ICAO Required Operational Procedures for the Use of ACAS II
Just a couple of years ago, ICAO reviewed the contents of PANS-OPS and subsequently published advice as shown in the excerpt from Part III Chapter 3that follows:
3.2 USE OF ACAS INDICATIONS
The indications generated by ACAS shall be used by pilots in conformity with the following safety considerations:
a) pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only;
Note 1.- TAs are intended to alert pilots to the possibility of a resolution advisory (RA), to enhance situational awareness, and to assist in visual acquisition of conflicting traffic. However, visually acquired traffic may not be the same traffic causing a TA. Visual perception of an encounter may be misleading, particularly at night.
Note 2.- The above restriction in the use of TAs is due to the limited bearing accuracy and to the difficulty in interpreting altitude rate from displayed traffic information.
b) on receipt of a TA, pilots shall use all information to prepare for appropriate action if an RA occurs; and
c) in the event of an RA, pilots shall:
1) respond immediately by following the RA as indicated, unless doing so would jeopardise the safety of the aeroplane;
etc, etc (seven additional instructions)
It might help if I explained that when I was writing guidance for UK flight crews (published in CAP 579) before the first UK aeroplanes actually had TCAS II installed, we felt it was important to ensure that pilots responded to any TAs that were posted ONLY (a) by preparing for the RA that might follow (eg hands on control column/ sidestick and on throttles/power levers) so that no time would be lost initiating the advised manouevre if an RA was posted, and (b) by looking out for the potentially conflicting traffic.
Because we pilots of civil aircraft are taught to assess abnormal situations that come to our attention before we react, rather than by simply responding without first assimilating all information available, the CAP advice included looking outside to possibly locate the intruder. If there was going to be any inaccuracy in the sense posted by the RA (unlikely, but possible IN THOSE EARLY YEARS), it would help the pilot flying the aeroplane to satisfy him or herself that any RA that followed the TA was not incorrect.
When TCAS II versions 6.02. 04 and 04A were installed (ie before version 7, henceforward described as ACAS II), there was a fairly high proportion of what could later be described as 'nuisance' RAs. Later, with version 7/ACAS II installed, this proportion reduced significantly. Thus, it now became more important to put the emphasis on responding without delay to RAs, having been primed to initiate that response by the TA that would normally precede it. Hence the ICAO caution about the possibility of mis-identifying the traffic that was causing the TA/RA. It is is simply that the emphasis has shifted now towards 'trust ACAS II to get it right' rather than 'confirm in your mind that TCAS II hasn't got it wrong'!
Ultimately, however, the pilot may disregard the RA (subject to the very important seven additional instructions in subparagraph c)) only when in his/her opinion 'doing so would jeaopardise the safety of the aeroplane'.
I hope that this helps anyone who reads this to understand the relative importance of a TA as described by ICAO.