FLC OF CPR TURBO PROP IN CRUISE AT FL280 LOST PRIMARY FLT CTL INSTS. REVERTED TO NEEDLE BALL AND AIRSPD WITH WET FREE COMPASS FOR FLT CTL AND HDG.

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CORPORATE FLT WITH 6 PAX AT FL280 MY COPLT SAID LOOK AT MY ATTITUDE INDICATOR. THE AIR DRIVEN GYRO WAS FLOPPING FROM 30 DEG BANK EITHER SIDE AND 30 DEG PITCH IN BOTH DIRECTIONS. HAVING EFIS ON THE L SIDE OF THE COCKPIT AND (2) TURN AND SLIP INDICATORS WE DIDN'T CONCERN OURSELVES WITH THE LOSS OF 1 ATTITUDE GYRO. WHEN APPROX 100 MI S OF CGT WE WERE CLRED DIRECT TO CGT DIRECT TO BRAVE INTXN DIRECT TO UES. AS THE COPLT LOOKED UP THE COORDINATES FOR BRAVE I PROGRAMMED FOR CGT; BUT WAS UNABLE TO DO SO. THE RNAV GAVE FUNNY LOOKING INDICATIONS AND ERASED THE PREVIOUS WAY POINT. ABOUT THAT TIME I NOTICED THAT THE AUTOPLT WAS NOT MAINTAINING THE ASSIGNED HDG AND WE WERE IN A BANK TO THE L OR TURNING L. THE CENTER CTLR ASKED WHERE WE WERE GOING. I TOLD MY COPLT WE APPARENTLY HAD LOST THE SLAVED HDG INDICATORS AND TO GO TO FREE SLAVE; WHILE WE WERE DOING SO AND TRYING TO MAINTAIN HDG MY ALT VARIED 500 FT EITHER SIDE OF ASSIGNED FL280. THE COPLT TOLD ME WE HAD A 35 DEG SPLIT IN HDG INDICATORS; I TRIED TO MAINTAIN WINGS LEVEL WITH THE PLTS EFIS ATTITUDE INDICATOR BUT NOTICED THE TURN NEEDLE AND COMPASS KEPT TURNING. I INSTRUCTED THE COPLT TO FLY USING HIS TURN AND SLIP INDICATOR BUT HE TOO VARIED IN ALT BY 500-800 FT. I TOLD HIM I WOULD NOW TAKE IT USING MY TURN AND SLIP IF HE WOULD MONITOR COMPASS (MAG) AND ALT AT THAT TIME HE STATED THAT THE COMPASS SYS WAS ON FREE GYRO BUT MY ATTITUDE INDICATOR HAD FAILED. WE WERE ABLE TO REGAIN COMPLETE CTL OF ALT AND HDG BY USING NEEDLE BALL AND FREE GYRO COMPASS. OUR MAINT DEPT REPLACED COPLT'S ATTITUDE INDICATOR (MECHANICAL). PLT'S ATTITUDE INDICATOR (AC) AND PLT'S (DC PWRED COMPASS INDICATOR) AT NO TIME DID WE HAVE ANY FAIL FLAGS ON ANY SYS.