Bell 47D1 N93067 accident near Pell City, Alabama

Probable cause

The pilot’s use of an improper clutch engagement procedure, which resulted in clutch glazing, slippage, and, ultimately, the in-flight disengagement of the clutch and a loss of rotor rpm. Contributing to the accident were the installation of a light-weight clutch assembly and a higher horsepower engine, both of which increased the tendency for clutch slippage, and inadequate inspection of the clutch assembly.

NTSB analysis

The commercial pilot was completing a business flight in the helicopter. He reported that, while descending the helicopter for a confined off-airport landing, at 25 to 35 ft above ground level, he heard a "snap/pop" and that the engine suddenly began "over-revving." The helicopter started descending and then touched down hard, and one of the main rotor blades struck the helicopter's tail. After touchdown, the engine was still running, but the rotor blades were not moving.

Examination of the transmission revealed that it incorporated a light-weight clutch assembly, which was not authorized for use in the installed transmission model. Further examination revealed that the clutch shoe running surface on the inner diameter of the clutch drum exhibited a series of abnormal wear grooves along the entire surface of clutch shoe contact and that the clutch lining of each shoe exhibited glazing, which occurs when the clutch is improperly engaged. The contact surface of each clutch shoe also contained numerous metallic slivers that were atypical of a normal in-service clutch assembly and appeared to be material that had transferred from the drum to the shoes.

During the transmission examination, the pilot was asked to describe his normal engine start and clutch engagement procedure. The clutch engagement procedure described by the pilot was not in accordance with the published procedure. The pilot's procedure likely resulted in a lengthy clutch engagement and an extended period of clutch slippage during each start that led to the glazed condition of the clutch assembly. Once the glazed condition began to develop, the period of clutch slippage during each start lengthened, further contributing to the glazed condition. Ultimately, the glazed condition resulted in the in-flight disengagement of the clutch and a loss of rotor rpm.

The pilot reported that, about 15 flight hours before the accident, he had a mechanic install an engine in the helicopter that produced 25 more horsepower than the previously installed engine. The installation of the higher horsepower engine would have increased the tendency for slippage of a glazed clutch assembly.

The pilot also reported that the transmission had accumulated about 615 hours since overhaul. According to the helicopter's maintenance and overhaul instructions, the transmission should be disassembled every 600 hours to inspect all parts, including the clutch, for damage or excessive wear. Given that the engine had been installed about 15 flight hours before the accident, the 600-hour inspection could have been accomplished at that time. However, review of maintenance records did not indicate that the 600-hour transmission inspection had occurred, and the mechanic who installed the higher horsepower engine reported that he did not disassemble the transmission.

Text from the NTSB. Open the NTSB docket for ERA14LA099 for the full record.