Studies into the potential
effectiveness of downlinking
resolution advisory data from airborne collision-avoidance systems (ACAS)
indicate that it could prevent more than 90% of unintentional air traffic
controller intervention incidents.
Eurocontrol has been assessing the value of downlinking such information in
order to prevent controllers, during an airborne conflict, inadvertently
relaying instructions that contradict ACAS commands to pilots.
Contradictory instructions between
air traffic control and ACAS equipment were a contributing element to the
mid-air collision between a Tupolev
Tu-154 and a Boeing 757 over Lake
four years ago.
Following the collision Eurocontrol began examining the
potential of transmitting ACAS data, under the Feasibility of ACAS Resolution
Advisory Downlink Study (FARADS) programme, and presented its latest findings
at a Brussels
workshop on 31 May.
Part of the study, conducted by
UK-based Qinetiq, has
assessed the critical issue of whether ACAS downlink would increase
controllers’ situational awareness enough to prevent a possible inappropriate
Researchers examined both datalink candidates for
performing the task – Mode-S and 1090MHz extended squitter – and looked at scenarios in both en route
and terminal airspace.
Less than 40% of present cases
result in controllers becoming aware of ACAS resolution advisories before an
aircraft has noticeably deviated from its course.
But the study has found that downlinking ACAS data would allow
controllers to detect 95% of resolution advisories within 8.1s of their being
issued – a large improvement on the present figure of 28s.
“The downlink of resolution advisory
information virtually doubles the chances that the controller will be aware of
the [advisory] before any significant deviation occurs,” says the study.
“[This] indicates that the downlink
of resolution advisory information by either of the technologies…is
sufficiently timely to allow a significant increase of the situational
awareness of controllers in ACAS encounters.”
The study says this improvement
could prevent 91% of incidents in en route airspace in which controllers pass
instructions to an aircraft while unaware that it is subject to an ACAS
advisory. In terminal airspace the proportion is around 88%.
But the downlink is less effective
in allowing a controller to change a pilot’s response, because such
intervention requires a relatively long sequence of events to take place within
a limited time window. In en route airspace controllers would only be able to
affect pilot behaviour in 12% of encounters, and this falls to just 5% in