Narrative:

Aircraft equipped with IRS and FMC. I arrived at aircraft and completed preflight including a full alignment of IRS. Once airborne, confirmed IRS/FMC navigation functions were working normally by using fix page and raw data. Approximately 10-15 mins into the climb, both captain and myself noted the FMC was not automatic updating itself. About this time we received a 'verify position' message from the FMC. We reverted to raw data for navigation and I checked the IRS and FMC position page. Both the left and right IRS claimed the actual position was 3.8 and 3.4 NM (respectively), 90 degrees to our left. We were going to complete a position shift exercise when we noted we had automatic updating functions back and the FMC position was correcting itself. Later in the flight, a flight attendant called and asked our ground speed. I told her it was 389 KTS. She stated she had lost a bet because a passenger guessed 388 KTS. I asked her to ask the passenger if he had a GPS. The passenger said yes. I asked her to tell the passenger to turn it off and keep it turned off. The flight continued uneventfully. At deplaning, I found out the passenger was employed by another airline. Flight in question was to phx in a B737-300.

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Original NASA ASRS Text

Title: FLC OF B737-300 HAVE NAV PROBS INFLT WHEN BOTH IRS AND FMC DO NOT UPDATE INFO. THEY REVERT TO RAW DATA AND LATER THE PROB CLRS ITSELF. A GPS IS FOUND TO BE IN USE IN THE CABIN.

Narrative: ACFT EQUIPPED WITH IRS AND FMC. I ARRIVED AT ACFT AND COMPLETED PREFLT INCLUDING A FULL ALIGNMENT OF IRS. ONCE AIRBORNE, CONFIRMED IRS/FMC NAV FUNCTIONS WERE WORKING NORMALLY BY USING FIX PAGE AND RAW DATA. APPROX 10-15 MINS INTO THE CLB, BOTH CAPT AND MYSELF NOTED THE FMC WAS NOT AUTO UPDATING ITSELF. ABOUT THIS TIME WE RECEIVED A 'VERIFY POS' MESSAGE FROM THE FMC. WE REVERTED TO RAW DATA FOR NAV AND I CHKED THE IRS AND FMC POS PAGE. BOTH THE L AND R IRS CLAIMED THE ACTUAL POS WAS 3.8 AND 3.4 NM (RESPECTIVELY), 90 DEGS TO OUR L. WE WERE GOING TO COMPLETE A POS SHIFT EXERCISE WHEN WE NOTED WE HAD AUTO UPDATING FUNCTIONS BACK AND THE FMC POS WAS CORRECTING ITSELF. LATER IN THE FLT, A FLT ATTENDANT CALLED AND ASKED OUR GND SPD. I TOLD HER IT WAS 389 KTS. SHE STATED SHE HAD LOST A BET BECAUSE A PAX GUESSED 388 KTS. I ASKED HER TO ASK THE PAX IF HE HAD A GPS. THE PAX SAID YES. I ASKED HER TO TELL THE PAX TO TURN IT OFF AND KEEP IT TURNED OFF. THE FLT CONTINUED UNEVENTFULLY. AT DEPLANING, I FOUND OUT THE PAX WAS EMPLOYED BY ANOTHER AIRLINE. FLT IN QUESTION WAS TO PHX IN A B737-300.

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.