A method and apparatus for establishing a justification basis to Aircraft Regulatory Authorities, to allow for a reduction in aircraft sink-speed assumptions. To further allow the aircraft to operate an increased maximum landing weight limitation. A system for use in measuring aircraft vertical velocity at initial contact with the ground, experienced while aircraft are executing either normal or hard landing events. Pressure sensors are attached to the working pressure within the landing gear strut, so to monitor in-flight landing gear strut pre-charge pressure, until such time as the pre-charge pressure suddenly increases, to detect the aircraft has come into initial contact with the ground. Rotation sensors are attached to the hinged elements of the landing gear strut, so to monitor in-flight landing gear strut extension, until such time as the strut extension suddenly decreases, to detect the aircraft has come into initial contact with the ground.