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ACAS/TCAS

ACAS/TCAS. And we wonder why we might need TCAS…. ICAO ACAS. The International Civil Aviation Organization (ICAO) is responsible for the global standardization of ACAS.

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ACAS/TCAS

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  1. ACAS/TCAS

  2. And we wonder why we might need TCAS….

  3. ICAO ACAS • The International Civil Aviation Organization (ICAO) is responsible for the global standardization of ACAS. • The equipment which meets the ACAS II Standards and Recommended Practices (SARPs) is known as TCAS II, version 7. A joint RTCA/EUROCAE working group is currently finalizing amendments to the MOPS, addressing three specific safety improvement changes related to the collision avoidance logic; these new MOPS will form TCAS II version 7.1.

  4. ICAO ACAS • The Airborne Collision Avoidance System II (ACAS II) was introduced in order to reduce the risk of mid-air collisions or near mid-air collisions between aircraft. It serves as a last-resort safety net irrespective of any separation standards. • ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. Non-transponding aircraft are not detected.

  5. ICAO ACAS • ACAS II works independently of the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems. While assessing threats it does not take into account the ATC clearance, pilot’s intentions or autopilot inputs. • Currently, the only commercially available implementation of ICAO standard for ACAS II (Airborne Collision Avoidance System) is TCAS II version 7.0 (Traffic alert and Collision Avoidance System).

  6. Types of ACAS (TCAS) ACAS I • Gives Traffic Advisories (TAs) but does not recommend any maneuvers. The only implementation of ACAS I concept is TCAS I. ICAO Standards and Recommended Practices (SARPs) for ACAS I are published in ICAO Annex 10, volume IV and are limited to interoperability and interference issues with ACAS II. ACAS I is mandated in the United States for certain smaller aircraft (TCAS version 6.04a) • TCAS I was developed to accommodate the general aviation (GA) community and the regional airlines. This system issues ‘Traffic Advisories’ (TAs) to assist pilots in visual acquisition of intruder aircraft. TCAS I is mandated on aircraft with 10 to 30 seats, although TCAS II may be installed instead. Uses a directional antenna to view Mode A, C, or S transponders on other aircraft to generate a situation display and "Traffic Advisory" (TA) for nearby targets. This TA is used to help pilots visually locate nearby co-altitude traffic (Mode C) or unknown altitude traffic (Mode A).

  7. ACAS II • Gives Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical sense (direction). The only implementation of ACAS II concept is TCAS II Version 7. ICAO SARPs published in Annex 10. ACAS II (TCAS II version 7.0) is mandated in Europe. • TCAS II is a more sophisticated system which provides the information of TCAS I, and also analyzes the projected flight path of approaching aircraft and issues ‘Resolution Advisories’ (RAs) to the pilot to resolve potential mid-air collisions. TCAS II is required internationally in aircraft with more than 30 seats or weighing more than 15,000 kg.

  8. The TCAS II Minimum Operational Performance Standards (MOPS) were updated and approved by RTCA, Inc. Special Committee 147 and published in document DO-185A in December, 1997 and available through RTCA, Inc. This latest revision to the system is referred to as "Version 7" in the United States. The international community adopted these standards for the Airborne Collision Avoidance System (ACAS) and it is now mandated throughout most of Europe and other countries such as Japan and Australia. • The vertical sense (direction) of the RA is coordinated with other ACAS II equipped aircraft via a mode S link, so that two aircraft choose complementary maneuvers. RAs aim for collision avoidance by establishing a safe vertical separation (300 – 700 feet), rather than restoring a prescribed ATC separation.

