It is disturbing that there appears to be a disparity in international rules for airborne collision avoidance system (ACAS) avoidance manoeuvres (Flight International, 23-29 July).
Your item on the Japan Airlines near-collision in January last year indicates that International Civil Aviation Organisation documents give an air traffic control (ATC) instruction priority over an ACAS Resolution Advisory (RA). For the European Joint Aviation Authorities and the UK Civil Aviation Authority (CAA), the opposite priority is the rule.
The JAA's JAR-OPS Temporary Guidance Leaflet No. 11 of 1 October 1998 states that "if pilots simultaneously receive instructions to manoeuvre from ATC and an RA which are in conflict, the pilot should follow the RA".
UK AIC 54/1999 states that "manoeuvres should never be made in a direction opposite to that given in an RA" and clarifies the legality of departing from an ATC clearance to avoid immediate danger. My experience using ACAS worldwide suggests to me that the JAA and CAA have it right.
ACAS rarely gives an RA without a preceding Traffic Advisory (TA), typically 15-48 seconds before a predicted closest point of approach (CPA), whereas an RA is issued between 15 and 35 seconds before a CPA. Pilots should never manoeuvre solely in response to a TA, but I think pilots might use their discretion to alert ATC if a TA looks likely to become an RA.
When any TA is received, if the crew assesses it and prepares to react, if it becomes an RA, the action is immediate and follows the guidance given on the display indicator.
R W Gamblin
Barmouth, UK
Source: Flight International
