Accident Overview
On March 14, 2013, at approximately 1445 mountain daylight time, a Robinson R-44 helicopter (registration N392GP) experienced a loss of engine power while en route from Dalhart, Texas, to Monte Vista, Colorado. The flight was conducted under 14 CFR Part 91 as an instructional flight, operated by Leading Edge Aviation, Inc., of Bend, Oregon. Visual meteorological conditions prevailed, and a visual flight rules flight plan had been filed but not activated.
Sequence of Events
According to the flight instructor's accident report, the helicopter was cruising at 800 feet above ground level near Eagle Nest, New Mexico, when a loud "bang" was heard. Immediately thereafter, the low rotor RPM horn activated, the warning light illuminated, and rotor RPM decreased rapidly. The instructor initiated an autorotation to an open field. During touchdown, the helicopter 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; however, the main rotor blades struck and severed the tail boom. The helicopter sustained substantial damage. The flight instructor and the second pilot were not injured.
Post-Accident Examination
The helicopter was transported to the operator's facility in Bend, Oregon, where the engine was functionally tested. The engine started and ran normally, with all parameters 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. Scaring was found on one of the sensors opposite this magnet, indicating contact while the yoke was rotating. The missing magnet was later located affixed to a bolt just aft of the yoke, and a small dent was found on the horizontal firewall.
Additional Information
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 company also stated that rapid throttle changes in the R44 can result in the fuel-air ratio becoming too rich or too lean to sustain engine operation, potentially leading to engine failure, particularly at higher density altitudes.