Thursday, May 9, 2013


Mode ‘C’ and Mode ‘S’ transponders, all types and models utilising Gilham code altitude input.

                      During the 1990’s, the FAA received reports of eleven incidents involving certain transport category airplanes equipped with Mode "C" transponder(s) with single Gilham code altitude input. These reports indicated that, during level flight, the TCAS II issued false advisories that directed the flight crew to either climb or descend. Further investigation showed that these events were caused by incorrect Gilham coded altitude input. The instances reported in the USA all involved communication between aeroplanes with the TCAS II and aeroplanes having the Mode "C" transponder(s). Aeroplanes having Mode "C" transponders installed are predominantly older, out-of-production transport category (JAR/FAR/CS 25) aeroplanes.
Such inaccurate altitude reporting and consequent false TCAS II advisories, if not prevented, could cause the flight crew to manoeuvre the aeroplane from itsassigned flight path, possibly resulting in a mid-air collision.
                  To address this potential unsafe condition, on 12 November 1999, FAA issued AD 99-23-22 to require repetitive testing of the affected transponders. This AD was later revised (R1 dated 16 December 1999) and subsequently cancelled (rescinded) by AD 99-23-22R2 on 20 April 2000. The reason to rescind that AD were that test data had been collected that demonstrated that the repetitive tests are unnecessary.
       Prompted by the original FAA AD 99-23-22 and some similar reported occurrences in the UK, the CAA UK issued AD 002-12-99 (later revised twice), which was made applicable to a much wider range of aircraft, i.e. not restricted to aircraft with TCAS installed and also applicable to aircraft fitted with Mode “S” transponders, if using Gilham code altitude input.
        In 2006, EASA adopted the CAA UK requirements as EASA AD 2006-0265 to require the identification of incorrect transmission of altitude data from transponders which utilise Gilham coded altitude encoders as a sensor input and, where aircraft transponders accept dual Gilham coded altitude encoders, the transponder altitude data comparator must be checked for correct operation. That AD had been published as PAD 06-170 for consultation on 07 August 2006 with a comment period until 21 August 2006 and no comments were received during the consultation period.
                       Since that AD was issued, many comments have been received on the impact of the repetitive requirements, particularly on general aviation (JAR/FAR/CS 23) aeroplanes. These comments together with the comments received during the consultation process confirm the conclusion of the FAA that the repetitive tests, relating to the Gilham code altitude input, are no longer necessary to ensure safe operation. Although Maintenance Organisations reported problems with the altimetry systems of aircraft affected by EASA AD 2006-0265, many of these problems related to failures which were not associated with the Gilham code altitude input.
To account for these non Gilham related transponder problems, EASA has decided to revise Safety Information Bulletin (SIB) 2011-15 to include a check of Mode C transponder systems (the original SIB only applied to Mode S Transponder Systems). SIB 2011-15R1 recommends a test interval of 24 months, which was contained as requirement in EASA AD 2006-0265.
                           In addition, the European Commission has recently published Implementing Regulation (EU) No. 1207/2011 which contains the requirements for the performance and interoperability of surveillance for the Single European Sky, which includes mandated repetitive testing of transponders. This regulation only applies to aeroplanes with a take-off mass exceeding 5 700 kilograms.
        After review of all available information and for the reasons described above, this Notice cancels EASA AD 2006-0265. At the same time, SIB 2011-15R1 is published to recommend certain actions for general aviation aeroplanes and helicopters that still have the affected transponders installed, but who do not have to comply with European Commission Implementing Regulation (EU) No. 1207/2011.

EASA AD 2006-0265-CN

No comments: