47TH ANNUAL CONFERENCE, Arusha, Tanzania, 10-14 March 2008
WP No. 89
Investigate ADS-B Applications – Single Emergency Code Management
Presented by TOC
For the 2007/08 Work program, TOC was tasked with investigating single emergency code management with ADS-B. It was documented in the 2006/2007 work program that the problem exists and that further research would be needed. The working paper describes the hardware issue, dealing with the exact problem and why. It then begins exploring ICAO for support in relevant documentation and concludes with the implementation issues in Australia.
TOC considers the issue(s) to be of concern particularly if introduced to replace radar as a sole means of surveillance in high density airspace. For these reasons, TOC has decided to propose draft recommendations, to provide guidance for when this technology is introduced.
1.1. At the 2007 IFATCA conference in Istanbul, the IFATCA Technical Operations Committee (TOC) presented a working paper on “Surveillance Applications Policy – Operational Applications of ADS-B” (working paper 94). In this paper, TOC raised the issue of limitations with the data capability of Automatic Dependant Surveillance – Broadcast (ADS-B) to transmit individual emergency codes (or notification). For the 2007/08 work programme, TOC was tasked to investigate this further.
1.2. The following is an extract from working paper 94 providing some more detail:
“Some emergency functionality when using ADS-B may have limitations. Most ADS-B aircraft will only have a general alert ‘EMG’ regardless of the code selected. Some aircraft (rare, Airbus A380) have avionics that will allow for display of Emergency, Hijack and Radio failures (EMG, HIJ or RAD).”
1.3 These limitations are due to the aircraft transponder that transmits the data. Newer aircraft compliant with Radio Technical Commission for Aeronautical Services (RTCA) standard DO260A are capable of transmitting individual emergencies.
1.4 Several other issues will be investigated in that paper including the following:
- Hardware issues;
- Losing the capability to transmit Squawk Pulse Indicator (SPI)while an emergency code is being transmitted;
- Issues with some emergency phraseology;
- The generic Automatic Dependant Surveillance (ADS) issue with emergencies; and
- Amended procedures from Australia and Europe to manage this issue.
RTCA has developed specific Minimum Operational Performance Standards (MOPS) for the implementation of ADS-B using 1090ES MHz. DO-260 MOPS were issued in September 2000 but have since been superseded by DO-260A MOPS for 1090 MHz Automatic Dependent Surveillance – Broadcast (ADS-B) and Traffic Information Services (TIS-B).
DO-260A was issued in October 2003 and includes:
- Enhanced reception techniques;
- 1090 MHz support for TIS-B;
- Changes to the ADS-B Minimum Aircraft System Performance Specifications (MASPS) (DO-242) including the separation of accuracy and integrity values of the broadcast data;
- Reorganization of the ADS-B reports, and changes as to how intent information is broadcast; and
- Open issues from original 1090 MHz development.
It is however to be noted that DO260A allows the co-existence and continued use of DO260 equipment by using a revision numbering system (DO260A version “0” is exactly DO260).
2.1.1. Most Boeing and Airbus transponders (through a service bulletin) are capable of meeting the RTCA standard DO260. The DO260A standard, while rarely in existence today, will correct the mandate of transmitting the aircraft status message which allows for transmission of individual emergencies, when selected on the transponder (7500, 7600 and 7700).
2.1.2. The discrete emergency mode issue is not a limitation of the DO260 versions as they are correctly described in both standards (transponder register 6.1). The implementation option that manufacturers took was to not implement the 6.1 register even though it was described in the MOPS (their argument being that they elected not to implement as there was no application described), but this issue will be rectified over time (with the DO260A standard). How long this will take is a very important consideration.
2.1.3. A possible solution was to take the Mode A code directly from the squitter message and use that to transmit the discrete emergency code (rather than use the dedicated emergency mode fields of the message) to the ground. However, at some point during the drafting of DO260A, a geographic filter was established for the Mode A code ‘TEST’ message which blocked the code from being broadcast in any region that is outside of coordinates set in that filter. The current coordinates are located around North America thus resulting in aircraft broadcasting the Mode A code in the message within that region and not in Europe (or anywhere else in the world for that matter). There is no documented reason for the creation of this filter.
2.1.4. At this point in time only a few avionics are capable of transmitting this individual emergency data. These may include the following:
- Rockwell Collins TDR94D-108 regional aircraft transponder.
- The Honeywell KT73 does support the Aircraft status message at least for 7500, 7600 and 7700.
