Comments by "Axxe" (@Axxe80) on "'It was like a rollercoaster': Passenger describes moment after planes nearly collide midair" video.

  1. The devices in both planes "scan" for a counterpart. Upon detection the distance and course can be calculated. If both planes are in danger of a collision the devices exchange data so that one of them orders the pilots to climb and the other its pilots to sink. So the short answer to your question: Both planes "talk" to each other so that they don't give the same order.
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