Narrative:

Departed las. Received instructions to climb to 7000 ft on an assigned heading by las departure control. I turned to the heading and selected the assigned altitude (7000 ft) in my altitude controller. I engaged the heading bug -- with autoplt 'on.' aircraft turned to heading. I was controling my rate of ascent with the electric trim. At this point, was about 1500-1700 FPM. Upon reaching 7000 ft, I engaged the altitude hold. The annunciator light showed 'engaged/captured.' the aircraft began to level, then unexpectedly, the aircraft pitched up very sharply. I immediately grabbed the control wheel and pushed the wheel forward and disconnected the autoplt at the same time. I pulled back power and did a very rapid descent to my assigned altitude, 7000 ft. I flew the remainder of the trip with autoplt disengaged. I believe that I gained about 700 ft before I was able to stop the deviation. I spoke with the departure controller about this. I also called ATC, upon landing at my destination, and had a discussion with departure control via telephone at las. I know that there was traffic at 8000 ft at about my 8-9 O'clock position and at about 2 mi when this occurred. I wrote up the autoplt action on the flight log. Our maintenance facility will look into this upon my return to prescott. I have not been using the autoplt since this occurrence.

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Original NASA ASRS Text

Title: SMT FAILED TO MAINTAIN CLRED ALT WHEN ALT HOLD FUNCTION OF AUTOPLT MALFUNCTIONED ON CLBOUT FROM LAS.

Narrative: DEPARTED LAS. RECEIVED INSTRUCTIONS TO CLB TO 7000 FT ON AN ASSIGNED HDG BY LAS DEP CTL. I TURNED TO THE HDG AND SELECTED THE ASSIGNED ALT (7000 FT) IN MY ALT CTLR. I ENGAGED THE HDG BUG -- WITH AUTOPLT 'ON.' ACFT TURNED TO HDG. I WAS CTLING MY RATE OF ASCENT WITH THE ELECTRIC TRIM. AT THIS POINT, WAS ABOUT 1500-1700 FPM. UPON REACHING 7000 FT, I ENGAGED THE ALT HOLD. THE ANNUNCIATOR LIGHT SHOWED 'ENGAGED/CAPTURED.' THE ACFT BEGAN TO LEVEL, THEN UNEXPECTEDLY, THE ACFT PITCHED UP VERY SHARPLY. I IMMEDIATELY GRABBED THE CTL WHEEL AND PUSHED THE WHEEL FORWARD AND DISCONNECTED THE AUTOPLT AT THE SAME TIME. I PULLED BACK PWR AND DID A VERY RAPID DSCNT TO MY ASSIGNED ALT, 7000 FT. I FLEW THE REMAINDER OF THE TRIP WITH AUTOPLT DISENGAGED. I BELIEVE THAT I GAINED ABOUT 700 FT BEFORE I WAS ABLE TO STOP THE DEV. I SPOKE WITH THE DEP CTLR ABOUT THIS. I ALSO CALLED ATC, UPON LNDG AT MY DEST, AND HAD A DISCUSSION WITH DEP CTL VIA TELEPHONE AT LAS. I KNOW THAT THERE WAS TFC AT 8000 FT AT ABOUT MY 8-9 O'CLOCK POS AND AT ABOUT 2 MI WHEN THIS OCCURRED. I WROTE UP THE AUTOPLT ACTION ON THE FLT LOG. OUR MAINT FACILITY WILL LOOK INTO THIS UPON MY RETURN TO PRESCOTT. I HAVE NOT BEEN USING THE AUTOPLT SINCE THIS OCCURRENCE.

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.