Accident Details
Probable Cause and Findings
The fatigue failure of a second stage compressor vane due to erosion, which resulted in a catastrophic engine failure. An inadequate inspection of the compressor section by company maintenance personnel during the last 300-hour inspection was also causal in this accident. The maintenance personnel's failure to conduct the periodic recommended compressor washes was a factor.
Aircraft Information
Registered Owner (Current)
Analysis
On November 30, 1999, at 1345 hours Pacific standard time, a MDHI 369E, N5238C, collided with a standpipe during a forced, autorotative landing near Taft, California, following a loss of engine power. The helicopter sustained substantial damage; however, the certificated commercial pilot, the sole occupant, was not injured. The helicopter was being operated as an external load flight by Omni Energy Services under 14 CFR Part 133 when the accident occurred. The flight originated from a private helipad near Taft at 1135. Visual meteorological conditions prevailed at the time and no flight plan was filed.
The pilot reported that he was in the process of picking up an 800-pound sling load on a 100-foot-long line. As his load reached about 90 to 100 feet agl, the engine quit. He released his load, turned to the right to avoid people and equipment on the ground, and entered autorotation. After touchdown, the left rear cross tube struck a standpipe, shearing the left skid tube and strut off the helicopter. As the main rotor slowed, the helicopter rolled onto its left side.
The ground at the accident site had been disturbed by vehicles and construction equipment, producing dusty and gritty conditions.
A representative of the engine manufacturer, Rolls-Royce Allison, participated in the investigation. The engine teardown revealed that the second stage compressor vanes had failed. All first stage vanes were intact, and all third, fourth, fifth, and sixth stage vanes exhibited some degree of foreign object damage (FOD). Additionally, erosion of the plastic coating on first and second stage vanes exceeded the maximum allowable limits specified by the Rolls-Royce Allison 250-C20 Series Operation and Maintenance Manual. The first stage compressor blades were found eroded with the leading edges rolled over and scalloped. Metallurgical examination of the failed vanes revealed that they conformed to all chemical and process specifications for the components.
According to maintenance records, a 300-hour inspection had been performed on the engine at 2,418.8 flight hours, about 130 hours prior to the accident. The manufacturer recommends that an inspection of the compressor case be performed every 300 flight hours when operating in an erosive environment. In addition, there were no records found for compressor washes.
Data Source
Data provided by the National Transportation Safety Board (NTSB). For more information on this event, visit the NTSB Records Search website. NTSB# LAX00LA047