By Davide Crivelli, The Conversation From Business Insider

The sky is a crowded place. In June 2017, there were on average 33,000 flights every day over Europe alone.

Surely then, it’s not surprising that we hear of “near-misses” between passenger jets, and of “hero pilots” avoiding “mid-air collisions?” Well yes, actually it is, because that can’t happen. These so-called “near-misses” are still a long way off from being a collision.

Take the recently reported news of an Emirates Airbus A380 and an Air Seychelles Airbus A330 traveling from Mauritius to the Seychelles, for example. The two planes came closer than the safety margin required by regulations: all aircraft must have a 14km radius, 300m in height “bubble” around them, in which no others are allowed to fly.

Planes tend to follow similar paths, called airways, which are like roads. Flights are directed at different altitudes, on different layers – called flight levels – which ensure that two crossing airplanes are never at the same altitude, and these bubbles are kept clear.

Air traffic controllers also have radars and computer systems which help them keep planes far apart when descending, climbing, or changing route. In this instance, the Emirates crew misunderstood the air traffic controller’s instructions, and started descending through the Air Seychelles flight path, while the controller did not correct them.

Pilots, air traffic controllers, maintenance staff and everyone else involved in air transport do sometimes make mistakes but it is very rare. Safety has been increasing steadily, from 12 fatalities per 10 million flights in 2006, down to four in 2016. Everyone working in aviation is highly skilled and regularly trained and, on top of that, the technology used to fly aircraft is very sophisticated and can quickly overcome human error.

In this instance, the traffic collision avoidance system fitted in both aircraft quickly spotted the mistake and instructed both crews to adjust their flight path. Rather than it being a narrow escape, the closest the two planes ever got was 14km apart – and more than 30 seconds from any remote risk of collision. Hardly a “near miss.”

Avoiding collisions

Since the early 1990s in the USA – and since 2000 in Europe – passenger aircraft cannot fly without a traffic collision avoidance system (TCAS).

This is a relatively simple but clever radar system that helps keep the sky safe. The aircraft constantly transmits its identifier via radio, along with some other information such as altitude, speed and direction.

If an aircraft’s TCAS picks up a signal from another plane, it starts a conversation with the other aircraft, working out how far away they are from one another based on the time it takes to receive a response.

The system also works out which direction the transmission is coming from using an antenna and then calculates if there is going to be an unsafe situation. It also builds a display of surrounding aircraft to let the pilot monitor the situation.

If an aircraft is about to burst the safety bubble around another aircraft, the two TCAS units start coordinating a solution. At first, usually between 20 and 48 seconds from the risk of collision, pilots of both aircraft will hear a concerned voice alerting “traffic! traffic!” in the cockpit. The intruder aircraft will be displayed in yellow on the display and pilots will usually start looking outside for lights or visual clues.

As the two aircraft get closer – within 20 to 25 seconds of one another – the TCAS issues a “resolution advisory.” It automatically coordinates a smooth evasive maneuver with the other aircraft’s unit and the systems tell the pilots what to do: descend, climb, or make other altitude changes.