A United Boeing 777-200 and a FedEx 777-200 freighter experienced a loss of separation incident over the Atlantic Ocean on Saturday, prompting a Traffic Collision Avoidance System (TCAS) advisory.
The Transportation Safety Board of Canada and AvHerald reported that United flight 879, traveling from London Heathrow to Houston, was cruising at 34,000 feet when its crew requested to climb to 36,000 feet.
Meanwhile, FedEx flight 10, operating from Memphis to Paris, was eastbound at 35,000 feet in the opposite direction.
According to the TSB report, the incident occurred when “the controller issued a CPDLC clearance direct to TOPPS, which inadvertently included a level change to FL360.” Recognizing the error immediately, the controller issued voice instructions to the United crew to maintain 34,000 feet and disregard the Controller-Pilot Data Link Communications clearance.
Despite these corrective instructions, approximately two minutes later, UA879 began climbing. Air traffic control again instructed the crew to descend back to FL340, emphasizing that the level change clearance had been canceled.
The United crew reported receiving a TCAS alert during their climb as they approached the FedEx 777’s altitude.
The TSB documentation indicates UA879 reached 500 feet above its cleared altitude before returning to its assigned level. This reduced the required 1,000-foot vertical separation in Reduced Vertical Separation Minimum airspace to only 500 feet between the two aircraft.
The safety system functioned as designed, with vertical separation quickly re-established within seconds. Both flights continued to their destinations without further incident.
The incident occurred approximately 160 nautical miles east-northeast of Gander, Newfoundland, in Canadian-controlled oceanic airspace.
The Canadian TSB classified this occurrence as a Class 5 incident, and “data on Class 5 occurrences are recorded in suitable scope for possible future safety analysis, statistical reporting, or archival purposes,” an agency spokesperson said.
