Mid-air collisions are really scary. Usually, chances of survival are slim during these incidents, especially if the collision happened during cruise. Aside from the recent American Eagle flight AA5342 collision with a Black Hawk helicopter over the Potomac river, there were other worse mid-air collisions such as the 1996 Saudia flight 763 and Kazakhstan Airlines mid-air collision. As there are many aircraft already in the air at the same time, Traffic Alert and Collision Avoidance System, or TCAS was invented. Basically, this helps prevent midair collisions by monitoring nearby aircraft. It works by detecting signals from other planes’ transponders and issuing alerts when a collision risk arises.
Pilots receive traffic advisories that show the location of nearby aircraft. If the risk increases, TCAS provides a resolution advisory, instructing the pilot on how to maneuver to avoid a collision.
So how does TCAS work? TCAS-equipped aircraft constantly communicate with others by sending and receiving signals. These signals are transmitted at 1030 MHz and 1090 MHz frequencies. By analyzing these exchanges, TCAS creates a real-time map of surrounding aircraft and predicts possible conflicts.
This system only works with aircraft that have operational Mode C or Mode S transponders, which transmit altitude and identification data. If a potential collision is detected, TCAS automatically coordinates avoidance maneuvers between affected aircraft. These maneuvers are mainly vertical and are displayed visually in the cockpit, accompanied by voice instructions.
Components that Make Up TCAS
TCAS has several components, including a computer unit, antennas, and cockpit displays. The computer unit continuously monitors air traffic, assesses risks, and issues advisories. Antennas on the aircraft’s top and bottom enable the system to send and receive signals. Cockpit displays provide pilots with visual and audio alerts for quick response.
TCAS operates in different modes: Standby, Transponder, Traffic Advisories Only, and Automatic. In Standby mode, the system remains inactive, while Transponder mode allows signal responses without active TCAS monitoring. Traffic Advisories Only mode provides alerts without resolution advisories, and Automatic mode fully activates the system, issuing both traffic and resolution advisories when needed.
When a traffic advisory appears, pilots visually locate the aircraft and ensure safe separation. However, they do not rely solely on TCAS displays for maneuvering. If a resolution advisory is issued, pilots must follow TCAS instructions immediately, even if air traffic control has provided different commands. In such cases, controllers wait until the situation resolves before issuing new instructions.
Some air traffic control systems have RA downlink capabilities, allowing controllers to see resolution advisories in real time. However, ICAO does not require this feature. Pilot training focuses on following RAs (Resolution Advisories) correctly, notifying controllers promptly, and minimizing altitude deviations before returning to assigned flight levels.
Some Limitations
While TCAS is a valuable safety tool, it does have limitations. The system depends on accurate altitude reporting from other aircraft and assumes no sudden changes that could affect the resolution advisory. In rare cases, TCAS may even contribute to near-miss collisions. To address these concerns, recent regulations have introduced ground proximity alerts to prevent unsafe descent maneuvers.
TCAS II Version 7.0 improves safety by adding corrective RAs and green arc indicators on cockpit displays. These enhancements guide pilots on safe climb and descent rates, helping to reduce the risk of errors and improve overall safety.
Airbus’ AP/FD TCAS
Research indicates that a resolution advisory (RA) occurs approximately once every 1,000 flight hours on short- and medium-haul flights, and about every 3,000 hours on long-haul flights. A 2017 study by Eurocontrol revealed that in 25% of cases, pilots did not respond correctly to RAs. To address this, Airbus developed an autopilot/flight director TCAS function. This system automates avoidance maneuvers, enhancing response accuracy and ensuring better compliance with the RA instructions. By improving how pilots interact with TCAS, this feature contributes to safer operations and reduces the potential for errors during critical situations.
Available on the A320, A330, A350, and A380 aircraft, the AP/FD TCAS function enables pilots to maintain autopilot engagement during Resolution Advisories (RAs) or to follow the RA manually with flight director guidance. This system ensures precise responses to TCAS alerts while preventing unnecessary or excessive maneuvers.
An Airbus study of over 130,000 A350 and A380 flights showed that 91% of pilots preferred keeping the autopilot engaged during RAs. However, activation rates remain low for A320 and A330 aircraft.
As more aircraft take to the skies and air traffic becomes increasingly congested, manufacturers and avionics suppliers continue refining safety technologies like TCAS to prevent mid-air collisions. While accidents and incidents can still occur, the primary goal is to minimize the loss of human lives. Advancements in TCAS technology aim to enhance the accuracy of traffic advisories and resolution advisories under all conditions.
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