Bell 206 N777CP accident near Zephyrhills, Florida

Probable cause

Improper torquing of the “B” nut at the fuel control unit, which led to a total loss of engine power and subsequent autorotation that resulted in substantial damage.

NTSB analysis

While in cruise flight, the pilot heard a binding sound and felt a reduction in engine power. The low rotor rpm aural alarm then sounded, and he initiated an autorotation to swampy terrain. The helicopter came to rest amongst pine trees and undergrowth. The tailboom was impactseparated and the main rotors were fractured about midspan.

During a detailed examination of the helicopter, the “B” nut at the engine fuel control unit was found loose with no thread engagement. During a test run of the engine, the “B” nut was tightened and torqued to proper specification. The engine ran smoothly and continuously at all power settings.

A review of the helicopter’s maintenance records revealed the last annual inspection was completed about 75 hours prior to the accident. The pilot, who also holds an airframe and powerplant certificate, was the last person to perform maintenance on the engine.

According to the Rolls-Royce maintenance manual, torque paint must be applied to all the rigid tube “B” nuts to ensure proper tightening. According to the maintenance manual, “failure to properly install, align, and tighten fuel, oil, and air fittings could result in an engine failure.”

Text from the NTSB. Open the NTSB docket for ERA21LA023 for the full record.