The Traffic Collision Avoidance System (TCAS) was introduced to reduce the risk of mid-air collisions and near mid-air collisions of aircraft. Separate from other standards, the TCAS serves as a last-resort safety measure. It is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. The system operates by interroganding the Mode C and S transponders of nearby aircraft and then tracks their altitude and range, issuing alerts to the pilots as needed. Unfortunately, TCAS will not detect aircraft without transponders nor issue the warning of nearby traffic. It is also not linked to aircraft navigation, flight management systems, Air Traffic Control systems, or autopilot systems, instead working independently of these. While assessing potential threats, TCAS does not take into account ATC clearance, pilot intentions, or flight management system inputs.
There are two types of alerts issued by TCAS: traffic advisory (TA) and resolution advisory (RA). The TA is issued to help the pilot in the visual acquisition of the conflicting aircraft and prepare the pilot for an RA. When TCAS establishes a potential risk, an RA is generated. In most cases, the RA informs the pilot of the vertical speed at which the aircraft should be flown to avoid the other aircraft. This is shown on the flight instruments and often accompanied by an audio message indicating the same. Once the RA is issued, it is coordinated with the other aircraft (assuming it has TCAS technology equipped) and both aircraft are instructed of the proper evasive maneuvers.
There are currently three types of TCAS: TCAS I, II, and III. TCAS I provides traffic advisories but does not recommend any maneuvers. In the United States, certain types of smaller aircraft are mandated to have TCAS I. TCAS II provides both traffic advisories and resolution advisories in the vertical direction, but not horizontally. Lastly, TCAS III provides TAs and RAs in both vertical and horizontal directions. It is sometimes referred to as TCAS IV and is not currently implemented and not expected soon. Standards and recommended practices (SARPs) have not yet been developed and there are currently no plans for such development.
International standards for TCAS are set forth by ICAO, the International Civil Aviation Organization and are based on the minimum operational performance standards prepared by entities such as RTCA and EUROCAE (European Organization for Civil Aviation Equipment). TCAS equipment is currently available from four vendors: ACSS, Garmin, Honeywell, and Rockwell Collins. Though the equipment from each vendor varies, they all provide the same basic functions and collision avoidance & coordination logic.
TCAS’ safety benefits are usually represented by risk ratio. Studies have estimated that TCAS makes flying up to 21.7% safer. At any given time, regardless of the type or level of TCAS equipment on other aircraft, the risk of collision for a specific aircraft can be reduced by a factor greater than three by utilizing TCAS II. However, ultimately, the single most important factor in the safety performance of TCAS comes down to the pilot’s response time to RAs.
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