Summary
This document proposes to supersede Airworthiness Directive (AD) 91-24-15, which currently requires modifying the instrument air plumbing system on Beech Aircraft Corporation (Beech) Models 1900 and 1900C airplanes. The proposed action would require an additional modification to the plumbing of the instrument air system on the Models 1900 and 1900C airplanes, and would add the Model 1900D airplanes to the applicability. Eight reports of moisture freezing in this system on airplanes with the modification required by AD 91-24-15 incorporated prompted the proposed action. In addition, recent testing on the Model 1900D indicates that the design of the instrument air system on these airplanes is also conducive to moisture freeze-ups. The actions specified by the proposed AD are intended to prevent ice formation in the plumbing of the instrument air system, which, if not detected and corrected, could result in aerodynamic problems and subsequent loss of control of the airplane.