Incident
On 17 November 2003 at 04:49 UTC, a serious incident (airprox) occurred in the Zurich Terminal Control Area, approximately 10 nautical miles west-northwest of the Kloten DVOR/DME, involving three aircraft: Polar Air Cargo flight 529 (PAC529), a Boeing 747-259B (registration N924FT); Swiss International Air Lines flight 289 (SWR289), a McDonnell Douglas MD-11 (registration HB-IWO); and Swiss International Air Lines flight 2591 (SWR2591), an Airbus A319 (registration HB-IPV). All three flights were destined for Zurich Airport.
Sequence of Events
PAC529 was a cargo flight from Liège, Belgium, to Zurich, continuing to Delhi. SWR289 was arriving from Johannesburg, South Africa, and SWR2591 from Benghazi, Libya. All three were instructed by Zurich Area Control Centre (ACC) to hold at the EKRIT waypoint due to a curfew preventing landings before 05:00 UTC. PAC529 was cleared to flight level (FL) 140, SWR289 to FL130, and SWR2591 to FL150.
At 04:47:45, the crew of SWR289 reported a Traffic Advisory (TA) from their Traffic Collision Avoidance System (TCAS). The approach controller (APW) responded that the aircraft above (PAC529) was cleared to FL140. The SWR289 crew noted the other aircraft was descending and only 700 ft above them.
Shortly after, at 04:48:44, the controller informed SWR2591 of traffic below at FL144, cleared to maintain FL140 in the holding. SWR2591 reported a TCAS Resolution Advisory (RA) but stated they did not execute an evasive climb and remained at FL150; their TCAS indicated only 500 ft vertical separation from the B747.
Radar recordings showed that PAC529, after overflying waypoint GOLKE, did not turn left toward EKRIT as expected. Instead, it continued on a southeast course, then turned left to a northeast course about 3.7 NM south of EKRIT, and later turned right to enter the holding. During this phase, the autopilot was disengaged. PAC529 had reached FL140 but then descended to FL138. The controller queried the PAC529 crew about their altitude; they affirmed FL140, but the controller reported the radar showed FL144. The controller urged PAC529 to maintain FL140 precisely due to traffic above and below.
The minimum separation between PAC529 and SWR2591 was reduced to 600 ft vertically when PAC529 crossed SWR2591's flight path while in a right turn. The PAC529 crew took nearly half a minute to stabilize at FL140 after the RA.
Weather
According to MeteoSwiss ATIS information ALPHA, the weather at Zurich at 04:20 UTC was: wind 220° at 6 kt, visibility 20 km, rain, few clouds at 1300 ft, scattered at 2400 ft, overcast at 4800 ft, temperature 5°C, dew point 4°C, QNH 1023 hPa.
Findings
The investigation did not assign a probable cause. The report noted that the PAC529 crew felt overloaded due to the late holding instruction, and that a possible trim condition could have contributed to the altitude deviation, but no explicit cause was stated.