What happened
On June 6, 2010, at 19:46 UTC, a high-risk airprox incident occurred in Swiss airspace, approximately 22 NM east of Zurich Airport. An Airbus A319-112, operated by Hamburg International with registration D-AHIO, was descending toward Friedrichshafen. Simultaneously, an AVRO 146-RJ100, registered as HB-IXX and operated by Swiss International Air Lines, was climbing out of Zurich toward Hannover.
The crew of D-AHIO had received clearance to descend to flight level 130. However, during the descent, the aircraft's flight control unit was incorrectly set to flight level 120. This error caused the aircraft to descend below its assigned altitude and enter the same flight level as the climbing HB-IXX. The two aircraft reached a minimum horizontal separation of 1.4 NM and a vertical separation of only 200 ft, representing a high collision risk.
The investigation
The investigation focused on the sequence of events leading to the loss of separation. Investigators examined radar data, radio communications, and statements from both flight crews and the air traffic controller. The investigation established that while the controller had coordinated a descent to FL 120 with the Hamburg International crew, the crew of D-AHIO mistakenly believed they had been cleared to FL 120, which led them to report their incorrect altitude as if it were intentional.
Findings
- The primary cause of the incident was that the crew of D-AHIO set flight level 120 on the flight control unit instead of the authorized flight level 130 and failed to notice the error.
- A lack of effective cross-checking between the pilot flying and the pilot not flying on D-AHIO allowed the incorrect altitude setting to persist.
- The air traffic controller failed to intervene when the crew of D-AHIO explicitly reported they were descending to FL 120, likely because the controller was under the impression that FL 130 had been coordinated.
- The risk of misunderstanding was increased because the crew of D-AHIO was not wearing headsets, a practice permitted by their company regulations only below FL 100, which was inappropriate for the high-traffic environment at the time.