The supreme danger of a mid-air collision between aircraft is best represented by the 1996 crash of Saudi Arabian Airlines Flight 763 and Kazakhstan Airlines Flight 1907 over Charkhi Dadri, India. The cause was a confluence of errors and lack of adequate technologies.

The Kazakh pilots did not speak English and relied on a radio operator with no instruments to relay instructions. Those pilots also flew unpredictably on their approach to New Delhi, descending well below the prescribed 15,000 ft altitude, due to either turbulence or communication problems. Meanwhile, the Saudi plane was leaving New Delhi in a narrow air corridor, as most of the surrounding air space was owned by the Indian military. Also, the New Delhi airport lacked secondary radar — the type that provides altitude information.

And as a result, 349 people perished.

The incident motivated many regulations and technology changes, and brought the entire aviation industry in compliance with traffic collision avoidance systems (TCASs).

Development

The TCAS was an initiative of the U.S. FAA and the International Civil Aviation Organization to make sure that catastrophic mid-air collisions are avoided. The dangers of them have been evident since two planes first flew in the same airspace.

A TCAS relies on queries and replies of aircraft transponders by onboard antennas. A TCAS computer tracks aircraft altitudes, builds a 3D map of the airspace and issues advisories to pilots. From the start of the development of TCASs in 1981, it has seen an evolution that allowed the system to improve and be more effective.

  • TCAS I: The most simple TCAS to be made available, it only delivers a "traffic advisory" to the pilot, telling him or her that another airplane has entered the detection range. This information is offered on a visual display in the cockpit. The pilots will query air traffic control to ensure planned flight paths are not intersecting. It will issue a "clear of conflict" advisory once the plane has passed. TCAS is most common on small passenger aircraft.
  • TCAS II: In addition to all the features of TCAS I, this version offers "resolution advisories" — that is, it provides real-time instructions to pilots to avoid mid-air crashes. As planes approach, aural and visual instructions such as "climb, climb" or "descend, descend now" will be issued, with the phrasing becoming more assertive as the urgency increases. This is minimum standard TCAS for airliners.
  • TCAS III: Whereas TCAS II offers only vertical axis guidance ("level off"), TCAS can also tell pilots to turn. However, it is discontinued, as studies proved that the antenna on the aircraft were not accurate enough to predict a plane's horizontal trajectory.
  • TCAS IV: Once geolocation systems such as GPS become widespread, TCASs were able to use this information to determine aircraft bearing and provide X axis resolution advisories.

TCASs are under iterative development. Changes are continually proposed to the systems by users and engineers. The European Organization of the Safety of Air Control computed the probability of mid-air flight collisions equates to one every three years — a risk factor that is still too high.

Additionally, even TCAS IV is poised for obsolescence, as other technologies that rely on satellite tracking or machine learning are under research. There is also a need to integrate ground proximity warnings, or more intelligent reasoning for when a plane cannot exert a maneuver, such as if it is told to climb but it is at its rated ceiling.

TCAS components

To enable the TCAS to detect planes and provide evasion maneuvers, a set of computers, displays and antenna are installed. There are two directional antennas located above and below the aircraft, which transmit the aircraft's current position and detects other aircraft.

The most important component is the TCAS transmitter-receiver computer, which sends and receives signals from the directional antennas and computes the trajectory of other aircraft that it has detected.

In flight

Suppose Flight 222 is on a flight path that might intersect with that of Flight 444. If equipped with a TCAS II system, the pilot of Flight 222 would experience the following.

  1. An "other alert" would appear, likely in the visual speed indicator, once Flight 444 is within 12 NM and a vertical separate of 2,700 ft or less.
  2. Once Flight 444 is within 6 NM and 1,200 ft, the TCAS will issue a visual "proximity alert."
  3. A traffic advisory prompts both a visual and aural warning once a plane is detected within 35 to 45 seconds of flight time and within 1,200 ft. The pilots should not make any maneuvers at this time — doing so may actually cause a collision. Instead, they should prepare to respond to the next warning, should that come.
  4. A resolution advisory supplies both a visual and aural instruction to pilots of both Flight 222 and Flight 444. Flight 222 will be told to descend and 444 will be told to climb. Pilots must follow the instructions of an RA, even over the instructions of an air traffic controller, unless doing so would be of obvious danger.

Once the planes pass each other, each will be granted a "clear of conflict" message.

Conclusion

Unfortunately, aircraft collisions still occur. The most recent occurred in May 2019, between two sightseeing float planes in Alaska. TCASs are not a perfect technology, but are a substantial improvement over the dark ages of flight plan coordination.