What happened
On July 16, 2002, at approximately 19:39 UTC, a near-collision occurred northwest of the Zurich East (ZUE) radio range involving an Air France A320, registration F-GFKM, and a Lufthansa City Line CRJ1, registration D-ACLW.
The Air France aircraft was en route from Paris to Munich and had received clearance from Zurich Upper 2/3 Sector air traffic control to descend to flight level (FL) 310. However, instead of descending, the aircraft began an unintended climb, reaching at least FL 334. Simultaneously, the Lufthansa aircraft was climbing through FL 340.
The crew of the F-GFKM only realized the error when their airborne collision avoidance system (ACAS) issued a traffic advisory (TA). The pilot flying on the Air France flight immediately deactivated the autopilot and initiated a manual descent to avoid the approaching traffic. The Lufthansa crew also received an ACAS TA, followed by a resolution advisory (RA) instructing them to climb. The Lufthansa pilots chose not to follow the RA climb instruction because they had established visual contact with the Air and feared losing that contact; they believed the Air France aircraft would pass behind them.
Radar data indicated that the two aircraft came within approximately 0.6 NM of each other with a vertical separation of only 600 feet, representing a significant violation of separation rules.
The investigation
The investigation focused on the flight crew's execution of the descent and the air traffic control environment. Investigators examined the flight control unit (FCU) inputs on the A320. It was determined that while the first officer had correctly entered the target altitude of FL 310, they had mistakenly selected a climb instead of a descent by turning the vertical speed selector knob in the wrong direction (clockwise instead of counter-clockwise).
Furthermore, the investigation noted that the first officer was distracted by searching for the next sector's name on a navigation chart and failed to monitor the flight mode annunciator (FMA) to verify the descent had been properly initiated. The investigation also noted a brief moment where the air traffic controller misidentified a different Lufthansa aircraft (DLH5808) as the aircraft in question, though this was not a contributing factor to the near-collision.
Findings
- The primary cause was the erroneous initiation of a climb by the Air France crew due to an incorrect manual input on the vertical speed selector.
- The crew failed to perform reciprocal monitoring of the flight mode annunciator, meaning the unintended climb went unnoticed by both pilots until the ACAS alert.
- The use of vertical speed mode instead of a managed descent mode increased the risk of such an input error.
- The Lufthansa crew's decision to prioritize visual contact over the ACAS resolution advisory contributed to the close proximity of the two aircraft.