What aircraft was involved and where did it happen?

The accident on 2014-10-30 involved a B-737-800, registration EI-EKS, at TMA Sevilla (Sevilla), ES.

What was the probable cause of the accident?

The incident was caused by the Vueling crew's failure to maintain the authorized descent rate of 2000 ft/min, compounded by a lack of explicit rate restriction reinforcement by air traffic control during subsequent clearances.

TCAS Resolution Advisory triggered by descent rate violation in…

A near-midair collision involving a Vueling Airbus A320 and a Ryanair Boeing 737 occurred in Spanish airspace after the A320 crew exceeded authorized descent rate limits.

What happened

On 30 October 2014, an Airbus A320-216, registration EC-KCU, operated by Vueling, was performing an instrument approach to runway 27 at Seville Airport (LEZL). While descending, the crew requested a runway change due to favorable winds. Seville Air Traffic Control (LECS) authorized the change and issued successive descent clearances to various flight levels, specifically instructing the crew to maintain a descent rate of 2000 ft/min or less.

Simultaneously, a Boeing 737-800, registration EI-EKS, operated by Ryanair, was descending through the same sector from flight level 410 to 150, with an authorized descent rate of 2000 ft/min or higher.

In the vicinity of the VULPE reporting point, the vertical separation between the two aircraft dropped to just 100 feet, with a horizontal separation of 1.4 NM. The Airborne Collision Avoidance System (TCAS) on both aircraft issued Traffic Advisories (TA) followed by Resolution Advisories (RA). The crew of EC-KCU initially reacted to an "Adjust Vertical Speed" RA by disengaging the autopilot and increasing their descent rate to 4400 ft/min, a maneuver contrary to the initial instruction. The situation only stabilized when the TCAS issued a subsequent "Climb, Climb" RA, which the crew then followed.

The investigation

The CIAIAC investigation examined the flight data, air traffic control communications, and the actions of both flight crews. The investigation focused on why the Vueling crew increased their descent rate despite existing restrictions and why they initially responded to the TCAS RA with a maneuver that increased the risk of conflict. The investigators also reviewed the radar capabilities of the Seville controller and the functionality of the TCAS systems on both aircraft.

Findings

The primary cause of the incident was that the crew of EC-KCU failed to comply with the descent rate instructions provided by the controller.
The controller cleared the aircraft to a lower flight level without explicitly restating that the previous descent rate restrictions were still in effect.
The controller provided a direct routing to point ROTEX without receiving a full acknowledgment of the existing speed and rate restrictions from the crew.
The air traffic controller's radar display did not provide real-time descent rate information, preventing the timely detection of the rate violation.
The crew of EC-KCU failed to adhere to the initial TCAS RA "Adjust Vertical Speed" instruction, instead increasing their descent rate.
The investigation noted that the crew was at the end of their duty period, which may have impacted flight management.

Safety action

Following the investigation, safety recommendations were proposed to AESA and ICAO to ensure that controllers explicitly repeat descent rate restrictions when clearing aircraft to new flight levels or during sector transfers to prevent similar misunderstandings.

TCAS Resolution Advisory triggered by descent rate violation in Seville TMA