What was found
(CAN) THE RETAINING BOLT FOR THE AIR FILTER HOUSING BACKED OFF FROM THE TORQUED POSITION AND ALLOWED THE HOUSING TO MOVE FORWARD AND STRIKE THE PROPELLER DURING FLIGHT. A VIBRATION WAS FELT BY THE PILOT AND HE RETURNED THE AIRCRAFT TO BASE AND LANDED SAFELY. MAINTENANCE INSPECTION REVEALED THAT THE FIBER LOCK ANCHOR NUT IN THE AIR FILTER HOUSING WAS IN WORN CONDITION WITH LITTLE OR NO LOCK FEATURE REMAINING. A FLEET CAMPAIGN TO CHECK ALL SIMILAR INSTALLATION IS IN PROGRESS. (ENGINE/PROP REMOVED FOR REPAIR.)