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ATC Avoiding Instructions Opposite to TCAS RA

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Category: Safety Nets Safety Nets
Content source: SKYbrary About SKYbrary

When a loss of separation is likely to occur or has occurred, the controller will issue instruction(s) to one or both pilots. These instructions may conflict with Airborne Collision Avoidance System (ACAS) indications, creating confusion and the possibility for error.

This conflict may arise because of the much quicker reaction time of TCAS compared to the controller, who has to:

  • assess the situation;
  • develop a solution in a very short period of time; and,
  • communicate this solution to the aircrew as quickly and clearly as possible.

The detection of the conflict may be delayed due to tasks with other aircraft under his control. Communications with conflicting aircraft may also be delayed due to RTF congestion or misunderstandings between the controller and the pilots.

Airborne Collision Avoidance System (ACAS) II automatically detects any risk of collision. When a risk of collision is detected, it calculates the necessary manoeuvre and communicates the solution directly to the flight crew via the RA display and an aural message attention getter. It does this in less than one second.

Whenever both aircraft are operating TCAS II in RA mode, TCAS II coordinates the RAs. So if TCAS indicates a descent to one aircraft, but the pilot climbs in response to the controller's advice, the risk of collision is increased.

Therefore, whenever TCAS II indicates a RA, the pilot should follow the RA immediately, even if it is contrary to the instruction received from the controller.

This advice is supported by ICAO and JAR-OPS 1, which requires that "An operator shall establish procedures to ensure that:

... When undue proximity to another aeroplane (RA) is detected by ACAS, the commander or the pilot to whom conduct of the flight has been delegated must ensure that any corrective action indicated by the RA is initiated immediately, unless doing so would jeopardize the safety of the aeroplane;
The corrective action must:
(i) Never be in a sense opposite to that indicated by the RA
(ii) Be in the correct sense indicated by the RA even if this is in conflict with the vertical element of an ATC instruction."

(JAR-OPS 1.398 - Use of Airborne Collision Avoidance System (ACAS))

Further Reading

ICAO Doc 4444: PANS-ATM Chapter 15;

ICAO Doc 8168: PANS-OPS Volume 1, Chapter 3;

JAR-OPS 1.398