Traffic Alert and Collision Avoidance System

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TCAS display

The Traffic Alert and Collision Avoidance System ( TCAS ) is an implementation of the Airborne Collision Avoidance System ( ACAS ) on board an aircraft . The functionality of the ACAS is defined by the ICAO . It is used to avoid aircraft collisions in the air .

General and history

After a series of mid-air collisions in the 1950s, a call for a collision warning system arose in 1956.

TCAS was developed over a period of around 30 years and was finally introduced in the USA at the end of the 1980s. The decisive factor for the obligation to equip with a collision warning system was the collision of a small aircraft with a DC-9 on August 31, 1986 near Los Angeles ( Aeroméxico flight 498 ). A law of December 1987 required the equipping of all commercial aircraft with more than 30 seats in US airspace with a TCAS II by December 1993 (and TCAS I for 10 to 30-seaters). This meant that all airlines around the world had to equip their aircraft accordingly if they wanted to serve US airspace.

The ICAO , in which the system, like Eurocontrol , is called ACAS ( Airborne Collision Avoidance System ), in 1998 has an obligation to equip aircraft with ACAS II for aircraft with a take-off mass of more than 17 t from 2003 (Eurocontrol already from 2000) and Aircraft with a take-off mass of more than 5.7 t were decided from 2005.

The NASA helped in improving the TCAS. NASA used flight simulators and tested how well commercial pilots understood the TCAS and how long it took them to react to different situations. Based on these findings, the TCAS software has been improved with the aim of making it easier and faster to operate.


There are three TCAS (or ACAS) versions:

TCAS I: Is only traffic information ( Traffic Advisory , TA). These only include the distance, rough direction and altitude difference to your own aircraft.

TCAS II: Resolve recommendations ( Resolution Advisory , RA) are also calculated. These are purely vertical ( rise , fall , specification of the vertical speed).

TCAS III: Still in development and testing. It should also be able to provide lateral (sideways) avoidance recommendations (course changes). This requirement may not be able to be met with the existing technology; It is therefore expected that TCAS III will not be used, but that the next version, TCAS IV, will follow immediately.

TCAS IV: Also in development. TCAS IV will evaluate the information from newly developed transponders and therefore has the prerequisites to also calculate lateral (lateral) avoidance recommendations (course changes).

In addition, research is being carried out into displaying the TCAS avoidance recommendations on the air traffic controller's radar screen (this is technically possible, but expensive). It is required that TCAS automatically notify the air traffic controller's workstation about resolution advisories (RA) that have been issued (ICAO 08/2004) so ​​that contradicting evasive instructions cannot arise in the first place (see  aircraft collision in Überlingen ).

A Portable Collision Avoidance System was developed for small aircraft .


TCAS queries both Mode S and Mode A / C - capable transponders of other aircraft in the surrounding airspace and uses their answers to determine a "safety picture". This situation report roughly and depending on the system covers a range of horizontally 20–40  NM (corresponds to approx .: 37–74 km) and vertically ± 10,000 ft (± 3,048 m). From the data received ( direction, distance, approach speed , altitude and rate of climb / descent ), the system calculates whether, and if so, when a collision with an aircraft is to be expected.

Depending on the data received from the possible collision opponent, the system issues a TA (Traffic Advisory) or an RA (Resolution Advisory). This recommendation is based on

  1. the calculation of the point of closest point of approach ( CPA = Closest Point of Approach )
  2. the duration TAU until this point is reached, and
  3. whether there would be a distance violation at all.

The responses to TCAS queries are sent in mode S in format DF0 (56 bit short message) or in format DF16 (112 bit).


The TAU area is a specific “protection zone” around your own aircraft. If an aircraft enters the TAU area, the TCAS triggers an alarm. The boundaries of the TAU area are defined by a period of time (time-to-go). TAU is the time required to get to the Closest Point of Approach (CPA; point of closest approach). The length of time is a distance divided by the rate of approach - both vertically and horizontally.


There are cases in which the approach rate is so low (e.g. aircraft overtaking from behind; aircraft flying in parallel is slowly approaching more and more) that the border of the TAU area is not exceeded. Nevertheless, the distance can shrink below an NM. In this case, the calculated approach rate is not very useful, since a sudden increase in the approach rate (e.g. an aircraft flying in parallel suddenly changes course in the direction of one's own aircraft) would leave no scope for a timely warning.

This problem is resolved by the additional distance modification (DMOD). The DMOD only affects the physical distance between your aircraft and the target. The time until the collision or the rate of approach play no role.

The DMOD values ​​also change with altitude.

These TAU values ​​vary depending on the altitude. For example, no more RA is issued below a certain height in order not to endanger the ground clearance through a possibly required vertical evasive recommendation downwards (Descend). A TA or RA is issued earlier and earlier. In addition to the TAU criterion, the aircraft is also surrounded by a virtual airspace in which no aircraft may be located, otherwise an RA will also occur here. The reason for this are approaches with extremely low approach speeds.

