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.

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.

TCAS does provide many benefits when it comes to increasing situational awareness for flight crews and along with the benefits come the challenges to people such as AMTs that are tasked with keeping it working.

Having just encountered a three-hour ATC delay on a recent airline trip, I did go to the Amtrak schedule and found that it would take the train about 10 hours (not considering Federal Railroad Administration (FRA) Rail Traffic Controller induced delays) to travel between my required city pairs. Even with a three-hour delay I am still convinced travel by air is the only way to go.

Jim Sparks has been in aviation for 30 years and is a licensed A&P. His career began in general aviation as a mechanic, electrician, and avionics technician. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. You can reach him at