Airworthiness Directives; Hartzell Model HC-B4 Series Propellers Installed on Mitsubishi MU-2 Series Aircraft

This amendment supersedes three existing airworthiness directives (AD), applicable to Hartzell Model HC-B4TN-5(D,G,J)L/ LT10282(B,K)-5.3R and HC-B4TN-5(D,G,J)L/LT10282N(B,K)-5.3R propellers installed on Mitsubishi MU-2 series aircraft. These AD's currently require replacement of existing LT10282(B,K)-5.3R propeller blades with LT10282N(B,K)-5.3R improved ``N'' configuration propeller blades, and repetitive inspection and rework when required of the inner hub arm bore. This amendment requires new repair limits, shot peening procedures, and retirement at 10,000 hours time in service for the ``N'' configuration blades. Additionally, this action requires replacement of existing propeller hubs with new improved fatigue strength steel hubs and requires inspection, and specified rework as necessary, of the new steel hubs at a repetitive interval of 3,000 hours time in service. This amendment is prompted by a determination that the current hub design and blade repair limits do not adequately protect against initiation of fatigue cracks in the propeller hub arm bore and do not prevent the resonant speed of the propeller from shifting into the permitted ground idle operating range. The actions specified by this AD are intended to prevent initiation of fatigue cracks in propeller assemblies and subsequent progression to propeller failure, with departure of the blade, or hub arm and blade, that may result in loss of aircraft control.