Pilots of a business jet obeyed an air traffic control instruction to descend, despite a contradictory collision-avoidance warning, aggravating a serious airprox incident in Swiss airspace.

As a result the Beechcraft Premier 1 faced a “high risk of collision” with an Airbus A319, says Swiss accident investigation authority BFU.

The incident is reminiscent of the fatal July 2002 mid-air collision – also in Swiss airspace – which occurred after a Tupolev Tu-154 crew followed air traffic control directions instead of a resolution advisory.

BFU says Geneva area control had cleared the Hahn Air Premier 1 to climb to 27,000ft after it departed Zurich on 10 June 2011.

Geneva had already cleared the Germanwings A319, inbound to Stuttgart on a virtually opposite heading, to descend to 25,000ft before handing the flight to Zurich area control.

But the A319’s cleared altitude had been entered into the Geneva system as 28,000ft, in line with an agreement between Geneva and Zurich area control which allowed handovers at this altitude without additional co-ordination.

The discrepancy went unnoticed by the Geneva sector controllers – which included a trainee being instructed by a coach – while information from Mode-S radar stations, which had detected the A319 crew’s selected altitude, was unavailable to Geneva and Zurich.

Controllers at both area centres identified the conflict as the A319 passed through 28,000ft, converging with the climbing business jet, and Geneva asked Zurich to stop the A319’s descent.

Within 12s a short-term conflict alert system activated in both centres and on-board collision-avoidance warnings sounded in both aircraft. Each crew immediately followed the resolution advisories, says BFU.

Controllers in both centres attempted to intervene by ordering each aircraft to make immediate left turns, but the Geneva controller handling the business jet – believing there was no time for consultation with other sectors – instructed it to descend to 26,000ft.

The Premier 1 crew confirmed the descent instruction. Despite the resolution advisory, active for 14s, ordering the jet to climb, the pilots broke off the climb and began to descend.

BFU says the resolution advisory remained valid for another 18s. The Premier 1 was descending as demanded by air traffic control, it states, while the collision-avoidance system was still instructing the pilots to climb at 1,500-2,000ft/min.

While the aircraft, at their crossing point, were separated by 1.3nm laterally and 75ft vertically, they had been just 0.6nm and 50ft apart at the closest point.

Although BFU highlights the worrying decision to prioritise an air traffic instruction over the collision-avoidance system, it points out that five air traffic controllers failed to notice the discrepancy between the A319’s cleared altitude and the clearance entered into the system, from which the conflict evolved. It adds that military exercises had led to a heavier workload and a “more difficult overview” for civil air traffic control.

Source: Cirium Dashboard