What happened
On 01 January 2017, an Egypt Air Cargo Airbus A300B4-622R(F) and an Air France Airbus A320-214 experienced a near mid-air collision (MAC) while operating in the vicinity of the Belgian border. The cargo flight, MSX541, was climbing through FL210 after departing from EBOS, while the Air France passenger flight, AFR640F, was maintaining FL220.
As the two aircraft approached one another, the Traffic Collision Avoidance System (TCAS) triggered Traffic Advisories (TA) followed by Resolution Advisories (RA) in both cockpits. While the Air France crew responded to a climb instruction, the crew of the Egypt Air aircraft misperceived a "Level Off" instruction as a command to climb. Consequently, the captain of MSX541 took manual control and increased the climb rate to 3500 ft/min. This maneuver brought the aircraft within 427 feet of vertical separation and 0.69 NM of horizontal separation at the closest point of approach.
The investigation
The AAIU(Be) examined flight data, radar records, and communications from both operators, Belgocontrol, and Eurocontrol. The investigation focused on the discrepancy between the TCAS downlink message, which indicated a "Level Off" (LVL) command, and the actual flight path of the cargo aircraft. Eurocontrol performed "What-if" simulations to determine if timely responses to the initial RA would have prevented the loss of separation.
Findings
- The Egypt Air cargo aircraft was climbing at a higher rate than anticipated due to a light payload.
- The crew of MSX541 misinterpreted the TCAS "Level Off" instruction as a command to climb.
- The closest point of approach occurred at 11:47:11 UTC, with a vertical separation of only 427 feet.
Safety action
- The investigation noted that misinterpretations of TCAS instructions have occurred historically, particularly when older "Adjust Vertical Speed" messages were used.
- While the TCAS system has since been updated to use the clearer "Level Off" command, the investigation highlighted that because RA events are rare, crews may lack the instinctive response required. Some airlines have addressed this by incorporating TCAS event training into flight simulator exercises.