B737 PLT ON IOE HAS LNAV DISENGAGE DURING DSCNT RESULTING IN HDG DEV.

Narrative

I WAS ON MY IOE TRIP ON THE B737 AFTER COMPLETING SIMULATOR TRAINING AND 1 ACFT TRIP; AS PART OF THAT TRAINING. SO I HAD 17 1/2 HRS IN THE ACFT AT THE START OF THIS TRIP. I HAD SPENT THE PAST 7 YRS AS A FE ON THE DC10. THE INSTRUCTOR AND I WERE DISCUSSING THE PROCS FOR USING LNAV AND VNAV COUPLED TO FLY THE CIVET 1 ARR INTO LAX. WHILE I WAS INVOLVED WITH SETTING THE VNAV ALT; I DID NOT NOTICE THAT LNAV HAD DISENGAGED AS WE PASSED CIVET FOR BREMR AND DSNDING FROM 14000 FT TO 12000 FT. WE WERE 2 DOTS TO THE L OF TRACK WHEN LAX APCH CTL QUESTIONED OUR POS. I MADE A CORRECTION BACK TO THE R AND RE-ENGAGED LNAV. THE CORRECTION WAS COMPLETE BY BREMR OR MAYBE JUST PAST THAT POINT. LNAV WAS THEN CLOSELY MONITORED AND COMPARED WITH RAW DATA. PROPER GND TRACK WAS THEN MAINTAINED AND THE REMAINDER OF THE ARR AND APCH TO LAND AT LAX RWY 25L WAS COMPLETED UNEVENTFULLY. THIS WAS A VALUABLE LESSON LEARNED IN THE IMPORTANCE OF MONITORING THE FMA TO BE ASSURED THAT WHAT HAD BEEN SELECTED AND DESIRED WAS IN FACT ENGAGED. THE APCH CTLR SAID TO REVIEW THE APCH ARR BEFORE A FUTURE FLT TO LAX. YES SIR!