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

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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.