TCAS

There are two levels of TCAS: the first or type 1 is for general aviation use. The objective is to provide “Traffic Advisories” to assist pilots in visually locating other aircraft that are intruding in their airspace.

TCAS II is a bit more sophisticated, in addition to locating intruders, type II can provide “Resolution Advisories” (RA) which issue guidance commands both aurally and visually on maneuvers to avoid a potential conflict that could lead to a mid-air collision.

The performance standards of type II were updated and approved in December 1997 and are referred to as “Version 7” by the Radio Technical Commission for Aeronautics (RTCA). TCAS II Version 7 has been scrutinized since its introduction and several anomalies were noted. Events had been reported where flight crews did not respond correctly to the “Adjust Vertical Speed” advisory where they increased rather than decreased the rate of change. In other situations Version 7 software failed to revise the RA when converging aircraft remained within a 100-foot vertical separation. The likelihood of this is aggravated when one aircraft is not adhering to a standard RA maneuver or is following the instructions of a ground-based air traffic controller.

Safety margins

A feature has been added to TCAS II Version 7.1 that will increase safety margins when an intruder aircraft is either not TCAS II equipped or is not following the system generated instructions to avoid a potential collision. Now, when it is detected that an intruder is not responding to the standard evasive action, a reversed RA will be issued by the new logic to steer the alerted aircraft away from the conflict. In addition, Version 7.1 provides a new “Level off” alert which will recommend a 0 feet/minute vertical rate and will not be tied to standard flight levels.

Version 7.1 has been mandated this past March (2012) by Eurocontrol for all aircraft utilizing European Union Airspace that are more than 5,700 kg (12,500 pounds) takeoff weight or those authorized to carry more than 19 passengers. An extended deadline of December 2015 for compliance exists for aircraft certified prior to March 1, 2012 and are equipped with Version 7.0.

Real-time traffic info

Automatic Dependent Surveillance-Broadcast (ADS-B) messages can now be sent from suitable transponders with parameters that contain aircraft identification, present location, and even speed. This technology is referred to as hybrid surveillance and enables TCAS equipment capable of processing this data to significantly enhance the performance of collision avoidance systems. In addition, ADS-B will provide real-time traffic information on the flight deck of even small aircraft but will not include RA or any other recommendation for collision avoidance.

The identity information that ADS-B provides can be used to make the flight deck display resemble the picture viewed by an air traffic controller and will potentially increase situational awareness. With the onset of ADS-B, future plans for TCAS including level III and IV have been shelved.

The FAA has issued Advisory Circular AC 20-151A which provides guidance for obtaining airworthiness approval plus includes definition and description of current and future plans for TCAS.

Ground/flight issues

Operation of on-board collision avoidance systems is predicated on a properly operating transponder. Anytime an aircraft is on the ground, Mode “A” (identification), Mode “C”, and Mode “S” should be automatically inhibited. Ground/flight issues are among the more common dilemmas encountered by transponders and subsequently collision avoidance systems. In addition radar altimeters will provide relative information pertaining to height above the ground and are used to inhibit aural TCAS alerts below 400 feet above ground level (AGL) while the aircraft is descending and below 600 feet AGL when climbing.

Most systems do include a self-test feature capable of being initiated by a flight crew member. This will verify the integrity of components, annunciators, symbols, and aural alerts. That TCAS comprise digital technology, built-in tests along with fault logs can often be a friend when diagnosing malfunctions.