Airworthiness Directives; GARMIN International Inc. GTX 330 Mode S Transponders and GTX 330D Diversity Mode S Transponders

The FAA adopts a new airworthiness directive (AD) for certain GARMIN International Inc. GTX 330/GTX 330D Mode S transponders that are installed on aircraft. This AD requires you to install GTX 330/330D Software Upgrade Version 3.03, 3.04, or 3.05. This AD is the result of observations that the GTX 330 and GTX 330D may detect, from other aircraft, the S1 (suppression) interrogating pulse below the Minimum Trigger Level (MTL) and, in some circumstances, not reply. The GTX 330/ 330D should still reply even if it detects S1 interrogating pulses below the MTL. We are issuing this AD to prevent interrogating aircraft from possibly receiving inaccurate replies due to suppression from aircraft equipped with the GTX 330/330D Mode S Transponders when the pulses are below the MTL. The inaccurrate replies could result in reduced vertical separation or unsafe TCAS resolution advisories.