Summary
This document proposes to adopt a new airworthiness directive (AD) that would apply to certain Rockwell Collins TDR-94 and TDR-94D Mode S transponders that derive altitude information from a Gillham (gray code) encoded pressure altitude source and are installed on airplanes. The proposed AD would require you to have the unit modified to prevent erroneous altitude reporting. The proposed AD is the result of reports that erroneous altitude resolutions could occur when the affected transponders are utilized in areas with other airplanes equipped with certain aircraft collision avoidance system (ACAS) or traffic alert and collision avoidance system (TCAS) configurations. The actions specified by the proposed AD are intended to prevent these erroneous altitude resolutions from causing a reduction in the intended ACAS or TCAS Change 7 separation margins. Such a condition could result in air traffic control or the pilot making flight decisions that put the airplane in unsafe flight conditions.