A B747's INS failed in cruise at FL310. The aircraft began a turning descent. The captain disconnect...

Attributes
ACN	863770
Time
Date	200912
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	INS / IRS / IRU
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 9000 Flight Crew Type 5600
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

While in cruise the aircraft went into a descending right turn unscheduled. I took control after identifying that the first officer's ADI had tumbled. I verified my ADI was still good with the standby ADI and outside reference. By the time corrective action was taken; the aircraft had lost about 500 ft and turned 30-40 degrees off course. I returned the aircraft to proper position after disconnecting the #2 autopilot. After returning to proper altitude and heading; further investigation revealed the #2 INS had a warning light and message which read 'remove inu.' remainder of flight was uneventful. ATC did not call me.

Original NASA ASRS Text

Title: A B747's INS failed in cruise at FL310. The aircraft began a turning descent. The Captain disconnected the autopilot and returned the aircraft to level flight on track.

Narrative: While in cruise the aircraft went into a descending right turn unscheduled. I took control after identifying that the First Officer's ADI had tumbled. I verified my ADI was still good with the standby ADI and outside reference. By the time corrective action was taken; the aircraft had lost about 500 FT and turned 30-40 degrees off course. I returned the aircraft to proper position after disconnecting the #2 autopilot. After returning to proper altitude and heading; further investigation revealed the #2 INS had a warning light and message which read 'remove INU.' Remainder of flight was uneventful. ATC did not call me.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

A B747's INS failed in cruise at FL310. The aircraft began a turning descent. The captain disconnected the autopilot and returned the aircraft to level flight on track.