Narrative:

We were established at FL350 approximately 50 mi southeast of the bty VOR and proceeding direct to bty. ZLA gave us a crossing restr to cross 10 mi northwest of bty (the other side) at FL310. I had learned a new technique several weeks ago on how to use the fix page to install a crossing restr such as this. I input the restr and had the captain verify it and then executed it and then switched to the cruise page on the FMC. Shortly after, I started a descent for the crossing restr. During the descent, the aircraft started a left turn. I was not expecting a left turn and I checked the CDI and it was 60 degrees off center and the needle was fully deflected. I then disconnected the LNAV and told the captain we were off course. I started a turn to the right back to bty and ZLA queried us on our heading and the captain replied we were unable RNAV. We picked up a vector direct to lidat and the LNAV worked normally after that point. Looking back, I believe the problem was that a discontinuity was introduced into the route. We both verified the routing prior to executing, but after executing, I immediately switched the FMC to the cruise page to establish my cruise power setting. I should have stayed on the legs page for a short time to ensure that a route discontinuity was not introduced. This error was my fault, but I feel that the single FMC system in the 300 series was a contributing factor. However, I will be more observant after inputting changes in the route to prevent this occurrence again. Supplemental information for acn 616225: what apparently happened, after a discussion with my first officer, was that a fix was incorrectly entered into the FMC earlier in the flight without either of us catching it prior to execution. We both learned to really pay attention when either pilot creates a new waypoint in the FMC.

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Original NASA ASRS Text

Title: B737-300 CREW HAD A TRACK DEV IN ZLA CLASS A. THE ACFT IS ONLY EQUIPPED WITH A SINGLE FMS, AND HAS NO NAV DISPLAY.

Narrative: WE WERE ESTABLISHED AT FL350 APPROX 50 MI SE OF THE BTY VOR AND PROCEEDING DIRECT TO BTY. ZLA GAVE US A XING RESTR TO CROSS 10 MI NW OF BTY (THE OTHER SIDE) AT FL310. I HAD LEARNED A NEW TECHNIQUE SEVERAL WKS AGO ON HOW TO USE THE FIX PAGE TO INSTALL A XING RESTR SUCH AS THIS. I INPUT THE RESTR AND HAD THE CAPT VERIFY IT AND THEN EXECUTED IT AND THEN SWITCHED TO THE CRUISE PAGE ON THE FMC. SHORTLY AFTER, I STARTED A DSCNT FOR THE XING RESTR. DURING THE DSCNT, THE ACFT STARTED A L TURN. I WAS NOT EXPECTING A L TURN AND I CHKED THE CDI AND IT WAS 60 DEGS OFF CTR AND THE NEEDLE WAS FULLY DEFLECTED. I THEN DISCONNECTED THE LNAV AND TOLD THE CAPT WE WERE OFF COURSE. I STARTED A TURN TO THE R BACK TO BTY AND ZLA QUERIED US ON OUR HDG AND THE CAPT REPLIED WE WERE UNABLE RNAV. WE PICKED UP A VECTOR DIRECT TO LIDAT AND THE LNAV WORKED NORMALLY AFTER THAT POINT. LOOKING BACK, I BELIEVE THE PROB WAS THAT A DISCONTINUITY WAS INTRODUCED INTO THE RTE. WE BOTH VERIFIED THE ROUTING PRIOR TO EXECUTING, BUT AFTER EXECUTING, I IMMEDIATELY SWITCHED THE FMC TO THE CRUISE PAGE TO ESTABLISH MY CRUISE PWR SETTING. I SHOULD HAVE STAYED ON THE LEGS PAGE FOR A SHORT TIME TO ENSURE THAT A RTE DISCONTINUITY WAS NOT INTRODUCED. THIS ERROR WAS MY FAULT, BUT I FEEL THAT THE SINGLE FMC SYS IN THE 300 SERIES WAS A CONTRIBUTING FACTOR. HOWEVER, I WILL BE MORE OBSERVANT AFTER INPUTTING CHANGES IN THE RTE TO PREVENT THIS OCCURRENCE AGAIN. SUPPLEMENTAL INFO FOR ACN 616225: WHAT APPARENTLY HAPPENED, AFTER A DISCUSSION WITH MY FO, WAS THAT A FIX WAS INCORRECTLY ENTERED INTO THE FMC EARLIER IN THE FLT WITHOUT EITHER OF US CATCHING IT PRIOR TO EXECUTION. WE BOTH LEARNED TO REALLY PAY ATTN WHEN EITHER PLT CREATES A NEW WAYPOINT IN THE FMC.

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.