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Wednesday, 3 July, 2002, 08:51 GMT 09:51 UK
Q&A: Mid-air crash
Investigators are trying to find out how two planes collided over Germany in clear skies. Air safety expert Chris Yates of Jane's Aviation gives answers to some of the key questions they will be looking at.

Why might the Russian crew have failed to respond to the first instruction to descend?

There are several possibilities. There could have been some form of communication failure on board the aircraft, for example a radio operating intermittently.

Or the pilot may have been talking to another air traffic control unit or his operational headquarters at the time that Swiss air traffic control was trying to contact him to take evasive action.

Why was the DHL plane not advised to change course?

We will have to wait until the investigation is completed before we are certain what advice was given. But with the evidence that exists at the moment, it could be that the controllers were attempting to alert the Russian aircraft first, it being on an incorrect flight level, or heading towards an incorrect flight level.

Eventually the DHL aircraft took evasive action when warned by air traffic control and/or its on-board alerting system - but the warnings came seconds too late.

If one plane does fail to respond, how quickly do air traffic control warn the other plane?

In theory it should be a quick process. If the primary aircraft cannot be contacted then the other aircraft will be alerted as promptly as possible. Perhaps there was a degree of delay of doing that in this instance.

Are air traffic controllers implicated in the crash?

I think it is far too early to speculate one way or the other. We will have to wait for the final analysis of the air accident investigators before we can point any finger of blame on individuals.

What is the quality of Swiss Air Traffic Control?

It is exceptionally good, as are all European air traffic control operators. It was a state-run operation with a shift towards privatisation. I doubt that this has had an effect on this particular incident.

There is one thing to add here. During daylight hours, a country's air space is divided into sectors with a given number of controllers controlling the sectors and a certain number of aircraft. But during the night hours as the traffic levels diminish sectors are often grouped together with one or two controllers operating them.

A possibility, however vague, exists that the attention of a controller was taken to something happening elsewhere in his area of control and he did not notice what was going on until it was too late when he had to start making contact with the aircraft himself.

What devices do planes have to warn of mid-air collisions?

The technology involved here is a system called Traffic Alert Avoidance System (TCAS) and is a mandatory requirement for aircraft in most countries around the world.

However, TCAS is being fitted retrospectively in certain aircraft fleets. It is uncertain whether TCAS was on board the Russian plane but there is evidence to show that it was available technology on board the DHL airplane.

Could the different air space sectors be a factor?

I think it is one of the factors that will be given some close consideration in the air accident investigation; the fact that the two aircraft were coming close, if not on to, the handover between two adjacent flight information regions.

As often happens at such handovers, pilots are instructed to switch to a new radio frequency to contact the next radio controller. In that switch over there can be a degree of confusion.

The pilot may not take down the frequency correctly and will call back on the previous frequency to ask who should be contacting them and what the right numbers are. So there can be confusion that creeps in at that moment in time.

How quickly can pilots react to emergency situations when in automatic pilot?

It is fairly easy to disengage the auto pilot. It depends on the configuration of the cockpit. Not all cockpits and control systems are the same but typically on the control stick there is a release button which fairly instantaneously disengages the auto pilot system and allows the pilot to take back control of the aircraft from the computers on board.

Could language problems have been a factor?

It is a possibility. English is the language of air traffic control but it has to be said that some pilots have a better command of English than others. This has been a problem that has exhibited itself in instances around the world over the years.

Another possible scenario exits. These two aircraft were at the boundary of two flight information regions, one for Germany and the other for Switzerland. It is possible that confusion could have arisen because one or other aircraft was in a handover between one control sector and another.

Thank you for e-mailing us your questions about the German air disaster. We are getting the answers and will publish a selection here.

Disclaimer: The BBC will use as many of your questions as possible but we cannot guarantee that all e-mails will be published. The BBC reserves the right to edit comments that are published.

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26 May 02 | In Depth
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