The Traffic alert and Collision Avoidance System (TCAS) has been introduced in order to reduce the risk of mid-air collisions or near mid-air collisions between aircraft. When TCAS II version 7.1 is implemented the probability of mid-air collisions will be reduced significantly. *

The introduction of TCAS change 7.1 software is available for Fokker 50, Fokker 70 and Fokker 100 aircraft as well as for other types of aircraft like Airbus A318, A319, A320, A321, Boeing 737 series, 757, 767,  Bombardier Dash 8 and CRJ. (* source: www.eurocontrol.int)

The Traffic alert and Collision Avoidance System (TCAS) has been introduced in order to reduce the risk of mid-air collisions or near mid-air collisions between aircraft. Studies conducted for Eurocontrol, using recently recorded operational data, indicate that currently the probability of a mid-air collision in European airspace equates to one in every 3 years. When TCAS II version 7.1 is implemented that probability will reduce by a factor of 4. *

EASA issued Safety Information Bulletin EASA SIB No: 2009-16, in which they indicated that there have been a number of instances of incorrect pilot response to TCAS Resolution Advisories (RA) due to misinterpretation of the RA aural annunciations and RA displays, which have resulted in serious incidents.

The “Adjust Vertical Speed, Adjust” resolution advisory requires the reduction of vertical speed to 2000, 1000, 500, or 0 feet/min. In the cases of incorrect responses, the pilots increased their vertical speed instead of reducing it, consequently, causing the situation to deteriorate.

Enhancements in training alone can improve the behavior of flight crews when an “Adjust Vertical Speed, Adjust” resolution advisory is issued, but is not sufficient to avoid all the opposite reactions. A second reason to update the current TCAS software is a series of mid-air encounters in which safety margins have been lost (including 2 accidents), due to the failure of the TCAS system to reverse some Resolution Advisories (RA) when a reversal is more efficient to resolve the threat of a collision.

EASA has mandated this change 7.1 software update as an Airspace Usage Requirement per December 01/2015. (* source: www.eurocontrol.int)

Several other authorities have other Mandates (planned).

TCAS software change 7.1 improves safety by implementing the following improvements:

• The “Adjust Vertical Speed, Adjust” (AVSA) RA of TCAS software change 7.0. Several AVSA RA versions have been replaced by a single “Level-off” RA. The associated aural message is straightforward and the associated man oeuvre corresponds to the standard manoeuvre already performed in critical Situations. 
• The reversal logic of TCAS software change 7.0 by detecting geometries by easing the triggering thresholds of reversal RAs in encounters in which the aircraft remain within 100 feet vertical separation to each other. Furthermore the change adds detection of RA compliance by the own aircraft and prediction of vertical separation at the closed point of approach to detect the need of reversal RA.