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MAINTENANCE MANUAL
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REFERENCE UNIT TCAS CMPTR(E1-6)
SELECT SW RELAYS
IRS Interface Schematic
Figure 2
E20408
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747-400
MAINTENANCE MANUAL
2. Inertial Reference Unit (IRU) (Fig. 3)
_______________________
A. General
(1)
Each IRU provides the guidance reference for the airplane by sensing airplane angular rate and linear acceleration. Position, velocity, attitude, heading, acceleration and body angular rates are computed from the sensor data. A digital computer in each IRU performs all the calculations for IRS. The left IRU is shown on the schematic. The right and center units are similar.
(2)
The IRU weighs approximately 40 pounds. It consists of 3 laser gyros, 3 accelerometers, circuit cards, a sensor assembly, a power supply and a chassis assembly. The three IRUs are located in the main equipment center, E1-6. Two holddown hooks are used to secure the IRU on its mount.
(3)
The front panel of the IRU has a fault ball indicator, an interface test switch, and on some models, a total time indicator.
(a)
The BITE fault ball indicator shows black for an operational IRU and yellow for a failed IRU. The fault ball indicates a failure that could cause erroneous outputs. Other failures detected by BITE, which do not affect the outputs are stored in BITE memory. Pushing the TEST switch has no affect on the status of the BITE fault ball.
(b)
Pushing the TEST switch activates the IRU to output test data. During the first 2 seconds, the analog fault discrete is turned on. For the next 8 seconds, the analog fault discrete is turned off and the digital outputs are transmitted as failed. This causes warning flags to appear on the flight displays. After 10 seconds and until the switch is released, the IRU outputs test data for observation in the flight deck. This test can be performed in the NAV or ALIGN mode. The CMC can also initiate this test. The test is inhibited in the ATT mode and is inhibited in the NAV mode whenever the ground speed is greater than 20 knots.
(c)
The total time indicator (if present) displays accumulated operating time.
B. The IRU contains the following major functional units:
(1) Digital Inputs
(a) The IRU has two low speed ARINC 429 input ports for air data. Normally, the primary port is used for IRU calculations. When the ADC port select discrete is grounded (by moving the ADC source select switch to the alternative position) the IRU uses the alternate port. The selected port sends ARINC 429 data to the I/O computer.
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115V AC
AC28V DCDC
MSUCMC
AIR DATACOMPUTERS
ADC PORT
SELECT
FMC-L
FMC-R INERTIAL REF UNIT (E1-6)
IRU Internal SchematicFigure 3
R EIU,C EIU,L EIU
AIR/GNDRELAY
(2)
(3)
(4)
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747-400 MAINTENANCE MANUAL
(b)
The IRU has two low speed Arinc 429 input ports for FMC initialization data. Both these ports send ARINC 429 data to the I/O computer. The FMCs send initial latitude/longitude (align mode only) and heading (ATT mode only) to the IRUs. This data is transferred to the IRU memory. It is then placed on the CPU input bus for further processing.
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