On November 8, 2011, about 1425 Pacific standard time, a Bell 47D1 helicopter, N4251A, sustained substantial damage during a hard landing at Brown Field Municipal Airport, San Diego, California. The helicopter was being operated as a visual flight rules (VFR) instructional flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91, when the accident occurred. The certified flight instructor (CFI) and the private pilot student/owner were not injured. The flight originated from Brown Field Municipal Airport, about 1355. Visual meteorological conditions prevailed, and no flight plan had been filed.
The CFI reported that they had just performed a set of uneventful maneuvers in the traffic pattern, and decided to execute a practice autorotation from 1,000 feet above ground level (agl) 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 then began to reapply engine power at 250 feet agl, however, the engine did not respond. He then relinquished the helicopter controls to the CFI, and the helicopter landed hard. The helicopter sustained substantial damage to the lower fuselage during the landing, and the main rotor blades struck and severed the tailboom.
The helicopter was powered by a Franklin, six-cylinder, normally aspirated engine, and equipped with a float-type carburetor. The engine was examined by an inspector from the Federal Aviation Administration (FAA), and an Airframe and Powerplant Mechanic. The accessory case had become cracked during the impact sequence, and as such, the engine could not be operated. The fuel tank and carburetor bowl both contained fuel. A post impact examination did not reveal any anomalies with the engine that would have precluded normal operation.
The carburetor icing probability chart from Federal Aviation Administration (FAA) Special Airworthiness Information Bulletin (SAIB): CE-09-35 Carburetor Icing Prevention, indicated a probability of serious icing at glide power at the temperature and dew point reported at the time of the accident.
The CFI reported that he monitored the carburetor temperature gauge throughout the flight, and that its indicated temperature did not warrant the use of carburetor heat at any point leading up to the practice autorotation. Neither he nor the student applied carburetor heat during the autorotation, nor did they monitor the temperature during the maneuver.
The CFI reported 510 flight hours providing flight training in rotorcraft. His total experience in the Bell 47 series was 6 hours, all of which was performed in the accident helicopter, with the owner present. CFI’s prior helicopter experience occurred exclusively in turbine-powered helicopters, and this was the first time he had performed an autorotation in a Bell 47.