The French civil aviation safety investigation authority is calling on the international safety community to consider modifications to existing airborne collision avoidance system (ACAS) processes, as well as to study the use of the autopilot to resolve close encounters.

BEA's recommendations are included in its final report on a June 2010 “serious incident” in which a controller trainee erroneously put a departing Air France A319 and an inbound Easyjet Switzerland A319 on converging flight paths near the Basel Mulhouse airport in eastern France. The ACAS on both aircraft alerted the pilots with resolution advisories multiple times in rapid succession. One cabin crew member on the Easyjet aircraft, which was approaching the airport to land, was injured as the pilot manually pulled as much as +2g and -0.2G as he attempted to follow ACAS resolution advisories.

An ACAS system provides several layers of traffic advisories, the most benign of which is the traffic alert, which triggers 35-48 sec. before the calculated point of closest approach but does not require action. If the threat of a collision becomes more imminent, the system can issue “preventive” or “corrective” resolution advisories (RA) when the two aircraft are 15-35 sec. from the point of closest approach. Per Airbus instructions, both types of RA require the pilots of the A320, A330 and A340 to turn off the flight directors and disconnect the autopilot and hand-fly the aircraft while monitoring or adjusting vertical speed according to ACAS audible and visual instructions on the flight instruments. A preventive alert requires no immediate action while a corrective alert does.

The incident took place as both aircraft were mistakenly commanded by the controller trainee to level off at 11,000 ft. while both were diverting around a thunderstorm. Upon realizing the error, the controller requested that the departing Air France A319 descend back to 10,000 ft.; however the aircraft was already at 10,600 ft. and climbing at a rate of 3,000 ft./min., well above Air France’s recommendation of 1,000 ft./min. when within 1,000 ft. of the final altitude. The pilot began to descend, but the ACAS alerted and called for a continued climb. The BEA says the high rate of climb “may have further shortened the lead-time” between triggering of a traffic advisory (TA) followed by an RA.

The Easyjet A319 pilots, already flying at 11,000 ft., received a preventative RA, prompting the crew not to climb. However after turning off the autopilot, the pilot applied a series of nose-up and nose-down inputs, an action the BEA says was an instinctive reaction an RA. The ACAS later issued a series of corrective RA commands.

While an “error in speech” by the trainee controller was at the root of the loss of separation event, BEA says the ACAS design and procedures exacerbated the pilots’ responses.

Recommendations include having the International Civil Aviation Organization (ICAO) study “resolution automation” with the autopilot engaged as an ACAS standard, a system that is already available on the A380 and has been proposed by Airbus for retrofit on the A320, A330 and A340. To prevent pilots from instinctively taking action when none is required, BEA is asking ICAO to study the impact on safety of classifying all preventive RAs as TAs only.

Regarding the high vertical speed of the Air France A319, BEA is recommending that the European Aviation Safety Agency study “setting a standard for smooth vertical flight paths when approaching a level selected by the crew”.

The agency notes that Airbus is already studying an automatic system that would gradually decrease the vertical speed of an aircraft as it approaches the altitude or flight level selected by the crew.