In the event of a conflict, a TA occurs first. This emits an acoustic signal “TRAFFIC, TRAFFIC”, and the aircraft in question usually appears on the multifunction display (MFD) as a yellow symbol.

The next step is an RA: The aircraft in danger of a collision now changes from a yellow to a red symbol. In addition, the pilot receives the acoustic signal "CLIMB, CLIMB" from the system at this moment, ie the request to go straight into the climb. Meanwhile, the pilot of the other aircraft receives the signal "DESCEND, DESCEND", that is, the request to go into descent immediately. So that both pilots do not receive the same recommendation to avoid, the system determines the recommendation of the other system from its transponder signal. In addition to the acoustic signals, a red and a green band is often shown on the variometer : The green area indicates the range of vertical speed at which a collision is very unlikely; the red band, on the other hand, is an area that makes a collision likely. As soon as the collision situation has been resolved, the acoustic signal "CLEAR OF CONFLICT" is given.

RA and TA are only notes to the pilots. The information about the impending collision is not fed into the autopilot. The pilots must carry out the evasive maneuver themselves.

If an aircraft has a transponder that does not provide any altitude information ("Mode A" transponder, mainly found in small aircraft), only a TA is generated. There can be no vertical avoidance recommendation, only a warning is given. The TCAS computer can track several aircraft at the same time (in the double-digit range) and, in the case of many systems, generate a combined recommendation of avoidance for several simultaneous threats. However, significantly fewer aircraft can be considered (in the single-digit to low double-digit range).

Passive TCAS

PCAS device from Zaon Flight Systems, Inc.

Passive TCAS, also known as TCAD or PCAS for portable devices, does not have the option of actively triggering transponders. Instead, the device listens for transponder responses that are sent on board other aircraft on the basis of ground penetrating radar or TCAS devices, and evaluates these response signals. Passive TCAS can only work in radar-monitored areas or in the vicinity of TCAS-equipped aircraft. Its range is also less. However, this is offset by significantly lower costs (approx. 1/10 to 1/20), so that this system is more likely to be found in aircraft used in general aviation.

Example limits of the Boeing 747-400

rel. height 
rel. height 
foot Seconds  Seconds  NM lateral NM lateral foot foot
0 - 500 ft
radar altitude 
20th blocked 0.1 blocked 1200 blocked
501 - 2500 ft
radar altitude 
35 20th 0.1 0.066 1200 300
2501 - 10000 ft
40 25th 0.3 0.066 1200 300
10001 - 20000 ft
45 30th 1.0 0.082 1200 300
20001 - 30000 ft
45 30th 1.105 0.082 1200 300
over 30000 ft
45 30th 1.122 0.082 1200 300


The FAA has issued a regulation that all aircraft with 10-30 seats must be equipped with TCAS I and all aircraft with more than 30 seats must be equipped with TCAS II as soon as they fly into the USA.

The European Aviation Safety Agency (EASA) issued a regulation for the whole of Europe on January 1, 2005, according to which all aircraft with more than 19 seats or more than 5700 kg must be equipped with TCAS II version 7.0. Since March 1, 2012, aircraft in this group must be equipped with version 7.1. For aircraft with an individual certificate of airworthiness issued before March 1, 2012, this regulation has been in effect since December 1, 2015.


ACAS is the name of the basic concept. The ICAO uses this term when determining the standards that the system must meet.

TCAS is a concrete implementation of this concept. TCAS II Version 7.0 and TCAS II Version 7.1 are the only implementations that meet the requirements of ACAS II.


If there is an undercut with an already existing, ground-based warning ( Short Term Conflict Alert , STCA), air traffic controllers are required to order clear horizontal course changes for those involved in the absence of information on a possible TCAS avoidance recommendation (RA) so as not to conflict with the TCAS recommendations to guess.

Critical events related to TCAS

Bashkirian Airlines Flight 2937 / DHL Flight 611

Main article: Airplane collision from Überlingen

On the night of July 1, 2002, an air collision occurred near Überlingen between a DHL freighter on its way to Brussels and a passenger aircraft of the Russian Bashkirian Airlines on its way to Barcelona. Both crews received correct avoidance recommendations from their respective TCAS. The TCAS of the DHL plane recommended sinking, which was immediately followed. At the same time, the TCAS advised the Russian aircraft to climb. However, the Russian crew ignored this and instead responded to an almost simultaneous sink instruction from the air traffic controller at Swiss air traffic control Skyguide . The controller had also given this request, which conflicted with the TCAS avoidance recommendation, to avoid a collision, but for system-related reasons without knowledge of the TCAS activation. In the end, both aircraft were thus descending and collided a little later.

The reason for this is probably the lack of experience with TCAS on the one hand, and the inadequate international coordination of the regulations for prioritizing air traffic controllers and TCAS instructions on the other. While there were several regional recommendations prioritizing TCAS, none had official or regulatory status.

