AVIONICS Training by OSTROUMOV Ivan
Chapter 7. Navigation and surveillance systems

7.8. TCAS II

Provides overview of the surrounding airspace in case of detecting other aircraft with display them on the screen (TA). Moreover, TCAS II assesses the potential collision in the air with other aircraft in case of presence such a threat generates a message (RA) for both pilots, allowing aircraft to breed in the vertical plane and avoid a collision.

TCAS II Versions

In Europe (TCAS II v.7.1) from 01.01.2005 for the aircraft with a greater number of passengers than 11 or a maximum take more than 5700 kg;

In the United States (TCAS II v.6.04) since 1994. with a total of more than 30 passengers.

ICAO recommends the use of TCAS II system from 01.01.2003 to fly globally.

The principle of operation of TCAS ІІ

At the heart of the system is contained computer unit (Fig. 92). It generates signals of request on frequency 1030 MHz similar to signals of request of the secondary radar (radar) and emits them via two antennas placed above and below the plane. These signals are received on a different plane, which is covered by the TCAS, using aircraft responder antennas. Aircraft responder generates a signal of response and emits it at a frequency of 1090 MHz. TCAS antenna receives the signal response and determine the direction of its location. TCAS evaluator determines the time for which the signal is back, and estimates the distance to aircraft. The accuracy of the distance is 4,2 • 10-6 m.


Fig. 92. The structure of TCAS

TCAS calculator monitors the movements of neighboring aircraft and predicts their trajectory of movement. The next step is checking the opportunity of collision of own aircraft with predicted trajectories. Data about TCAS altitude is received from barometric altimeter of system of air signals, and about the height of other planes from aircraft transponder answers.

TCAS system forms some imaginary space area around the aircraft, which is called collision area. The dimensions of this area change depending on the altitude of flight. TCAS calculations with the help of algorithms ensure the protection of this area from reaching other aircraft.

TCAS always makes surveillance of airspace around the aircraft through the measurements of request signals from aircraft transponders and receiving of response signals from other aircraft. From the received data about air traffic TCAS makes the prediction of trajectory of flight for each detected airplane. The time of updating TCAS information is 1 s.

TCAS forms two imaginary airspace areas around aircraft (Fig.93):
- Warning Area;
- Caution Area.


Fig.93. Caution Area and increased attention [9]

The base of TCAS working algorithms is time criteria, corresponding to which in case if the conflict situation is detected the necessary cautions about air situation are given (TA), depending from time when it occurred, or recommendations how to avoid this conflict situation (RA). Herewith the time which the pilot needs for making a maneuver for avoiding conflict situations is taken into account.

The length of Caution Area is 20-48 s of flight from TCAS estimated time of intrusion of conflicting aircraft to collision area.

The length of Warning Area is 15-35 s of flight from time of intrusion into collision area.

Time and height dimensions of these areas are different, what depends on altitude of flight. (Table.4) [31].

Table 4 Dimensions of protection areas for TCAS II version 7

If aircraft is in Caution Area, then TCAS considers it like a conflicting aircraft and give the warning about air situation (Traffic Advisory – TA). This message consists of voice and visual warning, which help the pilot to make the visual search and identify the aircraft.

If the conflicting aircraft crosses the Warning Area, TCAS makes a recommendation (Resolution Advisore – RA) about how to solve this conflicting situation. Recommendations RA can be corrective, warning or cathartic.

Corrective RA points to making a definite maneuver in vertical plane for solving the conflict situation. The messages show the pilot unacceptable values of vertical speed.

Warning and cathartic RA tries to attract pilot’s attention to tracking the vertical speed. The pilot must receive the values of vertical speed in green area and as little as possible to deviate from planned trajectory of flight.

Information about placement of other aircraft, which are in TCAS area, can be displayed at: - combined indicator of vertical peed on TCAS display (VSI/TDI) (Fig.94); - enhanced display of meteorological radiolocation station (Fig.95); - display of electronic system of indication (Fig.96).


Fig.94. Indication of information on VSI/TDI display [9]
Fig.95.Indication of information on weather radar display
Fig.96. Indication of information on EFIS [9]

If the trajectories of movement of aircraft do not intersect, those aircraft are pointed as blue rhomb; aircraft, vertical distance between which id 1200 ft, are pointed as blank rhomb; aircraft, vertical distance between which is less than 1200 ft, and distance is less than 6 miles – a solid blue rhomb.

If TCAS calculator finds the threat of collision with aircraft the nearest point of convergence is estimated and the time to fly to it is calculated. If the time of possible collision is to 20-48 s, this aircraft is marked as a yellow circle. In this case the pilot should visually find aircraft and try to avoid collision manually.

In case if aircraft continue to move in then by 15-35 s before the possible collision the TCAS calculator creates a recommendation to a pilot about separation of aircraft in vertical plane. For coordination of recommendations TCAS systems of different airplanes exchange the information with the help of transponder mode S. Aircraft which provoked RA on display is marked as a red square. Vertical speed needed for solving the conflict situation is shown on the scale of variometer by green zone, and unacceptable – red.

Recommendations are formed in that way so to ensure the safe separation of aircraft in vertical plane, herewith the distance between aircraft must be not less than 300-700 ft (fig.97).


Fig.97. Separation of aircraft in vertical plane

Accept the visual alarm on a display TCAS shows the corresponding voice messages (only on English):

  1. Messages about air traffic – «Traffic – Traffic».
  2. Warning RA - «Monitor vertical speed».
  3. Corrective RA:
    • «Climb – Climb»;
    • «Descend – Descend»;
    • «Climb, crossing climb – Climb, crossing climb»;
    • «Descend, crossing descend – Descend, crossing descend».
  4. Corrective RA with enhancing or weakening:
    • «Increase climb – Increase climb»;
    • «Increase descent – Increase descent».
  5. RA which changes the direction of maneuver:
    • «Climb, climb now! – Climb, climb now!»;
    • «Descent, descent now! – Descent, descent now!».
  6. laxative and restrictive RA:
    • «Adjust vertical speed, adjust»;
    • «Maintain vertical speed, maintain»;
    • «Maintain vertical speed, crossing maintain».

Algorithm of working TCAS II estimates the necessary vertical speed for making the corresponding maneuver in vertical plane. This safe speed allows to separate aircraft with enough altitude interval. For each safe speed corresponds the exact voice message, which tells pilot about the necessity of acting for its abidance. The least allowable vertical speeds and corresponding voice messages for TCAS II version 7 are shown in table 5 [31].

The voice message «Clear of conflict» appears after successful solving of conflict situation.

Recommendations RA are necessary for pilots’ carrying. Besides, in case of giving a recommendation RA pilot of aircraft must answer the ATC by phrase: «TCAS RA».

After issuance of voice massage «Clear of conflict» pilot must return aircraft to planned trajectory of flight and answer the ATC by phrase: «Clear of conflict (number of preset flight level) Resumed» or «Clear of conflict, returning to (number of preset flight level)».

After issuance of first recommendation RA the pilot must go to the performance of required maneuver in not less than 5 s from the moment of issuance of voiced message. A safe time of pilot’s reaction for the next RA is two times less and is equal to 2.5 s.

AVIONICS training course materials represented only in demonstrative mode. If it useful, all grammar mistakes will be corrected and more documents will be added. Let me know if you interest in avionics.
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Avionics training in details by
Ostroumov IvanOSTROUMOV Ivan, PhD
Associate Professor, www.sciary.com/ostroumov