An investigation of the static longitudinal stability characteristics of a MD 500E helicopter

A MD 500E helicopter, registration number N5144Q, was flight tested to determine its compliance with the FAA's airworthiness criteria for helicopter single-pilot instrument flight. Upon completion of the flight test program, the FAA found questionable results with regard to the helicopter's static longitudinal stability. Specifically, the longitudinal cyclic stick force gradients for certain critical configurations were claimed to be "not perceivable". A review of the flight test results was made and the configurations critical to the helicopter's static longitudinal stability identified. Methods were then examined for improving or assisting the longitudinal cyclic stick force gradients. Modifications to the helicopter's operational envelope and "equivalent levels of safety" findings were considered. The only envelope modification found necessary was a density altitude limitation of 8,000 feet for IFR operations. With this limitation, the static longitudinal stability stick forces have been found to remain greater than the longitudinal cyclic stick friction forces for positive static longitudinal stability results. Furthermore, an equivalent level of safety is believed to be provided by the helicopter's oversized 5-inch ADI and 3-axis autopilot. In the author's opinion, the on-board systems and the recommended IFR operating limitations should alleviate any of the longitudinal cyclic stick force gradient concerns held by the FAA.