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.