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.