ERA10TA261 · Bell 407

The certified flight instructor (CFI) was providing aircraft orientation training for the commercially rated pilot. Visual meteorological conditions prevailed and no flight plan was filed for the public aircraft flight. About 7 minutes into the flight, while enroute to practice confined area operations at a field about 3 miles from the helicopter base, the crew heard an unusual noise from the engine compartment. About 250 feet above the targeted field, the crew heard a louder noise, and the engine surged twice before ceasing to develop power. The CFI then conducted an autorotation to the sloping terrain below. The helicopter sustained substantial damage, which included fuselage crushing and the partial loss of one vertical stabilizer. Data downloaded from the engine control unit revealed an overtemperature fault indication; the engine was then removed and shipped to the engine manufacturer's facility for a detailed examination. A circular metal deflector plate, which was normally affixed to the aft end of the combustion chamber liner, was found fragmented in the turbine section. The turbine blades and vanes exhibited significant damage, which resulted from the deflector plate's release into the gas path. Metallurgical analysis of the combustion chamber liner revealed that the required circumferential fillet weld between the liner and the deflector plate had not been performed; only the preliminary positioning welds attached the deflector plate to the liner, and those welds failed during normal engine operation.

Maintenance records indicated that the liner had accumulated about 158 hours in service since its overhaul and reinstallation. The liner overhaul included replacement of the deflector plate; the replacement was accomplished by a repair facility that was not authorized to conduct that procedure and that also did not possess the applicable guidance. The investigation was unable to determine the specifics of why the repair facility replaced, inspected, and approved the deflector plate. Although 19 months had transpired between the improper repair and the liner's failure, the investigation did not locate any information that indicated that either the repair facility or the Federal Aviation Administration principal maintenance inspector for the repair facility was aware that maintenance personnel at the repair facility had accomplished a procedure that it was not authorized to conduct. The repair facility identified 19 other assemblies that had a known or suspected improper repair, recalled those assemblies, and no additional in-service failures occurred. The engine manufacturer subsequently modified its overhaul manual to clarify the relevant repair and replacement procedures.

Verbatim NTSB analysis and probable cause from the NTSB dataset