Europe’s air transport regulator is formally recommending adoption of rules enabling aircraft to use the latest collision-avoidance technology within the continent’s airspace.

Aircraft with a maximum take-off weight above 5.7t are mandated to carry the TCAS 7.1 collision-avoidance system.

But a follow-on concept, known as ACAS Xa, has been developed to improve upon the system’s performance and capabilities, while reducing the possibility of nuisance alerts.

The European Union Aviation Safety Agency has drawn up a formal opinion, which it will submit to the European Commission, supporting the use of ACAS Xa and preventing restriction of Xa-equipped aircraft.

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Source: Gabriela Natiello/Unsplash

ACAS Xa would facilitate new airspace concepts involving closer aircraft spacing

ACAS Xa is compatible with TCAS 7.1. But while TCAS relies exclusively on transponder interrogation to determine and project positions of intruder aircraft, ACAS Xa does not use hard-coded rules, employing instead a probabilistic airspace model.

It detects threats using a Markov decision process, using a statistical representation of where the intruder aircraft is likely to be in the future in order to determine the best course of action.

Alerts are based on “perceived risk” and “filter out” many potential resolution advisories where this risk is low, says pan-European navigation organisation Eurocontrol.

ACAS Xa is essential for the evolution of airspace concepts – both in Europe and the USA – allowing reduced spacing between aircraft. TCAS is not compatible with such concepts, as it would generate too many unnecessary alerts.

Variants of ACAS Xa will also allow collision-avoidance systems to be extended to other aircraft categories, unlike TCAS which is restricted to aircraft capable of meeting specific performance criteria, such as a 2,500ft/min climb rate.

ACAS Xa installation will not be mandated. EASA’s opinion states that operators should still have the choice whether to fit Xa or TCAS 7.1.

EASA adds that the opinion focuses solely on enabling safe use of ACAS Xa, although it acknowledges the potential of other variants such as ACAS Xo – intended for close-spaced parallel approaches – and broader concepts such as ACAS Xr and Xu, respectively for rotorcraft and unmanned aerial vehicles.