrete input by the central maintenance computer which causes the IRUs to output stored data. The CMC can also command each IRU into interface test individually.The MSU supplies system mode discretes to each IRU from their respective mode select switch.Each IRU has three identical high speed ARINC 429 output buses which transmit data to various airplane systems. 1) Each EIU receives data from all three IRUs which is
processed and retransmitted for use by the CMC, flight recorder and by the integrated display system on the PFD, ND, and EICAS.
2) Each FMC receives data from all three IRUs which is used for navigation and to calculate steering commands, pitch commands and thrust limits.
3) Each weather radar transceiver receives attitude inputs from two IRUs which is used for attitude stabilization of the antenna.
4) The yaw dampers use ground speed and body lateral acceleration from all three IRUs which are used for gain scheduling.
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5) The ground proximity warning computer is connected only to the left IRU. IRS position data is used to inhibit warnings in certain parts of the world.
6) Each flight control computer is connected to one IRU. IRS attitude, velocity and accelerations are used in flight control calculations.
(g) Each IRU's 28v DC FAULT output discrete is received by all three EIUs. These discretes activate the IRS LEFT, IRS CENTER and IRS RIGHT messages on the main EICAS display.
(7)
IRS Select Relays
(a) The data transmitted to the IRS select relays are routed to the brake system control unit and to the MAWEA card file. The brake system uses IRS generated ground speed, deceleration and pitch data for anti-skid calculations. The MAWEA uses IRS data to compute stall warning conditions. The IRS select relays are located on P415 Power Distribution Center - Right. Selection of the IRUs is controlled by the IRS source select switch. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 导航 NAVIGATION 1(148)
