Narrative:

I was on my way home from st george; ut; en route to provo; ut. I was at 31000 ft and had been cleared by ZLC to descend pilot's discretion to 24000 ft. Shortly after I contacted center and told them I was leaving FL310 for FL240. The controller said; thank you. I set 240 in the autoplt and pressed the select button arming the altitude capture feature of the autoplt. As I was descending to FL240 a passenger asked me a question. He then pointed out the window and said that a nearby plane looked close. As I looked to see the traffic I heard the traffic avoidance alert me to nearby traffic saying; 'traffic; traffic.' I realized the nearby plane the passenger had pointed out was the traffic that was making the alarm go off. I looked down at my altitude and realized I was going through FL237 and the altitude 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 climb to FL240. I acknowledged. I had descended to FL236 and immediately began climbing to FL240. As I was climbing to FL240; I tried to figure out why the altitude preselect feature on the autoplt had not captured. As I climbed to FL240 the autoplt again did not capture the leveloff. The controller then asked what altitude I was cleared to. I told him FL240. He then asked what altitude I had been cleared to before he cleared me to FL240. I responded FL240. He then told me that I needed to call ATC when I got on the ground and gave me a phone number. I was about 70 mi from provo when this took place. I was on the ground in about 15 mins and within 10 mins of being on the ground I called the number ATC had given me. The man I talked to said that an air carrier flight 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 altitude and that I went 300 ft or so below my assigned altitude. He then asked for my name; address; phone number; and pilot 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 altitude. In order to resolve this problem so it does not occur in the future I will have the autoplt looked at. I will also give a better briefing to my passenger so they do not distract me while flying. Previous to this occurrence I have never had an instance of altitude deviation and I now understand more fully the importance of staying within my assigned altitude. I will be more vigilant in watching and monitoring my instruments especially during an altitude change and leveloff. I realize that it is not the controller's job to alert me to altitude.

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Original NASA ASRS Text

Title: 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.

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.