Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to aircraft equipped with Garmin AT, Apollo GX series GPS navigation units with software versions 3.0 through 3.4 inclusive. This proposal would require modification and testing of the software for Apollo GX50/55/60/65 TSO-C129a GPS navigation units; and reidentification of the part. This action is necessary to prevent the GPS navigation unit, under certain conditions, from providing erroneous cross-deviation information, which could result in the aircraft deviating from its intended course for a brief period of time. Erroneous information may also place an excessive workload on the flightcrew while they monitor other available navigation data to avoid deviating off course. This action is intended to address the identified unsafe condition.