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 CAUTION: THE UNIT CONTAINS STATIC SENSITIVE DEVICES, AND PRECAUTIONS MUST
_______ BE USED WHEN HANDLING THE UNIT. FAILURE TO OBSERVE ELECTROSTATIC DISCHARGE PRECAUTIONARY PROCEDURES WHEN HANDLING THE IRU MAY RESULT IN DEGRADATION OR FAILURE OF THE IRU. THE UNIT CONTAINS HIGH VOLTAGE DEVICES. THE UNIT COVER SHOULD NOT BE REMOVED EXCEPT IN THE OVERHAUL SHOP
 B.  The IRU weighs 42.4 lbs. It consists of 3 laser gyros, 3 accelerometers, 9 circuit cards, a power supply and a chassis assembly. The two IRU's are located in the main equipment center, E3-5.
 C.  The IRU is interfaced with airplane wiring through a triple section connector of which only the two lower sections are used. Two holddown hooks are used to secure the IRU on its mount.
 D.  AIRPLANES WITH IRU -109 AND PREVIOUS; The front panel of the IRU has a fault ball indicator, an interface test switch, and, on some models, a total time indicator.
 E.  AIRPLANES WITH IRU -110 AND SUBSEQUENT; The front panel of the IRU has an interface test switch and on some models a total time indicator.
 F.  AIRPLANES WITH IRU -109 AND PREVIOUS; 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.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.

 

INERTIAL REF UNIT (E3-5)
ONLY ONE DADC IS CONNECTED

AIRPLANES WITH -110 IRU's AND SUBSEQUENT;
TO EACH IRU
NO FAULT BALL INDICATOR IS INSTALLED. THEGROUNDED IF IRU-2CHASSIS INDICATOR HOLE CONTAINS A PLUG.
IRU SchematicFigure 10
MSU

A 737-300/400/500MAINTENANCE MANUAL
 G.
H.
I.
J.
K.
A
737-300/400/500MAINTENANCE MANUAL
 (1)  
AIRPLANES WITH IRU -110 AND SUBSEQUENT; Failures detected by BITE which do not affect the outputs are stored in BITE memory.

 (2)  
Pushing the test switch activates the ISDU annunciator discretes for 2 seconds, then IRU digital failure words for 8 seconds. After 10 seconds and until the switch is released, the IRU outputs fixed value test data for observation in the flight deck. The test may be performed in the NAV or ALIGN mode. The test is inhibited in the ATT mode and in the NAV mode whenever the ground speed is greater than 20 knots. Some displayed test outputs are:

 (a)  
Drift Angle: -10° (L)

 (b)  
Ground Speed: 200 Kts

 (c)  
Inertial Vertical Speed: -600 ft/min

 (d)  
Magnetic heading: 15°

 (e)  
Pitch Angle: 5°

 (f)  
Present Position (Lat, Long): N22° 30', E22° 30'

 (g)  
Roll Angle: 45° (R)

 (h)  
True Heading: 10°

 (i)  
Wind Direction: 30°

 (j)  
Wind Speed: 100 Kts

 

 (3)
The total time indicator displays accumulated operating time.The IRU has two power sources, one a 115 volt ac source and one a 28 volt dc source. Either source is sufficient for operation but both are required for initial startup.Inside the IRU, the power control circuit automatically switches to a backup source (hot battery bus) when the normal 115 volt ac source is not available. During startup, the control circuit verifies that the battery is connected by switching off the 115 volt ac input. Inputs from the mode select switch controls the power supply switching circuits. The output of the power supply provides control voltage for the IRU and the ISDU and high voltage excitation to the three laser gyros.All IRU input data is routed into the computation circuit. The air data source supplies the ARINC 429 bus input containing air data. Initialization data originates from either the flight management computer or the ISDU. The IRU program pins tell the computation circuits which way the IRU is facing in the airplane.Software does all computations, including compensations, navigational calculations, and coordinate transformations. Gyro and accelerometer outputs are compensated for sensor bias, scale factor, misalignment, and thermal changes. The compensated signals are used in computing airplane pitch, roll and heading relative to the local navigation coordinates. Airplane attitude is used to resolve acceleration into components relative to the local navigation coordinate system. Barometric altitude from an air data computer is used to compensate the vertical speed computations.Three identical ARINC 429 data buses (high speed) provide digital data to flight management, autoflight, and flight instrument systems. A status discrete is applied to the MSU.
　
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