TCAS II Version 7.1

While TCAS II is not an integral component in the ADS-B systems on-board aircraft, the two systems are very closely linked. TCAS involves communication between all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft interrogates all other 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).

As previously discussed, ADS-B messages are transmitted from aircraft through 1090 (ES) transponders, containing the identity, location, and velocity information. The signals are broadcast on the 1090 MHz radio frequency.

TCAS equipment which is capable of processing ADS-B messages may use this information to enhance the performance of TCAS, using techniques known as "hybrid surveillance." Some TCAS manufacturers will incorporate this capability as a part of their version 7.1 upgrade. See FAA Advisory Circular AC 20-151A for more information on Hybrid Surveillance.

As currently implemented, hybrid surveillance uses reception of ADS-B messages from an aircraft to reduce the rate at which the TCAS equipment interrogates that aircraft. This reduction in interrogations reduces the use of the 1030/1090 MHz radio channel, and will over time extend the operationally useful life of TCAS technology. The ADS-B messages will also allow lower cost technology to provide real time traffic in the cockpit for small aircraft.

Hybrid surveillance does not include the use any of the aircraft flight information in the TCAS conflict detection algorithms; ADSB is used only to identify aircraft that can safely be interrogated at a lower rate.

In the future, prediction capabilities may be improved by using the state vector information present in ADS-B messages. Also, since ADS-B messages can be received at greater range than TCAS normally operates, aircraft can be acquired earlier by the TCAS tracking algorithms.

The identity information present in ADS-B messages can be used to label other aircraft on the cockpit display (where present), painting a picture similar to what an air traffic controller would see and improving situational awareness.

If a TCAS II system is installed in an aircraft to be fit 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."

TCAS version 7.1 will be offered as an upgrade by all of the major TCAS manufacturers, and also makes two important safety enhancements. Version 7.1 changes the current TCAS II aural 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, in particular the situation when one of the pilots decides not to follow the Resolution Advisory (RA), or is instructed by ATC to perform a particular maneuver. The solution in Change 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. Version 7.1 corrects an issue when descending through 1000 ft AGL. 7.1 also 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. And 7.1 also allows for the transmission of the TCAS processor part number and software level, and corrects TCAS multi-aircraft logic issues which reduces the risk of "closeencounters" of multiple aircraft in RVSM airspace.

The enhancements introduced in TCAS version 7.1 proved to be significant enough to warrant mandates by both the International Civil Aviation Organization (ICAO), and the European Aviation Safety Agency. ICAO has mandated TCAS version 7.1 by January 1, 2014 for forward fit aircraft, and January 1, 2017 for retrofit aircraft. EASA has stepped their time-table up by comparison with forward fit aircraft due by March 1, 2012, and retrofit aircraft by March 1, 2014.

The FAA is reportedly a "strong supporter" of TCAS version 7.1, and has issued the statement "The latest version of of software for TCAS II is version 7.1. To ensure compatibility with international standards, the FAA encourages the installation of this software as soon as practical." This statement has stirred thoughts among the avionics manufacturing community that version 7.1 may be mandated in the US.

The certification path for TCAS version 7.1-modified units will be via STC and TC. The change will be reflected in the aircraft TC for forward fit aircraft, and the system will need to be STC’d in a retrofit aircraft. This will add fairly significant cost to the upgrade. The original STC for the TCAS II system currently installed in your aircraft will need to be re-opened, amended for version 7.1, and recertified. STC’s require time, and money.

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