How do aircrafts avoid colliding in mid-air

Updated: Apr 1

Have you ever wondered how aircraft fly? Why do they all have a similar shape? Why most aircraft are white in color? do pilots sleep on duty? How do they avoid colliding in mid-air?

This blog is all about the last question. How do they avoid colliding in mid-air? There is one hero that does all the magic and takes all the responsibilities to alert the aircraft if there is another one in their way. This hero is called TCAS or Traffic Collision Avoidance System.

Before I start telling about TCAS, let me tell you about an incident that happened in Indian Airspace near New Delhi's Indira Gandhi International Airport. The incident is called the Charkhi Dadri mid-air collision.

Charkhi Dadri is a small village in the Indian state of Haryana about 100 KM from New Delhi. This incident involved two aircraft, a Saudi Arabian Airlines Flight 763, a Boeing 747, and a Kazakhstan Airlines Flight 1907, an Ilyushin II-76.

On 12th Nov 1996, Saudi Arabian Flight 763 was traveling to Dhahran, Saudi Arabia from New Delhi, India and Kazakhstan Flight 1907 was traveling to New Delhi from Shymkent, Kazakhstan. They collided over the village of Charkhi Dadri. The crash killed all 349 people on board both planes. This is still the World's deadliest mid-air collision and the deadliest aviation accident to occur in India.

Saudi Flight took off from New Delhi at 18:32 local time. At the same time, Kazak Flight was descending to land in New Delhi. Both the flights were controlled by approach controller VK Datta. At that time New Delhi airport had the Primary Radar system which gave the readings of distance and bearing but not altitude. The controller had to depend on the pilots to get the altitude information. Also, New Delhi uses two corridors for aircraft, one for take-off and another for landing. At that time Indian Air Force was using one of the corridors, leaving the controllers to use only the one available corridor.

Kazak flight was cleared to descend to 15000 feet and maintain the altitude. Saudi flight was asked to climb to 14000 feet and maintain. Both the flights were on the same path but in opposite direction. Controller VK Datta wanted a gap of 1000 feet between the flights when they passed each other. Kazak flight reported that they were at 15000 feet and maintaining but in reality, they were at 14500 feet and still descending.

Controller Datta called the Kazak flight again, he wanted to warn them about the flight level of the Saudi flight, he got no response, it was too late. The two aircrafts had collided, the Kazak flight's tail cutting through the left-wing and horizontal stabilizer of the Saudi flight.

Image Source:

All 312 people on board the Saudi flight and all 37 people on the Kazak flight were killed. Captain Timothy J. Place, a pilot for the United States Air Force, was the only eyewitness to the accident. He was making an initial approach to the New Delhi airport when he saw that "a large cloud lit up with an orange glow".

Both the flights were equipped with TCAS. But at that time installing a TCAS in aircraft wasn't compulsory and TCAS instructions were not necessary to follow. This incident made India install a Secondary Surveillance Radar System at the New Delhi airport. It was India's first radar upgrade. Also, after this incident, the installation of TCAS became mandatory in all aircraft with a maximum take-off mass of over 5700 kg or authorized to carry more than 19 souls.

TCAS or a traffic collision avoidance system is an aircraft collision avoidance system that is designed to reduce mid-air collisions between aircraft. TCAS monitors the airspace around an aircraft for other aircraft which is equipped with a corresponding active transponder. This is independent of air traffic control and warns the pilots of the presence of other transponder-equipped aircraft which may be a threat of mid-air collision. TCAS is based on the Secondary Surveillance Radar Transponder signals but it operates independently.

Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position and all other aircraft reply to other interrogations. This interrogation and response cycle may occur several times per second. TCAS system builds a 3D map of aircraft in the airspace, incorporating their range, altitude, and bearing. Then by extrapolating the current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.

Image source: Wikipedia

When the TCAS finds a potential threat, it issues the following types of alerts:

  1. Traffic Advisory (TA)

  2. Resolution Advisory (RA)

  3. Clear of Conflict (COC)

These alerts are aural.

When a TA is issued, pilots initiate a visual search for the intruder aircraft causing the TA. If the intruder is visually acquired, pilots are instructed to maintain visual separation from the intruder.

When an RA is issued, pilots respond immediately to the alert. If they don't it could jeopardize the safe operation of the flight. RA alert means that the aircraft will have to perform some manoeuvers which are contrary to ATC instructions. Pilots may assume that ATC is aware of the situation and is providing a better resolution if the ATC asks them not to change their path or altitude but it is not true. The reality is ATC is not aware of the RA until the RA is reported by the pilot. When such a case happens, pilots are supposed to do what RA alert wants them to do.

When TCAS issues COC, it means that the intruder has passed safely and now there is no threat of collision. At this point, pilots are required to contact the ATC again and correct their path and altitude.

The aviation industry has seen many deadly and horrible incidents, still, it is the safest way of traveling. More people die each year by a coconut falling on their head than in an aircraft crash. Aircraft have fascinated me since childhood. I read so much about them, played simulator games, watched them take off and land for hours. And when I got to fly a Robinson R-22 helicopter during my course, I knew that this love for flying and machines is endless.

Engineering!! literally no limits.

Ref: Wikipedia

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