RPTR LOSES CTL OF HIS ACFT AND ENTERS A SPIRAL WHEN THE ARTIFICIAL HORIZON FAILS.

Narrative

APPROX 5 MINS AFTER TKOF; THE ALT INDICATOR (ARTIFICIAL HORIZON) STARTED TO PRECESS. CONTINUED CLB ON INSTS; WITHOUT DIFFICULTY UNTIL STARTING LEVEL OFF PROC AT 17000 FT. AT THAT TIME CTR ADVISED THAT I WAS CLRED ON COURSE TO PNE (DEST ARPT). I WAS ON A HDG OF 010 DEGS AND STARTED A R TURN AT THE SAME TIME I WAS TRYING TO LEVEL OFF. WITHOUT THE ALT INDICATOR; I PUSHED TOO FAR FORWARD ON THE CTL WHEEL AND THE PLANE STARTED A DSCNT. WHEN THE RATE OF DSCNT WAS INCREASING PAST 1000 FPM; I PULLED BACK ON THE CTL WHEEL TO BRING THE NOSE UP. INSTEAD OF SLOWING THE RATE OF DSCNT; I MERELY INCREASED THE RATE OF TURN; SO THAT WE WERE NOW IN A DSNDING SPIRAL TURN. THE RATE OF DSCNT WAS PEGGED AT 5000 FPM. AT THAT TIME ATC QUERIED MY INTENTIONS WITH A REMARK LIKE 'WE CLRED YOU TO PNE; BUT DIDN'T EXPECT YOU TO GO BY WAY OF KANSAS CITY' (OR SOMETHING LIKE THAT). I CHOSE NOT TO RESPOND UNTIL I GOT THE ACFT UNDER CTL. I DECIDED TO LOOK OUT THE ACFT WINDOW TO GET A VISUAL FIX ON WHAT WAS HAPPENING. OBSERVING GND LIGHT IN THE WIND SCREEN MOVING TO MY L RAPIDLY; I PUSHED L RUDDER AND USED L AILERON TO STRAIGHTEN OUT OF THE TURN; THEN GRADUALLY BROUGHT THE NOSE UP TO PULL OUT OF THE DIVE. BY THAT TIME ATC AGAIN QUERIED ME AND I RESPONDED WITH THE RPT THAT MY GYROSCOPE HAD FAILED AND REQUESTED A 'NO-GYROSCOPE' VECTOR BACK TO SUS ARPT. IT TOOK ANOTHER MIN OR SO TO STABILIZE THE ACFT FOR DSCNT. I HAD DSNDED FROM FL120 TO FL130 IN ABOUT 20-30 SECONDS. THE RETURN TO SUS WAS ACCOMPLISHED WITHOUT FURTHER INCIDENT.