What happened
On 11 October 2012, at approximately 09:44 UTC, a Pilatus PC-12/47E, registration M-WINT, was involved in a near mid-air collision at Sabadella Airport (LELL). The aircraft was on a private flight from the United Kingdom and had been cleared by the control tower to join the right downwind leg for Runway 31, following a Cessna F152, registration EC-DMC.
However, the pilot of M-WINT misunderstood the instruction, believing he was cleared for a left-hand pattern. Consequently, the aircraft performed a series of turns that brought it onto the final approach for the opposite runway, Runway 13. At the same time, the Cessna F152 was executing a touch-and-go maneuver on Runway 31. The two aircraft crossed paths at a horizontal distance of 0 NM and a vertical separation of only 100 ft. The conflict was only averted when the instructor onboard the Cessna F152 visually detected the approaching Pilatus and instructed the student pilot to execute an evasive turn to the left.
The investigation
The CIAIAC investigation focused on the communications between the air traffic control (ATC) tower and the aircraft, as well as the operational environment during a transition period between service providers (AENA and ferroNATS). The investigation examined the control room, which at the time contained a student controller being evaluated, an instructor, an evaluator, and an instructor-assistant.
Investigators found that the student controller was operating without radar assistance as part of his evaluation process. The investigation also reviewed the radio transcripts, noting that the pilot of M-WINT provided an incomplete and non-standard acknowledgment of the initial clearance. Furthermore, the investigation looked into why the ATC personnel, including the evaluation team, failed to detect the incorrect flight path of the Pilatus until the aircraft was on final approach for Runway 13.