- The Airbus A380 has a Honeywell transponder which supports DO260A and hence may have the capability.
2.2. SPI functionality
2.2.1 Aircraft transponders that do not meet the requirements of DO260A will not be able to squawk ident when an emergency code is selected. This is because these two functions both share the same two bit (ADS-B) transmission. When an emergency SSR code is selected, then this is sent via this ‘bit’ of data, the same ’bit’ of data is also used for SPI so one cannot send both. It needs to be broken down into the next level of data bits The four options for the status field bit are indicated below:
Status field for the information (bit) sent in the enhanced message formats of ADS-B.
Aircraft avionics installed today do not implement aircraf status message (type 28) – therefore they tell us that there is an emergency but not what type.
The DO260A standard developed by RTCA separates out the SPI bit and hence can generate SPI during emergency.
2.2.2 Normal SPI identification functionality used in Mode A/C radar environments should not be required as often in a normal ADS-B operation. This is because of the unique information transmitted in an ADS-B message. This information includes the 24 bit code assigned to the airframe (globally unique, issued through its country of origin) and the flight ID. Identification is normally based on the Flight ID (input by the pilot) and correlation can be done automatically (based on coupling corridors) with an active flight plan and based on documented ICAO identification techniques.
2.3. Relevant ICAO documentation summary
2.3.1 The following are extracts from relevant ICAO documentation detailing the requirements and recommendations for emergency code management. The purpose of these is to highlight the importance of the individual emergency codes and the documented rules and procedures that govern ATC response.
2.3.2 The options of detailing any required change to current Standards and Recommended Practices (SARPS) in the Regional Supplementary (SUPPS) procedures Doc 7030 is not possible due to the following required conditions:
“1. SUPPS should indicate a mode of implementing procedural provisions in Annexes and PANS. SUPPS may also indicate permissible additions to provisions in Annexes and PANS, subject to the restrictions in b) and c);
2. SUPPS must not be in conflict with the provisions contained in the Annexes or PANS. They must either specify detailed procedural regional options of those provisions or promulgate a regional procedure of justifiable operational significance, additional to existing provisions in Annexes or PANS.
3. In the drafting of SUPPS, variations in the text of procedures with similar intent applicable to more than one area should be avoided.”
This will lead to States determining their individual requirements and filing differences in Aeronautical Information Publication (AIP).
2.3.3 Annex 2 Rules of the Air:
Appendix two – Interception of Civil Aircraft:
- “States ADS-B equipped traffic shall select the appropriate emergency functionality, if available to the pilot, unless other wise instructed by the appropriate ATS.
- Includes the use of ADS-B facilities to permit ‘recognition of aircraft identity and immediate recognition of any emergency or urgency conditions’ so as to ‘eliminate or reduce the hazards inherent interceptions undertaken as a last resort…”
2.3.4 Annex 10 Aeronautical Telecommunications, Volume III:
Annex 10 Aeronautical Telecommunications, Volume III had the technical provision for Mode S and Extended Squitter (ES) removed and collated into a separate document called “Technical Provisions for Mode S services and extended Squitter”.This document is yet to be numbered and is only posted on the ICAO website as a draft with the usual disclaimer.
Table B-2-97: BDS (Comm–B data registers) code 6,1 — Emergency/priority status (Subtype 1: Emergency/priority status) details that the subcode shall be coded as per the following table:
It is interesting to note that the word ‘General Emergency’ is used where we would expect Mayday (7700) emergency to read. This could be responsible for the technical issue of manufacturers not finding an application and therefore not producing this subset in the equipment. This could be read as possibly covering all emergencies.
2.3.5 Annex 11 Air Traffic Services:
Section 2.22 regarding service to aircraft in the event of an emergency, states that aircraft might operate the equipment as follows:
“…(c) activate the appropriate emergency and/or urgency capability of ADS-B…”
2.3.6 ICAO Doc 4444 Air Traffic Management:
220.127.116.11 Chapter 8 now details ‘surveillance’ and captures ADS-B operations, including using ADS-B for identification;
18.104.22.168.1 describes the use of the ‘ADS-B ident’ function.
…”To indicate that it is in a state of emergency or to transmit other urgent information, an aircraft equipped with ADS-B might operate the emergency and/or urgency mode as follows”…
22.214.171.124 SARPS still awaiting Air Navigation Commission (ANC) approval include:
In Section 8.2; Situation Display ADS-B emergency modes are proposed to be added to the existing paragraph which clearly states that:
“ADS-B emergency and/or urgency modes, safety related alerts and warnings as well as information related to automated coordination shall be presented in a ‘clear and distinct manner, providing for ease of recognition”.
