The BFU report says that the legal basis, procedures and instructions for traffic collision avoidance system (TCAS 2) Version 7 are intended as "a semi-automatic system which shall serve as a last line of defence in collision avoidance.
"Clear and unambiguous procedural instructions for the crews are an essential prerequisite. This prerequisite is so important, because the system philosophy of TCAS II Version 7 provides only one procedure after the issuance of an RA [resolution advisory] and that is to follow the generated RA.
"The decision to follow an RA without reservation could mean that up to the resolution of the conflict the crew has to divert from other obligatory standards for instance, from instructions for vertical separation issued by ATC and from other general right-of-way rules."
However, the Tupolev Tu-154M flight manual, Russian regulations at the time, and the TCAS manufacturer's guidance all stated that visual avoidance, rules of the air and controllers' instructions take precendence over an RA, and that TCAS is a back-up guidance system rather than the "final safety net" that Eurocontrol insists it is.
Source: Flight International
