What happened
On June 15, 2000, at approximately 06:35 UTC, an air traffic incident occurred near the KLO VOR/DME involving two commercial aircraft. The first aircraft, a CL-65 operating as LDA138, was performing a scheduled flight from Geneva to Vienna. The second aircraft, a B735 operating as DLH3HD, was flying from Düsseldorf to Milan Malpensa.
Initially, the air traffic controller at the Zurich Area Control Centre authorized LDA138 to climb to FL280. However, due to the aircraft's low rate of climb, the controller later issued a reclearance to FL260, specifically requesting a climb rate of at least 1500ft/min. Although the crew of LDA138 acknowledged this instruction to descend/level at FL260, the aircraft continued climbing toward FL280.
At the same time, DLH3886 was cruising at FL270. As the two aircraft approached each other, the controller noticed LDA138 had already reached FL264, exceeding the authorized altitude. The aircraft eventually crossed paths with a horizontal separation of 1.4NM and a vertical separation of only 700ft.
The investigation
Investigators examined radar recordings and radio communications from the Zurich Area Control Centre. The investigation established that the controller had attempted to manage a potential conflict by reassigning LDA138 to a lower altitude to ensure separation from DLH3886. The investigation also reviewed the flight data of both aircraft and the responses of the flight crews during the encounter.
Findings
- The crew of LDA138 failed to comply with an ATC instruction regarding their assigned altitude.
- The pilot of LDA138 acknowledged the instruction to stay at FL260 but continued the climb to FL280.
- The rate of climb for LDA138 had fluctuated, dropping to 500ft/min after passing FL257.
- The crew of DLH3886 experienced a TCAS-RA (Resolution Advisory), which prompted a temporary climb to FL272.
- Visual contact was maintained between both aircraft, and the pilot of LDA138 also observed the other aircraft on TCAS.
- The separation was maintained only because of the combination of visual contact, TCAS intervention, and the reduced climb rate of the ascending aircraft.