KC135 CREW DEVIATES FROM ASSIGNED TRACK DUE TO DATA INPUT ERROR IN NAV SYS IN CZQX AIRSPACE.

Narrative

WE WERE FLYING OVER THE NORTH ATLANTIC FROM KEFLAVIK NAS (BIKF) TO THE STATES IN NON RADAR ENVIRONMENT. APPROX 1/2 WAY ACROSS; AND AFTER XING A CHK POINT; THE ACFT MADE AN UNEXPECTED SIGNIFICANT L TURN (APPROX 30 DEG HDG CHANGE). THE AUTOPLT WAS COUPLED TO THE INS AT THE TIME. I UNCOUPLED THE INS AND RETURNED THE ACFT TO A HDG WE EXPECTED WOULD TAKE US TO THE NEXT FIX. HOWEVER; MY NAVIGATOR INSISTED WE NEEDED TO COME FURTHER L TO HEAD TOWARD THE NEXT FIX. THIS DIDN'T MAKE SENSE BECAUSE THERE SHOULD NEVER HAVE BEEN A TURN THAT SIGNIFICANT ALONG THE RTE OF FLT. AFTER ANALYZING THE PROB; THE NAVIGATOR DETERMINED THAT HE HAD ENTERED THE INCORRECT LATITUDE COORDINATES BY A MARGIN OF 1 DEG. THIS PLACED US 60 NM N OF THE CHKPOINT. ATC WAS PROMPTLY NOTIFIED OF THIS DEV AND REQUESTED THAT WE RPT WHEN WE RETURN TO COURSE. WE WERE BACK ON COURSE IN 15 MINS. THE HUMAN FACTORS UNDERLINING THIS SIT ARE HIGHLIGHTED BECAUSE OF THE METHOD BY WHICH THE COORDINATES ARE ENTERED; AND THE WAY THE INFO IS PRESENTED TO THE PLTS FOR XCHKING. THE COORDINATES FOR EACH CHKPOINT ARE MANUALLY PROGRAMMED INTO THE INS BY THE NAVIGATOR. AFTER THE NAVIGATOR ENTERS THE RTE OF FLT; THE PLT THEN CAN XCHK THOSE POINTS BY COMPARING WHAT HE SEES ON A SMALL DISPLAY TO WHAT HE READS ON THE CHART. THE POS OF THIS DISPLAY (WHICH IS CLOSER TO THE COPLT) AND THE SIZE OF THE LIGHTED LETTERING SOMETIMES MAKES IT DIFFICULT TO READ THE COORDINATE INFO THE NAVIGATOR HAS ENTERED. THE CREW'S ABILITY TO BACK EACH OTHER UP IS DEGRADED BY THIS DESIGN FLAW. DEPENDING ON EXTERNAL LIGHTING CONDITIONS; IT SOMETIMES TAKES A LOT OF EFFORT TO VISUALLY XCHK THE INFO; THEREBY ON OCCASION MAKING AN ERROR LIKELY.