Background

With the density of air traffic increasing around the world, it became apparent the the mid-air collision risk would rise also, and a system was developed to make pilots situationally aware of other nearby air traffic (TCAS 1), and in certain more sophisticated systems recommend an escape manoeuvre to best avoid an impending mid air collision (TCAS 2).

One accident which focussed attention on the need for an on-board traffic alert and avoidance system was that in which a Boeing B727 collided in midair with a Cessna 172 over San Deigo in the USA. See picture above. The C172 hit the B727’s right wing with disastrous results. All occupants of both aircraft died.

The collision occurred when both both aircraft were under ATC radar surveillance in the terminal area.

Method of operation

TCAS uses the transponder signals from nearby aircraft to determine their position "relative" to your aircraft. TCAS does not rely on the reflection of pulsed radio energy off a target as does radar.

Instead, the two transponders simply talk to each other.

It is vitally important that you realise that although your aircraft is fitted with TCAS, it will not be able to give you advice or warnings about other aircraft near you that are not transponder equipped, or have their transponder turned off.
(ie: Non-Transponder equipped aircraft are invisible to TCAS).

TCAS will NOT warn pilots of the aircrafts likely collision with terrain. That warning system is called the Ground Proximity Warning System (GPWS). After all, mountains are not fitted with transponders.

Basically the TCAS in your aircraft "interrogates" transponder signals from other aircraft near you, and presents these on a cockpit display panel such as the one in Fig 1. Your aircraft is always in the centre of the screen. Other aircraft are displayed relative to you in various symbologies depending on their level of threat to your aircraft.

Symbology

An open white diamond... signifies aircraft which though nearby, are not assessed as being a threat at that moment. It is called a "Surveillance target". 

A solid white diamond... If the target aircraft gets within 1, 200 ft and 6 miles of your aircraft it will become what is referred to as a "Proximity Target". No aural associated with this.

A solid yellow circle... signifies that the target aircraft is within 40-45 seconds of the point of closest approach, and to make you aware of this the crew are advised through an aural message of "Traffic, Traffic". The flight crew should now attempt to sight the traffic visually, using the relative position indicated on the display screen as a guide.
The "Traffic, Traffic" aural message will only be heard once, not continually.

A solid red square... signifies that the aircraft is a direct threat to your aircraft and the crew are alerted to this by an aural message of either "Climb, Climb" or "Descend, Descend", as appropriate to best avoid the target aircraft.
There are no turning manoeuvres issued with TCAS systems, only climbs or descents.

TCAS 1.

If your aircraft is fitted with this system you will receive only a "Traffic, Traffic" aural warning. This is called a "Traffic advisory" (TA). You will need to calculate the best avoidance manoeuvre yourself as no resolution advisories are given. This will either be a climb or descent as you feel appropriate. The other aircrafts position and height will be displayed next to the aircrafts symbol, as well as a trend arrow which tells you whether that aircraft is climbing or descending.

TCAS 2

Considerations and disadvantages of TCAS

There elements with TCAS which you should remember. Here are some...

1. When operating in a very dense traffic area, you can get multiple advisories at a time when your work load is already high. Departing and arriving at airports with multiple runways, especially where simultaneous runway operations (Simops) are in progress can cause a considerable amount of unwarranted TCAS advisories. It may be wise to select the traffic advisory (TA) function to assist reduce nuisance RA’s. Parallel runway operations can also cause nuisance TA’s and RA’s.
   
2. It may be advisable to de-select RA function when operating with one engine shut down. This is because a TCAS climb RA could demand a rate of climb greater than that available, with the risk of low speed stall as a result.
   
3. Descend RA’s are inhibited when the aircraft is below 700 ft AGL, to avoid a controlled flight into terrain (CFIT) from occurring, which would significantly ruin your day.
   
4. Both traffic and resolution aural advisories are inhibited when the aircraft is at or below 400 ft AGL.
   
5. Ground proximity warning system (GPWS) aurals have priority over TCAS aurals.
   
6. TCAS 1 and TCAS 2 can only display other nearby aircraft relative altitude and vertical speed if those aircraft are fitted with at mode "C" or mode "S" transponder. They can NOT identify the relative altitude and vertical speed of an aircraft equipped with only a mode "A" (non-altitude reporting) transponder.
   
7. Selecting TA on the TCAS 2 mode control panel will mean that both visual and aural RA’s will be suppressed. That is, there will be no "Climb" or "Descend" aurals, and RA red squares will no be displayed.

End of TCAS texts.

Pilots are the "ULTIMATE PROFESSIONALS"... Strive to be one !