The investigation report of the Federal Bureau of Aircraft Accident Investigation (BFU) on the air collision also states:

“In addition, Eurocontrol examined in a further analysis how TCAS II would have behaved in this case with the proposed amendment CP 112, which had already been developed before the accident. According to the results presented, after the appearance of the RA, TCAS would have generated a reversed RA, which, if the crew of the Boeing B757-200 had responded accordingly, would have resulted in a sufficient vertical distance between the two aircraft. "

Near collision on February 26, 1999

On 26 February 1999, there was in the UK to fall below the prescribed graduation from an approved in Germany Boeing B737-300 ( B737 ) and a Gulfstream IV (GIV). The loss of the separation triggered a SHORT TERM CONFLICT ALERT at air traffic control and a TCAS recommendation (Resolution Advisory (RA)) for both aircraft involved, which both pilots followed. A collision was prevented by the TCAS.

Germanwings flight 2529 / Hahn Air Lines flight 201

On the morning of June 10, 2011, 18 NM (approx. 33 km) north-east of the Friborg VOR / DME , the staggering of two commercial aircraft was not achieved. An Airbus 319 of the German Wings , traveling from Barcelona to Stuttgart received by the responsible air traffic controllers in Geneva permission to flight level (FL) 250 (about 7620 meters altitude ) to decline. At the same time, the air traffic controller entered a descent to FL 280 into the computer system. A short time later, the Geneva control center handed over this flight to the Zurich control center. For the transfer between Geneva and Zurich, an altitude of 28,000 feet = FL 280 = 8400 meters was planned for flights to Stuttgart.

At the same time, Hahn Air Lines flight 201 , a Raytheon 390 Premier 1 that took off from Zurich and destined for Palma de Mallorca, received approval to climb to FL 270. As a result, the two machines came closer to below the minimum graduation. In both aircraft, the crews initially received a traffic notice (TA) from the respective TCAS. A little later, correct alternative recommendations were given by both systems. The Airbus TCAS recommended sinking, which the crew immediately implemented. At the same time, the TCAS requested the Raytheon to climb. This recommendation was also initially implemented by the pilots of the Hahn Air Lines flight.

Nine seconds after the recommendation to avoid (RA), the pilots of the Raytheon received the instruction from the control center in Geneva, where they had become aware of the problem, to descend immediately to FL 260. The crew complied with this order, which was in conflict with the TCAS recommen- dation, without notifying the air traffic controller that the RA was still active.

Ten seconds after the request to descend, the aircraft were the closest to each other. Another 13 seconds later, at 08:37:31 UTC (10:37:31 CEST ), the HHN 201 was 100 feet (approx. 30 meters) lower than the A 319 (the horizontal distance at the time was about 0.6 NM (approx. 1 km). As a result, the TCAS of both machines changed directions. While the crew of the Hahn Air Lines flight was requested to maintain the rate of descent, the system at the same time ordered the other aircraft to issue a reverse recommendation (reversal RA) and thus the transition to the climb. At 08:37:39 UTC, the distance between the machines increased again, which is why the TCAS computers reported the solution to the conflict situation with "clear of conflict".

See also

Web links

Individual evidence

  1. Technology Transfer Program - At The Airport With NASA - Collision Avoidance ( en ) NASA . Archived from the original on April 9, 2009. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved August 8, 2009. @1@ 2Template: Webachiv / IABot /
  2. Airborne Collision Avoidance System (ACAS) Manual . (PDF; 1.0 MB) International Civil Aviation Organization, First Edition, 2006, 1.4 System Overview; Retrieved December 1, 2012
  3. The format of the Mode S response telegram, radar tutorial
  4. John D. Ruley: Zaon PCAS XRX · Portable collision avoidance with direction ( english ) Retrieved August 5, 2009.
  5. EASA Commission Regulation No 1332/2011. EASA, accessed July 8, 2017 .
  6. Frequently Asked Questions (FAQ) ( English ) EUROCONTROL . April 8, 2009. Archived from the original on May 12, 2008. Retrieved on August 8, 2009.
  7. ACAS II Bulletin - Issue 10. Eurocontrol , November 5, 2007 (English).
  8. Schöneberg: Investigation report AX001-1-2 / 02 (PDF; 4 MB) Federal Office for Aircraft Accident Investigation . May 2004. Accessed on August 8, 2009: "Accident · July 01, 2002 · near Überlingen / Lake Constance"
  9. Peters: Investigation report 6X005-1 / 2/99 (PDF; 20 kB) Federal Office for Aircraft Accident Investigation . April 2000. Retrieved on May 14, 2013: "Serious incident · February 26, 1999 · near Lambourne (UK) VOR"
  10. Final report No. 2165 of the Swiss Accident Investigation Board SUST (PDF; 3 MB) Swiss Accident Investigation Board . July 17, 2013. Archived from the original on December 3, 2013. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved November 30, 2013. @1@ 2Template: Webachiv / IABot /