What happened
On 7 September 2019, at approximately 14:00 UTC, a loss of separation occurred between an Airbus A320-214, registration EI-IKL, operated by Alitalia, and a Boeing 737-8AS, registration EI-ENN, operated by Ryanair. Both aircraft were operating under instrument flight rules (IFR) and were cleared to the ASBIN waypoint within the Madrid Terminal Control Area (TMA).
For several minutes, the two aircraft were flying in close proximity, essentially one above the other, while maintaining appropriate vertical separation. However, during their descent, the vertical distance between the aircraft began to decrease rapidly. At the point of closest approach, the aircraft were separated by only 1.4 NM horizontally and 3 and 300 ft vertically at flight level FL185.
The investigation
The investigation examined radar track data, flight recorder information, and air traffic control (ATC) communications. The analysis focused on the descent profiles of both aircraft and the effectiveness of the instructions provided by the Madrid approach unit (sector RNN).
Investigators reviewed the sequence of commands issued by the controller, noting that the controller had instructed the higher aircraft to maintain a specific rate of descent and the lower aircraft to descend at a rate of -2000 fpm or higher to establish vertical separation. The investigation also examined the automated conflict alerts, specifically the Short-Term Conflict Alert (STCA) system, which activated during the event.
Findings
- The primary cause of the loss of separation was the failure of both crews to comply with ATC instructions regarding their assigned rates of descent.
- The crew of the Airbus A320-214 failed to adhere to the prescribed descent rate of -2000 fpm, at times exceeding this rate by more than double.
- The crew of the Boeing 737-8AS failed to follow the instruction to maintain a descent rate of -2000 fpm or higher.
- The air traffic controller failed to actively monitor and enforce the descent rate instructions.
- The controller did not issue more forceful intervention, such as a heading change or a level-off instruction, despite the lack of compliance with the initial rate-of-descent instructions.
- Neither crew reported receiving any Traffic Collision Avoidance System (TCAS) Resolution Advisories (RA) during the incident.
- Neither crew filed a formal loss of separation report following the event.