What was found
DURING CRUISE FLIGHT PILOT REPORTED SMOKE IN COCKPIT. REPORTED THAT WHEN HE CLOSED THE BLEED AIR HEATER CONTROL VALVE THE SMOKE STOPPED. HE REOPENED VALVE AND IN A SHORT TIME SMOKE RETURNED. PILOT PERFORMED PRECAUTIONARY LANDING AT THAT POINT. WHEN MAINTENANCE ARRIVED THE COMPRESSOR INLET WAS INSPECTED FOR SIGNS OF OIL LEAKAGE FROM THE COMPRESSOR BEARING SEAL AND NO FAULTS WERE NOTED. THE BLEED AIR HEATER SYSTEM WAS INSPECTED AND NO FAULTS WERE NOTED. THE ENGINE WAS BORESCOPED AND NO FAULTS WERE NOTED. CAUSE HAS BEEN UNABLE TO BE DETERMINED AT THIS POINT.