Date: 201509
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Engine Air
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Not Flying
Reporter qualification: Flight Crew Private
Human factors: Training / Qualification; Situational Awareness
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Aircraft Damaged; Flight Crew Landed in Emergency Condition; Flight Crew Inflight Shutdown; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: I and a friend; the owner and builder of the plane; agreed a few days earlier to meet at the airport for a scenic flight over the ocean. The friend had recently lost his medical and needed me to go with him as PIC. I have flown with him before and found him to be both capable and diligent in his flying skills. The plane is an experimental 'tundra' plane powered by a liquid-cooled engine. When I arrived at the airport he was already seated in the left seat with the engine running; ready for departure. I got in the plane and readied myself for flight. Within about five minutes we taxied for departure on runway 31. The departure was uneventful; including a short climb out. We leveled at about 1500 feet and proceeded on a north-westerly heading.Within a few minutes I noticed an odor of something burning. I scanned the plane for any sign of smoke and saw none. I did not mention this to the pilot as the area we were flying over is an agricultural area and frequent burning of brush occurs. About 5-6 minutes into the flight the engine 'coughed' then began running smoothly for about 30 seconds. By then the pilot had already turned back toward the airport. He engine ran rough for a few seconds then seemed to smooth out. We continued towards the airport; about 3 miles away. The pilot radioed that we were returning to the airport and intended to land on runway 13. There was no response from anyone. About 2 miles from the airport the engine 'coughed' again and the RPMs drop significantly. At that point we both began scanning the ground for a place to land in case the engine completely failed. Almost immediately the engine stopped running. The pilot decided to land in an open field about a mile and a half from the airport. Upon landing; which was relative uneventful; we discovered that during the pre-flight; the pilot forgot to remove the plugs from the two air intakes located near the back of the fuselage. The air that enters the intakes goes directly to the radiator that cools the engine. This caused the engine to overheat and ultimately seize.This is my first off-field landing. In addition to better understanding the urgency of decision making in critical events like this; I now better understand that trusting even a competent pilot can lead to trouble.I should have recognized that my friend was eager (which he later told me he was) to go flying and could have been distracted. I should have done my own pre-flight of the plane before taking off; to mitigate any risk.Upon smelling something burning during the flight; I should have mentioned this to the pilot and gotten his opinion of the cause instead of assuming I knew the cause of the odor. He later told me the engine temperature gauge indicated the engine was running hot.There were no injuries to me; the pilot or anyone on the ground. We talked with the foreman of the field and he advised that the field was not planted so there was no damage to any crop.




Synopsis: A Tundra pilot failed to remove the engine cooling air intake plugs which caused an engine overheat and seizure. A safe off airport landing followed.