What aircraft was involved and where did it happen?

The accident on 2012-11-05 involved a A320-232, A320-214, registration N614NK, HK4817, at Rionegro, CO.

What was the probable cause of the accident?

The probable cause was an operational error involving the failure to verify the relative positions of the two aircraft before discontinuing vertical separation, compounded by a deficiency in radiotelephony procedures, specifically the failure to correct an incorrect altitude readback.

Near Mid-Air Collision Avoided During Approach at Rionegro

Two Airbus A320 aircraft experienced a loss of separation in the Rionegro TMA due to a readback error and degraded surveillance services.

What happened

On December 5, 2012, at 18:39 UTC, a serious incident involving a loss of separation occurred within the Rionegro TMA, Medellín, during the final approach to runway 36. Two Airbus A320 aircraft, HK-4817 (operated by Viva Colombia) and N614NK (operated by Spirit Airlines), were in the sequence for landing under Visual Meteorological Conditions (VMC).

At approximately 18:36 UTC, air traffic control (ATC) queried the position of HK-4817, which reported being four miles from the Rionegro (RNG) VOR at 10,000 feet. Shortly after, the controller authorized N614NK for the approach to runway 36, requesting the pilot report leaving the RNG VOR at 10,000 feet. However, the pilot of N614NK read back an altitude of 13,000 feet. The controller accepted this readback without correcting the discrepancy.

As both aircraft departed the VOR on final approach, N614NK was descending from 13,000 feet while HK-4817 was at 10,000 feet. The aircraft converged with a vertical separation of only 600 feet. The pilot of HK-4817 alerted the tower that N614NK had crossed closely overhead. Following instructions from the tower, the crew of N614NK initiated a missed approach, while HK-4817 landed safely. No injuries were reported among the passengers or crew of either aircraft.

The investigation

The GRIAA investigation established that surveillance services were degraded at the time of the event due to a radar head update at the Cerro Verde station, forcing controllers to rely on procedural separation. The investigation found that the controller failed to verify the relative positions of the two aircraft before authorizing the approach for N614NK. Furthermore, the investigation revealed that the controller's instruction for a missed approach did not provide immediate conflict resolution, as the prescribed path still directed the aircraft toward the conflict zone.

Regarding the flight crews, the investigation noted that the crew of N614NK experienced reduced situational awareness due to heavy frequency congestion and the fact that all other communications were in Spanish, while they were the only English-speaking crew. Although the crew of N614NK experienced both TCAS Traffic Advisories (TA) and Resolution Advisories (RA), they did not report these alerts to ATC or request separation assistance.

Findings

Improper readback verification: The controller failed to correct the pilot's incorrect readback of 13,000 feet, which led to the loss of vertical separation.
Degraded surveillance: The lack of radar service necessitated procedural separation, yet the controllers were not adequately trained or prepared to manage high-density traffic without radar support.
Operational error: The controller authorized the approach for N614NK under the assumption that the pilot would perform a procedure to descend over the VOR rather than flying a direct approach.
Crew situational awareness: The crew of N614NK did not notify ATC of their TCAS RA or the presence of the other aircraft despite the proximity.
Inadequate coordination: Coordination between Medellín APP South and the Rionegro tower was insufficient, with traffic information being provided only as the conflict was already in progress.