TCAS

Traffic Collision Avoidance System: Avoiding close encounters or worse!


Having accumulated several million miles traveling within the world’s airways, a frequent occurrence has been overhearing my fellow passengers voice a belief that the air traffic control system must be an organization funded by the railroads to discourage travel by air.

Yet given the total number of flights every year along with the forecast exponential growth in the total number of aircraft entering service, a system or method to help ensure two or more aircraft do not occupy the same space at any given time has to be a good thing and the fact that most close encounters are nothing more than that indicate something is working as advertised.

Midair collisions

Statistics point out the majority of midair collisions happen to those engaged in recreational flying without a flight plan in good weather conditions during weekend daylight hours and virtually all encounters occurred below 6,000 feet.

The flight decks of today provide pilots with an abundance of information which tends to lead to a heads down approach to flying. Undisputedly, many close calls and collisions could be avoided by simply spending more time looking out the windows.

An aircraft collision avoidance system is intended to independently monitor the airspace around aircraft and alert the crew to potential conflicts and has taken on the official title of “Traffic Collision Avoidance System” or TCAS. It involves communications between all in range aircraft equipped with an appropriate transponder and utilizes the same coding as the ground-based secondary surveillance radar (SSR) utilized by air traffic controllers.

The concept is to establish a three-dimensional zone surrounding the TCAS equipped aircraft capable of alerting the pilot(s) to any intrusion of their airspace by another transponder equipped machine. This technology is completely independent of ground-based directives and can monitor all aircraft with an operating Mode C (altitude reporting) or Mode S (enhanced surveillance) transponder. Transponders produce an “Interrogation” transmission utilizing a coded signal on the 1,030 MHz frequency and then wait for a coded response on 1,090 MHz.

A TCAS processor senses for the 1,030 MHz signal produced by all transponders and once this interrogation is noted, the processor will utilize its own aircraft altitude input compared to the decoded intruder altitude to make a determination if advising the flight crew is warranted. The equipment uses pressure altitude, radar altitude, and in some cases aircraft position information to detect threats and provide flight crews with evasive actions.

System installations will include several external antennae with one mounted on the top and another on the bottom of the fuselage. The upper antenna is frequently a directional type that is accurate within about 15 degrees and the lower may be either directional or omni directional. TCAS antennas are separate from the upper and lower blades required for the enhanced surveillance transponder (Mode S). It has been noted that propellers can effectively block at least part of the arc of the directional antenna(s) so a thorough investigation should be conducted prior to deciding on antenna location for new installations.

Hazardous warnings

Once the processor analyzes the transponder interrogations and correlates to aircraft parameters, a pictorial image is produced and sent to the flight deck display. Frequently this presentation will appear on an electronic vertical speed indicator (VSI) but with newer digital flight instruments it is possible to display traffic information in several locations.

Standardized audible advisories have been created and can be reproduced to the crew to draw further attention to a potential hazard. The intent is to produce an image with an alert to enlighten flight crews to all identified traffic within a certain radius of their aircraft. The second feature is to highlight any potential threats. That is, aircraft getting too close or on a collision course at the same altitude.

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