Summary
We propose to adopt a new airworthiness directive (AD) for certain Twin Commander Aircraft Corporation Models 690, 690A, and 690B airplanes. This proposed AD would require you to inspect between the surface of the left-hand (LH) and right-hand (RH) upper wing skins and the engine mount beam support straps for any signs of corrosion, replace the upper steel straps with parts of improved design, and modify both wings. This proposed AD results from reports that corrosion was found between the mating surfaces of the wing upper skin surface and the engine mount beam support straps. We are proposing this AD to detect and correct corrosion on the engine mount beam support straps and the upper wing skins, which could result in failure of the engine mount beam support straps. This failure could lead to loss of the engine and possible loss of control of the airplane.