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TCAS

If you have ever flown with FSInn,SquawkBox,PMDG,etc; you have probably seen the little boxes respresenting other aircrafts on the EICAS. You have probably noticed a letter combination on the EHSI (Electronic Horizontal Situation Indicator) called TCAS, this tutorial will introduce you to the TCAS.

Background
TCAS is short for Traffic alert and Collision Avoidance System (sometimes called Airborne Collision Avoidance System, ACAS). It was developed due to a group of nasty collisions between airliners in the US. In the middle of the 1970s the first version called Beacon Avoidance Collision System (BCAS) was developed, this system used the air traffic controls (ATC) radar beacon system to determined when aircraft got to close to each other. In the 1980s a development was made to make a system that was independent of ATC. The results were TCAS wich used aircrafts transponder to calculate the time to closest approach (CPA). It does this by simply asking the other aircrafts transponders. With the information received from these it can calculate a range, the direction and altitude of the other aircraft. It can then use this to warn the pilots when they get to close to each other. A newer version called TCAS II was developed in the early 1990s. The difference to the older system was that the new system is able to coordinate actions between the aircraft.

Function
The TCAS system uses four antennas, two are used by the TCAS and the other two are used by the transponder. It uses these antennas to ask and respond to requests by other aircraft. The TCAS also keeps track of the own aircrafts speed, direction and altitude in conjunction with the signals received from the other aircraft it can calculate a time to closest approach. It can then use this to either issue a traffic advisory (TA) or a resolution advisory (RA). The TA is issued when an aircraft is between 20 and 48 seconds away from colliding (depending on the aircrafts altitude). The RA is issued when an aircraft is between 15 and 35 seconds from colliding. Note that the system isn't fully activated when the aircraft is below 360ft of the ground, otherwise the system would warn a bit to often. In the TCAS II system the aircrafts communicate with eachother to determine which manouvers to take (if the aircraft don't communicate they might turn towards each other which isn't really the idea with avoidance).

Then how does it warn. It does it in two ways, the first is visual by displaying the other aircraft in the EHSI. There are a number of symbols displayed:

* Unfilled diamond, shown in either cyan or white and is a non-threat target (aircraft).
* Filled diamond, also shown in either cyan or white. This displays proximate targets, these targets are not a threat. The target is now closer than 6nm and +/- 1200ft from our aircraft.
* Filled circle, shown either in amber or yellow. This is a target close enough to cause a TA to be issued.
* Filled square, shown in red. This is a target close enough to cause a RA to be issued.

In connection with the symbols information can be displayed to let the pilot know what the target is doing. For example in figure below the first symbol (A) shows that we have a target, in this case really close, RA issued, 100ft below and climbing. In the second example we have a traffic alert for a target that's 700ft and level above us. In the third example (C) we have a proximate target 200ft below us and descending. In the last example (D) we have a non-threatening target.

The second way the TCAS will alert us is using audio signals. There exist a couple different ones and they can be accompanied by a instruction, see the table below.

There are a lot of signals above, note that none of them are used in squawkbox, if you get an alert there it will be a tone combination followed by "TCAS Alert". Of course if you want to change this you can replace the old file (tcas.wav) with a new one.

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