Accident Sequence
The pilot reported that after rotation, a positive rate of climb was established. At 110 knots, with the landing gear retracted and wing flaps at 10 degrees, he noticed a right roll, a drop in climb performance, and a decrease in manifold pressure on the right engine to at least 34 inches, while the left engine maintained 42 inches. Citing a "very minimum climb rate, rising terrain ahead, [and] airspeed not increasing," the pilot decided to land the airplane in a small field located about 1/4 mile away and 50 degrees to the left. He abruptly lowered the nose and raised the flaps to gain airspeed, then landed with a nose-high attitude and the landing gear partially extended.
Post-Accident Examination
Examination of the airplane revealed vertical compression to the belly area, a split in the fuselage across the top at the aft end of the cabin, and damage to both wings, with the left wing buckled downward just inboard of the engine. Additionally, a clamp on the right engine intake manifold was found loose.
Weight and Performance
The estimated takeoff weight placed the airplane 74 pounds over the maximum allowed weight of 6,200 pounds. The type certificate holder estimated that with the airplane at 6,400 pounds, climbing at 110 knots, and experiencing a partial power loss down to 26 inches on one engine, the rate of climb should have been 1,150 feet per minute with flaps and landing gear up, and 830 feet per minute with flaps at 10 degrees and landing gear down.
Terrain and Pilot Experience
Higher terrain was located to the east, and lower terrain to the west. For a straight-out departure, terrain elevation was 25 feet above the runway at 0.5 nautical miles, and 70 feet above the runway at 2.8 nautical miles. The pilot reported a total flight experience of 15,000 hours, including 13,000 hours in multi-engine airplanes, and 30 hours in the accident make and model, all within the preceding 90 days.
Probable Cause
The official probable cause was the pilot's improper in-flight decision to perform a precautionary landing and his failure to maintain airspeed after experiencing a partial loss of power on one engine. A contributing factor was the partial loss of power on one engine due to an induction air leak.