Switzerland’s air traffic control company on Wednesday admitted its automatic collision warning system was turned off for maintenance on Monday when a Boeing 757 and a Russian Tupolev 154 slammed into one another and exploded in a fireball above Lake Constance.
The revelation raised fresh concerns about the role of controllers from Skyguide, the private company that provides Swiss air traffic control, in the moments before the accident.
Russian officials have claimed ground staff in Zurich did not react fast enough to prevent the DHL cargo carrier and the Bashkirian Airlines passenger jet crashing at more than 10 500m with the loss of 71 lives.
Five hundred rescue workers and investigators searched on Wednesday to gather debris, baggage and body parts scattered across the countryside by the force of the impact. The remains of the planes were transported to a disused hangar in Friedrichshafen where aviation experts started to reconstruct them.
The sole Swiss air traffic controller on duty at the time of the accident, who told the Russian pilot to dive a mere 50 seconds before the impact, was reportedly too traumatised to talk to investigators on Wednesday.
In Zurich, Roger Gaberelle, a Skyguide official, confirmed that “the [automatic collision warning] system was not working at the time”.
However, he denied that the two controllers on duty that night — one of whom was on a break at the time of the incident — acted irresponsibly.
The company insisted one air traffic controller was adequate to supervise the five planes crossing Swiss airspace at that time and denied that their behaviour breached regulations. Maintenance staff had turned off the short-term collision alert (STCA) system for routine work.
“It is not against regulations to take a break at night when the traffic is light, regardless of whether the STCA system is off and the other controller is in agreement,” another Skyguide official, Philipp Seiler, said.
Compounding problems for the Swiss, details emerged on Wednesday of a report that described Skyguide’s radar system as falling below European safety standards. The study, by the Swiss aircraft accident investigation bureau, was based on three near-misses between 1998 and 2000. It said that discrepancies of several seconds were possible between Swiss radar readings and those of neighbouring countries. Such timing could put the location of a plane on the radar screens out by up to 500m.
But Skyguide, which became a limited company owned by the Swiss government in 1998, attempted again to shift blame to the Russian Tupolev 154, claiming the plane did not have a working traffic collision avoidance system (TCAS) as required by international law.
“Accident investigators need to look into why it took three warnings before the pilot of the Tupolev responded and why the Tupolev’s [collision avoidance system] wasn’t responding,” said Seiler.
Meanwhile local politicians warned that congestion in the skies in the area, where at least three air traffic control zones converge at the crossroads of Europe’s airspace, had created a disaster waiting to happen. Reductions in vertical separation distances between planes, others suggested, illustrated the commercial pressures on air safety.
Moscow angrily dismissed allegations that Russian pilot error — specifically delays in acknowledging requests from Swiss air traffic control to take evasive action — caused the collision. Stressing the experience of the Bashkirian Airlines crew and the airworthiness of their Tupolev 154, officials pointed the finger at Swiss air traffic control.
The Russians said Swiss controllers had radioed their first warning for the passenger plane to dive 300m under the path of the approaching Boeing 757 only 50 seconds before the two aircraft exploded in a fireball. “The human factor is to blame,” Russia’s Deputy Transport Minister Alexander Neradko said. “And it is influenced by several factors, including those on the ground.”
Earlier Anton Maag, chief of the Zurich airport control tower, revealed the Russian captain had been told three times to dive by an increasingly frantic Swiss controller but the pilot responded only in the final moments — about 25 seconds before impact.
At the moment the Russian jet began its descent, the captain of the Boeing reported his automatic traffic TCAS was commanding him to perform an emergency dive. He followed the instruction.
The planes slammed into each other seconds later at 10 590m, killing everyone on both aircraft. If the DHL cargo carrier had held its height at 10 800m it would have passed harmlessly over the passenger plane.
Additional reporting by Kate Connolly, Andrew Clark, Nick Paton Walsh, Ian Traynor and agencies