Narrative:

On descent into lax on the civet one arrival. ATC reclred us to fly direct to snrkl intersection and fly the mitts one arrival. ATC also gave us an airspeed change, altitude crossing at snrkl and to descend via the mitts one arrival. We quickly reprogrammed the FMS (LNAV with the new arrival). I went off the ATC radio to talk to company and get a gate. I was heads down for 2 or 3 mins. When I came back up to the ATC radio, I noticed we had drifted south off course. I had the localizer for runway 24R dialed up to identify snrkl. I noticed full scale deflection on my localizer needle and almost full-scale deflection on the first officer's LNAV CDI. We had been cleared direct snrkl and I was not sure how far south we were of our intended course. During the course of our FMS reprogramming, the LNAV had become disengaged or we had not re-engaged it after putting the new arrival in. In any case, I immediately turned the aircraft right to intercept the arrival at intersection south or just prior to it. The approach controller never said anything to us, so I don't think we had any traffic conflicts. Obviously, the moral of the story is, after reprogramming the FMS/LNAV, always verify the LNAV is still engaged if it is required. The rest of the arrival and approach were uneventful.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737 FLC WERE CHANGED TO THE MITTS ARR FOR LNDG LAX AND DID NOT ENGAGE THE LNAVE AFTER REPROGRAMMING THE FMS, CAUSING A TRACK DEV.

Narrative: ON DSCNT INTO LAX ON THE CIVET ONE ARR. ATC RECLRED US TO FLY DIRECT TO SNRKL INTXN AND FLY THE MITTS ONE ARR. ATC ALSO GAVE US AN AIRSPD CHANGE, ALT XING AT SNRKL AND TO DSND VIA THE MITTS ONE ARR. WE QUICKLY REPROGRAMMED THE FMS (LNAV WITH THE NEW ARR). I WENT OFF THE ATC RADIO TO TALK TO COMPANY AND GET A GATE. I WAS HEADS DOWN FOR 2 OR 3 MINS. WHEN I CAME BACK UP TO THE ATC RADIO, I NOTICED WE HAD DRIFTED S OFF COURSE. I HAD THE LOC FOR RWY 24R DIALED UP TO IDENT SNRKL. I NOTICED FULL SCALE DEFLECTION ON MY LOC NEEDLE AND ALMOST FULL-SCALE DEFLECTION ON THE FO'S LNAV CDI. WE HAD BEEN CLRED DIRECT SNRKL AND I WAS NOT SURE HOW FAR S WE WERE OF OUR INTENDED COURSE. DURING THE COURSE OF OUR FMS REPROGRAMMING, THE LNAV HAD BECOME DISENGAGED OR WE HAD NOT RE-ENGAGED IT AFTER PUTTING THE NEW ARR IN. IN ANY CASE, I IMMEDIATELY TURNED THE ACFT R TO INTERCEPT THE ARR AT INTXN S OR JUST PRIOR TO IT. THE APCH CTLR NEVER SAID ANYTHING TO US, SO I DON'T THINK WE HAD ANY TFC CONFLICTS. OBVIOUSLY, THE MORAL OF THE STORY IS, AFTER REPROGRAMMING THE FMS/LNAV, ALWAYS VERIFY THE LNAV IS STILL ENGAGED IF IT IS REQUIRED. THE REST OF THE ARR AND APCH WERE UNEVENTFUL.

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.