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Traffic Alert and Collision Avoidance System (TCAS)

Surveillance

TCAS has made a measurable impact on reducing the number of midair and near midair collisions worldwide; but there have been collisions that resulted from an inconsistent application of TCAS procedures. See: DHL 611 & Bashkirskie Avialinii 2937 (Midair) for a 2002 example.


 

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Photo: Chicago O'Hare Arrival TCAS, (Eddie Photo)

General Description

[AIM, ¶4-4-16]

  1. TCAS I provides proximity warning only, to assist the pilot in the visual acquisition of intruder aircraft. No recommended avoidance maneuvers are provided nor authorized as a direct result of a TCAS I warning. It is intended for use by smaller commuter aircraft holding 10 to 30 passenger seats, and general aviation aircraft.
  2. TCAS II provides traffic advisories (TAs) and resolution advisories (RAs). Resolution advisories provide recommended maneuvers in a vertical direction (climb or descend only) to avoid conflicting traffic. Airline aircraft, and larger commuter and business aircraft holding 31 passenger seats or more, use TCAS II equipment.
    1. Each pilot who deviates from an ATC clearance in response to a TCAS II RA must notify ATC of that deviation as soon as practicable and expeditiously return to the current ATC clearance when the traffic conflict is resolved.
    2. Deviations from rules, policies, or clearances should be kept to the minimum necessary to satisfy a TCAS II RA.
    3. The serving IFR air traffic facility is not responsible to provide approved standard IFR separation to an aircraft after a TCAS II RA maneuver until one of the following conditions exists:
    4. (a) The aircraft has returned to its assigned altitude and course.

      (b) Alternate ATC instructions have been issued.

  3. TCAS does not alter or diminish the pilot’s basic authority and responsibility to ensure safe flight. Since TCAS does not respond to aircraft which are not transponder equipped or aircraft with a transponder failure, TCAS alone does not ensure safe separation in every case.
  4. At this time, no air traffic service nor handling is predicated on the availability of TCAS equipment in the aircraft.

Regulatory

[14 CFR 91, Appendix G, §2, ¶(g)] After March 31, 2002, unless otherwise authorized by the Administrator, if you operate an aircraft that is equipped with TCAS II in RVSM airspace, it must be a TCAS II that meets TSO C–119b (Version 7.0), or a later version.

Pilot Procedures

[ICAO Doc 8168 Vol 1 PANS OPS, §III, Chapter 3]

3.1.1 Airborne collision avoidance system (ACAS) indications shall be used by pilots in the avoidance of potential collisions, the enhancement of situational awareness, and the active search for, and visual acquisition of, conflicting traffic.

3.1.2 Nothing in the procedures specified in 3.2, “Use of ACAS indicators”, shall prevent pilots-in-command from exercising their best judgement and full authority in the choice of the best course of action to resolve a traffic conflict or avert a potential collision.

3.2 The indications generated by ACAS shall be used by pilots in conformity with the following safety considerations:

a) pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;

Note 1.— TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.

Note 2.— The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.

b) on receipt of a TA, pilots shall use all available information to prepare for appropriate action if an RA occurs; and

c) in the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;

Note 1.— Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS.

Note 2.— Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularly at night.

2) follow the RA even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre;

3) not manoeuvre in the opposite sense to an RA;

Note.— In the case of an ACAS-ACAS coordinated encounter, the RAs complement each other in order to reduce the potential for collision. Manoeuvres, or lack of manoeuvres, that result in vertical rates opposite to the sense of an RA could result in a collision with the threat aircraft.

4) as soon as possible, as permitted by flight crew workload, notify the appropriate ATC unit of the RA, including the direction of any deviation from the current ATC instruction or clearance;

Note.— Unless informed by the pilot, ATC does not know when ACAS issues RAs. It is possible for ATC to issue instructions that are unknowingly contrary to ACAS RA indications. Therefore, it is important that ATC be notified when an ATC instruction or clearance is not being followed because it conflicts with an RA.

5) promptly comply with any modified RAs;

6) limit the alterations of the flight path to the minimum extent necessary to comply with the RAs;

7) promptly return to the terms of the ATC instruction or clearance when the conflict is resolved; and

8) notify ATC when returning to the current clearance.

