Accident Overview
On January 27, 2016, about 1157 eastern standard time, a Robinson R22 Beta (registration N404LE) was substantially damaged during an accident near Lindenhurst, New York. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight with a flight instructor and a student pilot on board; neither occupant was injured.
Sequence of Events
According to the flight instructor, the helicopter had been airborne for about two hours and was returning to Republic Airport (FRG) in Farmingdale, New York. During the return, the helicopter began to experience vibrations that increased in intensity. As the instructor reduced power, the vibrations became "very violent," and the low rotor rpm horn and light warning system activated. The instructor performed an autorotation to a street in a residential neighborhood. Upon touchdown, the helicopter bounced, and the main rotor blades struck a sign. The helicopter subsequently rolled over onto its right side. The flight instructor stated that he was using full carburetor heat throughout the flight.
Examination Findings
An on-site examination by a Federal Aviation Administration (FAA) inspector revealed that the tailboom had separated just before the vertical fin. The tail rotor gear box and tail rotor blades displayed damage consistent with ground contact. An unquantified amount of fuel was observed in the helicopter’s fuel tanks, and the carburetor heat control was noted in the "ON" position.
Engine Testing
A subsequent engine test run showed that the engine started and operated smoothly at idle and when rpm was increased. However, during a magneto check, selecting the left magneto caused the engine to shut down, while the engine continued to operate when the right or both magnetos were selected.
Further examination of the left and right magnetos revealed that each magneto’s terminals produced a blue spark across a 7-millimeter gap throughout all magneto rpms when tested on a stand. The torque putty on the left magneto’s case half mating screw was compromised. Disassembly of the left magneto disclosed that its internal magneto timing was off by two teeth, a condition that would have increased the gap between the distributor gear finger and the distributor block’s stationary electrodes.