Notice of Final Rule Making
- Proposed Amendment To An Airworthiness Directive (AD) Requiring Additional Maintenance To Aircraft Transponder Systems
As part of the output of a transponder there is a series of digital pulses (pulse train) carrying identification information in the form of a code known as Mode A. This pulse train is transmitted in response to an interrogation by a Secondary Surveillance Radar (SSR) used by Air Traffic Control (ATC) to identify all transponder equipped aircraft within a particular control zone.
Analysis of Service Difficulty Reports (SDRs) submitted by Airservices Australia (AsA) has identified that some older transponders (particularly the models utilising electron tube technology eg. Bendix/King KT76A) are not achieving the performance specifications to which they were originally manufactured and certified.
Note: For the purposes of this NFRM, Electron Tube Technology (ETT) is defined as technology that utilises the physical and electrical characteristics of a physical body to oscillate and amplify a signal, at its resonant frequency, for subsequent broadcast. This includes thermionic valves, klystrons or cavity oscillators etc. As these components age the characteristics which provide the signal stability vary which affects the output signal.
AsA is progressively replacing ageing terminal area radars with the new Australian Mode S Terminal Area Radar (AMSTAR) equipment which utilises solid-state primary surveillance radar and Mode A/C and S capable SSR systems. The rollout commenced late 2008 with radars now operational at Coolangatta and Melbourne. New radars are also to be installed at Adelaide, Sydney, Cairns, Brisbane, Canberra, Darwin and Perth. The replacement program is expected to be completed by late 2011.
These new radars are detecting anomalies with the Mode A code transmitted from a small number of transponders and these anomalies are resulting in Short Term Conflict Alerts being displayed on ATC consoles.
The anomalies result from the detection of an uncommanded and random changing of the Mode A code being transmitted. This is being caused by the varying pulse shape and relative amplitudes of the data pulses in successive replies. To date approximately 60 SDRs have been received from AsA reporting these anomalies since 28 January 2009.
With the introduction of the AMSTAR Project these defects, having previously been ignored, or not detected by the older Radar Sensor Procurement Project (RASPP) radar, are now being identified. The older radar data processor did not rely on the pulse shape and relative amplitude characteristics of these pulses to extract the required data. Although the AMSTAR system still requires the performance characteristics used previously, it also relies on extra parameters (pulse rise time, pulse decay time, pulse width and pulse amplitude variance) to provide better resolution and discrimination of the aircraft responses.
The standard to which all transponders are approved has not changed since the publishing of the transponder minimum operational performance specification in RTCA/DO-112 Minimum Performance Standards Airborne ATC Transponder Equipment (RTCA/DO-112) in 1961. Whilst this document has been superseded a number of times the technical specifications remain unchanged, the latest being RTCA/DO-181D.
A significant number of aircraft in Australia were, and still are, maintained in accordance with a maintenance schedule derived from Civil Aviation Regulation (CAR) Schedule 5. The introduction of the RASPP radars in the early 1990s highlighted the fact that the current maintenance requirement for transponders under this basis was inadequate. There were no performance metrics identified for the transponders to be tested against. The requirement, as stated in CAR Schedule 5, is:
"(k) check the ATC transponder system for correct performance in all modes using the self test facility: select code 0101 for this test;"
This maintenance task did not provide any verification that the output signal was compliant with the design specifications, but simply confirmed that selected sections of internal circuitry were functioning. It did not ensure the validity of the transmitted pulse train. AD/RAD/47 was prepared in response to these issues and originally became effective on 23 July 1994. The AD provided the minimum performance specifications to which transponders should be tested, to align with the minimum maintenance tests required by the Title 14 of the United States of America’s Code of Federal Regulations Part 43 Appendix F.
The CARs require that all equipment fitted to an aircraft must be either serviceable or have an approval from CASA to operate with unserviceable equipment. The Australian Aeronautical Information Publication, General, Section 1.5 Paragraph 6, deals with the requirements for the carriage of transponders in Australia. The proposed AD expands the testing to be conducted to confirm that the transponder signal has the characteristics required by the ATC SSR to provide the aircraft identification and positional data necessary for the safe separation required for operations within a control zone. This is not a change to the equipment specification, but rather it is ensuring that the equipment is still performing to the standard to which it was originally certified.
Contact: Charles Lenarcic, Project Officer