SMA PLT IN NIGHT OP; EXPERIENCES HDG TRACK ALTDEV. NON ADHERENCE TO ATC CLRNC.

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GRADUAL GYROSCOPE FAILURE AT 11000 FT IMC IN TURB. DURING THE 6 1/2 HRS OF A ROUND TRIP FLT FROM NASHUA; NH; TO CLEVELAND; OH; AND BACK; WE HAD BEEN PLAGUED BY A CONSTANT PROB OF THE PLANE DRIFTING OFF TO THE L. WE BLAMED EVERYTHING FROM OURSELVES; TO FUEL BAL; AILERON TRIM AND RUDDER TRIM. IT DIDN'T SEEM TO MATTER WHAT WE DID; WE KEPT CREEPING TO THE L. ABOUT 45 MINS FROM NASHUA ON OUR RETURN TRIP; DARKNESS HAD SETTLED IN LEAVING THE COCKPIT PITCH BLACK EXCEPT FOR THE DIMLY LIT INST PANEL. WE WERE TAKING VERY LIGHT CHOP FROM TIME TO TIME AND WE HAD BEEN IMC FOR MAYBE 10-15 MINS WHEN I FOUND MYSELF IN A CLBING TURN TO THE R. I ATTEMPTED TO CORRECT FOR THIS BY LOWERING THE NOSE AND TURNING SLIGHTLY TO THE L. CHKING WITH OTHER INSTS I QUICKLY FOUND I WAS BELOW ASSIGNED ALT AND DSNDING AND MY AIRSPD WAS INCREASING. A GLANCE AT THE ATTITUDE INDICATOR CONTRADICTED THIS AND SHOWED THAT I WAS IN EVEN A STEEPER CLB. I CHKED THE TURN AND BANK. IT SHOWED A L TURN AT ABOUT 1/2 STANDARD RATE. FOR A FEW SECONDS THIS WAS CONFUSING. THEN; AT JUST ABOUT THE SAME INSTANT IN TIME MY PAX (A PLT) AND I FIGURED OUT THAT WE HAD GYROSCOPE FAILURE. KNOWING THE GYROSCOPE WAS GOING ITS OWN WAY; I CONCENTRATED ON THE ALTIMETER AND TURN AND BANK FOR ATTITUDE. LEVELING THE PLANE OUT; I STARTED TO REGAIN MY ALT TO OUR ASSIGNED 11000 FT. WE ADVISED OUR ZBW CTLR THAT WE WERE HAVING SOME GYROSCOPE PROBS AND NEEDED AN EXPEDITED DSCNT TO GET DOWN UNDER THE OVCST WHICH WAS RPTED IN THE AREA TO BE ABOUT 4000 FT. AS IT TURNED OUT; WE WERE TO BE STARTING OUR DSCNT ANYWAY. CTLR DID AN EXCELLENT JOB; GIVING US A 'NO GYROSCOPE' DSCNT DOWN THROUGH THE OVCST. IN OUR POST-MORTEM OF THE FLT WE LOOKED BACK ON THE 6 HRS OF FLYING AND WONDERED HOW COULD WE NOT HAVE SEEN IT. IT WAS SO OBVIOUS. THE INSIDIOUS INSIGNIFICANT ERROR IN THE GYROSCOPE. IN THIS CASE WE WOULD HAVE BEEN FAR BETTER OFF HAD THE GYROSCOPE JUST QUIT RUNNING. UNFORTUNATELY; GYROSCOPE FAILURES ARE GRADUAL AND OFTEN NEAR IMPOSSIBLE TO DETECT IN THE EARLY STAGES. THE 3 CLUES WE SHOULD HAVE PICKED UP ON WERE: 1) I HAD FLOWN THIS PLANE ON A DOZEN PRIOR TRAINING FLTS WITH NO EVIDENCE OF ANY PROB IN DIRECTIONAL CTL; 2) THIS IS AN OLD PLANE AND I HAD JUST RECENTLY REPLACED THE OTHER 2 GYROSCOPES AND SHOULD HAVE BEEN AT LEAST A LITTLE SUSPICIOUS; AND 3) BOTH MY PAX AND I HAD RECALLED THAT EVERY ONCE IN A WHILE THE ATTITUDE INDICATOR WOULD VIBRATE A LITTLE. THIS LATTER ITEM SHOULD HAVE BEEN A DEAD GIVEAWAY THAT SOMETHING WAS GOING WRONG.