31 July 2011 C1 CIVIL AVIATION DEPARTMENT HONG KONG ...

AIRWORTHINESS NOTICE NO. 12APPENDIX NO. 67Issue 331 July 2011MODE 'S' TRANSPONDER ICAO 24-BIT AIRCRAFT ADDRESSES1 A foreign CAA has become aware of incorrect 24-bit addresses being installed / hardwired on individual aircraft. This has happened not only on first installation of aMode 'S' transponder but also when a modification has been made or following achange of State of Registry. Incorrect installation, such as setting the address to allzeros or inadvertent duplication of an address, can pose a risk to flight safety. Inparticular, the airborne collision avoidance system (ACAS) operates on theassumption that only a single, and therefore unique 24-bit aircraft address exists perairframe. The performance of ACAS can be seriously degraded and in some casesdisabled if an incorrect or duplicate address is installed on an aircraft.2 ICAO has recognised that the present management methodology of aircraft 24-bitaddresses presents a genuine safety hazard that needs to be addressed and suitablymitigated in any system that is to make use of the Mode 'S' address.3 In order to ensure that the 24-bit Mode 'S' address is installed correctly at the time ofinitial Certificate of Airworthiness issue, as well as throughout the in-service life ofthe aircraft and at the time it leaves the Hong Kong register, the following should beaccomplished:(a)(b)(c)(d)A positive check that the correct Mode 'S' address is assigned for eachtransponder installed on the aircraft.The correct Mode 'S' address is periodically confirmed and recorded for eachtransponder installed on the aircraft, via a field test set at an appropriatemaintenance opportunity (not to exceed a two year periodicity). This taskshould be incorporated into the Approved Maintenance Schedule orProgramme.Ensure whenever the aircraft is subject to modification that the Mode 'S'address has not been changed.The Hong Kong assigned Mode 'S' address is removed when the aircraftleaves the Hong Kong register.4 Operators are requested to review their documented procedures and update them, asappropriate, to ensure the above points are addressed. These procedures should alsoinclude a method to record that the applicable actions have been accomplished.AN-12 A67 P.131 July 2011