One of the consequences of the Potomac mid-air crash in Washington DC, a year ago, has been to raise the profile of efforts to rethink the logical – and technological – strategy behind airborne collision avoidance.

Proposed legislation drawn up in the aftermath of the collision, but still awaiting enactment, presses the US FAA to develop a plan for widespread adoption of ACAS X, the technology intended to supersede TCAS as the last line of defence.

Although TCAS has undergone multiple improvements since first being mandated, ACAS X effectively overhauls the concept with a new approach intended to offer compatibility with higher-density airspace and changes in aircraft performance, taking advantage of better surveillance data while reducing nuisance alerts.

ACAS X uses Markov statistical and probabilistic decision processes, rather than hard-coded rules, to assess likely conflict geometry and determine the best course of action – filtering out potential resolution in low-risk situations.

Decoupling from TCAS’s rigid logic not only enables ACAS X to be flexible, but offers the ability to be modular – with separate variants such as ‘Xa’ for commercial aircraft, ‘Xr’ for helicopters, and ‘Xo’ for specific operations like closely-space parallel approaches.

Investigation into the Potomac accident, between a CRJ700 and military UH-60 helicopter, offered an insight into ACAS X’s capabilities through simulation of the collision.

At a National Transportation Safety Board hearing last July, the FAA’s TCAS programme manager, Neal Suchy, showed that a prototype of ACAS Xr on the helicopter would have given its crew a traffic advisory 73s before the accident.

“Use of the prototype [ACAS Xr] on the helicopter did significantly reduce the collision risk without actually having to change the existing design of TCAS or [ACAS Xa],” he added.

During the hearing, MIT Lincoln Lab transportation safety leader Wes Olson pointed out that ACAS X could also better handle unexpected manoeuvring by aircraft in conflict.

This might have been a crucial advantage 25 years ago, when a Japan Airlines 747-400 crew initially complied with air traffic control’s attempt to resolve a conflict with a JAL DC-10-40 over Shizuoka, rather than follow their TCAS resolution advisory.

The two aircraft were carrying 677 occupants, and sudden evasive action by the 747 pilots averted – by literally a few metres – what might have been the greatest recorded catastrophe in civil aviation.

Such fortune was absent the following year when a similar scenario resulted in the Uberlingen collision between a Tu-154 and a 757 freighter.

TCAS has nevertheless justified its development in countless instances. US and European regulators have acted to permit ACAS X operations in commercial airspace, and the potential for ACAS X to build on the TCAS legacy is clear – although limitations remain.

Suchy reminded the NTSB hearing that resolving conflicts at low height – as was the case over the Potomac – is problematic, given that systems inhibit resolution advisories to crews.

“In general both TCAS and [ACAS Xa], as designed with current inhibits, would have only provided a traffic alert prior to the collision,” he said. “Neither would have provided [a resolution advisory]. In order to get that…you’d have to lower the inhibits.”