Investigators focus on conflicting orders

Wreckage from the planes is being analysed

By Tom Symonds BBC Transport correspondent

Airline accidents are rarely the result of one error or equipment breakdown. The worst disasters are usually the result of a series of errors or problems.

This one is likely to be no different.

Investigators will have a lot of information to work through in their search for answers. The aircrafts' so-called black boxes will provide cockpit voice recordings and data from instruments on board.

There are two key questions to answer:

• Why were the planes warned so late they were on a collision course?

• Why did both planes dive, making a crash more likely?

The last-minute warning device available to pilots in their cockpit is known as TCAS - the Traffic Alert and Collision Avoidance System.

Using radio signals from other planes in the area, it decides whether any are too close for comfort.

	:	Mid-air collision
How the crash happened

If they are, first pilots are given an audible warning - "traffic, traffic". Then they are told to take avoiding action - to "climb" or "descend".

Nearby aircraft show up as diamonds on a display, although the position of the planes cannot be relied on as completely accurate.

TCAS can tell if another plane is descending, and if appropriate, warn the pilot to climb.

Russian Captain Alexander Gross took his plane into a fatal dive

In this case, both captains decided to make the same manoeuvre - losing height, with tragic consequences.

The latest evidence from the crash inquiry has revealed one of the two pilots - the Russian - was given conflicting advice by air traffic control and his onboard safety systems.

A Swiss controller realised what was about to happen and told the pilot to descend - it's thought the warning came less than a minute before the accident.

But at almost exactly the same time the Russian plane's TCAS, also registered that a collision was possible. A computerised voice advised the pilot to climb.

The pilot would have had almost no time to decide between the two courses of action. He may have delayed, making the crash more likely.

The actions of air traffic control is under intense scrutiny

He decided to obey air traffic control, and descend. It was probably the wrong decision.

The basic rule of the skies is that the pilot is captain of his own craft, and always makes the final decision about how to fly his plane.

But the Civil Aviation Authority in the UK says there's another rule - always follow the advice of TCAS, and then tell air traffic control what you're doing.

Other factors

There were other factors in this accident. A key device for warning controllers of a potential problem was not working.

The Short-Term Conflict Alert (STCA) system makes aircraft turn magenta or red on the radar screen if they are to close to one another.

Whatever happened the pilots would have had less than a minute to react

On the night of the accident, STCA at air traffic control was switched off for maintenance.

It has also been confirmed that one of the controllers on duty had taken a break.

Swiss air traffic control has said the pilot of the Russian jet was given between 50 seconds and a minute to take avoiding action.

Perhaps, given the problems in air traffic control, that was not enough. Whatever the pilots did it would have been important not to overreact to the situation.

If either had pushed his stick forward too hard, he could have lost several thousand feet in a matter of seconds, putting him in danger of colliding with other aircraft further below.

Inquiry questions

Apart from the problem of pilots being given conflicting information, changes to air traffic control procedures will be examined too.

Relatives desperately want questions over the crash answered

Europe has recently reduced the separation between aircraft from 600 metres (2,000 feet) to 300 (1,000 feet).

Whatever happened the pilots would have had less than a minute to react. In the darkness it is possible they never even saw each other.

There have been at least 12 mid-air collisions in the past 10 years.

But most involved small aircraft, military planes or cargo jets. This accident will be regarded as a rare tragedy.