  9. ACAS II operates on relatively short time scales. The maximum generation time for a TA is 48 seconds before the Closest Point of Approach (CPA). • For an RA the time is 35 seconds. The time scales are shorter at lower altitudes (where aircraft typically fly slower). Unexpected or rapid aircraft maneuvers may cause an RA to be generated with much less lead time. It is possible that an RA will not be preceded by a TA if a threat is imminent. The effectiveness of an RA is evaluated by the ACAS equipment every second and, if necessary, the RA may be strengthened, weakened, reversed, or terminated. Uses a directional antenna to view Mode A, C, or S transponders on other aircraft to generate a situation display and a TA for nearby targets. For target aircraft with Mode C or S transponders, the TCAS display can generate a "Resolution Advisory" (RA), which commanded vertical maneuver (climb/descent) to avoid nearby co-altitude traffic. For target aircraft with Mode S transponders *AND* TCAS II equipment, RAs will be coordinated between aircraft (e.g. the two TCAS processors will cooperatively agree to send one aircraft in a climb and the other in a descent.) Note: aircraft equipped with TCAS II must have Mode S transponders installed.

  10. RA Advisories

  11. ACAS III • Gives TAs and RAs in vertical and/or horizontal directions. Also referred to as TCAS III and TCAS IV. Not currently implemented and unlikely to be in the near future. ICAO SARPs for ACAS III have not been developed. Currently, there are no plans to proceed with such a development. • ADS-B will likely replace TCAS III & IV

  12. From the controller side… 2-1-27. TCAS RESOLUTION ADVISORIES • 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. • 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. • 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:

  13. 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. NOTE- 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- 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.

  14. Tradewind GOM TCAS Alerts and Warnings • TCAS should be operated in the TA/RA mode, if equipped during all operations in the RVSM airspace. • In the even that a Traffic Advisory(TA) is received, flight crews should begin a visual search and prepare to respond to a Resolution Advisory (RA). • Maneuvers should never be made in a direction opposite to that given in an RA; this is because the sense may have been determined following an exchange of data with the established threat. Note: for this reason, if the pilots simultaneously receive an instruction from ATC and an RA, and they conflict, the advice given by the TCAS RA should be followed.

  15. Tradewind GOM • Pilots should be aware that if the intruder is also TCAS equipped, the direction not to follow an RA may result in a decrease in separation at Closest Point of Approach (CPA) because of the intruder's RA response. A decision not to follow an RA also renders the other aircraft's TCAS less effective than if your own aircraft was not TCAS equipped. • In the event that an RA is issued, the required maneuver should be initiated immediately. An RA should be disregarded only when the potentially conflicting traffic has been positively identified. • Reporting Requirements: (1). All RAs should be reported to ATC. (2). A written report is needed for all RAs requiring a change in the existing vertical speed while operating in the RVSM airspace and Transition Areas. (3). An Altitude Deviation Report should also be submitted as soon as possible to the appropriate airspace controlling agency. (4). Crews should submit an OIR to the Chief Pilot.

  16. Safety Benefits • The safety benefits delivered by ACAS are usually expressed in terms of the risk ratio (does ACAS make safety better or worse?). For Europe, ACAS is estimated to reduce the risk of mid-air collision by a factor of about 5 (i.e. a risk ratio of 22%). • The importance of correctly following RAs is illustrated by the fact that a pilot who never follows RAs faces three times the risk that is faced by a pilot who always follows RAs. The human is the weakest element in the ACAS control loop; without “human in the loop” the risk ratio would improve by a factor of 10.

  17. Ten Fundamental Dos and Don’ts 1) TCAS II must be operated in RA mode to provide full safety benefits 2) Pilots must follow all RAs promptly and accurately 3) Pilots most never maneuver in the opposite sense of an RA 4) Pilots must report RAs to controllers as soon as possible 5) Controllers must not interfere with pilots' reactions to RAs 6) Vertical speed must be reduced in response to 'Adjust Vertical Speed' RAs 7) TCAS traffic displays must not be used for self-separation 8) Vertical speed must be reduced when approaching the cleared flight level. 9) VFR pilots must operate their altitude reporting transponder. 10) Pilots and controllers must be recurrently trained on ACAS II operations.

  18. Case Study: • Accident paper • http://asasi.org/papers/2004/Reuss_ACAS_ISASI04.pdf • Accident Video

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