What aircraft was involved and where did it happen?

The accident on null involved a aircraft, registration EI-DCZ, at IE.

What was the probable cause of the accident?

The incident was caused by the Radar Controller's failure to maintain required vertical and lateral separation, compounded by a loss of situational awareness regarding the closing speeds of the two aircraft.

TCAS Prevents Mid-Air Collision Near Irish Coast

A potential collision between two passenger aircraft was narrowly avoided off the southeast coast of Ireland when onboard collision avoidance systems activated following an air traffic control error.

What happened

On the evening of 23 September 2007, two commercial passenger aircraft entered Irish-controlled airspace near the 'BANBA' reporting point, located off the southeast coast of Ireland. The first aircraft, a B737-8AS with registration EI-DCZ, was traveling west toward Cork. The second aircraft, an MD-83 registered as OE-IKB, was flying north toward Dublin.

While the MD-83 was cruising at Flight Level 280, the Shannon-based Air Traffic Control (ATC) radar controller issued a series of clearances to the B737-8AS. Initially cleared to descend to Flight Level 290, the aircraft was subsequently instructed to descend much further to Flight Level 100. This descent placed the B737-8AS directly into the flight path of the northbound MD-83.

As the aircraft closed in on each other at a combined speed of approximately 630 knots, the situation became critical. The aircraft came within just 600 feet of vertical separation and 3 nautical miles of lateral separation. The potential impact was only averted when the onboard Traffic Alert and Collision Avoidance System (TCAS) on both aircraft triggered Resolution Advisories (RA), commanding one pilot to climb and the other to descend. Both flight crews responded immediately and correctly to the automated commands.

The investigation

The Air Accident Investigation Unit (AAIU) examined radar data, audio recordings, and pilot reports. The investigation focused on the actions of the Shannon Upper Air Control team, which consisted of a Radar Controller and a Planning Controller.

Investigators found that while the Planning Controller had noted the significant speed difference between the two aircraft, the Radar Controller failed to act on this information. The investigation also reviewed the performance of the Short Term Conflict Alert (STCA) system, which had provided three distinct electronic warnings to the controller, including audio alarms and visual red labels on the radar screen. The investigation concluded that the ATC system's human element failed to rectify the conflict despite these automated alerts.

Findings

The primary cause of the incident was the failure of the Radar Controller to maintain safe separation between the two aircraft.
The Radar Controller experienced a partial loss of situational awareness, leading to dangerous tactical decisions.
Although the ATC system provided multiple electronic warnings (STCA), the controller did not respond appropriately to these unambiguous cues.
The safety of the B737-8AS and the MD-83 was only preserved because the aircraft's TCAS functioned as the final line of defense.
There were no technical or mechanical failures identified in the aircraft or the air traffic control equipment.