Some important points on use of ACAS

  • Pilots need to be aware of the ACAS version they use, and its limitations; and be trained according that version.

  • To support the effectiveness of ACAS, the pilot of any aircraft that is fitted with a transponder (even when flying VFR and not ACAS equipped) should select the transponder ON with an appropriate code, with altitude reporting function (ALT, Mode C) if possible, at all times unless specifically requested otherwise.

  • Some of the simpler ACAS systems, including TCAS I, can give only Traffic Alerts (TA) and cannot generate Resolution Advisories (RA).

  • The objective of the TA is to assist the pilot in visual acquisition of conflicting traffic, i.e. where to look for the other aircraft.

  • Visually acquired traffic may not be the same traffic as that causing a TA.

  • The bearing displayed by ACAS is not sufficiently accurate to support the initiation of horizontal manoeuvres based solely on the traffic display. Pilots should be aware that the bearing indication may be up to 30 degrees in error.

  • Pilots must not manoeuvre in response to a Traffic Alert (TA) only .

  • With TCAS II, the purpose of the TA is to alert the pilot so that he can be prepared to respond to any RA that might follow.

  • Pilots must not deviate from an assigned clearance on the basis of a TA only.

  • TAs should not be reported to air traffic controllers. Similarly, "TCAS contact" should not be reported, e.g. after receiving traffic information from ATC.

  • Pilots of aircraft fitted with TCAS II version 6.04A need to be aware that they will experience a much greater number of TAs and RAs, e.g. when flying in RVSM airspace. It is important they follow all RAs, even though the number is greater than with TCAS II version 7.0.

  • Reducing vertical rate before level-off will reduce the number of RAs due to high vertical rate encounters, i.e. <1500 feet per minute by 1000 feet from the assigned level (and monitor autopilot performance during altitude capture).

  • Pilots must always respond promptly and accurately to all RAs as indicated, whatever the airspace, unless doing so would jeopardise the safety of the aircraft.

  • Visually acquired traffic may not be the same traffic as that causing an RA.

  • When following an RA, the alteration of the flight path must be limited to what is necessary to comply with the RA; and pilots must never manoeuvre in the opposite sense to an RA.

  • Vertical speed must be reduced in response to "Adjust Vertical Speed" RAs.

  • Pilots who deviate from an air traffic control instruction or clearance in response to an RA must promptly return to the terms of that instruction or clearance when the conflict is resolved. ATC must be notified, as soon as practicable of the deviation and its direction, and when the deviation has ended.


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