European air transport legislation has been revised to enable introduction of airborne collision-avoidance systems based on alternative conflict-resolution modelling.
The European Union legislation – which enters into force on 10 March – requires aircraft authorised to carry more than 19 passengers to be equipped either with the new ACAS Xa system or the current ACAS II version 7.1.
This regulation, which follows a European Union Aviation Safety Agency recommendation last year, also applies the aircraft with a maximum certified take-off weight above 5.7t.
It marks the first major evolution in European collision-avoidance avionics regulation since ACAS II version 7.1 was mandated in 2015.
The legislation says the regulation to “allow access” of ACAS Xa-equipped aircraft to European airspace is in line with requirements to “reflect the state of the art and the best practices” in aviation, taking into account “scientific and technical progress”.
Under development by the US FAA since 2008, ACAS Xa takes advantage of computing techniques unavailable at the time when ACAS II was introduced.
The new system is also intended to reduce unnecessary alerts and facilitate implementation of future operating environments, including extending collision-avoidance capability to different categories of aircraft.
ACAS Xa is designed to be interoperable with ACAS II 7.1, and present familiar displays and alerts, but the two differ in their collision-avoidance logic and sources of surveillance data.
While ACAS II exclusively relies on transponder interrogation to determine an intruder’s current and future position – based on hard-coded fixed rules – the ACAS Xa logic continually assesses millions of possible future states, and uses a probabilistic model and a numeric look-up table to decide on the best resolution.
This means ACAS Xa will consider a different risk profile to ACAS II, alerting in cases where ACAS II might not or filtering out low-risk encounters to avoid nuisance alarms.
