Irish investigators are attributing a serious air proximity incident between a Flightline Boeing MD-83 and a Ryanair Boeing 737-800 to a breakdown in communications between two air traffic controllers, last September.
The 737 had been westbound from London Stansted to Cork, cruising at flight level 300 (30,000ft/9,150m), while the MD-83 was flying north on the Faro-Dublin route at the lower altitude of FL280.
Two Shannon-based controllers were overseeing the aircraft, which were off Ireland's south-east coast at the time. The planning controller informed the radar controller that the two aircraft had substantially different ground speeds, with the MD-83 closing rapidly from the south.
After the 737 crew requested a descent towards Cork, the radar controller cleared the aircraft to FL100. This descent instruction was not noticed by the planning controller, who was distracted by an operational telephone call.
The distance between the two aircraft at the time was 14nm (26km) and the short-term conflict-alert system activated. As the distance reduced to 8nm, the radar controller attempted to halt the 737's descent by instructing the crew to level at FL290 - by which time the jet had already descended below this altitude.
Shortly afterwards the MD-83 crew, which had twice been told to change heading as the aircraft closed on one another, then initiated a descent following a resolution advisory from the collision-avoidance system. The 737 crew similarly responded to an advisory to climb.
Ireland's Air Accident Investigation Unit (AAIU) says the aircraft were separated by 3nm laterally and 600ft vertically at their closest point. "Neither controller played any part in the ultimate safe resolution of this avoidable air proximity," it states, adding that there was a "critical breakdown" in follow-up communication between the two.
"In spite of at least four warnings - one verbal and three electronic - the radar controller appeared not to comprehend the closing speeds of the two aircraft and allowed the higher one to descend and lose the required minimum vertical and lateral separation from the other. What ensued was a critical failure of the human element of the air traffic control system to rectify this situation."
In its inquiry into the 23 September 2007 event the AAIU points out the large difference in experience levels between the two controllers.
It says the more-experienced planner, who did not intervene after the initial caution over the aircraft speeds, might have offered greater assistance by monitoring the aircraft and exercising greater assertiveness.
But it adds that there was "no possibility" of controller confusion because the radar display was uncluttered and electronic warnings were clearly displayed.
The AAIU has advised that the Irish Aviation Authority review its training and team resource management processes to take the incident into account.