FMC LNAV MODE WAS READING 190 DISTANCE WHEN ACTUAL DISTANCE 30 MILES.

Narrative

WE WERE IN LEVEL FLT AT 15000' MSL AND NAVING THE ROCKDALE 5 ARR TO HARTFORD; CT (BDL). WE WERE USING THE FLT MGMNT SYS NAV FUNCTION WITH THE AUTOPLT ENGAGED IN THE L-NAV MODE. I NOTICED THAT THE MILEAGE ON THE FMC SCREEN INDICATED THAT WE WERE 190 MI FROM BDL. WE WERE ATTEMPTING TO FIND THE ERROR WHEN THE CENTER CTLR TOLD US TO INTERCEPT THE STAR. THE FMC INDICATED THAT WE WERE ON THE ARR ROUTE. I SWITCHED TO VOR/DME NAV AND FOUND THAT WE WERE W OF COURSE AND 30 MI FROM THE BDL VORTAC. AS I WAS TURNING TO THE E TO INTERCEPT THE ARR ROUTE; THE CTLR INSTRUCTED US TO FLY A 090 DEG HDG AND CONTACT HARTFORD APCH. WE WERE VECTORED TO THE FINAL APCH AND LNDG W/O FURTHER INCIDENT. EVERYTHING APPEARED IN ORDER ON THE FMCS EXCEPT FOR THE MILEAGE. THE ROUTE WAS DROPPED FROM THE SYS AFTER WE LANDED; SO WE COULD NOT FIND THE ERROR. A PROC THAT WOULD PREVENT THIS FROM HAPPENING WOULD BE TO HAVE THE NON FLYING PLT MONITOR THE ARR BY USE OF THE VOR/DME NAV. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. WAS NOT ABLE TO DETERMINE WHAT CAUSED THE ERROR IN THE COMPUTER. COMPANY HAS ISSUED A MEMO THAT THIS TYPE OF ERROR CAN HAPPEN AND HAS REQUESTED FLT CREWS TO SET UP RAW DATA FOR BACKUP ON ALL APCHS. THE FMC IS NORMALLY POWERED FROM AC WITH DC AS A BACKUP. THE CAPT CHKED THE ELECTRICAL SOURCE AND EVERYTHING WAS NORMAL. BECAUSE OF THE SYS CAPABILITIES OF BUILDING YOUR ROUTE PLUS ALL THE STORED ROUTES; MANY PLTS DO NOT HAVE THEIR CHARTS AVAILABLE. HE HAS NOW NOTICED MANY PLTS AT LEAST HAVE THE CHARTS READILY AVAILABLE.