Description (AMM 34-28-31)

 

A- The inertial reference unit (IRU) provides attitude, acceleration, angular rates, velocity, true and magnetic headings, positional data, absolute altitude and wind data signals. The signals are developed from a set of three laser gyros and three accelerometers mounted to airplane reference. The signals are provided to other systems including the flight management computer system, the digital flight control system, the electronic flight instruments system, the

 

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 hold down 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.

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 discrete 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:

 

• Drift Angle: -10¡ (L)

• Ground Speed: 200 Kts

• Inertial Vertical Speed: -600 ft/min

• Magnetic heading: 15¡

• Pitch Angle: 5¡

• Present Position (Lat, Long): N22¡ 30', E22¡ 30'

• Roll Angle: 45¡ (R)

• True Heading: 10¡

• Wind Direction: 30¡

• Wind Speed: 100 Kts

• The total time indicator displays accumulated operating time.

 

G- 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.

 

H- 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.

 

I- 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.

 

J- 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.

 

K- 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.

 

L- AIRPLANES WITH IRU -109 AND PREVIOUS; Built-in test equipment (BITE) circuits isolate faults to the LRU level. Faults are indicated by the yellow fault ball on the front of the IRU, the amber FAULT light on the MSU, and a status bit on the data buses to the user systems. The user systems may record an inflight fault based upon the fault from the IRU. In addition, status words are stored in a non-volatile memory in the IRU for at least the previous nine flights. Contents of the non-volatile memory can be extracted on the ground for maintenance.

 

M- AIRPLANES WITH IRU -110 AND SUBSEQUENT; Built-in test equipment (BITE) circuits isolate faults to the LRU level. Faults are indicated by the amber FAULT light on the MSU, and a status bit on the data buses to the user systems. The user systems may record an inflight fault based upon the fault from the IRU. In addition, status words are stored in a non-volatile memory in the IRU for at least the previous nine flights. Contents of the non-volatile memory can be extracted on the ground for maintenance.

 

N-Parameters continuously monitored by the BITE include the laser gyros, accelerometer and power supply outputs, computer and memory operation, and temperature. When the IRU is initially turned on, the battery power is tested. The ON DC indication displays during the battery check.

O- The manual test is initiated by pressing the test switch on the IRU or by holding the display selector in TEST position. The test is also performed as part of the IRS BITE from the FMCS CDU. The test can be done in ALIGN mode, or in NAV mode when ground speed is less than 20 knots. It is inhibited in ATT mode.