through a pilot making a mistake. but remember, there is avionics on board, particularly one called collision avoidance assistance. reporter: the faa says it has taken steps to prevent any similar occurrences in the future but they didn t say what those steps were. these air traffic controllers are trained in real-time. we do know an instructor was there at the moment this incident happened ready to step in, if needed. but the trainee realized the mistake immediately and corrected the problem immediately. we do know that the flights continued safely to their destinations. still, when you hear this sort of thing, it s very disturbing. it s horrifying. four in one month. is there any repurchaercussions? is. any accountability behind the scenes? there was a controller caught sleeping and that controller in
they re unacceptable. they re worked on. but there is a backup system and people are safe. reporter: this is what can happen when planes get too close. in 1991, this usair 737 at l.a.x. landed on top of a skywest commuter plane preparing for takeoff. 34 people died. accidents like this are incredibly rare because of collision avoidance systems now mandated in planes. these mistakes can occur through a controller making an error, through a pilot making a mistake. but remember, there is avionics on board, particularly one called tcas, which is collision avoidance assistance. reporter: the faa says it s taken steps to prevent any similar occurrences in the future, but they didn t say what those steps were. well, we know that air traffic controllers, they re usually trained in realtime. now according to the faa, there was an instructor who was