[ICAO Doc 8168, Vol 1, III-3-3-2] The indications generated by ACAS shall be used by pilots in conformity with the following safety considerations:

a) pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;

Note 1.— TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.

Note 2.— The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.

b) on receipt of a TA, pilots shall use all available information to prepare for appropriate action if an RA occurs; and

c) in the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;

Note 1.— Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS.

Note 2.— Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularly at night.

2) follow the RA even if there is a conflict between the RA and an air traffic control (ATC) instruction to manoeuvre;

3) not manoeuvre in the opposite sense to an RA;

Note.— In the case of an ACAS-ACAS coordinated encounter, the RAs complement each other in order to reduce the potential for collision. Manoeuvres, or lack of manoeuvres, that result in vertical rates opposite to the sense of an RA could result in a collision with the threat aircraft.

4) as soon as possible, as permitted by flight crew workload, notify the appropriate ATC unit of the RA, including the direction of any deviation from the current ATC instruction or clearance;

Note.— Unless informed by the pilot, ATC does not know when ACAS issues RAs. It is possible for ATC to issue instructions that are unknowingly contrary to ACAS RA indications. Therefore, it is important that ATC be notified when an ATC instruction or clearance is not being followed because it conflicts with an RA.

5) promptly comply with any modified RAs;

6) limit the alterations of the flight path to the minimum extent necessary to comply with the RAs;

7) promptly return to the terms of the ATC instruction or clearance when the conflict is resolved; and

8) notify ATC when returning to the current clearance.

Note.— Procedures in regard to ACAS-equipped aircraft and the phraseology to be used for the notification of manoeuvres in response to an RA are contained in the PANS-ATM (Doc 4444), Chapters 15 and 12 respectively.

[ICAO Doc 444, Chapter 12, ¶12.3.1.2]

Circumstances Phraseologies
... after modifying vertical speed to comply with an ACAS resolution advisory (Pilot and controller interchange) TCAS CLIMB (or DESCENT)
... after ACAS “Clear of Conflict” is annunciated (Pilot and controller interchange) RETURNING TO (assigned clearance)

Air Traffic Control Response to a Pilot's Resolution Advisory (RA) Response

[FAA JO 7110.65V, ¶2-1-27]

  1. When an aircraft under your control jurisdiction informs you that it is responding to a TCAS Resolution Advisory (RA), do not issue control instructions that are contrary to the RA procedure that a crew member has advised you that they are executing. Provide safety alerts regarding terrain or obstructions and traffic advisories for the aircraft responding to the RA and all other aircraft under your control jurisdiction, as appropriate.
  2. Unless advised by other aircraft that they are also responding to a TCAS RA, do not assume that other aircraft in the proximity of the responding aircraft are involved in the RA maneuver or are aware of the responding aircraft’s intended maneuvers. Continue to provide control instructions, safety alerts, and traffic advisories as appropriate to such aircraft.
  3. Once the responding aircraft has begun a maneuver in response to an RA, the controller is not responsible for providing standard separation between the aircraft that is responding to an RA and any other aircraft, airspace, terrain or obstructions. Responsibility for standard separation resumes when one of the following conditions are met:
    1. The responding aircraft has returned to its assigned altitude, or
    2. A crew member informs you that the TCAS maneuver is completed and you observe that standard separation has been reestablished, or
    3. The responding aircraft has executed an alternate clearance and you observe that standard separation has been reestablished.
    4. NOTE− 1. AC 120−55A, Air Carrier Operational Approval and Use of TCAS II, suggests pilots use the following phraseology to notify controllers during TCAS events. When a TCAS RA may affect an ATC clearance, inform ATC when beginning the maneuver, or as soon as workload permits.

      EXAMPLE− 1. “New York Center, United 321, TCAS climb.”

      NOTE− 2. When the RA has been resolved, the flight crew should advise ATC they are returning to their previously assigned clearance or subsequent amended clearance.

      EXAMPLE− 2. “New York Center, United 321, clear of conflict, returning to assigned altitude.”

