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In the 2014 film The Amazing Spider-Man 2, there's a sequence in which a villain disables the city's power grid. All over town, stuff loses power... including the local airport and its air traffic controllers. This is a problem because there are currently two planes inbound, on courses that will cause them to collide with each other in midair unless the tower can warn them in time!

Luckily, Spider-Man finds a way to defeat the villain and the ATC gets their power back just in time to radio the planes to change course, and they miss each other by an extremely narrow margin.

Putting aside for the moment the question of whether or not a real ATC would have their own independent backup power source, how realistic are the two premises of this crisis?

1) That two airplanes would be inbound towards a major airport on courses that would cause them to both occupy the same position in 3D space at the same time, and

2) That neither pilot would actually notice this until either the tower told them or something suddenly goes CRUNCH! Do commercial jet liners not have their own radar at the very least? (Heck, my car has little radar emitters that warn me if something is too close!)

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    $\begingroup$ only one word: TCAS $\endgroup$ – Federico Mar 24 '15 at 11:18
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    $\begingroup$ related: What happens when the airport shuts down $\endgroup$ – ratchet freak Mar 24 '15 at 11:32
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    $\begingroup$ If only one person in the whole airport had thought to buy a handheld radio! $217 on Amazon. Shoulda, coulda, woulda.... $\endgroup$ – A E Mar 24 '15 at 16:00
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    $\begingroup$ @AE You mean like the ones they have on charging racks in the tower for just that eventuality? $\endgroup$ – Simon Mar 24 '15 at 20:44
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    $\begingroup$ @AE Or they could run out onto the ramp and climb aboard any airplane... $\endgroup$ – Harper - Reinstate Monica Nov 24 at 6:26
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This depends on the aircraft type, the flight rules and weather conditions as contributing factors.

Larger aircraft or medium sized aircraft operating under general aviation or commercial aviation will in many cases be equipped with a technology called Traffic Collision Avoidance System (TCAS) (See: How does TCAS work?), which can instruct both aircraft to take evasive action if a collision is imminent. This is especially important in Instrument Meteorological Conditions, where pilots do not rely on outside visibility, but on instrument flight only operating under Instrument Flight Rules.

In Visual Meteorological Conditions, aircraft would likely be able to see each other and take evasive manueouvers without ATC or TCAS having to warn them, e.g. this is the case for smaller aircraft operating under Visual Flight Rules.

Aircraft receive clearances and instructions to occupy a certain flight level or altitude, sometimes coupled with clearances for approaches or arrival/departure procedures. If pilots do not receive further instructions, they will start to hold over their last clearance limit and commence lost radio/comms procedures, which include holding for a given amount of time and then beginning the descent towards the airport on defined and published approaches. If the power outage is long enough, the airspace is quite busy and all other frequencies are also unavailable, one of the being the distress or guard frequency, there is a probability that two aircraft on the same level will reach their clearance limit at some point and enter a hold at the same flight level, which could result in a collision.

The scenario however assumes that really all failsafes and contingency procedures have failed.

See also ths related question: What happens when an airport shuts down?

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It can happen. The last line of defense is "See and Avoid".

Associated Press: September 04, 1987

WASHINGTON — The Federal Aviation Administration said today that it is working on new navigation procedures for ocean flights in the wake of a July incident in which a Delta Air Lines jetliner came within 30 feet of colliding with another jet.

Investigators say the Delta airliner, flying 60 miles off course, came much closer to colliding with another jet than was previously thought.

The Delta plane was within Canadian air traffic control when the near-collision occurred, and the Canadian Aviation Safety Board said Thursday that it wants immediate changes in that nation's airline safety rules to reduce the chance of additional incidents.

However, the FAA said today that it already has a program under way to improve navigation procedures used by American aircraft on transatlantic and transpacific routes.

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ATC facilities all have backup power generators.

And aircraft have TCAS.

And if one ATC facility goes down, others are out there to assume control.

Chicago Center went down several months ago due to sabotage (a fire in a closet with much of the network/switching equipment), and while it wasn't an efficient operation for some time, the other layers of ATC picked up the load and things kept working.

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