What happened
On 30 August 2003, at approximately 08:27 UTC, an air traffic incident occurred near the KLO VOR, 6 NM southeast of Zurich. An EasyJet Switzerland Boeing 737, registration HB-IIT, was on its final approach to runway 14 when the crew initiated a go-around due to an unstable approach. Simultaneously, a Swiss International Air Lines Embraer E145, registration HB-JAG, was taking off from runway 16.
To prevent a potential collision, the aerodrome controller instructed the Boeing 737 to climb rapidly and directed the departing E145 to maintain its runway heading rather than following its standard departure route. However, the Boeing 737 failed to complete the instructed left turn, instead maintaining a heading of approximately 120 degrees. As both aircraft entered cloud cover, they closed laterally and vertically, resulting in a separation infringement where the aircraft reached a lateral distance of only 2 NM and zero altitude difference. The conflict was eventually resolved after the controller issued emergency heading changes to both aircraft.
The investigation
The investigation examined the actions of both flight crews and the air traffic controllers involved. It was established that the aerodrome controller had mistakenly interpreted a slight radar movement as confirmation that the Boeing 737 was complying with the left-turn instruction. The investigation also looked into the weather conditions and the decision-making processes regarding separation standards. It was found that while the tower logbook indicated instrument meteorological conditions (IMC) were in effect, the controllers had unilaterally decided to apply visual meteorological conditions (VMC) procedures for departures, believing the weather had improved, without informing the approach coordinator.
Findings
- The flight crew of the Boeing 737 did not execute the instructed left turn to heading 020°; the co-pilot mistakenly set a heading of 120° on the mode control panel, likely confusing the instruction with a 20-degree turn.
- High workload during the manual go-around procedure contributed to the crew's failure to perceive the specific heading instruction.
- The aerodrome controller's decision to revert to VMC procedures for departures was made without notifying the approach coordinator and was based on a subjective assessment of the weather.
- The E145 crew responded promptly to all ATC instructions, including an immediate right turn to avoid the encroaching Boeing 737.
- The separation infringement was a result of the Boeing 737's failure to comply with the assigned heading instruction.