What happened
On August 23, 2000, at approximately 08:58 UTC, an airprox incident occurred near the Saffa intersection, roughly 18 NM east of the location. An Airbus A320, operating as SWR213 for SWISSAIR, was on a scheduled flight from Vienna to Zurich. During its descent, the crew received instructions from the Zurich Area Control Centre (ACC) to descend to flight level 140. However, the crew incorrectly read back a descent to flight level 120. The air traffic controller failed to notice this erroneous readback.
Simultaneously, an ATR42-500, operating as SWR3003 for AIR LITTORAL, was flying from Innsbruck to Zurich. The controller subsequently cleared SWR3003 to descend to flight level 120. Because the SWR213 crew believed they were authorized to descend to the lower altitude, the two aircraft converged. The aircraft eventually reached a vertical separation of only 700 feet and a horizontal distance of approximately 0.5 NM. The situation was resolved when the Traffic Collision Avoidance System (TCAS) on the Airbus A320 issued a Resolution Advisory (RA), commanding the pilot to climb.
The investigation
Investigators examined the communications between the Zurich Radar East Sector and both aircraft. The investigation established that the SWR213 crew had initially maintained visual contact with the slower SWR3003, but as the faster aircraft overtook the slower one, they relied on TCAS. The investigation also looked into the working environment of the air traffic controller, noting the presence of a group of visitors in the radar sector.
Findings
- The primary cause of the incident was a misunderstanding by the SWR213 crew regarding their authorized flight level, compounded by the controller's failure to correct an incorrect readback.
- The controller's failure to detect the error may have been contributed to by an increased noise level at the workstation caused by a group of 4-5 visitors present in the radar sector.
- The TCAS RA on the Airbus A320 was the critical factor that prevented a collision, as the crew immediately followed the command to climb.
Safety action
To prevent similar communication errors caused by environmental noise, the investigation recommended the introduction of headsets for air traffic controllers at their workstations.