Background Information

• The European Aviation Safety Agency (EASA) Airworthiness Directives (AD) 2008 - 0158R1, which revised AD 2008 - 0158 concerning a design deficiency in Funkwerk TRT-600 transponders, has been replaced by AD 2008 - 0158R2.
• Air Navigation Service Providers (ANSPs) have reported intermittent loss of detection on Mode S Secondary Surveillance Radars (SSR) with aircraft equipped with Funkwerk Avionics TRT-600 transponders.
• These transponders do, however, reply correctly to SSR Mode A/C only interrogations.

Impact on ATS Operations

• In an ATC surveillance environment where there is sole use of an SSR Mode S or an SSR Mode S radar operating in mixed mode (Modes S and A/C), or a combination thereof, TRT-600 transponders may not reply correctly to interrogations, and detection of aircraft may be lost for prolonged periods.
• In an ATC surveillance environment based on interlacing of replies received by combinations of SSR Mode S operating in mixed-mode and SSR Mode A/C, TRT-600 transponders may be detected by SSR radars that operate in Mode A/C only.