Handling non-equipped Mode S and ADS-B State aircraft

Operating non-equipped State aircraft requires specific arrangements to ensure safety and efficiency.

In Mode S and ADS-B environment, Military Authorities need to take account of the principles for the assignment of 24-bit aircraft addresses as established by ICAO (the International Civil Aviation Organization) and ensure the uniqueness of the aircraft addresses used at any time by State aircraft.

We help Member States and military authorities clarify the technical and procedural aspects of operating non-Mode S and ADS-B equipped State aircraft in European airspace in the form of openly available publications and guidance materials.

In accordance with Annex 10 to the ICAO Convention (Volume III, Chapter 9, entitled Aircraft Addressing System), aircraft addresses are to be assigned to aircraft in accordance with the following principles:

• at any one time, no address shall be assigned to more than one aircraft;
• only one address shall be assigned to an aircraft, irrespective of the composition of on-board equipment. The address shall not be changed except under exceptional circumstances and shall not be changed during flight;
• addresses composed of 24 ZEROs or 24 ONEs shall not be assigned to aircraft.

For operations in Mode S and ADS-B designated airspace, Military use a proportion of the 24-bit aircraft addresses available to them. The relatively large number of aircraft addresses earmarked for military use allows frequent rotation of the assignment of 24-bit addresses to military aircraft. The purpose of this random changing of addresses is to conceal information concerning military airframes. The 24-bit addresses must not, however, be rotated during flight.

When Mode S transponders are used in flight, unique and correctly assigned aircraft addresses must be used. The presence of duplicate 24-bit addresses can jeopardise the safety of aircraft and the operational efficiency of ATM (air traffic management) functions, and must therefore be avoided at all times.

The uniqueness of the 24-bit aircraft address is important for the unambiguous identification of aircraft. The effects of duplicate addresses are unpredictable. They can cause synchronous garbling, radar track swapping or dropping.

Aircraft Collision Avoidance System (ACAS) II equipment uses Mode S protocols and transponders to maintain safe separation between aircraft. Duplicate 24-bit addresses can have serious safety implications. For example, one or more ‘threat’ aircraft can remain partially or totally undetected.

The carriage of Mode S and ADS-B equipment applies to State aircraft operating in European airspace on the basis of obligations defined in the National context and/or in the sequence of SES regulatory initiatives.

The performance and interoperability requirements related with the carriage of Mode S (ELS and EHS) and ADS-B OUT in European Union airspace, for flights under GAT/IFR status, are regulated by the Commission Regulation (EU) No 1207/2011 (SPI IR3 ) amended by Regulation (EU) 2020/587 of 29 April 2020.

Article 8 of the SPI IR mandates, from 07 December 2020, the carriage of Mode S ELS for all State aircraft and Mode S EHS and ADS-B OUT for Transport type State aircraft^[1][2] when operating as GAT/IFR. Article 8 applies only to EU State aircraft, thus non-EU State aircraft are not covered by that Article 8 (nor by the SPI IR).

For EU State aircraft, Member States had until 1 January 2019 to communicate to the European Commission which State aircraft cannot be equipped with the Mode S or ADS-B OUT capabilities as per Article 8.3 of the SPI IR. Such exemptions were limited to three reasons justifying non-equipage, in Article 8(3) of Regulation (EU) No 1207/2011.

Non-EU State aircraft operators that plan to conduct flights with non-Mode S or non-ADS-B OUT transport type State aircraft in European airspace shall consult the National AIPs of the States to be overflown, and submit case by case requests for dispensation/waivers for such particular flights in line with arrangements and procedures published therein.

[1] State aircraft with a maximum certified take-off mass exceeding 5 700 kg or having a maximum cruising true airspeed capability greater than 250 knots. It applies for aircraft with an individual certificate of airworthiness first issued on or after 7 June 1995.
[2] As per SPI IR, transport-type state aircraft means: “fixed wing State aircraft that are designed for the purpose of transporting persons and/or cargo.”