Lack of situational awareness after controllers changed shift led to a serious three-way air proximity incident in the vicinity of Milan nearly two years ago, Italian investigators have concluded.
The incident centred on a BMI Airbus A320 and an Air France A320 travelling on separate opposite-direction airways, at adjacent flight levels 350 and 360, and a White Airways A319 that was approaching their crossing point on a near-perpendicular course at level 360.
While the Air France and BMI aircraft were being handled by the Milan East air traffic control sector, the Milan West sector was initially overseeing the White jet. Milan West cleared the White flight direct to a waypoint within the Milan East sector.
Just before the White aircraft transferred to Milan East, there was a change of executive controller on the Milan East sector. The incoming executive controller was initially dealing with an EasyJet Switzerland flight as the White aircraft transferred.
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Italian investigation agency ANSV says that, 78s after this transfer, the short-term conflict alert triggered, warning that the White A319 was approaching the Air France A320 at the same altitude, flight level 360.
The White aircraft was instructed to descend to level 350 but the controller almost immediately saw a chance to resolve the conflict horizontally and told the White and Air France flights to adjust their headings.
But straight afterwards a second alarm alerted the controller to a new conflict between the White A319 and the BMI A320, triggered because the White aircraft had been descending - as requested - to level 350, at which the BMI jet was cruising.
ANSV says their closest approach was 2.18nm (4km) horizontally and 400ft (120m) vertically. Forty seconds later the White aircraft, having returned to level 360, passed 3.8nm behind the Air France A320 at the same altitude.
The traffic collision-avoidance systems on the aircraft involved did not trigger any advisory because the aircraft were not on a collision trajectory. ANSV nevertheless describes the loss of separation in the 9 July 2009 incident as "serious".
It says the lack of situational awareness was "encouraged" by a shortfall in formality during the exchange of controllers. ANSV says the change was "essentially based on mutual trust" and conducted with "excessive confidence", with no detailed breakdown of the air traffic situation.
Source: Flight International