Limitations

Your TCAS will only spot other aircraft with TCAS that are operating. Many aircraft still do not have TCAS and in some parts of the world it may be common practice to turn the transponder off:

[AC 91-70A, ¶8-16.b.] The TCAS system depends upon signals from the transponders of other aircraft and vice versa. Some foreign carriers actually turn their transponders off when they leave a Caribbean or South American radar environment. The International Air Transport Association (IATA) is making an effort to correct this situation. This is another reason to maintain extra vigilance during night operations. Look for instructions on the Latin America/South America High Altitude Chart. In South America, there are no alternate instructions in the AIP. Therefore, use code 2000 when beyond radar coverage if there is no specification for another code.

Typical Installation (Gulfstream G450)

[G450 OM, ¶2A-34-60]

  • The Honeywell Traffic and Collision Avoidance System (TCAS-2000) is production installed on the G-450 airplane. TCAS provides the flight crew with notifications of the presence of other transponder equipped traffic in the vicinity that may present a collision hazard. TCAS can track up to fifty (50) airplanes simultaneously, however the display can accommodate only thirty-two (32) traffic symbols. The system provides aural alerts to the presence of traffic, visual plots of the relative location of other airplanes on cockpit displays, and aural and visual cues for evasive maneuvers if collision is imminent. If both converging airplanes are equipped with TCAS and Mode S transponders, the systems mutually coordinate evasive maneuvers to ensure diverging flight paths.
  • TCAS can exchange data with other airplanes having Mode S transponders at a range of forty nautical miles (40 NM) and can detect (but not communicate with) Mode S equipped targets up to one hundred twenty miles (120 NM). For targets with Mode A or Mode C transponders, the range is twenty nautical miles (20 NM).
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Photo: G450 Antennas, (G450 AOM, §2A-06-00, figure 1)

Symbology:

TCAS symbology is the same on both the HSI, 1/6 window and MAP display. Traffic is represented in different icon shapes and colors corresponding to the potential threat of collision:

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Photo: G450 TCAS symbol definition, (G450 AOM, §2B-19-00, Table 1)

Control:

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Photo: G450 MCDU, (G450 AOM, §2B-19-20, pg. 2)

Relative Altitude Limits:

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Photo: G450 MCDU, (G450 AOM, §2B-19-00, Table 2)

Do you have to have it?

MEL Says No...

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Photo: G450 MEL, TCAS, (G450 MEL, pg. 34-13)

ICAO Says Yes.

[ICAO Annex 6, §6.18.]

6.18.1 From 1 January 2003, all turbine-engined aeroplanes of a maximum certificated take-off mass in excess of 15 000 kg or authorized to carry more than 30 passengers shall be equipped with an airborne collision avoidance system (ACAS II).

6.18.2 From 1 January 2005, all turbine-engined aeroplanes of a maximum certificated take-off mass in excess of 5 700 kg or authorized to carry more than 19 passengers shall be equipped with an airborne collision avoidance system (ACAS II).

6.18.3 Recommendation.— All aeroplanes should be equipped with an airborne collision avoidance system (ACAS II). 6.18.4 An airborne collision avoidance system shall operate in accordance with the relevant provisions of Annex 10, Volume IV.

References

14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation

Advisory Circular 90-48D, Pilot's Role in Collision Avoidance, 4/19/16, U.S. Department of Transportation

Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

Aeronautical Information Manual

FAA JO 7110.65V Air Traffic Control, U.S. Department of Transportation, April 3, 2014

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

ICAO Annex 6 - Operation of Aircraft - Part 1 Commercial Aircraft, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part I, 9th edition, July 2010

ICAO Annex 6 - Operation of Aircraft - Part 2 General Aviation, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part II, 8th edition, July 2014

ICAO Annex 6 - Operation of Aircraft - Part 3 Helicopters, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part III, 7th edition, July 2010

ICAO Doc 4444 - Air Traffic Management, Fourteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2001 *

ICAO Doc 4444 - Air Traffic Management, Fifteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2007 *

* Not all of Doc 4444 seems to have been reproduced in the 15th edition, so you might need to look at the 15th edition and then then 14th edition for some sections.

ICAO Doc 4444 - Amendment No. 1, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 1, 2007

ICAO Doc 4444 - Amendment No. 2, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 2, 19/11/09

ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 5th edition, 2006

Revision: 20161203
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