THE AIRPLANE CRASHED ON APCH 3/4 MI SW OF THE ARPT. A WITNESS HEARD SOUND HE ASSOCIATED WITH PROPS GOING INTO REVERSE, THEN OBSERVED AIRPLANE EMERGE FROM CLOUDS IN A NOSE-LOW ATTITUDE. CVR TAPE REVEALED SOUND OF AN ABRUPT CHANGE IN FREQ & AMPLITUDE 10 SECS PRIOR TO IMPACT. EXAM OF THE LEFT ENG BETA INDICATOR LAMP REVEALED HEAVY OXIDE DEPOSIT & STRETCHED COILS INDICATIVE OF THE LAMP BEING ILLUMINATED AT IMPACT; RIGHT ENG BETA LAMP WAS DESTROYED. POWER LEVERS AND (BETA MODE) TRIGGER LOCKS OPERATED NORMALLY; TRIGGER RETURN SPRINGS INTACT. FLIGHT IDLE STOPS SHOWED NO EVIDENCE OF EXCESSIVE WEAR OR DEFORMATION. THE POWER LEVER (BETA) BLOCKING DEVICE LOCKOUT SOLENOID WAS TESTED ELECTRICALLY AND FUNCTIONED NORMALLY. THE RIGGING OF THE BETA LOCKOUT DEVICE COULD NOT BE CHECKED DUE TO IMPACT DAMAGE. THE OPERATOR CONDUCTED A FUNCT TEST OF THE ELECTRICAL OR BACKUP BETA BLOCKING DEVICES ON THE REMAINING 8 CASA 212 AIRPLANES; 3 WERE FOUND TO BE INOPERATIVE. NEITHER THE MFGR NOR OPERATOR HAD AN INSPECTION OR FUNCT TEST REQUIREMENT FOR THE BLOCKING DEVICES.