Narrative:

During cruise flight, my copilot noticed that the FMS display was calculating the wind speed at almost 300 KTS, obviously too high to be correct. We then spent several mins 'playing' with the FMS computer, trying to determine and correct the problem. During this time, the autoplt was flying and navigating. After several mins of involvement with the computer, center queried us on our route of flight. Upon checking our flight instruments, we realized that the FMS had reverted to 'dead reckoning' and that the aircraft had drifted off the airway. We explained our navigational problem to center, while we returned to tracking the correct radial with the VOR and CDI. Next time that the FMS is giving us winds of 300 KTS, we will immediately think about returning to basic navigation, before we worry about what's wrong with the computer!

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Original NASA ASRS Text

Title: HDG TRACK DEV.

Narrative: DURING CRUISE FLT, MY COPLT NOTICED THAT THE FMS DISPLAY WAS CALCULATING THE WIND SPD AT ALMOST 300 KTS, OBVIOUSLY TOO HIGH TO BE CORRECT. WE THEN SPENT SEVERAL MINS 'PLAYING' WITH THE FMS COMPUTER, TRYING TO DETERMINE AND CORRECT THE PROBLEM. DURING THIS TIME, THE AUTOPLT WAS FLYING AND NAVING. AFTER SEVERAL MINS OF INVOLVEMENT WITH THE COMPUTER, CTR QUERIED US ON OUR RTE OF FLT. UPON CHKING OUR FLT INSTS, WE REALIZED THAT THE FMS HAD REVERTED TO 'DEAD RECKONING' AND THAT THE ACFT HAD DRIFTED OFF THE AIRWAY. WE EXPLAINED OUR NAVIGATIONAL PROBLEM TO CTR, WHILE WE RETURNED TO TRACKING THE CORRECT RADIAL WITH THE VOR AND CDI. NEXT TIME THAT THE FMS IS GIVING US WINDS OF 300 KTS, WE WILL IMMEDIATELY THINK ABOUT RETURNING TO BASIC NAV, BEFORE WE WORRY ABOUT WHAT'S WRONG WITH THE COMPUTER!

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.