From our Operations Manual Part A (EU-OPS)
8.3.6 POLICY AND PROCEDURES FOR THE USE OF TCAS / ACAS
Traffic and Collision Avoidance System (TCAS) / Airborne Collision Avoidance
System (ACAS) description is given in the FCOM Aircraft System.
Associated procedures are given in QRH/FCOM "Abnormal and Emergency
procedures" and in FCOM Supplementary Procedures".
Airborne Collision and Avoidance Systems (ACAS) provide Flight Crew with an
independent back up to visual search and the ATC system by alerting the crew to
collision hazards, independent of any ground-based aids which may be used by
air traffic control for such purposes. ACAS II (Traffic Alert and Collision Avoidance
System Type II) is the specific equipment which is currently available to meet this
requirement, as detailed in the following paragraphs.
ACAS II
Provides collision avoidance manoeuvre advice in the vertical plane, in either of
two forms:
• Traffic Advisories (TA), which indicate the approximate position relative to the
subject aircraft, either in azimuth only, or azimuth and altitude, of nearby
transponding aircraft which may become a threat;
• Resolution Advisories (RAs) which command manoeuvres or manoeuvre
restrictions in the vertical plane to resolve conflicts with aircraft transponding
SSR Mode C altitude.
If a TA or a RA is received, the following action should be taken:
Traffic Advisory; intended to alert the crew that a RA, requiring a change in flight
path, may follow. A visual search should immediately be concentrated on that
part of the sky where the TA indicates the conflicting traffic to be. If the potential
threat gives cause for concern, air traffic control assistance should be requested in
deciding whether a change of flight path is required. Do not manoeuvre based on
a TA only.
Resolution Advisory; pilots shall:
• respond immediately by following the RA as indicated, unless doing so would
jeopardize the safety of the aeroplane;
NOTE:
1
Stall warning, windshear, and Ground Proximity Warning System alerts
have precedence over ACAS.
2 Visually acquired traffic may not be the same traffic causing an RA. Visual
perception of an encounter may be misleading, particularly at night.
• follow the RA even if there is a conflict between the RA and an air traffic
control (ATC) instruction to manoeuvre;
• not manoeuvre in the opposite sense to a 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.);
• as soon as possible, as permitted by flight crew workload, notify the
appropriate ATC unit of the RA using the appropriate RT phraseology;
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 is not being followed because it conflicts with an RA.
• promptly comply with any modified RAs;
• limit the alterations of the flight path to the minimum extent necessary to
comply with the RAs;
• promptly return to the terms of the ATC instruction or clearance when the
conflict is resolved; and
• notify ATC when returning to the current clearance.