ATC Error Creates Dangerous Near-Miss Between United and FedEx Over North Atlantic
A critical air traffic control error nearly resulted in a midair collision between United Airlines Flight 879 and a FedEx cargo flight over the North Atlantic on June 21, 2025, when both Boeing 777s came within just 500 feet of each other approximately 160 nautical miles northeast of Gander. The incident, caused by an erroneous Controller-Pilot Data Link Communication (CPDLC) message, highlights both the vulnerabilities and robust safety backup systems in oceanic airspace management.
Incident Timeline and Location
The near-miss occurred in Canadian-controlled oceanic airspace as United Airlines Flight 879, a Boeing 777-200 en route from London Heathrow to Houston George Bush Intercontinental, was cruising at 34,000 feet and requesting clearance to climb to 36,000 feet. The conflicting aircraft was a FedEx Boeing 777F flying in the opposite direction at 35,000 feet on a Memphis-to-Paris Charles de Gaulle route.
The incident location—160 nautical miles northeast of Gander—places it in a critical section of North Atlantic airspace where multiple heavily-traveled routes converge, making precise altitude separation essential for safety. This area typically sees hundreds of aircraft daily following organized track systems across the Atlantic.
Controller Error and CPDLC Miscommunication
The dangerous situation began when Oceanic Control initially responded correctly to United’s climb request with a “standby” instruction, recognizing the potential conflict with the oncoming FedEx aircraft. However, the controller subsequently issued a CPDLC message directing United Flight 879 to the TOPPS waypoint that erroneously included authorization to climb to 36,000 feet.
According to the Canadian Transportation Safety Board (TSB) investigation, this clearance put the United aircraft directly into the path of the approaching FedEx flight. The controller immediately recognized the error and issued voice instructions for United to maintain 34,000 feet and disregard the CPDLC clearance, but the correction came after the dangerous climb authorization had already been transmitted.
Critical Safety System Response
Despite the controller’s immediate voice correction, Flight 879 was observed climbing through 34,500 feet approximately two minutes later, violating the mandatory 1,000-foot separation requirement between the aircraft. This triggered a cascade of safety responses designed to prevent collision in oceanic airspace.
The United aircraft’s Traffic Collision Avoidance System (TCAS) activated, generating urgent audio alerts: “Traffic! Traffic! Traffic!” followed by “Descend… Descend.” These automated warnings provided the flight crew with independent guidance to avoid collision, operating separately from air traffic control instructions.
TCAS Technology and Collision Avoidance
The incident demonstrates the critical importance of TCAS as aviation’s last line of defense against midair collisions. The system operates independently of air traffic control, using transponder signals from nearby aircraft to construct a three-dimensional map of surrounding airspace and calculate potential collision trajectories.
In this case, TCAS aboard the United 777-200 detected that the initiated climb would intersect with the FedEx aircraft’s flight path and immediately commanded a descent to avoid collision. Pilots receive extensive training to respond immediately to TCAS instructions, even when they conflict with ATC clearances, as occurred in this incident.
Crew Response and Resolution
The United flight crew responded appropriately to both the repeated ATC instructions and TCAS warnings, immediately aborting their climb and returning to 34,000 feet. The Canadian TSB reports that proper 1,000-foot vertical separation was reestablished within seconds, allowing both flights to continue safely to their destinations.
The rapid resolution demonstrates the effectiveness of multiple safety layers in commercial aviation, where controller recognition, voice communication backup, and automated collision avoidance systems work together to prevent accidents even when initial errors occur.
Oceanic Airspace Challenges
This incident highlights the unique challenges of oceanic air traffic management, where aircraft operate beyond conventional radar coverage and rely heavily on precise controller instructions and pilot compliance. The North Atlantic tracks system accommodates hundreds of daily flights in organized streams, requiring exact altitude and lateral separation to prevent conflicts.
CPDLC technology, while generally reliable, introduces potential for miscommunication when controllers inadvertently include incorrect information in data link messages. The incident underscores why voice communication and TCAS systems remain essential backup safety measures in oceanic operations.
Investigation and Classification
The Canadian TSB has classified this occurrence as a Class 5 incident, with data recorded for “possible future safety analysis, statistical reporting, or archival purposes.” While not the most severe classification, the incident provides valuable information for improving oceanic air traffic procedures and controller training.
The investigation will likely examine controller workload factors, CPDLC message protocols, and crew response procedures to identify potential improvements in oceanic airspace safety systems.
Industry Safety Implications
The near-miss demonstrates both the vulnerabilities and strengths of current aviation safety systems. While the initial controller error created a dangerous situation, the multi-layered safety approach—including immediate error recognition, voice communication backup, and automated collision avoidance—successfully prevented a catastrophic outcome.
The incident may prompt reviews of CPDLC procedures and controller training, particularly regarding the integration of data link communications with voice instructions in complex oceanic environments where rapid corrections are essential.
Future Safety Enhancements
The aviation industry continues developing advanced technologies to prevent such incidents, including enhanced surveillance systems for oceanic airspace and improved controller decision support tools. However, this incident demonstrates that existing safety systems, when properly implemented and maintained, can successfully prevent accidents even when initial errors occur.
The effectiveness of TCAS in this situation reinforces its value as an independent safety system, while the rapid controller response highlights the importance of human oversight in automated communication systems. As oceanic airspace becomes increasingly congested, such multi-layered safety approaches remain essential for maintaining aviation’s exceptional safety record.
