Incident Overview
During cruise flight in an experimentally certified airplane, the crankcase breather, which was not pressurized and lacked a whistle slot, froze up. This caused the crankcase to pressurize, dislodging the crankcase seal. The pilot initiated a precautionary landing in a field of limited size, surrounded by trees.
Approach and Touchdown
Due to oil on the pilot's goggles and windshield, visibility was restricted. On final approach, the airplane struck a tree and subsequently made an uncontrolled touchdown in the field.
Investigation Findings
The investigation revealed that there is a requirement for a whistle slot on normal, utility, acrobatic, and commuter category airplanes, but no such requirement exists for experimental category airplanes.