FAA FLC CHK ACFT WITH 2 CHK AIRMEN ON BOARD IN POSSIBLE VIOLATION OF FAR 91 PT 13 AND 91 PT 301. AS ACFT GETS INTO SPIN DURING PLT CERTIFICATION FLT.

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MY SAFETY SUGGESTION IS THAT LIMITATIONS BE PLACED IN ALL FLT HANDBOOKS OR MANUALS PROHIBITING FLT MANEUVERS WITH YAW DAMPER APPLIED WITHOUT AUTOPLT OP. ON AUG/WED/91; WHILE ON A PROFICIENCY FLT WITH MR. X IN THE PLT'S SEAT OF THE TURBO SMT; I REQUESTED HIM TO DEMONSTRATE A STALL STRAIGHT AHEAD. HE SLOWED THE ACFT TO FULL FLAP SPD 130 KTS AND APPLIED 100 PERCENT FLAPS WITH A PWR SETTING OF 400 POUNDS PER ENG. HE THEN BEGAN TO PITCH THE ACFT UPWARD AND WHEN THE AIRSPD BEGAN TO BLEED OFF APPLIED INCREASING PWR AND PITCH UNTIL MAX PWR 1315 POUNDS WAS ATTAINED. THE AIRSPD FELL BELOW 60 KTS UPON WHICH TIME I WAS GOING TO PITCH THE NOSE OVER TO PREVENT A STALL IN SUCH A STEEP ATTITUDE. BEFORE I COULD REACT; THE ACFT PITCHED FORWARD AND THE L WING BEGAN TO FALL. MR. X THEN APPLIED FULL R AILERON AND THE ACFT STARTED TURNING L. AFTER ABOUT 1/3 OF A REVOLUTION; MR. X RETARDED THE THROTTLES TO IDLE AND IMMEDIATELY FEATHERED BOTH PROPS LEAVING NO OPTION FOR PWR MANIPULATION. HE YELLED AT ME TO DISCONNECT THE YAW DAMPER WHICH I COULD NOT REACH SINCE THE DISCONNECT BUTTON WAS ON HIS YOKE. THE ROTATION TIGHTENED UP ON THE SECOND TURN; AND SLOWED ON THE THIRD TURN TO A RECOVERY ON THE ENTRY HDG AT THE END OF 3 FULL TURNS. THE AIRSPD INDICATED 90 KTS ON RECOVERY FROM ROTATION NOSE DOWN SO I PUSHED FORWARD TO ATTAIN 120 KTS; INSTRUCTING MR. X TO UNFEATHER THE PROPS AND BRING UP THE PWR. WE RETRACTED THE FLAPS AND THEN THE LNDG GEAR; RETURNING TO LEVEL FLT. ALL INSTS REMAINED ERECT EXCEPT FOR THE FLT DIRECTOR SHOWING ABOUT 5 DEGS UP AND 5 DEGS R BANK IN LEVEL FLT. THE G LOADING ON PULL OUT I ESTIMATE TO BE LESS THAN 1 1/2 GS. ALSO THE FLT DIRECTOR WAS SLOW TO ERECT PRIOR TO DEP ON FLT. SUPPLEMENTAL INFO FROM ACN 186843: THE GREAT FALLS ATCT ASKED IF WE COULD TAKE AN FAA ATCT CTLR AS A PAX WITH US AND WE AGREED. THE CHKPLT THEN REQUESTED A STALL AND STALL RECOVERY. I ASSUMED THAT THE FIRST STALL BEING REQUESTED WAS AN APCH TO LNDG STALL. WHEN THE STALL WARNING HORN ACTIVATED; I STARTED TO INITIATE A NORMAL STALL RECOVERY BY LOWERING THE NOSE AND ADVANCING THE PWR LEVERS. I ASKED THE CHKPLT IF THAT WAS OK? HE SAID HE WOULD LIKE TO SEE OR FEEL THE ONSET OF THE STALL; IE; THE INITIAL BUFFET OR BURBLE. I SAID OK AND RETARDED THE PWR LEVERS TO THE PREVIOUSLY SET 600-700 POUNDS OF TORQUE; REESTABLISHED THE 10 PERCENT NOSE UP PITCH; AND ALLOWED THE AIRSPD TO BLEED OFF. SOMEWHERE BTWN 60-70 KTS OF AIRSPD; AND WITH NO ADVANCE WARNING (IE; NO PREBUFFET OR BURBLE) THE AIRPLANE ABRUPTLY AND RAPIDLY ROLLED TO THE L AND INTO A L TURNING SPIN. AT THIS POINT; IMMEDIATE STANDARD SPIN RECOVERY PROCS WERE STARTED; IE; PWR LEVERS TO IDLE; NOSE DOWN; YAW DAMPER OFF; FULL R RUDDER. AFTER APPROX 1 1/2 TURNS; THE RATE OF TURN DID NOT APPEAR TO BE SLOWING DOWN. I THEN FEATHERED BOTH PROPS AND THE RATE OF TURN IMMEDIATELY STARTED SLOWING DOWN AND STOPPED AFTER THE COMPLETION OF 1 1/2 ADDITIONAL TURNS. THE ALT LOSS WAS ESTIMATED AT 1500 FT.