AN ACR DC8 FREIGHTER FLC FLEW THE ACFT ON AN OCEANIC RTE WITH THE FO'S HORIZ SIT INDICATOR INOP. THE FACT THAT THE ACFT WAS EQUIPPED WITH A GPS SYS MADE THE OVERWATER OP SAFE.

Narrative

ON NOV/XA/97; FLT FROM LAX TO HNL; WE DISCOVERED AFTER LNDG THAT THE FO'S HSI COURSE INDICATOR WOULD NOT TURN WHEN THE SET KNOB WAS TURNED. WE WROTE THE DISCREPANCY UP AND WENT TO THE HOTEL FOR OUR REQUIRED REST. UPON RETURNING TO THE ACFT THAT EVENING WE FOUND THAT HNL MAINT HAD DEFERRED THE HSI UNDER MEL. THE INST WAS UNUSABLE. I COULDN'T BELIEVE THAT A PRIMARY NAV FLT INST COULD BE DEFERRED. BUT THERE IT WAS IN BLACK AND WHITE IN THE MEL. NOTE THAT IN THE REMARKS OF THIS MEL ITEM; 1 HSI (ILS NAV SYS) MAY BE INOP PROVIDED THAT IT IS 'NOT PWRED BY AN EMER BUS OR EQUIVALENT; AND IS NOT REQUIRED TO ACCOMPLISH AN EMER PROC.' SINCE THE CAPT'S FLT INSTS ARE ON THE EMER BUS THE CAPT'S HSI MUST BE OPERATIVE FOR FLT; BUT THE FO'S CAN BE INOP. THERE IS NO STIPULATION TO REQUIRE A WX RESTR IF AN HSI IS INOP SUCH AS 'VFR CONDITIONS ONLY -- RPTED AND FORECAST AT THE DEST.' INSTEAD; THE MEL ALLOWS A CREW TO NAV AND FLY APCHS SOLELY WITH THE USE OF ADF NAV. ON OUR HNL TO LAX FLT; ADF NAV OVERWATER WAS NOT POSSIBLE. EVEN THOUGH IT WAS NOT SPECIFICALLY REFED IN THE MEL ITEM REMARKS; OUR APPROVED GPS WAS SUFFICIENT FOR THE ENRTE NAV PORTION OF THE FLT. HOWEVER; AN ADF APCH INTO LAX; THOUGH POSSIBLE; WOULD HAVE BEEN HIGHLY IMPRACTICABLE AS LAX TFC WOULD HAVE BEEN SEVERELY IMPACTED WITH THE POSSIBLE CESSATION OF PARALLEL ILS APCHS DURING OUR APCH. IF AN ILS OR LOC APCH WERE FLOWN WITH EITHER HSI INOP THERE WOULD BE NO BACKUP VERIFICATION OF COURSE AND GS GUIDANCE FROM THE PNF -- YET THERE IS NO RESTR FOR ILS/LOC APCHS IN THE MEL ITEM. IN THE INTEREST OF SAFETY THIS MEL ITEM SHOULD BE MODIFIED TO REQUIRE THAT BOTH HSI'S (OR PDI'S AS APPROPRIATE) BE OPERATIVE FOR FLT. AT THE VERY LEAST; THE MEL REMARKS SHOULD BE MODIFIED TO REQUIRE MINIMUM VFR CONDITIONS -- RPTED AND FORECAST AT THE DEST OR THAT A SUITABLE VOR APCH IS AVAILABLE AND BOTH PLTS HAVE OPERABLE RMI/VOR RECEIVERS AND THAT ILS/LOC APCHS ARE NOT AUTH.