ACR WDB DEVIATED FROM CLRNC ROUTE WHEN FLT CREW FAILED TO PROPERLY PROGRAM OMEGA NAVIGATION MODE.

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WE CHKED OVER EBBER AND ATC ADVISED RADAR CHK OVER EBBER AND CLRED ENRTE FREQS (HNL RADIO); RADAR SVCS TERMINATED. ACFT WAS ON AUTOPLT AND OMEGA #2 WAS PROVIDING INPUTS TO AUTOPLT. I FAILED TO NOTICE OMEGA SWITCH WAS IN MANUAL POS AND SO ACFT DID NOT TURN TO NEW COURSE OUTBND. BOTH PLTS WERE BUSY FILLING OUT AIREPS AND NOTING FUEL READINGS AND COMPASS CHKS. ALSO DOING REQUIRED NAV SYS CHKS. IN APPROX 2-3 MINS RECEIVED SELCAL FROM HNL RADIO AND TOLD WE WERE 10 MI OFF COURSE AND TO REATTEMPT CONTACT WITH HNL CTR. CALLED HNL CTR AND THEY ADVISED WE WERE APPROX 17 MI RIGHT OF COURSE. A QUICK CHK REVEALED THE PROB AND CORRECTIVE ACTION TAKEN. CTR ADVISED WE WERE APPROX 21 MI OFF COURSE AND THEY LOST RADAR CONTACT. WE CORRECTED TO COURSE AND CHKED RIGHT OVER ERWEN. NO FURTHER PROBS. FE'S INSTRUMENTS WAS INDICATING PROPERLY; BUT FE WAS BUSY WITH AN AIR-CONDITIONING PROB. CAPT HAD SELECTED RADIO ON HIS HSI TO XCHK VOR POS OVER EBBER AND SO IT WAS OF NO USE IN ALERTING US TO OFF COURSE. MY HSI WAS NATURALLY CTRED SHOWING CONTINUATION OF LAST COURSE. POSSIBLE ADDITIONAL FACTOR IS BACKSIDE OF CLOCK OPERATION. I HAD BEEN UP FOR 17 HRS AT TIME WITH ONLY A SHORT NAP BEFORE DEP. ALSO; I FEEL OMEGA SHOULD HAVE AN INDICATOR LIGHT TO SHOW WHEN SELECTOR IS IN MANUAL.