Airworthiness Directives; GARMIN International Inc. GTX 33, GTX 33D, GTX 330, and GTX 330D Mode S Transponders

The FAA proposes to supersede Airworthiness Directive (AD) 2004-10-15, which applies to certain GTX 330 and GTX 330D Mode S transponders that are installed on airplanes. AD 2004-10-15 currently requires you to install GTX 330/330D Software Upgrade Version 3.03, 3.04, or 3.05. This proposed AD applies to certain GTX 33, GTX 33D, GTX 330, and GTX 330D Mode S transponders that are installed on airplanes and is the result of observations that the GTX 33/33D/330/330D may detect, from other airplanes, the S1 (suppression) interrogating pulse below the minimum trigger level (MTL) and, in some circumstances, not reply. The GTX 33/33D/330/330D should still reply even if it detects S1 interrogating pulses below the MTL. Consequently, this proposed AD would require you to install GTX 33/33D/330/330D Software Upgrade Version 3.03 or 3.06. Software Upgrade Versions 3.03 and 3.06 correct a TAS, TCAD, and TCAS I system ``whisper-shout'' problem that could potentially lead to the aircraft not being visible at certain ranges. TCAS II systems are not affected. We are issuing this proposed AD to prevent interrogating aircraft from possibly receiving inaccurate replies due to suppression from aircraft equipped with the GTX 33/33D/ 330/330D Mode S transponders when the pulses are below the MTL. The inaccurate replies could result in reduced vertical separation.

Agency Contact: Roger A. Souter, FAA, Wichita Aircraft Certification Office (ACO), 1801 Airport Road, Room 100, Wichita, Kansas 67209; telephone: 316-946-4134; facsimile: 316-946-4107; email