Event Summary
Shortly after takeoff, the pilot declared an emergency with air traffic control due to an in-flight fire. An airborne witness in another aircraft observed the accident airplane in level flight with black smoke emanating from it. The witness then reported the airplane banking left and entering a nose-down spiral toward the ground. The airplane impacted flat terrain in a steep nose-down attitude, and a post-impact fire consumed most of the airplane. The flight was airborne for approximately four minutes before the crash.
Post-Recovery Examination
On-scene and detailed post-recovery examinations revealed evidence consistent with an in-flight fire in the lower aft engine compartment area, near the turbocharger and adjacent firewall. Evidence on the nose landing gear strut cylinder indicated that temperatures in that area exceeded the melting point of aluminum while the airplane was in flight.
Component Analysis
During the engine examination, investigators found that the tension ring of the turbocharger exhaust clamp—which retains the exhaust duct to the turbine side of the turbocharger—had fractured at the 11 o'clock position. The exhaust clamp bolt remained connected and properly safetied. With the clamp broken and the exhaust ducting loose, hot exhaust gases would have been directed against the lower firewall. The examination of the turbocharger exhaust clamp showed a high-temperature creep/stress rupture, initiating from a crack at one of the resistance welds that join the sheet metal retainers to the tension ring. Turbocharger gases are approximately 1,200 to 1,500 degrees Fahrenheit. The tension ring fracture surface exhibited intergranular brittle fracture across about 80 percent of the cross-sectional area, originating at the resistance weld. The heavily oxidized resistance weld region, compared to the lighter oxidized sheet metal, suggested that the weld crack was pre-existing.
Historical Context
A review of past National Transportation Safety Board data revealed three other accidents (FTW98FA325, FTW99LA241, and CHI02FA042) with similar fractures of exhaust clamps. Other referenced data indicated that Inconel 718 could crack in an intergranular mode in a time-dependent creep/stress rupture fashion within this temperature range. Additionally, cockpit/cabin material that showed exposure to fire was found in the wreckage debris field away from any areas affected by the ground fire.