Narrative:

Due to a navigation equipment failure (#1 INS, omega had already failed) we were diverting into heathrow for repairs. We were cleared direct bnn to hold, ETA XA18. We were descending to FL160 when my attention was diverted by the master caution light illuminating. I checked the cue lights and found the fuel cue light on. We were dumping fuel at the time. I turned to the fuel panel. I saw that #3 fuel quantity was down to stand pipe level and had activated the fuel xfeeds to close and caused the fuel cue light. I noticed that the fuel was not balanced properly and was asking the flight engineer about that when the first officer said he had slipped below our cleared to flight level or FL160, but that he had caught it and was climbing back. I looked at my altimeter and saw 15700 ft and we were climbing. At that point, ATC came on and asked if we were level at FL160. As we were very nearly there by this time I said we were. ATC said they showed us at 15600 ft, so I admitted we had slipped below a little bit but were correcting. Supplemental information from acn 227618: the #1 INS gyroscope failed causing failure of the #1 INS navigation set, #1 attitude indicator, #1 HSI, #1 autoplt, and #1 automatic-throttles, and #2 rmdi compass card. I turned toward bovington, set 16000 ft in the altitude alert and, I thought, armed the altitude capture mode as well. I listened for a few seconds then scanned the instruments. I saw we were at 15750 ft descending and that the altitude capture did not occur.

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Original NASA ASRS Text

Title: ALTDEV WHILE DIVERTING TO LHR FOR NAV EQUIP REPAIRS.

Narrative: DUE TO A NAV EQUIP FAILURE (#1 INS, OMEGA HAD ALREADY FAILED) WE WERE DIVERTING INTO HEATHROW FOR REPAIRS. WE WERE CLRED DIRECT BNN TO HOLD, ETA XA18. WE WERE DSNDING TO FL160 WHEN MY ATTN WAS DIVERTED BY THE MASTER CAUTION LIGHT ILLUMINATING. I CHKED THE CUE LIGHTS AND FOUND THE FUEL CUE LIGHT ON. WE WERE DUMPING FUEL AT THE TIME. I TURNED TO THE FUEL PANEL. I SAW THAT #3 FUEL QUANTITY WAS DOWN TO STAND PIPE LEVEL AND HAD ACTIVATED THE FUEL XFEEDS TO CLOSE AND CAUSED THE FUEL CUE LIGHT. I NOTICED THAT THE FUEL WAS NOT BALANCED PROPERLY AND WAS ASKING THE FE ABOUT THAT WHEN THE FO SAID HE HAD SLIPPED BELOW OUR CLRED TO FLT LEVEL OR FL160, BUT THAT HE HAD CAUGHT IT AND WAS CLBING BACK. I LOOKED AT MY ALTIMETER AND SAW 15700 FT AND WE WERE CLBING. AT THAT POINT, ATC CAME ON AND ASKED IF WE WERE LEVEL AT FL160. AS WE WERE VERY NEARLY THERE BY THIS TIME I SAID WE WERE. ATC SAID THEY SHOWED US AT 15600 FT, SO I ADMITTED WE HAD SLIPPED BELOW A LITTLE BIT BUT WERE CORRECTING. SUPPLEMENTAL INFO FROM ACN 227618: THE #1 INS GYROSCOPE FAILED CAUSING FAILURE OF THE #1 INS NAV SET, #1 ATTITUDE INDICATOR, #1 HSI, #1 AUTOPLT, AND #1 AUTO-THROTTLES, AND #2 RMDI COMPASS CARD. I TURNED TOWARD BOVINGTON, SET 16000 FT IN THE ALT ALERT AND, I THOUGHT, ARMED THE ALT CAPTURE MODE AS WELL. I LISTENED FOR A FEW SECONDS THEN SCANNED THE INSTS. I SAW WE WERE AT 15750 FT DSNDING AND THAT THE ALT CAPTURE DID NOT OCCUR.

Data retrieved from NASA's ASRS site as of July 2007 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.