The 1 July mid-air collision between a Bashkirian Airlines Tu-154 and a DHL Boeing 757-200F, both with working airborne collision avoidance systems (ACAS), did not reveal equipment flaws, but highlighted a failure of worldwide air traffic management (ATM) to agree on standard operating procedures (SOP) for pilots who receive an ACAS resolution advisory (RA).
In this case the DHL pilot followed his RA, whereas the Bashkirian pilot followed air traffic control instructions that contradicted his RA. Both pilots were following standard operating procedures for their own countries.
Neither pilot departed from the International Civil Aviation Organisation's Annex 2 standards and recommended practices (SARP) on using ACAS, because a crucial clause in it says: "Nothing in the procedures specified shall prevent pilots in command from exercising their best judgement and full authority in the choice of the best course of action to resolve a traffic conflict."
So even if the world's pilots are provided with SOPs, they can ignore them. This phraseology may survive, but a change in emphasis clarifying normal procedure is expected.
In July last year Japan reported on a conflict in 2001 that came within 10m (33ft) of collision. A 747-400 pilot, in a cleared descent, ignored an RA to climb. The other - a McDonnell Douglas DC-10-40 pilot - complied with his RA and descended. Because they were in visual contact, the compliant pilot reversed his action and the non-compliant pilot steepened his descent so they avoided collision. The final avoidance manoeuvres were violent. The Japanese report to ICAO, reversing its original national advice on ACAS SOPs, says: "Pilots should comply with an RA when pilots receive simultaneously an instruction to manoeuvre from ATC and an RA." It calls upon ICAO to clarify RA procedures. Before the July collision, the Russian SOP was that ATC instructions take precedence over an RA.
European Joint Aviation Authorities procedures say an RA takes precedence over an ATC instruction, but the JAA is seeking clarification on how pilots should inform ATC that they are following an RA. Voice and/or Mode S datalink are the choices. All instructions make it clear that pilots should resume their cleared level or trajectory as soon as the conflict has been resolved, telling ATC.
ICAO guidance says that, if ATC is aware that a pilot is following an RA, "the controller should not attempt to modify the flightpath until the pilot reports returning to the terms of the current ATC instruction or clearance, but shall provide traffic information as appropriate".
ICAO has sent out to all its member states a proposal to clarify Annex 2 on ACAS procedures, and the organisation says its Air Navigation Commission will meet in March to consider state responses. The proposals that emerge will go to the council shortly afterwards, and implementation of the new ACAS SARP is expected around October.
From 1 January, ICAO rules specify that all aircraft above 15,000kg (33,000lb) shall be fitted with ACAS version II. From January 2005 the same SARP will apply for all aircraft above 5,700kg or capable of carrying 19 or more passengers.
Equipment is being standardised: now procedures remain to be resolved.
Source: Flight International
