Overview
The flight departed and was flying at 6,000 feet mean sea level when, about 7 minutes into the flight, the pilot contacted the controller stating the intention to shut off the master switch due to an electrical fire. The controller acknowledged and provided the location of a nearby airport. The pilot replied with "standby" and then declared an emergency, reporting the flight was descending. The controller informed the pilot of a nearby grass airstrip, but the pilot questioned the location and stated, "we need to land immediately." No further communications were received from the pilot.
Witness and Radar Observations
Witnesses near the accident site reported seeing smoke, a "vapor trail," or "dust" trailing the airplane before it struck the water of Arkabutla Lake. Radar data indicated transponder beacon returns continued every 4.6 seconds from 1154:06 until 1201:54, when the last return placed the airplane at 5,400 feet msl. A primary uncorrelated radar return was observed near the last transponder return and continued every 4-5 seconds until 1203:31. Between 1201:59 and 1203:21, the radar target was seen banking left from a southerly to a northeasterly heading; the final two returns showed a right bank, with the last return over a section of Arkabutla Lake.
Wreckage Examination
Before recovery, a section of the horizontal stabilizer was visible above the water's surface. The right wing and cockpit forward of the carry-thru spar were fragmented. The left wing was separated but largely intact. In the cockpit, heat damage was localized on the pilot's side behind the instrument panel. The battery switch was found in the off position, while both alternator switches were on. A 10-gauge electrical wire that jumps between the W15 and W11 bus bars on the left subpanel was fractured and showed evidence of arcing on the supply side. A pilot air control surface near the wire exhibited arcing damage composed partly of tin; undamaged areas of the same control lacked tin, while the damaged wire also contained tin. A rigid aluminum line for the left fuel flow and pressure indicator was cracked, sooted, and bulged near the combination oil pressure/oil temperature/cylinder temperature gauge, displaying features of overstress fracture at elevated temperature. Another rigid aluminum line for left oil pressure had a missing section and similar overstress fracture features.
Electrical System Analysis
Post-accident testing replicated the condition of the 10-gauge wire, producing a similar large ball on the wire's end when a compressive load shorted it to ground and current from the batteries flowed through it. Additional testing of current limiters with various loads showed that they opened more quickly as current increased. The emergency checklist for electrical smoke or fire directs pilots to shut off the battery and both alternator switches.