What happened
On 20 April 2017, a Boeing 737-800, registration EC-JBK, operated by Air Europa, was performing a standard arrival into Palma de Mallorca Airport. Simultaneously, a Cessna Citation C510, registration EC-LCX, operated by Caladero Aviation, was climbing out of the same airport.
Air traffic controllers in the Palma ACC sector F1X issued instructions to maintain vertical separation, directing the Boeing 737-800 to descend to FL120 and the Cessna Citation C510 to climb only to FL110. However, the crew of the Cessna Citation C510 incorrectly acknowledged the altitude as FL120. Because the controller did not correct this erroneous read-back, the aircraft continued climbing to FL120, placing both aircraft on the same flight level on converging paths.
The crew of the Boeing 737-800 noticed the conflict visually and via TCAS, which issued a descent resolution advisory. The crew of the Cessna Citation C510, equipped only with ACAS I (which provides traffic alerts but no resolution advisories), performed a manual evasive descent. At the closest point of approach, the aircraft were separated by only 0.9 NM horizontally and 0 ft vertically.
The investigation
The CIAIAC investigation focused on the communication breakdown and the monitoring capabilities of the air traffic control unit. Investigators examined radar data, voice recordings, and flight recorder information from the Boeing 737-800.
At the time of the incident, the sector was being managed by a controller receiving on-the-job training, overseen by an executive instructor. The investigation found that the controller under instruction was simultaneously managing other traffic, which diverted attention away from the incorrect altitude acknowledgment. Furthermore, the Short-Term Conflict Alert (STCA) system had not yet been implemented in that specific airspace volume at the time of the event.
Findings
- The primary cause of the incident was the failure of the air traffic controller to correct a faulty altitude acknowledgment by the crew of EC-LCX.
- The crew of EC-LCX mistakenly acknowledged FL120 instead of the instructed FL110, and both pilots on that aircraft subsequently set the incorrect altitude in the autopilot.
- The air traffic controllers were distracted by high workloads and the simultaneous task of instructing a trainee, which prevented them from noticing the incorrect read-back or the subsequent altitude deviation on their radar displays.
- The STCA system, which could have provided an automated alert, was not yet active in this sector; it was implemented in April 2018.