Incident Summary
The airplane was returning to the airport for landing when it began flying erratically. A witness reported seeing the aircraft streaming a whitish "vapor trail" from the left wing. Another witness observed the airplane bank abruptly into a steep left turn, followed by the nose pitching up. The aircraft then sank from view behind trees, after which the witness heard a crash and saw smoke.
Examination Findings
Post-accident examination found no evidence of flight control, engine, or propeller malfunction. However, the left inboard fuel cap was absent from the filler port. A ground search discovered the left inboard fuel cap in the grass beside the runway. The JetProp LLC, JetProp DLX Supplemental Flight Manual, Section 4, Normal Procedures Checklist, includes the instruction: "Left Wing 4.9e, ... Inboard Fuel Tank ... CHECK Supply Visually & SECURE CAP ..." The manual also states: "Replace cap securely." No mechanical malfunction with the fuel cap or filler port was identified.
Probable Cause
The official probable cause is the pilot's failure to maintain control of the airplane during a VFR pattern for a precautionary landing, which resulted in an uncontrolled descent and subsequent collision with terrain. Also causal was the pilot's inadequate preflight inspection of the aircraft, which resulted in his failure to secure the fuel cap.