What might have gone wrong?

The planes crashed in flames

The investigation into the chain of events behind mid-air collision over southern Germany has increasingly focused on the Swiss air traffic control agency Skyguide.

Intially Skyguide blamed the Russian crew of one of the two aircraft for ignoring warnings to dive.

But since then new important information has come to light:

• The pilot of the Russian Tu-154 was given conflicting instructions by air traffic control and his onboard computer
• The Russian pilot was given only 44 seconds warning
• A warning system at the control centre was switched off for maintenance
• Only one controller was on duty at the time
• The centre's radar system does not meet EU standards

Conflicting instructions: Cockpit voice recorders have revealed that air traffic controllers told the Russian airliner to dive, contradicting a warning given by the plane's onboard anti-collision system to climb.

The Traffic Alert and Collision (TCAS) system fitted to both aircraft should have directed them away from one another by advising one to descend and the other to climb.

	:	Mid-air collision
How the crash happened

European aviation authorities say pilots are trained to follow TCAS instructions at all times, even if they contradict air traffic control.

But there are no internationally established regulations. Russian officials say that their pilots are trained to follow TCAS, but to also take air traffic control instructions into account.

In this case it appears that the Tu-154 pilot followed the warning from the ground, while the Boeing 757 pilot followed the TCAS instructions to descend.

Warning time: The collision occurred at a standard crossing point between two air corridors, which the two planes approached at right angles, both of them at 36,000 feet.

Normally one would be instructed to descend to pass across the junction. In this case it appears the instruction was given unusually late.

Tragically, within seconds of the Russian pilot heeding an instruction from Skyguide to dive, a collision avoidance computer on the other aircraft, a DHL cargo plane, suddenly told its pilot to do the same.

In other words, verbal instructions from air traffic controllers to the Russian crew, and computer instructions on the 757, put the planes on a deadly new collision course.

Skyguide originally said it had warned the Russian pilot 90 seconds before impact, and that he had only responded to the third request to dive. Later it withdrew this version of events, acknowledging that he responded to the second request, which came 30 seconds before the crash.

German investigators say 90 seconds warning would have been needed to avert the crash.

No alarm: Skyguide has acknowledged that a system, which should have warned ground control that the two planes were on a collision course, was out of action for routine maintenance at the time of the collision.

Swiss officials say the controllers were told that the system was out of service, and would not warn them as usual of planes on a collision course.

Only one controller: At the time of the disaster, only one air traffic controller was at his desk, as another had gone on a break.

The flight recorders will reveal cockpit conversations

There have been conflicting reports as to whether this violated normal procedures for periods when the warning system was down.

The company says there were only five planes crossing Swiss airspace at that time and that one controller would be sufficient to handle them.

Radar: A report published just last week by the Swiss office for aviation safety has raised questions about the centre's radar system.

According to the report, which was commissioned after three near misses in Swiss airspace, the accuracy of pictures produced by the system is below EU standards.

It raises particular concerns about the near miss registered on 15 June 1998, when pictures on radar from control centres in both Geneva and Zurich showed the same plane in different positions.

Other issues that could be crucial to the inquiry concern:

• The vertical distance that should have divided the planes: Under a new system of Reduced Vertical Separation Minimums (RVSM) planes can now fly only 1,000 feet apart, rather than 2,000 feet, which was the minimum in the past. This should mean that pilots do not need so much time to descend or ascend in order to avoid collision.
• The handover of the flights from different national air traffic control centres: The Russian plane was handed over from German traffic controllers only five minutes before the crash, while the DHL plane had been handed over by Italian controllers just a few minutes before that. If the handover does turn out to be a factor in the crash, it will strengthen the argument for Europe's proposed Single Skies system, which would replace individual countries' airspace with an entire European Union zone.
• The need for pilots to change radio frequencies as they travel between national airspaces: Investigators will want to be sure that the Russian plane was listening to the right air traffic control frequency when controllers first tried to make contact.

Whatever the final conclusions, it is likely that a sequence of events - rather than a single catastrophic mistake - was to blame for the tragedy.