Airworthiness Directives; Various Transport Category Airplanes Equipped With Mode ``C'' Transponder(s) With Single Gillham Code Altitude Input

This amendment revises an existing airworthiness directive (AD), applicable to various transport category airplanes equipped with certain Mode ``C'' transponder(s) with single Gillham code altitude input. That AD currently requires repetitive tests to detect discrepancies of the Mode ``C'' transponder(s), air data computer, and certain wiring connections; and corrective actions, if necessary. The existing AD is prompted by reports that, during level flight, the Traffic Alert Collision Avoidance System (TCAS II) issued false advisories that directed the flightcrew to change course and either climb or descend. The actions specified by that AD are intended to prevent such false advisories due to inaccurate airplane altitude reporting, which could result in the flightcrew deviating the airplane from its assigned flight path and a possible mid-air collision. This new action revises certain compliance times and limits the applicability of the existing AD.