Airworthiness Directives; Burkhart Grob, Luft- und Raumfahrt, Model G 109 Sailplanes

This amendment adopts a new airworthiness directive (AD) that applies to certain Burkhart Grob, Luft- und Raumfahrt (Grob) Model G 109 sailplanes. This action requires installing a damper and new bell crank lever on the rudder, in addition to adjusting the weight and balance of the sailplane, to correct the tendency of flutter at specific excitation frequencies. For those Grob Model G 109 sailplanes that have previously accomplished this installation, a modification to the damper and bell crank lever, and adjustment to the weight and balance is required. These actions are prompted by the discovery of rudder vibration problems during testing of two Grob Model G 109 sailplanes. The actions specified by this AD are intended to prevent vibration of the rudder, which could result in structural damage and eventual loss of control of the sailplane.