IRS FAULT

Condition: One or more of these occurs:
An IRS fault occurs
On the ground, if the ALIGN light is also illuminated, the present position entry is possibly incorrect.

1. Choose one:
   On the ground:
   Go to step 2
   In flight:
   Go to step 6

On the ground

1. Choose one:
   ALIGN light is extinguished:
   Do not takeoff.
   COMPLETE
   ALIGN light is also illuminated:
   IRS mode selector (failed side) OFF
   The FAULT light extinguishes immediately and the ALIGN light extinguishes after approximately 30 seconds.
   Go to step 3

2. After the ALIGN light extinguishes:
   IRS mode selector (failed side) NAV

3. Choose one:
   ALIGN light is flashing:
   Re-enter present position.
   Go to step 5
   ALIGN light is not flashing:
   Go to step 5

4. Choose one:
   FAULT light illuminates again:
   Do not takeoff.
   COMPLETE
   FAULT light does not illuminate again:
   COMPLETE

In Flight

1. The IRS ATT and/or NAV mode(s) can be inoperative.

2. Partial capability can be restored by selecting attitude mode on the failed IRS. Straight and level, constant airspeed flight must be maintained for at least 30 seconds.

3. Choose one:
   Selecting attitude mode on the failed IRS is needed:
   Go to step 9
   Selecting attitude mode on the failed IRS is not needed:
   Go to step 12

4. Do the next step only if the captain's or first officer's primary attitude display is failed.

5. IRS mode selector (failed side) Confirm ATT (Action is irreversible.)
   Maintain straight and level, constant airspeed flight until the attitude display recovers (approximately 30 seconds).
   The primary attitude display stays failed and the SET IRS HDG prompt on the POS INIT page is blank until the attitude mode alignment is complete.

6. Choose one:
   FAULT light extinguishes:
   Enter magnetic heading on the POS INIT page or on the overhead IRS display unit by selecting HDG/STS.
   Note : Periodically enter updated heading on the POS INIT page or on the overhead IRS display unit by selecting HDG/STS.
   Do not engage either autopilot.
   COMPLETE
   FAULT light stays illuminated:
   Go to step 12

7. IRS transfer switch BOTH ON L or BOTH ON R
   Note: Autopilot(s) cannot be engaged.

IRS FAULT

On GND
If two consecutive entries of the same position do not pass the IRS latitude comparison test, the FAULT amber light illuminates steady and the IRS cannot be used for navigation.
If this was not the case and the FAULT amber light illuminates (typically at the first alignment of the day), this 'glitch' can be resolved by placing the LIGHT TEST switch into Test 10 seconds, then back to Dim or Bright.

Cycle the IRS to OFF and back to NAV for a new alignment, do not taxi. If the fault persists, refer to MEL 34-35 [Inertial Reference system]

In FLT
For commonality between Litton and Honeywell IRU systems, the NNC does not differentiate between FAULT steady and FAULT flashing.

ATT mode failure can be recognized by loss of pitch and roll indication. NAV mode failure is recognized by loss of groundspeed and present position data.

If the ATT mode has failed, basic attitude data may be recovered by forcing the IRS to ATT. If recovery is not successful, the IRS is inoperative. In this case, transfer IRS BOTH on X.

The autopilot FCC utilizes pitch data from the onside IRU and roll data from the other IRU. If attitude recovery was successful, technically speaking and not in line with the NNC, both autopilots are again functional, but there is no hard-over protection!

If only the NAV mode has failed, technically speaking and again not in line with the NNC, both autopilots are functional, except for navigation modes LNAV and VNAV. The opposite autopilot is functional in APP mode, if there was no IRS transfer.

SOP B-RNAV requires only one IRS in NAV mode. For P-RNAV, both IRS must be in NAV mode. Dual channel CAT II-III approach is no longer possible. Verify the Lower Landing Minimum Requirements Table.

IRS MODE selector(s) ATT

Alignment : Climb and descent are allowed, but the wings must be kept level and the airplane may not accelerate or decelerate for 30 to 60 seconds.

Heading : Obtain the heading from the remaining IRS or use the magnetic compass reading corrected for deviation. The HDG> prompt on the FMC POS INIT page is only displayed after attitude recovery. Enter the magnetic heading in the FMC with 3 digits or via the ISDU keyboard on the aft OVHD panel :

• place the SYS DSPL selector to the failed IRS (Left or Right)
• place the DSPL SEL selector to HDG/STS
• enter magnetic heading with 4 digits (Litton) or H+3 digits (Honeywell)

Updating : The IRS drifts like a conventional gyro. The heading should be updated every 15 minutes and prior to starting the approach.

Note: If the FAULT amber light remains illuminated in ATT, consider cycling the IRS to OFF and back to ATT in-flight. (alignment takes up to 120 seconds)