What happened
On the evening of 8 March 2015, a loss of separation occurred between two Boeing 737-8AS aircraft at Dublin Airport. The first aircraft, EI-EFF, was preparing for a scheduled departure to Luton, while the second, EI-DHA, was on final approach for a scheduled landing.
During the taxi phase, the Air Movements Controller (AMC) queried if the crew of EI-EFF would be ready for an immediate departure. Although the flight crew replied with a "negative" due to the cabin not yet being secured, the controller misunderstood this as an affirmative response. Consequently, the controller cleared the aircraft to line up and take off.
As the aircraft approached the runway, it slowed significantly, prompting the controller to attempt to instruct the crew to hold short. However, this instruction was lost due to a blocked radio transmission. The crew of EI-SSD, having just received the cabin secure signal and a confirmation of their takeoff clearance, proceeded to enter the runway. The departing aircraft became airborne just before the arriving EI-DHA touched down, with the two planes coming within approximately 807 meters of each other.
The investigation
The AAIU examined the communications between the Air Traffic Control Officer (ATCO) and both flight crews, as well as the airport's Advanced Surface Movements Guidance and Control System (A-SMGCS) data. The investigation reviewed the sequence of radio transmissions, noting that a simultaneous transmission blocked the controller's attempt to halt the departure. The investigation also looked into the operational status of the taxiway stop bar and the readiness of the aircraft crews.
Findings
- The controller's decision to clear the aircraft for takeoff was based on a misunderstanding of an indistinct radio reply.
- The flight crew of EI-EFF did not explicitly communicate to the controller that they were waiting for a cabin secure signal, which would have altered the departure sequence.
- A blocked transmission prevented the controller's instruction to hold short from being heard by the departing crew.
- The departing aircraft slowed to a very low taxi speed (5 knots) unexpectedly, which disrupted the controller's planned arrival/departure spacing.
- The runway occupation alert system (RIMCAS) activated due to the reduced separation between the two aircraft.
Safety action
Following the incident, the Irish Aviation Authority (IAA) implemented a "Time to Touchdown" (TTT) facility for the Tower Controller. This system displays the estimated time until the next aircraft touches down on final approach, providing controllers with better situational awareness to prevent similar separation losses.