Incident Overview
An aircraft was destroyed during an on-ground fire following an emergency descent and landing prompted by an in-flight fire. The pilot reported that smoke was detected by smell, after which the cabin rapidly filled with heavy black smoke. As the pilot maneuvered the aircraft for a landing in an agricultural field, flames began to enter the cockpit. During the landing, the aircraft nosed over.
Investigation Findings
Examination of the left exhaust stack assembly revealed a fatigue fracture adjacent to the main collector welded joint. The fracture was observed in both the heat affected zone and the parent material. It initiated in multiple locations on the outside surface of the tube and progressed through a fatigue mechanism. Energy dispersive spectroscopy (EDS) confirmed that both the weld and tube material met design specifications, and the weld profile contained no apparent flaws. The left exhaust stack assembly had accumulated approximately 100 hours since new.
Design Differences
The design of the Cessna-supplied left exhaust stack assembly differs from the comparable Knisley PMA assembly. The Cessna assembly features a two-piece main collector, with the two halves stamped out and joined by a seam weld. The Knisley main collector is manufactured by welding several tube sections together. Notably, the weight of the turbocharger assembly is supported in part by the left exhaust stack assembly.
Service Bulletin
Knisley issued a mandatory service bulletin calling for the removal of their left exhaust stack assembly and its replacement with the current Cessna assembly or an FAA-approved equivalent. The bulletin stated that the replacement was instituted because the Knisley assembly was not designed to carry the load of the turbocharger and compressor.