Flight Details
Aircraft Type: Boeing 737-800 Operator: Ryanair Registration: EI-ENM Flight Number: FR9177 Route: Dublin Airport (DUB), Ireland to Ibiza Airport (IBZ), Spain Date of Incident: 10 August 2023 Total Occupants on Ryanair Flight: 176 (170 passengers, 6 crew) Airspace Sector: Palma Terminal Control Area (TMA), sectors GXX and F1X
Introduction
On 10 August 2023, a loss of separation occurred between a Ryanair Boeing 737-800 and a Cessna Citation business jet operating under the callsign HERON while both aircraft were under air traffic control in Spanish airspace. The two aircraft—one inbound to Ibiza and the other outbound—came within 500 feet vertically and 0.9 nautical miles laterally, triggering a Traffic Collision Avoidance System (TCAS) Resolution Advisory on board the Ryanair aircraft. Both aircraft continued to their destinations without further incident.
On 24 April 2025, the Comisión de Investigación de Accidentes e Incidentes de Aviación Civil (CIAIAC) released its final report, concluding that the incident resulted from controller misjudgement, insufficient monitoring, and system limitations in Spain’s STCA (Short-Term Conflict Alert) software.
Sequence of Events
At 08:27 UTC, multiple aircraft, including the Ryanair flight FR9177, TAP Air Portugal traffic, and a Citation business jet (HERON), were operating in converging trajectories in the GXX sector of Palma’s terminal airspace.
The HERON Citation was flying eastbound after departing Ibiza, the TAP aircraft was flying a similar easterly route inbound to Palma, and the Ryanair was descending southward into Ibiza. All three aircraft were in close lateral proximity and were converging near the Andratx Non-Directional Beacon (NDB).
The GXX sector controller, misinterpreting the separation priorities, transferred the HERON aircraft to the adjacent F1X sector while it was still at FL196 and 23 nautical miles from the sector boundary—a premature handover that occurred without confirming HERON’s descent rate or level compliance.
The controller then issued descent clearance to the Ryanair aircraft into airspace that was presumed clear, assuming the TAP aircraft had vacated it. In reality, the HERON aircraft was still occupying that same vertical band, descending more slowly than expected.
This resulted in the Ryanair aircraft descending into airspace still occupied by the HERON Citation, and within moments, the two aircraft came into proximity below minimum separation standards—triggering a TCAS Resolution Advisory for the Ryanair crew.
Crew & Communication
The flight crews of both aircraft were operating on different radio frequencies, due to the HERON aircraft being under F1X control while still flying in GXX airspace. While this is not unusual, it reduced situational awareness across traffic involved in the convergence.
Despite this, the phraseology and communication between ATC and both aircraft were deemed clear, correct, and compliant with standard ICAO protocol. The Ryanair crew responded promptly to the TCAS RA, which resolved the conflict with no further hazard.
There was no indication of high workload in the GXX control sector based on recorded transmissions and task management, although the system recorded the sector’s workload at 93.5%. However, audio and data analysis showed that controller capacity was not directly overloaded, pointing instead to poor conflict detection.
Air Traffic Control & System Analysis
The CIAIAC identified three primary causes for the loss of separation:
Ineffective and untimely conflict prevention by the GXX sector controllers. Incorrect analysis of the scenario, including failure to recognise HERON’s trajectory as a conflict with Ryanair. Limitations in the STCA software used by ENAIRE, Spain’s air navigation provider, which failed to generate a timely conflict alert.
The investigation found that:
The HERON aircraft was still within GXX airspace but already communicating with F1X due to early sector transfer. The GXX controller incorrectly perceived TAP as the sole potential conflict for Ryanair and did not monitor the HERON aircraft closely. When Ryanair was cleared to descend, the flight level thought to be vacated by TAP was in fact still occupied by HERON, which was not descending at a rate in line with ATC expectations. The controller failed to verify HERON’s descent rate or level before issuing descent instructions to the Ryanair flight.
Visual analysis of the radar displays and controller consoles revealed that the HERON label had turned blue, indicating it was out of GXX’s frequency but still physically within GXX airspace—a known limitation of the radar interface, which can lead to misprioritisation during traffic sequencing.
STCA System Limitations
The Short-Term Conflict Alert (STCA) software, which should have identified the convergence risk between HERON and Ryanair earlier, did not trigger an early warning due to known limitations in ENAIRE’s implementation of the system. This further delayed controller recognition of the developing hazard.
Investigation Findings
The premature transfer of the HERON aircraft to the F1X sector was unjustified and based on flawed assumptions about descent progress. The GXX controller focused only on the TAP–Ryanair conflict, believing TAP’s departure from a flight level had made it available for Ryanair’s descent—an error that disregarded HERON’s presence. The HERON aircraft was still flying at FL196 near the F1X boundary, when by expectation, it should have been nearing FL145. Situational analysis by GXX was incomplete, lacking real-time verification of actual flight levels and rates of descent.
Root Cause & Contributing Factors
Primary Cause:
The failure of GXX sector controllers to detect and manage the developing proximity conflict between the Ryanair and HERON aircraft in time.
Contributing Factors:
Erroneous assumptions regarding descent rates and separation of HERON. Premature sector hand-off of HERON while it was still in conflict. STCA system limitations that did not identify the threat early enough. Focus on a misidentified conflict (TAP vs. Ryanair) instead of the actual hazard (HERON vs. Ryanair).
Safety Recommendations & Industry Impact
Following the investigation, Spain’s CIAIAC may issue or has issued recommendations to:
ENAIRE: Upgrade or revise STCA system parameters to enhance early detection of convergence threats, particularly in multi-sector environments. ATC training bodies: Reinforce procedures around cross-sector coordination and transfer timing, ensuring controllers delay sector transfers until true traffic separation is confirmed. Air traffic units: Emphasise descent rate monitoring as part of real-time conflict assessment. Encourage system upgrades that better highlight aircraft still within physical airspace despite being on another frequency, reducing controller misidentification.
Conclusion
This incident involving Ryanair flight FR9177 and a Citation business jet near Ibiza exemplifies the risk posed by controller assumptions, early hand-offs, and incomplete situational awareness in terminal sectors with complex converging traffic. Fortunately, the onboard TCAS system intervened, enabling safe conflict resolution.
Spain’s CIAIAC report sheds light on systemic weaknesses in air traffic monitoring protocols and highlights the importance of precise aircraft sequencing, controller vigilance, and reliable alerting software in high-density airspace.
Disclaimer
This article is based on publicly available information and reports at the time of writing. While every effort has been made to ensure accuracy, we cannot guarantee the completeness of the information provided.
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