Narrative:

We had just leveled at the assigned altitude, FL250. The autoplt was engaged, in the FMS, navigation mode, proceeding direct to the lit VOR. The autoplt altitude light was also illuminated, indicating that it was on altitude hold, at the preselected altitude, FL250. Within seconds of the leveloff, the aircraft began a left turn, approaching 30 degrees of bank. The turn over lit was to be a 15 degree left turn to the mgm VOR. Initially, the turn seemed too early, and the bank seemed excessive. It seemed probable that the autoplt turn knob on the center console had been bumped, so I depressed the tcs (autoplt interrupt) button and turned the aircraft back on course, and looked down at the turn knob. At that moment, the captain (who was putting approach plates away, and the probable cause of bumping the turn knob) also looked down at the turn knob and the autoplt control panel on the center console. We found the turn knob and the autoplt controls all in the correct position. Upon looking back up to my instruments, I realized that the aircraft was climbing through FL254. I immediately depressed the tcs button again and pushed the nose over returning to FL250. At the same time I began the descent back to FL250, ATC asked what our altitude was, and informed us that there was an air carrier aircraft off of our right side and 2 mi at FL260. The captain stated that we were at FL255 descending back to FL250, and that he had the traffic passing off of our right side in sight. The air carrier aircraft reported that they had an RA on their TCASII momentarily. The controller asked if they had to deviate, and they replied that they were about to, but the RA went away. The controller then gave us a phone number to call, and told us that there was a possible pilot deviation. We turned the autoplt off, and reset it, and continued normally. After review, we decided that the FMS was probably leading the turn over lit, though the initial part of the 15 degree turn was excessive. Also, by depressing the tcs button, the altitude hold was lost, though the altitude light was still illuminated. The altitude deviation could have been avoided if one of us would have flown the aircraft while the other looked for the problem. We both looked down at the same instant, and that few seconds was all it took for the deviation to occur.

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Original NASA ASRS Text

Title: A C650 FLC NEAR LIT FAILED TO MAINTAIN ASSIGNED ALT.

Narrative: WE HAD JUST LEVELED AT THE ASSIGNED ALT, FL250. THE AUTOPLT WAS ENGAGED, IN THE FMS, NAV MODE, PROCEEDING DIRECT TO THE LIT VOR. THE AUTOPLT ALT LIGHT WAS ALSO ILLUMINATED, INDICATING THAT IT WAS ON ALT HOLD, AT THE PRESELECTED ALT, FL250. WITHIN SECONDS OF THE LEVELOFF, THE ACFT BEGAN A L TURN, APCHING 30 DEGS OF BANK. THE TURN OVER LIT WAS TO BE A 15 DEG L TURN TO THE MGM VOR. INITIALLY, THE TURN SEEMED TOO EARLY, AND THE BANK SEEMED EXCESSIVE. IT SEEMED PROBABLE THAT THE AUTOPLT TURN KNOB ON THE CTR CONSOLE HAD BEEN BUMPED, SO I DEPRESSED THE TCS (AUTOPLT INTERRUPT) BUTTON AND TURNED THE ACFT BACK ON COURSE, AND LOOKED DOWN AT THE TURN KNOB. AT THAT MOMENT, THE CAPT (WHO WAS PUTTING APCH PLATES AWAY, AND THE PROBABLE CAUSE OF BUMPING THE TURN KNOB) ALSO LOOKED DOWN AT THE TURN KNOB AND THE AUTOPLT CTL PANEL ON THE CTR CONSOLE. WE FOUND THE TURN KNOB AND THE AUTOPLT CTLS ALL IN THE CORRECT POS. UPON LOOKING BACK UP TO MY INSTS, I REALIZED THAT THE ACFT WAS CLBING THROUGH FL254. I IMMEDIATELY DEPRESSED THE TCS BUTTON AGAIN AND PUSHED THE NOSE OVER RETURNING TO FL250. AT THE SAME TIME I BEGAN THE DSCNT BACK TO FL250, ATC ASKED WHAT OUR ALT WAS, AND INFORMED US THAT THERE WAS AN ACR ACFT OFF OF OUR R SIDE AND 2 MI AT FL260. THE CAPT STATED THAT WE WERE AT FL255 DSNDING BACK TO FL250, AND THAT HE HAD THE TFC PASSING OFF OF OUR R SIDE IN SIGHT. THE ACR ACFT RPTED THAT THEY HAD AN RA ON THEIR TCASII MOMENTARILY. THE CTLR ASKED IF THEY HAD TO DEVIATE, AND THEY REPLIED THAT THEY WERE ABOUT TO, BUT THE RA WENT AWAY. THE CTLR THEN GAVE US A PHONE NUMBER TO CALL, AND TOLD US THAT THERE WAS A POSSIBLE PLTDEV. WE TURNED THE AUTOPLT OFF, AND RESET IT, AND CONTINUED NORMALLY. AFTER REVIEW, WE DECIDED THAT THE FMS WAS PROBABLY LEADING THE TURN OVER LIT, THOUGH THE INITIAL PART OF THE 15 DEG TURN WAS EXCESSIVE. ALSO, BY DEPRESSING THE TCS BUTTON, THE ALT HOLD WAS LOST, THOUGH THE ALT LIGHT WAS STILL ILLUMINATED. THE ALTDEV COULD HAVE BEEN AVOIDED IF ONE OF US WOULD HAVE FLOWN THE ACFT WHILE THE OTHER LOOKED FOR THE PROB. WE BOTH LOOKED DOWN AT THE SAME INSTANT, AND THAT FEW SECONDS WAS ALL IT TOOK FOR THE DEV TO OCCUR.

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.