What happened
On the morning of November 17, 2003, a complex airprox incident occurred within the Zurich Terminal Control Area involving three aircraft: a Polar Air Cargo B747-200 (N924FT), a Swiss International Air Lines MD11 (HB-IWO), and a Swiss International Air and Lines A319 (HB-IPV).
All three aircraft were arriving at Zurich Airport and were instructed to enter the EKRIT holding pattern. The B747-200 was cleared to maintain flight level 140. However, during its approach, the aircraft deviated from its assigned altitude and path. The crew of the MD11 first reported a Traffic Advisory (TA) from their TCAS, noting an aircraft descending near them. Shortly thereafter, the crew of the A319 reported a TCAS Resolution Advisory (RA) after the B747-200 descended through its assigned level, reducing the vertical separation to approximately 500 feet.
The investigation
The investigation examined the flight paths and cockpit procedures of all involved aircraft. Radar data confirmed that the B747-200 did not follow the prescribed arrival route, bypassing the GOLKE waypoint before turning toward the holding pattern. The investigation also looked into the timing of air traffic control instructions, noting that the instruction to enter the EKRIT hold was issued to the cargo crew only minutes before they reached the waypoint.
Findings
- The primary cause of the separation loss was the altitude deviation of the B747-200, which descended as low as flight level 138 despite being cleared to flight level 140.
- The crew of the B747-200 may have experienced a temporary workload increase due to a late arrival of instructions from air traffic control and a deviation from the standard arrival route.
- The investigation noted that the B747-200 crew may have been managing a flight control imbalance or autopilot settings that contributed to the sudden descent when the autopilot was disengaged.
- The A319 experienced a TCAS RA because the vertical separation with the descending B747-200 had dropped to 500 feet.