PVT PLT OF AN SMT LOST CTL OF THE ACFT IN IMC CONDITIONS DUE TO AN AUTOPLT PROB RESULTING IN EXCURSION FROM ASSIGNED CRUISING ALT.

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ON DEC/XX/95 I FILED AND FLEW AN IFR FLT FROM FTG; CO; TO CPS; IL. THE ACFT WAS A C340. TKOF AND DEP WAS NORMAL. I WAS CLRED TO CLB TO OUR REQUESTED ALT OF FL210. THE CLB WAS MADE UNDER VFR NIGHT CONDITIONS. ALSO; THE CLB WAS MADE WITHOUT THE USE OF THE AUTOPLT. THE AUTOPLT IS A CESSNA 400B FLT DIRECTOR SYS. UPON REACHIING 21000 FT I LEVELED THE ACFT; ADJUSTED THE TRIM; AND SET THE PWR SETTINGS TO 30 INCHES MANIFOLD PRESSURE AND 2400 RPM. I ALSO CHKED THE OIL AND CYLINDER HEAD TEMP AND CLOSED THE COWL FLAPS. THE ACFT WAS IN LEVEL FLT AND I ENGAGED THE AUTOPLT PUTTING THE FLT DIRECTOR INTO AN ALT AND HDG MODE. IT MIGHT BE NOTED THAT JUST PRIOR TO TURNING ON THE AUTOPLT I NOTICED THE ARTIFICIAL HORIZON WAS JUMPING UP AND DOWN IN VERY SMALL RAPID MOVEMENTS. IMMEDIATELY THE ACFT WENT INTO A HARD R TURN AND RAPID DSCNT. I TURNED OFF THE AUTOPLT USING THE CTL YOKE SWITCH. BY THIS TIME THE ARTIFICIAL HORIZON GYRO HAD TUMBLED. I TRIED TO RECOVER USING THE L AILERON AND BACK PRESSURE ON THE YOKE. NO HELP. FINALLY I CUT THE PWR AND LET THE YOKE FREE. THE ACFT STOPPED THE SPIRAL BUT WAS DIRECTLY NOSE DOWN. I APPLIED BACK PRESSURE AND RECOVERED TO LEVEL FLT AT APPROX 13000 FT. I NOTIFIED ATC OF THE EMER. I CAREFULLY CHKED OUT THE FLT CTLS; ENG PARAMETERS; AND OTHER SYS AND DETERMINED THAT EVERYTHING APPEARED TO BE OPERATING NORMALLY. AFTER APPROX 3 MINS THE PLT'S GYRO CAME BACK ON-LINE AND I ENGAGED THE AUTOPLT AND CAREFULLY MONITORED ITS OP. I ELECTED TO CONTINUE ON WITH THE FLT AT 13000 FT AND DID NOT ENCOUNTER ANY MORE DIFFICULTIES WITH EITHER THE AUTOPLT OR ACFT. UPON LNDG IN CAHOKIA I VISUALLY INSPECTED THE ACFT AND FOUND NO VISUAL DAMAGE. I AM; HOWEVER; HAVING A DETAILED 100 HR INSPECTION PERFORMED ON THE ACFT. IN SUMMARY I REALLY DON'T KNOW WHAT HAPPENED. THE ACFT DID NOT RESPOND TO ANY FLT SURFACE MANIPULATION. COULD THIS HAVE BEEN CLR AIR TURB? COULD AN AUTOPLT MALFUNCTION HAVE BEEN THE CAUSE? WAS THE ARTIFICIAL HORIZON FAILING? I JUST DON'T KNOW THE ANSWERS TO THESE QUESTIONS. I WILL BE GLAD TO ASSIST IN ANY WAY I CAN TO HELP OTHER PLTS AVOID THIS DANGEROUS SIT. RECENTLY THE AUTOPLT WAS WORKED ON. THEY REPLACED THE ARTIFICIAL HORIZON WITH A NEW YELLOW TAGGED UNIT. I ALSO CONTEMPLATE HAVING A THOROUGH CHKOUT OF THIS SYS. THERE WERE 4 PAX ON BOARD. ALL PLTS; ONE OF THEM BEING A CFI. THIS WAS A PART 91 FLT. HOWEVER; THE ACFT IS CERTIFIED FOR PART 135 FLT. THE PIC HISTORY INCLUDES: DEC/90: ATTENDED FLT SAFETY FOR INST/SINGLE ENG FLT TRAINING. FEB/92: ATTENDED FLT SAFETY FOR CE-340A INITIAL ACFT SYS/FLT SIMULATOR TRAINING. NOV/94: ATTENDED CESSNA 340 RECURRENT TRAINING; INCLUDING ICC CHKOUT AND BI-ANNUAL CHK. 1993: HAD AEROBATICS TRAINING IN GLIDER ACFT. ALL THE ABOVE TRAINING PROVED INVALUABLE.