What was found
AIRPLANE MODIFIED PER SERV AERO STC SA3777NM WITH INSTALLATION OF PT6A-135 ENGINE. THE PROPELLER CONTROL REGULARLY BECOMES FROZEN WHEN THE ENGINE IS SHUT DOWN AND BECOMES COLD, THEN LOOSENS UP WHEN THE ENGINE IS STARTED AND WARMS UP. IN THIS CASE, THE CONTROL BECAME FROZEN AND IMMOVEABLE DURING FLIGHT. SUBMITTER STATES THIS IS A RECURRING PROBLEM WITH ALL AIRPLANES INCORPORATING THIS ENGINE MODIFICATION. CONTROLS OF AIRCRAFT QUALITY SHOULD BE INCORPORATED INTO THE MODIFICATION. NOTE: FIRE SLEEVE HAD BEEN INSTALLED ON CONTROL TO PREVENT HEAT DAMAGE, BUT WAS NOT EFFECTIVE.