TCAS II Version 7.1
Although a traffic alert and collision avoidance system (TCAS II) is not an integral component in the ADS-B systems on-board aircraft, the two systems are closely linked. Created to reduce the possibility of mid-air collisions, TCAS involves communication among all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft interrogates all other similarly equipped aircraft in a determined range about their position (via the 1030 MHz radio frequency), and all other aircraft reply to those interrogations (via 1090 MHz).
If a TCAS II system is installed in an aircraft that will eventually be outfitted with ADS-B equipment, the TCAS system will send a message to the ADS-B equipment telling the ADS-B system that the TCAS II system is installed and operational.
All major TCAS manufacturers offer an upgrade to TCAS version 7.1, which makes two important safety enhancements. Version 7.1 changes the current TCAS II audio warning from "Adjust Vertical Speed, Adjust" to "Level Off, Level Off." It also corrects missed and late TCAS reversals. TCAS reversals were introduced in TCAS version 7.0 to adapt to changing situations where the original sense had clearly become the wrong thing to do; for instance, when one of the pilots decides not to follow the Resolution Advisory (RA) or is instructed by ATC to perform a particular maneuver. 7.1 introduces improvements to the current reversal logic to address late issuance of reversal RAs and potential failures to initiate reversal RAs.
TCAS version 7.1 also makes four other minor enhancements to the system:
- Corrects an issue when an aircraft is descending through 1,000-feet above ground level (AGL)
- Modifies the Datalink Capability Report (the TCAS status report sent by the TCAS processor to the Mode S transponder) to tell the systems that the TCAS processor is Hybrid Surveillance-capable.
- Allows for the transmission of the TCAS processor part number and software level.
- Corrects TCAS multi-aircraft logic issues that reduces the risk of close-encounters of multiple aircraft in RVSM airspace.
The enhancements introduced in TCAS version 7.1 proved to be significant enough to warrant mandates by both ICAO and the EASA. According to the ICAO, all new-production aircraft had to have been upgraded to TCAS version 7.1 by January 1, 2014, and all retrofit aircraft by January 1, 2017. EASA required upgrades for forward-fit aircraft by March 1, 2012, and retrofit aircraft by December 1, 2015.
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