How does TCAS work?


How TCAS system works

The accident which resulted in a new aviation rule:

In 2002, two aircrafts crashed in mid-air over Überlingen due to the mismatch of commands of ATC (Air Traffic Controller) and TCAS. Russian Tupolev was given commands by the ATC to start descending whereas the TCAS system of that aircraft gave the resolution advisory to “Climb”. Due to this mismatch of commands, Russian Tupolev followed ATC and started descending. As ATC gave no commands to Boeing 757 which was on a collision course of Russian Tupolev, Boeing followed the resolution advisory of TCAS to “Descend”. This confusion caused both aircraft to crash in mid-air. This accident brought a new aviation rule in commercial aviation, according to which the TCAS command had to be given priority over that of ATC.

Introduction to TCAS:

TCAS stands for Traffic Alert and Collision Avoidance System. To understand this system, the working of a transponder should be considered first. There are mainly two types of transponders i.e. Mode Alpha and Mode Sierra. Mode Sierra transponder is mandatory to be fitted in all Commercial Jets but many glider planes and other smaller aircraft still use a Mode Alpha transponder.
Signals sent out by Mode Sierra transponder could show the aircraft's position, altitude, and current vertical speed, whereas the signals sent by Mode Alpha transponder only indicates the position of the aircraft and gives no information about the altitude. The TCAS has two antennas which could be used to scan the airspace around the aircraft at a 40 miles (64.4km) radius horizontally and 9900 feet (3km) upwards and downwards vertically so that it could search for a transponder of the intruders. These transponder signals are used to search for closure rate and relative altitude (in case it is fitted with Mode Sierra transponder) of the intruder.

Different indications on navigation display:

Traffic Collision Avoidance System
Once an aircraft comes into the TCAS airspace of another aircraft, there is a little diamond which pops up on the navigation display, showing the position and the vertical separation of the aircraft. If the other aircraft is within six nautical miles, this diamond becomes completely white. When the vertical separation of the aircraft is decreased, this diamond is changed into an amber dot.
The collision avoidance system becomes active at this moment. An aural warning Traffic! Traffic! is automatically called out because the other plane is considered to be a potential threat.

Traffic advisory provided by TCAS:

The perimeter of the caution area is 40 to 45 seconds from the time TCAS calculates the intruder’s penetration into the collision area. The perimeter of the warning area is 20 to 25 seconds away and the five-second variance is caused by other factors this system is also calculating if the intruder continues as predicted and enters the caution area the TCAS designates it as an intruder and issues a traffic advisory (TA). If the intruder continues to its entry into the warning area at 20-25 seconds prior to the collision area TCAS issues a resolution advisory.

The resolution advisory is of two types: corrective and preventive. Corrective resolution advisory instructs the pilot to make a specific change in the current vertical rate. Preventive resolution advisory instructs the pilot to avoid certain changes from the current vertical rate. To avoid the collision, one of the airplanes will get the resolution advisory to Climb! and the other to Descend!. In this way, the resolution advisory given by TCAS is helpful in avoidance of a collision.

Steps to be followed in case of a resolution advisory:

When there is a resolution advisory, the pilot has to switch off the autopilot and flight directors and follow the aural message and give the necessary input and monitor the vertical speed. The climb inputs should be smooth to avoid another TCAS alert. Normal flight with flight directors and autopilot activated is resumed after completion of resolution procedure.

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