Narrative:

I departed IFR from ZZZ in VMC with an initial clearance to 3000 ft. On departure; I contacted ny approach on 132.75 who confirmed my clearance to 3000 ft; then cleared me to 4000 ft a few mins later. I set my autoplt for altitude hold at 4000 ft and the aircraft climbed to 4000 ft. Shortly thereafter I was cleared direct weard and I coupled my autoplt for gpss to weard which had previously been programmed into my chelton flight system EFIS. The aircraft turned right toward weard but instead of rolling out on the correct heading; it continued to turn. Fearing an autoplt malfunction; I disconnected the autoplt and began to fly manually to weard. About the same time; I learned of traffic to my right (I am not sure if I heard it from ATC or from the 'tis' on my garmin GTX330 mode south transponder). I began looking for the traffic visually while resetting my autoplt for altitude hold at 4000 ft. At almost the same time; the controller angrily said I was above my assigned altitude of 4000 ft. My altimeter said 4400 ft. I manually flew down to 4000 ft and spotted the traffic above me and to the north. I kept visual contact with the traffic and continued resetting my autoplt for altitude hold at 4000 ft and a heading direct to weard. Unfortunately during this process; I again climbed to approximately 4400 ft and the controller angrily told me to get back down to 4000 ft. At that point I disconnected the autoplt; manually flew back down to 4000 ft and flew manually until I received a clearance to a higher altitude. I told ATC that I was having trouble with my autoplt. An hour later I was advised by ATC to phone ny TRACON. When I called them; I was advised that they had filed a pilot deviation describing the above situation. I subsequently discovered that the origin of my autoplt problem was that I had omitted to set the course to weard as direct and the autoplt was flying back to the course line between my departure pou and weard. I should have kept the autoplt in altitude hold mode and reset the track to weard.

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Original NASA ASRS Text

Title: LANCAIR PLT HAS AN ALTDEV DURING TRANSIT TO DEST.

Narrative: I DEPARTED IFR FROM ZZZ IN VMC WITH AN INITIAL CLRNC TO 3000 FT. ON DEP; I CONTACTED NY APCH ON 132.75 WHO CONFIRMED MY CLRNC TO 3000 FT; THEN CLRED ME TO 4000 FT A FEW MINS LATER. I SET MY AUTOPLT FOR ALT HOLD AT 4000 FT AND THE ACFT CLBED TO 4000 FT. SHORTLY THEREAFTER I WAS CLRED DIRECT WEARD AND I COUPLED MY AUTOPLT FOR GPSS TO WEARD WHICH HAD PREVIOUSLY BEEN PROGRAMMED INTO MY CHELTON FLT SYS EFIS. THE ACFT TURNED R TOWARD WEARD BUT INSTEAD OF ROLLING OUT ON THE CORRECT HDG; IT CONTINUED TO TURN. FEARING AN AUTOPLT MALFUNCTION; I DISCONNECTED THE AUTOPLT AND BEGAN TO FLY MANUALLY TO WEARD. ABOUT THE SAME TIME; I LEARNED OF TFC TO MY R (I AM NOT SURE IF I HEARD IT FROM ATC OR FROM THE 'TIS' ON MY GARMIN GTX330 MODE S XPONDER). I BEGAN LOOKING FOR THE TFC VISUALLY WHILE RESETTING MY AUTOPLT FOR ALT HOLD AT 4000 FT. AT ALMOST THE SAME TIME; THE CTLR ANGRILY SAID I WAS ABOVE MY ASSIGNED ALT OF 4000 FT. MY ALTIMETER SAID 4400 FT. I MANUALLY FLEW DOWN TO 4000 FT AND SPOTTED THE TFC ABOVE ME AND TO THE N. I KEPT VISUAL CONTACT WITH THE TFC AND CONTINUED RESETTING MY AUTOPLT FOR ALT HOLD AT 4000 FT AND A HDG DIRECT TO WEARD. UNFORTUNATELY DURING THIS PROCESS; I AGAIN CLBED TO APPROX 4400 FT AND THE CTLR ANGRILY TOLD ME TO GET BACK DOWN TO 4000 FT. AT THAT POINT I DISCONNECTED THE AUTOPLT; MANUALLY FLEW BACK DOWN TO 4000 FT AND FLEW MANUALLY UNTIL I RECEIVED A CLRNC TO A HIGHER ALT. I TOLD ATC THAT I WAS HAVING TROUBLE WITH MY AUTOPLT. AN HR LATER I WAS ADVISED BY ATC TO PHONE NY TRACON. WHEN I CALLED THEM; I WAS ADVISED THAT THEY HAD FILED A PLTDEV DESCRIBING THE ABOVE SITUATION. I SUBSEQUENTLY DISCOVERED THAT THE ORIGIN OF MY AUTOPLT PROB WAS THAT I HAD OMITTED TO SET THE COURSE TO WEARD AS DIRECT AND THE AUTOPLT WAS FLYING BACK TO THE COURSE LINE BTWN MY DEP POU AND WEARD. I SHOULD HAVE KEPT THE AUTOPLT IN ALT HOLD MODE AND RESET THE TRACK TO WEARD.

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.