FTW95LA056

During a maintenance test flight, the helicopter experienced a loss of engine power due to mechanical reasons. The maintenance history revealed that the engine had experienced a series of compressor stalls during the last 132 hours of flight. Company management elected to defer required maintenance until their 'busy season was over.' The metallurgical analysis showed that intergranular corrosion and erosion of the stator vanes combined to effectively weaken the individual vanes structurally. A compressor stall was experienced. The pilot performed an autorotation and during touchdown the main rotor blades severed the tail boom.

On November 29, 1994, at 1432 central standard time, a Bell 206B, N2183P, was destroyed during a forced landing near Natchitoches, Louisiana. The commercial pilot and the three passengers were not injured. Visual meteorological conditions prevailed for the local 14 CFR Part 91 flight.

In a written report submitted by the operator, they stated that the aircraft was on a maintenance test flight to evaluate the engine performance. They further stated that the reason for the test flight was to determine the effects of previously reported compressor stalls during the last 132 hours of operation. The operator stated that they had three maintenance personnel onboard, 2 were engine specialists, conducting the testing.

During an interview conducted by a Federal Aviation Administrator (FAA) Inspector, the pilot stated that one of the onboard technicians monitoring the test equipment requested that he move the starter/generator switch to the "OFF" position during one of the tests. After completion of the test the pilot "rolled" the throttle back to flight idle and moved the starter/generator switch to the "ON" position. The throttle was then "rolled" back to the full on and the pilot heard a "rapid popping" sound. The engine sustained a total power loss and the pilot initiated an autorotation from 1,200 above the ground, onto a golf course.

Maintenance personal reported that management had deferred maintenance on the aircraft, following the compressor stalls, "until the end of our busy season."

An FAA Inspector reported that during the forced landing the fuselage and dynamic components sustained damage and that the tailboom was severed by a main rotor strike.

The case halves were taken to Materials Analysis,Inc., Dallas, Texas, for metallurgical analysis. They determined that two dynamic processes had weakened the stator vanes - erosion and intergranular corrosion (see enclosed metallurgical report). The report further states "a compressor stall alone would not normally apply sufficient load to the stator vanes to cause an overload failure; however, the combined effects of vane erosion and pre-existing intergranular cracks in the vanes could effectively weaken the individual vanes to such a degree that unusual load applications could result in unanticipated vane overload failures."