Expert: Capt Paul - 11/28/2006

Question
hi again,
thank you for your answer, i understand now. Just wanted to add, that program i watched on discovery channel recently, they said that first they thought it was cause russians didnt listen, but after checking those recording machines from planes, they said it was a mistake of the person from the land, cause many things went wrong, and their computers down there were mostly off that night and were running with lower software on and also cause the person could not contact the DHL plane so had no idea they were following the instructions of TCAS and he sent russians the same way.
anyways, thanks for your answer

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The text above is a follow-up to ...

-----Question-----
hello Captain Paul Jeeves
Im curious about one thing. I watched a program about an airplane crash that happened a few years ago, when a russian plane with kids crashed into a DHL plane, what im curious about is this. They said every plane has a thing, dont know what its called, that allerts when there is another plane going right that way, your way. So in both planes this thing would turn on and alarm. And as they told there this thing would tell to go up or down. So im curious, how can this robot prevent a crash? I mean... what if in both planes it will tell to go up? or tell to go down for both? or are they connected somehow and always will tell different?
Thanks

-----Answer-----
Hi Lena,
The thing that you are talking about is called "TCAS".  Basically, it's a Traffic Alerting and Collision Avoidance" machine.  

When two or more aircraft are within certain boundaries of each other, the TCAS systems talk to one and other via electronic signals.

Typically, when the distance between them is quite large, they basically just say "hello" to each other and watch each other go their separate ways.

However, when they get to close (Close is defined in a number of different ways), they quickly work out a way for each other to avoid themselves.

Once they've done that, they send commands to the pilots instructing them which way to fly the aircraft.  Because the TCAS units have already devised an avoidance plan, all the pilots have to do is follow the given instructions to avoid the other aircraft.

In accident that you mention, the problem was that the Russian Crew did not follow the instructions of the TCAS system.  In fact, they went the opposite direction.  Because the Canadian Pilots in the DHL aircraft did what they were instructed to do, and the Russian aircraft went the wrong way, a collision resulted.

Typically, one system will decide to go up, while the other goes down or stays at the same level.  

Or, it could tell both systems to simply level off and maintain the altitude that they are currently at.

Possible, one system could immediately and rapidly descend, while the other rapidly climbs.  

There are any number of possibilities, but the system will NEVER tell the involved aircrafts to do something that WILL result in a collision.

I hope this answers your question.

Cheers

Paul

Answer
The accident was the result of an great number of failures within the ATC and other systems.  

However, TCAS was designed to be the final level of protection against failures within those systems.  

Because the Russians did not follow the TCAS instructions they took away the final chance to avoid the accident, regardless of the contrary instructions from ATC.

Cheers