PLT OF C501 EXPERIENCES TCAS TA WHEN HE DSNDS BELOW HIS CLRED ALT DUE TO FAILURE OF THE ALT CAPTURE FEATURE OF THE AUTOPLT.

Narrative

I WAS ON MY WAY HOME FROM ST GEORGE; UT; ENRTE TO PROVO; UT. I WAS AT 31000 FT AND HAD BEEN CLRED BY ZLC TO DSND PLT'S DISCRETION TO 24000 FT. SHORTLY AFTER I CONTACTED CTR AND TOLD THEM I WAS LEAVING FL310 FOR FL240. THE CTLR SAID; THANK YOU. I SET 240 IN THE AUTOPLT AND PRESSED THE SELECT BUTTON ARMING THE ALT CAPTURE FEATURE OF THE AUTOPLT. AS I WAS DSNDING TO FL240 A PAX ASKED ME A QUESTION. HE THEN POINTED OUT THE WINDOW AND SAID THAT A NEARBY PLANE LOOKED CLOSE. AS I LOOKED TO SEE THE TFC I HEARD THE TFC AVOIDANCE ALERT ME TO NEARBY TFC SAYING; 'TFC; TFC.' I REALIZED THE NEARBY PLANE THE PAX HAD POINTED OUT WAS THE TFC THAT WAS MAKING THE ALARM GO OFF. I LOOKED DOWN AT MY ALT AND REALIZED I WAS GOING THROUGH FL237 AND THE ALT CAPTURE FEATURE OF THE AUTOPLT HAD FAILED. AT THAT SAME TIME ATC CAME ON AND SAID; 'LEVEL 240?' I TOLD THEM I WAS BELOW FL240. HE TOLD ME TO CLB TO FL240. I ACKNOWLEDGED. I HAD DSNDED TO FL236 AND IMMEDIATELY BEGAN CLBING TO FL240. AS I WAS CLBING TO FL240; I TRIED TO FIGURE OUT WHY THE ALT PRESELECT FEATURE ON THE AUTOPLT HAD NOT CAPTURED. AS I CLBED TO FL240 THE AUTOPLT AGAIN DID NOT CAPTURE THE LEVELOFF. THE CTLR THEN ASKED WHAT ALT I WAS CLRED TO. I TOLD HIM FL240. HE THEN ASKED WHAT ALT I HAD BEEN CLRED TO BEFORE HE CLRED ME TO FL240. I RESPONDED FL240. HE THEN TOLD ME THAT I NEEDED TO CALL ATC WHEN I GOT ON THE GND AND GAVE ME A PHONE NUMBER. I WAS ABOUT 70 MI FROM PROVO WHEN THIS TOOK PLACE. I WAS ON THE GND IN ABOUT 15 MINS AND WITHIN 10 MINS OF BEING ON THE GND I CALLED THE NUMBER ATC HAD GIVEN ME. THE MAN I TALKED TO SAID THAT AN ACR FLT GOT A TA AS WELL. HE ASKED IF I WENT BELOW FL240. I TOLD HIM THAT YES I HAD. I ALSO TOLD HIM THAT MY AUTOPLT DID NOT CAPTURE MY ALT AND THAT I WENT 300 FT OR SO BELOW MY ASSIGNED ALT. HE THEN ASKED FOR MY NAME; ADDRESS; PHONE NUMBER; AND PLT LICENSE NUMBER. THE AUTOPLT HAD BEEN WORKING FINE ON THE TRIP DOWN TO ST GEORGE. I HAVE BEEN ABLE TO RELY ON THE AUTOPLT. I PLACED TOO MUCH TRUST IN THE AUTOPLT TO CAPTURE MY ALT. IN ORDER TO RESOLVE THIS PROB SO IT DOES NOT OCCUR IN THE FUTURE I WILL HAVE THE AUTOPLT LOOKED AT. I WILL ALSO GIVE A BETTER BRIEFING TO MY PAX SO THEY DO NOT DISTRACT ME WHILE FLYING. PREVIOUS TO THIS OCCURRENCE I HAVE NEVER HAD AN INSTANCE OF ALTDEV AND I NOW UNDERSTAND MORE FULLY THE IMPORTANCE OF STAYING WITHIN MY ASSIGNED ALT. I WILL BE MORE VIGILANT IN WATCHING AND MONITORING MY INSTS ESPECIALLY DURING AN ALT CHANGE AND LEVELOFF. I REALIZE THAT IT IS NOT THE CTLR'S JOB TO ALERT ME TO ALT.