NTSB Identification: LAX99LA047.
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Accident occurred Friday, December 11, 1998 in HELENDALE, CA
Probable Cause Approval Date: 03/31/2000
Aircraft: Bell 47G2, registration: N9021R
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The pilot encountered retreating blade stall while flying at 100 mph. He reported that the helicopter began to shake and it felt like an 'out of balance feeling, like when the washing machine gets unbalanced.' The rotor rpm began to decay. When he reduced power and stabilized the airspeed to about 60 to 65 mph, the shaking stopped. The pilot stated that he was not able to maintain lift, so he entered an autorotation over the hilly terrain. The tail rotor blades impacted an embankment and the main rotor blades severed the tail boom. The pilot's manual for the Bell 47G2 indicates that the 'never-exceed speed' (Vne) for the helicopter is 100 mph. According to the FAA's Basic Helicopter Handbook, the recovery procedure for retreating blade stall is to reduce the collective pitch, increase rotor rpm, reduce forward airspeed, and minimize maneuvering.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The pilot's failure to follow proper procedures to recover from main rotor blade aerodynamic retreating blade stall.

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