A survey of 22 AH-64A helicopters was made to determine the effects of age and use on the dynamic characteristics of the main rotor support system. Ten low-time and ten high-time production aircraft were tested, along with two production aircraft that are used by Mcdonnell Douglas Helicopter systems for flight test activities. The results of the survey indicated that, for the high-time aircraft, the frequency of the support system was statistically lower than for low-time aircraft; while the damping and response amplitude were statistically higher. In spite of the fact that such changes were observed, the magnitude of these changes is not sufficient to affect the fundamental dynamics of the main rotor support. In addition, it was determined that these changes were function of neither the production order of the aircraft nor the number of flight hours that the aircraft has accumulated. Instead, it appears that changes in the response amplitude, frequency, and damping may be simply the result of the aircraft being broken in, a process that probably takes less than 500 hours of flight time.
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