What happened
On September 13, 2000, a complex air traffic incident occurred in the Swiss Upper Information Region near Trasadingen, involving four aircraft: THY 1944 (Airbus A310), BAG 4608 (Boeing B733), IBE 3514 (Airbus A320), and AZA 467 (Boeing MD80).
The incident unfolded as a chain reaction of automated avoidance maneuvers. Initially, the THY 1944 was climbing through FL 255 toward its assigned level of FL 260. During this climb, the aircraft maintained a high rate of ascent of approximately 3,000 FPM. This rapid climb triggered a TCAS Resolution Advisory (RA) "climb" on BAG 4608, which was cruising at FL 270. Following the RA, the crew of BAG 4608 climbed briefly to FL 272.
This maneuver triggered further complications for the remaining aircraft. The crew of IBE 3514, flying at FL 280, responded to the changing traffic situation with an extremely aggressive climb of up to 3,500 FPM, reaching FL 291. This rapid ascent caused a significant reduction in vertical separation with AZA 467, which was also performing a TCAS-RA climb from FL 290 to FL 295. At the height of the conflict, the vertical separation between IBE 3514 and AZA 467 was reduced to only 400 FT while the aircraft were on a converging course.
The investigation
Investigators examined the flight data and radio communications from the Zurich Radar Upper Sector. The investigation established that air traffic controllers had correctly issued all necessary altitude assignments to maintain vertical separation for the crossing maneuvers. All flight crews had correctly acknowledged these instructions.
Control communications revealed that while the pilots promptly informed controllers of the TCAS-RAs, they often did so only after the maneuvers had already been initiated or completed. The investigation also noted a frequency error by the IBE 3514 crew, who mistakenly contacted Zurich Radar Upper Sector 3 instead of Rhine Control due to nearly identical frequencies.
Findings
- The primary trigger for the chain reaction was the high rate of climb exhibited by THY 1944, which prompted a TCAS alert on BAG 4608 despite no immediate collision risk.
- The subsequent response by IBE 3514 was deemed excessively aggressive, leading to the loss of required separation from AZA 467.
- The incident was characterized as a chain reaction where the execution of TCAS-RAs by one aircraft directly induced further avoidance maneuvers in others.
Safety action
Following the investigation, the Federal Office of Civil Aviation (FOCA) published AIC 15/01 on July 19, 2001, which established new guidelines regarding climb and descent rates to prevent similar TCAS-induced conflicts.