What happened
During a maintenance check flight conducted after an engine replacement, a fire erupted in the right engine nacelle. The pilot responded by shutting off the engine and feathering the propeller. Before the emergency checklist could be completed, black smoke entered the cockpit through a fresh air vent located in the wing's leading edge near the nacelle.
The resulting smoke density reduced visibility within the cockpit to approximately one foot, obscuring the instrument panel, controls, and the view outside. The pilot declared an emergency and opened the pilot-side window. While the pilot instructed a passenger to open the main door, the dense smoke prevented the passenger from locating the door handles. After some time, the door was partially opened, allowing the smoke to clear.
At an altitude of approximately 300 to 400 feet AGL, the pilot identified an open field and executed an emergency landing. The occupants were not injured.
The investigation
An examination of the right-hand engine showed it was mechanically intact. Investigators focused on a hose-end "B-nut" at the inlet to the engine-driven fuel pump, which displayed an unusual number of exposed threads. This B-nut was found to be loose; it required 1 1/8 turns to finger-tighten and an additional 1 1/4 turns with a wrench to reach a snug fit. Upon removal, neither the threads of the fitting nor the B-nut itself showed evidence of cracks or stripping. No other mechanical anomalies were identified.