Introduction
On November 8, 2011, about 1425 Pacific standard time, a Bell 47D1 helicopter, registration N4251A, sustained substantial damage during a hard landing at Brown Field Municipal Airport in San Diego, California. The helicopter was operated as a visual flight rules instructional flight under 14 CFR Part 91. The certified flight instructor and the private pilot student/owner were not injured. The flight originated from Brown Field about 1355. Visual meteorological conditions prevailed, and no flight plan had been filed.
Flight Details
The CFI reported that a set of uneventful maneuvers had just been performed in the traffic pattern. They decided to execute a practice autorotation from 1,000 feet above ground level to a power recovery at 200 feet agl. During the descent, the student maintained directional control while the CFI adjusted the rotor rpm. The student began to reapply engine power at 250 feet agl, but the engine did not respond. The student relinquished controls to the CFI, and the helicopter landed hard.
Accident Sequence
The hard landing resulted in substantial damage to the lower fuselage, and the main rotor blades struck and severed the tailboom.
Post-Accident Examination
The helicopter was powered by a Franklin six-cylinder normally aspirated engine with a float-type carburetor. An FAA inspector and an Airframe and Powerplant Mechanic examined the engine. The accessory case had cracked during impact, preventing engine operation. Fuel was present in the fuel tank and carburetor bowl. The post-impact examination revealed no anomalies with the engine that would have precluded normal operation.
Carburetor Icing Considerations
The carburetor icing probability chart from FAA SAIB CE-09-35 indicated a probability of serious icing at glide power given the reported temperature and dew point at the accident time. The CFI reported monitoring the carburetor temperature gauge throughout the flight and that its indicated temperature did not warrant carburetor heat use before the practice autorotation. Neither the CFI nor the student applied carburetor heat during the autorotation, nor did they monitor the temperature during the maneuver.
The CFI had 510 flight hours providing flight training in rotorcraft. His total experience in the Bell 47 series was 6 hours, all in the accident helicopter with the owner present. His prior helicopter experience was exclusively in turbine-powered helicopters, and this was his first autorotation in a Bell 47.