Synopsis
On 1 January 2017 at 11:47 UTC, an airprox (near mid-air collision) occurred between two commercial aircraft near Ghent, Belgium. The aircraft involved were an Airbus A320-214 operating Air France flight AFR640F from Paris Charles de Gaulle to Amsterdam Schiphol, and an Airbus A300B4-622R(F) operating Egypt Air Cargo flight MSX541 from Ostend-Bruges to Cairo. Both flights were under air traffic control surveillance.
Sequence of Events
After takeoff from Ostend, MSX541 was climbing to flight level 210 at a vertical rate of about 2500 ft/min. AFR640F was maintaining FL220. ATC instructed AFR640F to fly towards Hamstede before reaching the FERDI waypoint to expedite crossing. ATC intended for MSX541 to pass behind AFR640F, but MSX541 was climbing faster than usual due to a light load.
At 11:45 UTC, MSX541 received traffic information about AFR640F crossing from right to left above. MSX541 correctly read back maintaining FL210. Immediately after, AFR640F was advised about the climbing Egypt Air aircraft.
As the two aircraft approached each other, their Traffic Collision Avoidance Systems triggered alerts. A Traffic Advisory (TA) occurred at 11:46:33, but no visible change in MSX541's vertical speed followed. A Resolution Advisory (RA) was then generated. The downlink indicated a Level Off RA for MSX541 between 11:46:39 and 11:46:42. However, the MSX541 crew later stated they understood a "CLIMB" instruction. The captain took control from the autopilot and increased the rate of climb to approximately 3500 ft/min instead of levelling off.
On AFR640F, TCAS generated a CLIMB RA at 11:46:57, and the aircraft started climbing at 11:47:02 at 1500 ft/min as required. When MSX541 passed FL213, TCAS issued a "DESCEND" instruction, which was followed immediately by a reduction in climb rate (but not a descent). Both aircraft received Clear of Conflict messages at 11:47:16. The closest point of approach occurred at 11:47:11 with a separation of 0.69 nautical miles horizontally and 427 feet vertically. The tracks crossed at 11:47:06 with AFR640F passing right to left 1.2 NM ahead of MSX541 and 522 feet below, still climbing.
TCAS Modifications and Training
The investigation highlighted a safety issue: misinterpretation of TCAS RA instructions. Previously, the instruction to stop climbing was announced as "ADJUST VERTICAL SPEED," which was often misunderstood. The TCAS system was subsequently modified to use a clearer "LEVEL OFF" announcement. Both aircraft were equipped with TCAS II version 7.1, and all crew members had received training on the updated equipment. However, TCAS RA events are infrequent, and the investigation noted that some airlines, including most Belgian carriers, include TCAS events in flight simulator exercises.
Investigation Progress
The Air Accident Investigation Unit (Belgium) gathered operational and engineering information from Air France (via BEA), Egyptair (via Central Directorate of Aircraft Accident Investigation), Belgocontrol, and Eurocontrol. Eurocontrol performed "what-if" simulations using inCAS software:
- Scenario A: If MSX541 had responded timely to the Level Off RA (nominal 5-second delay), it would have levelled off at FL207, and no RA would have been issued for AFR640F.
- Scenario B: With a 10-second delay, MSX541 would have levelled off at FL209.
- Scenario C: If AFR640 had responded promptly to the Climb RA, vertical spacing at CPA would have increased by 71 feet to 497 feet.
- Scenario D: If AFR640 had not responded, vertical spacing would have reduced by 211 feet to 215 feet.
The final report is to be circulated among relevant authorities and stakeholders as per international standards.
Probable Cause
The interim statement does not identify a single probable cause; it notes a safety issue regarding misinterpretation of TCAS RA instructions.