Narrative:

Departed on time. Cleared direct cfood. The aircraft began going direct but started turing. I re-selected cfood and aircraft turned back. No more problems with LNAV until approaching fll. The original clearance was to cross fortl at 8000 ft. We were at FL220 and I stayed at altitude intending to descend on profile. ATC told us to begin our descent now and gave us a frequency change. The next controller cleared us to 7000 ft. I continued descending at 1100 FPM hoping to intercept the descent profile. The FMC showed 1.7 mi to touchdown so I started to increase my rate of descent. We were passing 17000 ft. Suddenly; when we intercepted the descent profile; the aircraft started turning to 090 degree heading. Instead of being approximately 40 mi from fortl; we were actually over fortl. The FMC was in error but the LNAV was correct. The controller said we were extremely high for his airspace and asked if we could hurry down. We hurried to 7000 ft. I asked the controller if I needed to call him back; and he said 'why?' apparently; everything was ok. Upon landing at fll; I felt like I might have misprogrammed the FMC and told the first officer that we would try it on the last leg to another city and one of us would fly out of the 'navigation' position to back up the LNAV. Everything seemed to work fine on that leg; but I radioed to maintenance that I wanted them to meet me when I landed to talk to them about the LNAV on this aircraft. They met me and I tried to explain what had happened and 'what should I put in the logbook to help them analyze the problem?' they said that they would take care of writing it up. While I was there; they ran a bite check on the FMC and discovered that there were several errors recorded over the last 2 days. One of the errors was my fortl crossing. This would have never happened with the -700 and its 2 system with the horizontal overlay. As things stand now; it's standard procedure for both pilots to fly in the navigation position; but since my bad experience; I question the safety of this procedure. Had I been in mountainous terrain; or even in the northeast united states; this could have been disastrous. Although I have mentally gone over this situation time and time again; I just can't see what I did wrong or how I could have prevented this situation. Now; I simply do not trust company's procedure for both pilots to fly in navigation. I don't want to deviation from our procedures and I won't; but I would like for us to take a closer look at the wisdom of trusting a system that can go bad with no warning.

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Original NASA ASRS Text

Title: A SINGLE FMC EQUIPPED B737-500 PERFORMED LNAV FUNCTIONS CORRECTLY BUT CALCULATED VNAV FUNCTIONS WITH A 40 NM ERROR RESULTING IN A MISSED XING RESTR.

Narrative: DEPARTED ON TIME. CLRED DIRECT CFOOD. THE ACFT BEGAN GOING DIRECT BUT STARTED TURING. I RE-SELECTED CFOOD AND ACFT TURNED BACK. NO MORE PROBS WITH LNAV UNTIL APCHING FLL. THE ORIGINAL CLRNC WAS TO CROSS FORTL AT 8000 FT. WE WERE AT FL220 AND I STAYED AT ALT INTENDING TO DSND ON PROFILE. ATC TOLD US TO BEGIN OUR DSCNT NOW AND GAVE US A FREQ CHANGE. THE NEXT CTLR CLRED US TO 7000 FT. I CONTINUED DSNDING AT 1100 FPM HOPING TO INTERCEPT THE DSCNT PROFILE. THE FMC SHOWED 1.7 MI TO TOUCHDOWN SO I STARTED TO INCREASE MY RATE OF DSCNT. WE WERE PASSING 17000 FT. SUDDENLY; WHEN WE INTERCEPTED THE DSCNT PROFILE; THE ACFT STARTED TURNING TO 090 DEG HDG. INSTEAD OF BEING APPROX 40 MI FROM FORTL; WE WERE ACTUALLY OVER FORTL. THE FMC WAS IN ERROR BUT THE LNAV WAS CORRECT. THE CTLR SAID WE WERE EXTREMELY HIGH FOR HIS AIRSPACE AND ASKED IF WE COULD HURRY DOWN. WE HURRIED TO 7000 FT. I ASKED THE CTLR IF I NEEDED TO CALL HIM BACK; AND HE SAID 'WHY?' APPARENTLY; EVERYTHING WAS OK. UPON LNDG AT FLL; I FELT LIKE I MIGHT HAVE MISPROGRAMMED THE FMC AND TOLD THE FO THAT WE WOULD TRY IT ON THE LAST LEG TO ANOTHER CITY AND ONE OF US WOULD FLY OUT OF THE 'NAV' POS TO BACK UP THE LNAV. EVERYTHING SEEMED TO WORK FINE ON THAT LEG; BUT I RADIOED TO MAINT THAT I WANTED THEM TO MEET ME WHEN I LANDED TO TALK TO THEM ABOUT THE LNAV ON THIS ACFT. THEY MET ME AND I TRIED TO EXPLAIN WHAT HAD HAPPENED AND 'WHAT SHOULD I PUT IN THE LOGBOOK TO HELP THEM ANALYZE THE PROB?' THEY SAID THAT THEY WOULD TAKE CARE OF WRITING IT UP. WHILE I WAS THERE; THEY RAN A BITE CHK ON THE FMC AND DISCOVERED THAT THERE WERE SEVERAL ERRORS RECORDED OVER THE LAST 2 DAYS. ONE OF THE ERRORS WAS MY FORTL XING. THIS WOULD HAVE NEVER HAPPENED WITH THE -700 AND ITS 2 SYS WITH THE HORIZ OVERLAY. AS THINGS STAND NOW; IT'S STANDARD PROC FOR BOTH PLTS TO FLY IN THE NAV POS; BUT SINCE MY BAD EXPERIENCE; I QUESTION THE SAFETY OF THIS PROC. HAD I BEEN IN MOUNTAINOUS TERRAIN; OR EVEN IN THE NE UNITED STATES; THIS COULD HAVE BEEN DISASTROUS. ALTHOUGH I HAVE MENTALLY GONE OVER THIS SITUATION TIME AND TIME AGAIN; I JUST CAN'T SEE WHAT I DID WRONG OR HOW I COULD HAVE PREVENTED THIS SITUATION. NOW; I SIMPLY DO NOT TRUST COMPANY'S PROC FOR BOTH PLTS TO FLY IN NAV. I DON'T WANT TO DEV FROM OUR PROCS AND I WON'T; BUT I WOULD LIKE FOR US TO TAKE A CLOSER LOOK AT THE WISDOM OF TRUSTING A SYS THAT CAN GO BAD WITH NO WARNING.

Data retrieved from NASA's ASRS site as of January 2009 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.