TCAS – How Aeroplanes Avoid Collision.

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Following major mid-air mishaps, it was evident that something needed to be done to avoid such accidents. In 1981, the Federal Aviation Administration, the regulating body of civil aviation in the USA, decided to come up with a concept which reduces such aviation mishaps. They named this as the TCAS- Traffic Collision Avoidance System. It is designed to increase the situational awareness of the pilots in that airspace, allowing the pilots to improvise accordingly. It operates independently from all the other radio equipment.

How it works?

When an aircraft is at the vicinity of the TCAS, the device emits a series of interrogation requests constantly. These requests are picked up by the transponders of other aircrafts in that airspace. The transponder then sends a beacon signal as a reply to the request of the TCAS, this emitted signal carries the altitude and other necessary data to the TCAS which then displays this data on the cockpit.

Based on the data gathered from the emission, the TCAS maps out a three-dimensional position of the approaching aircraft to the pilot. This data is then used to give vocal commands to the pilot, which is completely independent of the input given by the ATC. The TCAS would give the following three advisories based on the data gathered: Traffic Advisory, Resolution Advisory and Clear of Conflict.

Image Source: Wikipedia

Traffic Advisory: When this advisory is issued, pilots are trained to do a visual search in their vicinity and keep a visual separation from the target aircraft. This is the part where the pilot would hear, “traffic, traffic”.

Resolution Advisory: This advisory requires the pilot to take an immediate action to avoid conflict. The pilot is required to completely ignore the ATC instructions until clear of conflict. When this happens, the TCAS in the target aircraft will give an instruction opposite to the initial aircraft. This is done to gain maximum separation between the aircrafts. One aircraft is instructed, “climb, climb”, whereas the other aircraft is instructed, “descend, descend”.

Below is an image depicting an aircraft following TCAS instruction versus an aircraft following ATC instruction. Similar to the 1996 Charkhi Dadri incident.

Image Source: Honeywell Aerospace

All this is governed by the following components comprising the TCAS:

Antennas: TCAS operates in two frequencies, 1030MHz used to receive interrogations and 1090MHz used to transmit the reply. These antennas are placed on the top and the bottom of the aircraft and are generally omni-directional. The Mode S transponders are also placed beside the TCAS antennas. A Mode is defined as the interval between two or more interrogation pulses, Mode S is the type of interrogation used for civilian purposes.

The Computer Unit: This unit compiles the data altitude, change of path, calculations, advisories, and airspeed, from its own aircraft and the target aircraft, based on this data the processor calculates the flight path of both the aircrafts and predicts ahead if there would be a possible collision, the pilot is then instructed through the cockpit representation.

Cockpit Representation: The data after being compiled in the computer unit is then displayed to the pilots on a dedicated TCAS display.

Below is an image of TCAS and EHSI (Electronic Horizontal Situation Indicator) cockpit display.

Image Source: Wikipedia

There are various types of TCAS:

Passive: Smaller range of 7 nautical miles, sends queries to other radio systems to generate output.

TCAS I: Generates a TA to make the pilot aware of the situation, does not give an instruction to the pilot regarding change of course.

TCAS II: This version gives only vertical RAs’. It is mostly used in commercial airliners. The RA is coordinated between the aircrafts to provide maximum separation.

TCAS III: Had both vertical and horizontal RAs’, but it had inaccuracy in its results and the project was discontinued.

TCAS IV: It uses the data gained by the directional antennas and the Mode S transponder to generate an accurate output.

One of the major mid-air collision with the largest fatality is the 1996 Charkhi Dadri incident killing 349 people on-board both the aircrafts.

The Kazakhstan Airlines Ilyushin Il-76’s tail cut through the left wing and the horizontal stabilizer of the Saudia Airlines Boeing 747, sending the Boeing at a rapid spiral descent, killing all onboard.

Image Source: eInfochips