In Section 8.5; Operation of ADS-B Transmitters will provide for the same text as for current radar transponders whereby pilots might operate the emergency and/or urgency modes of emergency, communications failure, unlawful interference, minimum fuel and/or medical.
126.96.36.199 ICAO Doc 4444 Air Traffic Management included the following; inter alia, details on emergency management:
Chapter 5 Separation Methods and Minima:
188.8.131.52- states that larger separations than the specified minima should be applied in case of unlawful interference (and navigational difficulties) but this does not include the radio communication failure;
Chapter 8 Radar Services:
184.108.40.206.1 – states that transponder equipped aircraft experiencing radio-communications failure will operate the transponder on Mode A code 7600.
Chapter 15 Procedures related to Emergencies, Communication Failure and Contingencies states:
220.127.116.11- states that unless it is clearly stated from the flight crew or otherwise known the controller should take all necessary steps to ascertain the nature of the emergency and their intentions.;
18.104.22.168 (Note) – states that Aircraft equipped with an SSR transponder are expected to select 7500 during unlawful interference however they may select 7700. This is the only situation whereby the pilot may ‘choose’ between the selection of two mode A emergency codes. This does not apply for radio communication failure.
15.3.3. – states that in the case of radio communication failure, separation shall be maintained based on the assumption that the aircraft will:
(a) In airspace where procedural separation is being applied, maintain the last assigned speed and level for 20 minutes before adjusting level and speed in accordance with the filed flight plan, or
(b) In airspace where an ATS Surveillance system is used, maintain the last assigned speed and level for 7 minutes before adjusting level and speed in accordance with the filed flight plan. The 7 minutes starts, inter alia, after selection of the 7600 code which assumes that the ATCO immediately knows it is a radio fail and can prepare for the radio fail procedures to be followed.
2.3.7 ICAO Doc 8168 Aircraft Operations
Chapter 1 Operation of transponders provides for the operation of the discrete emergency codes and provides some detail on the controller response to these codes. The pilot would be expecting a level of agreed understanding and action from the controllers based on the assumption that the discrete code selected has been displayed to them. It describes the three Mode A codes for each of the discrete emergencies. Through the use of the word ‘shall’ it prescribes the use of 7700 for emergencies, 7600 for radio communications failure and allows the pilot to choose between 7500 and 7700 for unlawful interference.
- Section 1.5 for radio communications failure notes that once a controller observes the radio communications failure code, will perform a series of procedures to ascertain the extent of the failure.
- Section 1.6 states that controllers will request the pilot to confirm the code if they observe a 7500 code on their display and if no reply is received the ATC will take this to mean that the use of the code is NOT an inadvertent false code selection.
2.3.8 Summary of ICAO documentation
While the SPI function may certainly not be utilised as often in current Mode A/S environments as it once was (for identification) it is globally used for assessing radio failure/acknowledging receipt of ATC instructions when an aircraft’s transmitter has failed.
Supporting documentation on managing ADS-B radio failure will need to include new phraseologies and a change of expectations for pilots, once they experience this failure.
This may include phraseologies such as “If receiving this transmission stop squawking 7600 and squawk ident”. Initially you may need to use a manoeuvre (turn left/right 30 degrees etc) to establish that the aircraft is receiving only and while the initial investigation would support such a manoeuvre, ongoing acknowledgment in this way would seem impractical (although acceptable by ICAO). Pilots may need to realise that once ATC have established a ‘receive only’ communication loss that they will need to cease squawking 7600…. “radio transmission failure acknowledged, squawk normal, respond as required with ADS-B ident” so the SPI function can work.
These possible work-around solutions will not meet the criteria to be inserted into ICAO Doc 7030 Regional Supplementary Procedures as previously discussed. This will be established at a local level with local safety processes. The opportunity for globally applied methodology then deteriorates and will result in individual AIP differences.
2.4.1 Australia in its implementation of ADS-B has acknowledged and documented the issue of single emergency code management. It has developed procedures that default in a ‘worst case scenario’ for the management of emergencies. This process involves establishing, as soon as possible, the nature of the emergency (by voice/ Controller Pilot Data Link Communications (CPDLC) etc) so the correct procedures can be followed. The situation only becomes complex when you cannot contact the aircraft by voice/CPDLC to ascertain the nature of the emergency. When once the worst scenario would be to expect a 7700/Mayday emergency, the post 9/11 era has the default set as unlawful interference. This then becomes a State managed issue and individual responses will differ depending on the States governmental policy and threat management.
