Before TCAS, there was none; now there are many


In the beginning of civil aviation, the last-minute collision avoidance was performed by see-and-avoid, i.e. by the pilot watching out the window. After the collisions in Grand Canyon, 1956, and San Diego, 1978, the need for a more efficient collision avoidance system (CAS) became clear. This led to the development, in 1966, of the first version of what would later become TCAS. Nowadays, TCAS has proved its usefulness in multiple situations and is mandatory for most large aircraft (MTOM > 5700 kg or passengers > 19 in Europe).


One of the main benefits of TCAS is its coordinationscheme. When two TCAS equipped aircraft are in conflict, the TCASs exchange messages allowing them to choose complementary maneuvers. Complementarymaneuvers are coordinated maneuvers where a negotiation between the two systems allows choosing maneuvers in opposite directions.


However, TCAS has some limitations, on the safety level (e.g., induced collisions), operational level (e.g. parallel runways) and technical level (e.g., maintenance). To improve on TCAS, a new family of technologies has been developed: ACAS X. This family includes ACAS Xa (for large aircraft), and ACAS Xu (for drones). For the particular case of ACAS Xa, the technology is now mature, its standardization is in an advanced state, and it is soon to be introduced in the airspace (standardization finishing in 2018). Slightly delayed, the maturation and standardization processes of ACAS Xu will be coming shortly (standardization finishing in 2020).


Now when considering the introduction of ACAS Xu, things get complicated coordination-wise. Indeed, ACAS Xu is capable of vertical, horizontal, and mixed manoeuvres (horizontal + vertical component). Thus, when coordinating, two manoeuvres might not be complementary(i.e. in opposite directions) but still be compatible, it is to say they directions are not conflicting with each other. For example, two “climb” and “descend” coordinated resolutions are complementary, but a “climb” and “turn left” might not be complementary(e.g., if not coordinated), but still compatible.


The introduction of new CASs results in a multiplication of the number of different systems in the airspace. And this is not without consequences. During ACAS Xa integration tests, though the coordination was working perfectly, it was noted that the performances of TCAS were degraded. This highlighted the fact that CASs need not only to be able to coordinate but they also need to be interoperable. To be interoperable, two systems must conform to the following rules:

  • A new CAS entering the airspace should not degrade the performance of existing CASs.
  • Existing CASs should not be required to change (e.g., hardware, software, formats) to interoperate with a new CAS.
  • To the extent possible, a new CAS should not restrict future CASs from taking advantage of technological advances and innovative designs.

To ensure interoperability, the EUROCAE WG-75 is developing a common document with RTCA SC-228 specifying the requirements for CASs interoperability: the interoperability MASPS. When complete, it will provide the definitions and guidelines to ensure that the introduction of a new CAS will not interfere with the working of existing CASs.


To finish, a summary of short definitions for the main concepts seen above (note that they might evolve as the interoperability MASPS matures):

–      Coordinated: manoeuvres are coordinated if there is an explicit or implicit schema to decide on a joint solution for manoeuvring.

–      Compatible: manoeuvres are compatible if they directions are not conflicting, without necessarily being coordinated.

–      Complementary: manoeuvres are complementary if there is a negotiations process to choose opposite direction manoeuvres.

–      Interoperable: two systems are interoperable if one does not degrade the safety level provided by the other system, and vice-versa.

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