ICAO Annex 10, vol. IV (Aeronautical Telecommunications - Surveillance and Collision Avoidance Systems), Fourth Edition, July 2007
Airborne collision avoidance system (ACAS). An aircraft system based on secondary surveillance radar (SSR) transponder signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders.
Note.— SSR transponders referred to above are those operating in Mode C or Mode S.
Collision avoidance logic. The sub-system or part of ACAS that analyses data relating to an intruder and own aircraft, decides whether or not advisories are appropriate and, if so, generates the advisories. It includes the following functions: range and altitude tracking, threat detection and RA generation. It excludes surveillance.
Resolution advisory (RA) – an indication given to the flight crew recommending:
- a) a manoeuvre intended to provide separation from all threats; or
- b) a manoeuvre restriction intended to maintain existing separation.
Corrective RA. A resolution advisory that advises the pilot to deviate from the current flight path.
Preventive RA. A resolution advisory that advises the pilot to avoid certain deviations from the current flight path but does not require any change in the current flight path.
Traffic advisory (TA). An indication given to the flight crew that a certain intruder is a potential threat.
Contrary pilot response
22.214.171.124.3 Manoeuvres opposite to the sense of an RA may result in a reduction in vertical separation with the threat aircraft and therefore must be avoided. This is particularly true in the case of an ACAS-ACAS coordinated encounter.
126.96.36.199.1 New ACAS installations after 1 January 2014 shall monitor own aircraft’s vertical rate to verify compliance with the RA sense. If non-compliance is detected, ACAS shall stop assuming compliance, and instead shall assume the observed vertical rate.
Note 1.— This overcomes the retention of an RA sense that would work only if followed. The revised vertical rate assumption is more likely to allow the logic to select the opposite sense when it is consistent with the non-complying aircraft’s vertical rate.
Note 2.— Equipment complying with RTCA/DO-185 or DO-185A standards (also known as TCAS Version 6.04A or TCAS Version 7.0) do not comply with this requirement.
Note 3.— Compliance with this requirement can be achieved through the implementation of traffic alert and collision avoidance system (TCAS) Version 7.1 as specified in RTCA/DO-185B or EUROCAE/ED-143.
188.8.131.52.2 Recommendation.— All ACAS should be compliant with the requirement in 184.108.40.206.1.
220.127.116.11.3 After 1 January 2017, all ACAS units shall comply with the requirements stated in 18.104.22.168.1.