2.4.2 This Australian process is supported by the following documentation.
Manual of Air Traffic Services (Australia) ATS 7.2.18 ADS-B Emergencies:
“Emergency status undetermined
When an ADS-B emergency indication is received, without an accompanying voice confirmation or CPDLC emergency message, an Uncertainty Phase shall be declared and a check for covert or inadvertent activation of the ADS-B emergency mode shall be made by the controller as detailed at 22.214.171.124.
If the aircraft continues with the ADS-B emergency indication and there is no other voice or CPDLC confirmation that activation was in error, controllers should act on the basis that the aircraft is subject to unlawful interference conditions and follow standard Unlawful Interference procedures.”
2.5.1. Eurocontrol has been discussing this issue for a number of years; predominantly through the workings involved in the creation of the ADS-B Non-Radar Airspace (ADS- B NRA) joint European Organisation for Civil Aviation Equipment (EUROCAE)/RTCA standard ED126/DO303. The result of those discussions was that the ‘Co-operative Air Traffic Services through Surveillance and Communication Applications Deployed in ECAC’ (CASCADE) position is to require, as a minimum, aircraft to be able to transmit the discrete emergency modes. Nevertheless, the Notice of Proposed Amendment (NPA) 2007/05, which provides for the acceptable means of compliance for the ADS-B NRA application in Europe does allow for the generic flag. It does however, state that it is the implementers responsibility, as approved by the local authorities, for alternate procedures (such as the Australian use of unlawful interference) to deal with the equipment limitations.
2.5.2. CASCADE has recently simulated an alternate procedure for this issue and obtained controller feedback on it. This process hasn’t included a safety assessment, nor did it analyse issues with adjacent sectors (coordination) or assess pilot feedback aspects. This procedure as such will not be published by CASCADE but may be used to support implementers as input, provided of course they conduct their own analysis and apply appropriate safety case processes.
2.5.3. The Federal Aviation Administration (FAA) generally supports the same position as CASCADE (evidenced by their RTCA version of ED126 – DO303 – that also has the same requirement). The FAA has produced a Notice of Proposed rule Making (NPRM) 14 CFR Part 91 on ADS–B Out performance requirements to support ATC service. This document supports DO260A operations and proposes this for mandated introduction in 2020. It also proposes a network of ‘back-up’ radars to provide a redundancy to possible Global Positioning System (GPS)/ADS-B issues. This would then provide two options for receiving the emergency code:
- By ADS-B (both status field indicator and broadcast Mode A); and
- Mode A via legacy radar systems in the back up network.
2.6.1 The issue of single emergency code management comes down to exactly that – management. Eurocontrol and FAA may wish to have updated avionics in use before sanctioning ADS-B in their airspace. European Aviation Safety Agency (EASA) may however, allow individual states to manage this process. The question that belies us all is why implement ADS-B now? We have seen in Australia how it will greatly improve our ability to provide optimum trajectories for airlines and safety improvements due to increased conformance monitoring. The potential to affordably use 5NM provides enormous benefits to ATC, in airspace never destined to have radar.
2.6.2 In traditional non-surveillance airspace if an aircraft fails to report over a position, normal ‘out of communication (comms)’ procedures will commence. The assumption is, if the aircraft has not been contacted though all the normal channels that are available (unable to determine that it is only a receiver failure due to no surveillance) then we expect the pilot to fly in accordance with published ‘loss of comms’ procedures and we act accordingly. If this aircraft is subject to unlawful interference and was to descend or climb or track different than flight planned we would not know. The earliest we could expect to know what the intentions of the aircraft may have been would be if it didn’t appear on radar coverage when finally it should; or it turns up on radar where it shouldn’t; or perhaps another aircraft is able to identify it airborne at some stage.
The same scenario, this time in an ADS-B environment (assume continental Australian airspace); the aircraft fails to report at the same position… but 3 minutes prior we receive an ADS-B emergency and attempt to establish communication. We can’t – we notice that the aircraft commences descent and turns sharply… This example indicates how if an aircraft is subject to unlawful interference then having ADS-B is a great advantage.
