What happened
Shortly after takeoff, the pilot declared an emergency via air traffic control due to an in-flight fire. An airborne witness observed the aircraft flying in level flight while emitting black smoke. The witness then reported seeing the aircraft bank to the left and enter a nose-down spiral toward the ground. The aircraft impacted flat, level terrain in a steep, nose-down attitude. A post-impact fire consumed much of the wreckage. The total flight duration was approximately four minutes before the impact.
The investigation
Post-recovery examinations identified evidence of an in-flight fire located in the lower aft engine compartment near the turbocharger and adjacent firewall. Evidence on the nose landing gear strut cylinder indicated that temperatures in this area reached levels exceeding the melting point of aluminum during flight.
During the engine examination, investigators discovered that the tension ring of the turbocharger exhaust clamp, which retains the exhaust duct to the turbine side, had fractured at the 11 o'clock position. While the exhaust clamp bolt remained connected and properly safetied, the broken clamp allowed hot exhaust gases to be directed against the lower firewall.
Analysis of the clamp showed a high-temperature creep/stress rupture originating from a crack at one of the resistance welds joining the sheet metal retainers to the tension ring. The heavy oxidation of the weld region compared to the lighter oxidation of the sheet metal suggested the crack was pre-existing. Investigators also found cockpit and cabin materials in the debris field that showed evidence of fire exposure prior to the ground fire.