RPTR ERROR ADMITTED IN GROSS NAV ERROR; OVERWATER OP. AFTER AN AUTOPLT DISCONNECT; FLC FAILED TO RE-ENGAGE THE AUTOPLT MODE SELECTOR TO INS NAV POS FROM THE HDG POS.

Narrative

SHORTLY AFTER REACHING CRUISE AND SELECTING INS NAV; AN ELECTRICAL SYS SPIKE TRIPPED OFF THE AUTOPLT. THE AUTOPLT WAS RE-ENGAGED USING THE NORMAL INITIAL SETUP OF 'F-H-A' FLT DIRECTOR HDG ALT. SIMULTANEOUSLY; ATC ISSUED A FREQ CHANGE; AND WE WERE PASSING THE FIRST RPTING POINT. CONTACT WAS MADE WITH NY RADIO AND THE STANDARD POS RPT MADE. THOUGH WE FAILED TO RE-ENGAGE INS/NAV THE NEXT POS RPT CAME WITHIN 1 MIN OF THE ETA AT THAT POINT. ALL APPEARED NORMAL UNTIL NY RADIO ADVISED US TO CONTACT ZNY ON VHF. WE WERE ADVISED THAT IT APPEARED THAT WE WERE APPROX 45 MI E OF THE AIRWAY. A CHK OF INS COORDINATES CONFIRMED OUR POS. THE FLT WAS CLRED DIRECT TO THE NEXT WAYPOINT AND WAS COMPLETED WITHOUT FURTHER INCIDENT. ALTHOUGH THE AUTOPLT RE-ENGAGEMENT TO 'F-H-A' WAS ANNOUNCED THE IMMEDIATE FREQ CHANGE AND POS RPT DISTR THE CREW FROM MAKING THE USUAL XCHK AND THE SUBSEQUENT RE- ENGAGEMENT OF INS NAV. SUPPLEMENTAL INFO FROM ACN 361732: WHEN RE-ENGAGED AUTOPLT WAS IN HDG MODE AND NOT NAV MODE ALLOWING ACFT TO DRIFT OFF COURSE.