Narrative:

Flying FROGO6.fra departure followed by clearance to rebgr after fra. Company route loaded and verified during the preflight. After frogo fix, the FMS engaged, aircraft flew to rebrg instead of the fra VOR. Somehow, fra dropped out of the flight route. ATC noticed the deviation before we did. Once called by ATC, we inserted fra into the route and proceeded direct with a 15 degree heading change. Both pilots were in 'navigation' mode with no 'raw data' ground NAVAID backup. We failed to provide sufficient monitoring during this episode. I strongly believe 2 CDU's and a map presentation (as in the B737-700) would have prevented this error. Backing up the FMS route at all waypoints with the release would have prevented it as well. Supplemental information from acn 619522: both HSI, navigation selectors in navigation. The one CDU that we have was now being utilized for lax ATIS, cruise performance, etc. With both LNAV selectors in navigation. There is no VOR backup. More importantly, with only one CDU, which now makes it a time share item, the active waypoint is not always displayed for immediate viewing. When I fly the -700, I always have a CDU on the legs page and when the aircraft turns, I watch where it's going. This is not possible in an aircraft with only 1 CDU. It's a dumb confign in today's ATC environment. I remember seeing fra on the legs page. I have no clue why we were tracking frogo to rebrg. On the subsequent leg, the LNAV in the aircraft was tracking the course like it had a lock on a weak VOR. Wandering back and forth. I wrote up the discrepancy when we arrived. We need 2 CDU's in these airplanes.

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Original NASA ASRS Text

Title: B737-300 FLT CREW DEVIATES FROM DEP PROC ROUTING DUE TO UNDETECTED LOSS OF A WAYPOINT IN FMS.

Narrative: FLYING FROGO6.FRA DEP FOLLOWED BY CLRNC TO REBGR AFTER FRA. COMPANY RTE LOADED AND VERIFIED DURING THE PREFLT. AFTER FROGO FIX, THE FMS ENGAGED, ACFT FLEW TO REBRG INSTEAD OF THE FRA VOR. SOMEHOW, FRA DROPPED OUT OF THE FLT RTE. ATC NOTICED THE DEV BEFORE WE DID. ONCE CALLED BY ATC, WE INSERTED FRA INTO THE RTE AND PROCEEDED DIRECT WITH A 15 DEG HDG CHANGE. BOTH PLTS WERE IN 'NAV' MODE WITH NO 'RAW DATA' GND NAVAID BACKUP. WE FAILED TO PROVIDE SUFFICIENT MONITORING DURING THIS EPISODE. I STRONGLY BELIEVE 2 CDU'S AND A MAP PRESENTATION (AS IN THE B737-700) WOULD HAVE PREVENTED THIS ERROR. BACKING UP THE FMS RTE AT ALL WAYPOINTS WITH THE RELEASE WOULD HAVE PREVENTED IT AS WELL. SUPPLEMENTAL INFO FROM ACN 619522: BOTH HSI, NAV SELECTORS IN NAV. THE ONE CDU THAT WE HAVE WAS NOW BEING UTILIZED FOR LAX ATIS, CRUISE PERFORMANCE, ETC. WITH BOTH LNAV SELECTORS IN NAV. THERE IS NO VOR BACKUP. MORE IMPORTANTLY, WITH ONLY ONE CDU, WHICH NOW MAKES IT A TIME SHARE ITEM, THE ACTIVE WAYPOINT IS NOT ALWAYS DISPLAYED FOR IMMEDIATE VIEWING. WHEN I FLY THE -700, I ALWAYS HAVE A CDU ON THE LEGS PAGE AND WHEN THE ACFT TURNS, I WATCH WHERE IT'S GOING. THIS IS NOT POSSIBLE IN AN ACFT WITH ONLY 1 CDU. IT'S A DUMB CONFIGN IN TODAY'S ATC ENVIRONMENT. I REMEMBER SEEING FRA ON THE LEGS PAGE. I HAVE NO CLUE WHY WE WERE TRACKING FROGO TO REBRG. ON THE SUBSEQUENT LEG, THE LNAV IN THE ACFT WAS TRACKING THE COURSE LIKE IT HAD A LOCK ON A WEAK VOR. WANDERING BACK AND FORTH. I WROTE UP THE DISCREPANCY WHEN WE ARRIVED. WE NEED 2 CDU'S IN THESE AIRPLANES.

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.