CEN13LA194

The flight instructor and student pilot were cruising about 800 feet above ground level when they heard a loud “bang,” followed immediately by the low rotor rpm horn, a warning light illumination, and a rapid decrease in rotor rpm indication. In response, the instructor initiated an autorotation by lowering the collective, and the engine immediately lost power. The helicopter touched down and then rocked forward due to soft and downward-sloping terrain. The instructor applied slight aft cyclic to prevent the main rotor blades from contacting the ground; however, the main rotor blades struck and severed the tail boom. The engine was functionally tested, and it operated normally. However, one of the magnets used to provide rotor rpm indications was missing from the rotating transmission yoke and was found affixed to a bolt just aft of the yoke. It likely had become loose in flight, and its movement was the bang heard by the pilots. Scarring was found on one of the sensors opposite the magnet, indicating that the magnet had contacted the sensor. The separation of the magnet caused the rotor rpm indication to drop and the low rotor rpm warning horn and light to activate. Due to the control linkage between the collective and the throttle, when the instructor lowered the collective, the throttle closed rapidly. According to Robinson Helicopters, rapid throttle changes can result in a fuel-air ratio becoming too rich or too lean to sustain engine operation and result in an engine failure.

On March 14, 2013, about 1445 mountain daylight time, the flight instructor of a Robinson R-44, N392GP, was forced to make an autorotation to an open field after the engine lost power near Eagle Nest, New Mexico. The flight instructor and second pilot were not injured. The helicopter was substantially damaged. The helicopter was registered to Global Positioning Services, Inc., Anchorage, Alaska, and operated by Leading Edge Aviation, Inc., Bend, Oregon, under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed at the time of the accident, and a visual flight rules flight plan had been filed but not activated. The flight originated from Dalhart, Texas, and was en route to Monte Vista, Colorado. According to the instructor’s accident report, he and his student were cruising at 800 feet above the ground when they heard a loud “bang,” followed immediately by the low rotor RPM horn, warning light illumination, and a rapid decrease in rotor RPM. The instructor initiated an autorotation to an open field. The helicopter touched down and rocked forward due to the soft and downward sloping terrain. The pilot applied slight aft cyclic to prevent the main rotor blades from contacting the ground. The main rotor blades struck and severed the tail boom. The helicopter was later transported to the operator’s facility in Bend, Oregon, where the engine was functionally tested. The engine was started and ran normally, and all parameters where within normal limits. During the examination, it was discovered that one of the magnets used to provide rotor RPM indications was missing from the transmission yoke. There was scaring on one of the sensors opposite this magnet, indicating it had made contact with the magnet while the yoke was rotating. According to Robinson Helicopters, if one of the magnets or sensors opens, rotor RPM will drop and the low rotor RPM warning horn will activate. The magnet was later found affixed to a bolt just aft of the yoke, and a small dent was found on the horizontal firewall. According to Robinson Helicopter, rapid throttle changes in the R44 can result in the fuel-air ratio becoming too rich or too lean to sustain engine operation and result in an engine failure, particularly at higher density altitudes.