ACR WDB TRACK DEVIATION DUE TO INS FAILURE.

Narrative

MY CREW AND I HAD JUST MADE THE XING FROM LOS ANGELES TO HONOLULU IN THIS ACFT EARLIER THAT SAME DAY. IT WAS MY LEG (CAPT). THE #1 AUTOPLT WAS STILL PLACARDED INOP AND WE WERE USING THE #2 AUTOPLT CONNECTED TO THE #2 INS FOR THE XING AS WE HAD DONE ON THE PREVIOUS FLT. WE WERE ASSIGNED TRACK R577 FOR THE XING. THE FLT WAS ROUTINE UNTIL SOME POINT AFTER OUR POS RPT AT ESCRO INTXN. UPON XING EKTAS INTXN (NONREPORTING) A DIFFERENCE IN PLOTTED POS AND DME TO THE NEXT INTXN WAS NOTED. AT THIS POINT THE DIFFERENCE WAS MINOR. THE TRUE HDG BEING FLOWN BY THE AUTOPLT MATCHED THAT OF THE FLT PLAN AND WITH ONLY 2 INS UNITS ON BOARD WE WERE UNABLE TO DETERMINE WHICH UNIT WAS IN ERROR. AS THE FLT PROGRESSED WE NOTICED OPP DIRECTION TFC APCHING (ESTIMATE 10 MI SEP N OF OUR POS AND AT A HIGHER ALT). AT THIS POINT I BECAME OBVIOUS THAT WE WERE NOT ON TRACK. AT THIS POINT ONE INS SHOWED US 35 MI N OF COURSE WHEN COMPARED TO THE OTHER. I ASSUMED THE TFC WE SPOTTED WAS ON THE WBOUND TRACK R576 (IT'S A ONE WAY TRACK) AND WE IMMEDIATELY MADE A 45 DEG CORRECTION TOWARDS THE S. SHORTLY THEREAFTER WE WERE IN RADIO AND RADAR CONTACT WITH ATC. THEY INFORMED US THAT WE WERE IN FACT N OF COURSE AND CORRECTING. THEY THEN CLRED US DIRECT TO LAX AND WE REQUESTED RADAR VECTORS AND RPTED OUR NAV SYS AS UNRELIABLE. FLT WAS ROUTINE FROM THAT POINT ON. AN INS CHK WAS PERFORMED AT THE GATE AND #2 INS WAS FOUND TO HAVE A 65.8 MI ERROR AND WAS INDICATING A 17 KT GND SPD WITH THE BRAKES SET AT THE GATE. A LOGBOOK ENTRY WAS MADE TO RPT THE PROB. IT IS OPERATIONALLY APPROVED TO OPERATE WITH ONLY 2 INS UNITS INSTALLED. IF ONE STARTS TO DRIFT IT LEAVES YOU IN A QUANDARY AS TO WHICH UNIT IS CORRECT. THE BEST YOU CAN DO IS SPLIT THE DIFFERENCE. (NOTE: RUMOR HAS IT THAT ACR IS REPLACING ALL OF OUR DUAL INS UNITS WITH 3 OF THE GREATLY IMPROVED UNITS.) OUR COMPUTER GENERATED FLT PLANS GIVE A TRUE HDG FOR EACH LEG THAT WE CHK TO ASSURE THAT AFTER XING A FIX; THE INS HAS POINTED US IN THE RIGHT DIRECTION; HOWEVER THIS TRUE HDG IS AN AVERAGE MIDPOINT HDG BASED ON THE WINDS AT MIDPOINT ON THAT LEG. IN MY OPINION IT WOULD BE BETTER IF IT WERE BASED ON THE HDG REQUIRED AFTER XING THE FIX. AFTER ALL; THAT'S WHEN YOU MAKE THE XCHK. WE ARE SO USED TO SEEING SEVERAL DEGS OF DIFFERENCE AT THIS POINT THAT WE WERE UNABLE TO SPOT THE DEVIATION THAT PLACED US 35 MI OFF COURSE. AT THE TIME THE ERROR WAS DISCOVERED AND A CORRECTION WAS MADE THIS COCKPIT CREW HAD BEEN ON DUTY FOR MORE THAN 16 HRS!! THIS CREW WAS JUST SUPPOSED TO FLY A ROUND TRIP FROM LAX TO HNL AND BACK TO LAX; BUT THE RETURN FLT WAS DELAYED DUE TO A HIGH SPD REJECTED TKOF FROM HNL BECAUSE OF AN ENG FAILURE INDICATION REQUIRING AN ABORT. IT TOOK MAINT MORE THAT 4 HRS TO REPAIR THE ACFT AND CHANGE THE BLOWN TIRES. THIS SITUATION NOT ONLY EXTENDED OUR SCHEDULED DUTY DAY; BUT IT ALSO SUBJECTED THE CREW TO AN ADDITIONAL AMOUNT OF FATIGUE DUE TO THE STRESS RELATED TO THIS SITUATION. IT IS MY OPINION THAT A 17 1/2 HR DUTY DAY (INTL FLAG OPERATION) IS ASKING TOO MUCH FROM A NONSUPPLEMENTED CREW. I WONDER IF I WOULD HAVE NOTICED OUR EXCESSIVE COURSE DEVIATION SOONER IF I HAD NOT BEEN SO EXHAUSTED.