What happened
On November 21, 2001, at approximately 05:01 UTC, an air traffic incident occurred near the EKRIT holding point in Swiss airspace. A South African Airways Boeing 747-300, registration ZS-SAT, was operating a scheduled flight from Johannesburg to Zurich. While in the EKRIT holding pattern, the crew received instructions from Zurich Arrival West Sector to fly a heading of 050°, descend to flight level 100, and increase speed from 180 knots to 210 knots for an approach to runway 16.
Simultaneously, a Swissair Boeing (Douglas) MD-11, registration HB-IWQ, was descending from flight level 180 to 140 on a flight from Lagos to Zurich. As the Boeing 747-300 executed the requested changes, the aircraft experienced an uncommanded slight climb, reaching flight level 134. This deviation brought the two aircraft into close proximity. The crew of the HB-IWQ reported a TCAS climb, and the aircraft eventually passed within 0.3 NM horizontally and 600 ft vertically of the ZS-SAT.
The investigation
The investigation focused on the flight management of the ZS-SAT and the interaction between the crew and the automated systems. Investigators examined the settings on the Honeywell Triple Channel autopilot used by the Boeing 747-300. It was noted that the crew performed several complex inputs—including heading, altitude, vertical speed, and speed changes—within a very short timeframe.
Air traffic controllers also reviewed the radar data, which showed a Short Term Conflict Alert (STCA) triggered on the controller's monitor. The investigation confirmed that while the aircraft were in visual meteorological conditions and the pilots had visual contact with each other, the vertical separation was compromised due to the unexpected climb of the ZS-SAT.