2.6.3 One disadvantage would be the loss of communication of an ADS-B equipped aircraft and after due process, a transition to unlawful interference procedures. Local procedures determine a scramble of fighter planes for an intercept. Then after a period of time the aircraft regains communication (had a minor electrical issue) – The outcome, while serious, provided accurate tracking for the intercept and for planning of this aircraft’s possible intentions.
2.6.4 International Federation of AirLine Pilots Associations (IFALPA) was given a presentation on possible procedures to manage this scenario. It involved a complex set of instructions; at a time when cockpit workload management is already high (assuming an emergency of any kind). This was viewed as a further workload issue that would only serve to complicate the already very intense operation.
2.6.5 ADS-B with the limitations of DO260 is acceptable under certain conditions. IFATCA identifies the improvement to ATM that providing surveillance in areas of non surveillance potentially provides. This includes the key areas of efficiency, safety and environment.
3.1 Thorough researching of ICAO documentation (Annex 2 Rules of the Air, Annex 10 Aeronautical Telecommunications, Annex 11 Air Traffic Services, Doc 4444 Air Traffic Management, Doc 9694 Manual of Air Traffic Services Data Link Applications, and Doc 8168 Aircraft Operations) has not provided guidance or recommendations for the management of generic-single-emergencies. In has highlighted how important they are and how certain reactions can be expected from both pilots and ATC. It has also failed in some circumstances, to amend words such as ‘non-radar’ and ‘radar’ so some confusion may exist in ADS-B (or surveillance) airspace.
3.2 The limitations of ADS-B have been well documented through the many working papers and guidance material produced by ICAO and other international bodies. The standards are there to have these issues rectified; the issue is the required avionic updates. In the meantime individual States have to make the decision as to the importance of this known deficiency and must determine to manage it at the State level.
3.3 In Australia, having vast amounts of procedural continental airspace the decision to implement ADS-B and therefore improve efficiency and safety is one that is supported significantly by the industry. This includes the ATC managing the airspace.
3.4 The future is about more throughputs with better technology and process. To begin this journey with technology that cannot replicate fully what we have to day is not acceptable in areas currently serviced by radar.
3.5 IFATCA needs to monitor the developments carefully, and establish a position that carefully supports its use but with strict safety measures and an appreciation of its limitations.
3.6 Other States, Eurocontrol and the FAA wish to have the issue of single emergency code management resolved before implementation (understandable with 95% radar coverage) and will have a slower implementation as the necessary changes to avionics are completed.
3.7 Individual States with their relevant documentation will need to detail the intimate requirements on how to manage this issue. For some Air Traffic Service Providers (ATSP’s)/Air Navigation Service Providers (ANSP’s) the consistent issue of lost communications and the resulting issue of possible intercept and escort scenarios will weight their decision to mandate the individual codes. For Australia having lost communication with an International aircraft, 3 hours from any densely populated areas won’t result in fighter jets being scrambled.
3.8 The lack of internationally standardised procedures in handling single emergency code/SPI issues is a concern and needs to be addressed.
It is recommended that;
4.1 IFATCA Policy is:
If ADS-B is to replace radar then it must have the same or better functionality as radar, and this specifically includes discrete emergency codes and a SPI function.
and is included in the IFATCA Manual on page 3 2 6 2.
4.2 IFATCA Policy is:
In non-radar airspace DO260 ADS-B is considered an acceptable interim solution provided global procedures are established for handling single emergency codes and potentially decreased SPI functionality.
and is included in the IFATCA Manual on page 3 2 6 2.
4.3 IFATCA Policy is:
That ICAO establishes a global position and recommendations on the management of ADS-B operations without separate emergency codes and isolated SPI function.
and is included in the IFATCA Manual on page 3 2 6 2.
IP/12-ADS-B SITF/4 (Greg Dunstone).
ICAO Annex 2 Rules of the Air.
ICAO Annex 10 Aeronautical Telecommunications.
ICAO Annex 11 Air Traffic Services.
ICAO Doc 4444 Air Traffic Management.
ICAO Doc 7030 Regional Supplementary Procedures.
ICAO Doc 8168 Aircraft Operations.
ICAO Doc 9694 Manual of Air Traffic Services Data Link Applications.
ICAO State Letter 06/56.
DEPARTMENT OF TRANSPORTATION, Federal Aviation Administration, 14 CFR Part 91 [Docket No. FAA–2007–29305; Notice No.07–15] RIN 2120–AI92 Automatic Dependent Surveillance—Broadcast (ADS–B) Out Performance Requirements to Support Air Traffic Control (ATC) Service.
Last Update: September 29, 2020