Have you ever looked up at the sky, seen two planes cross paths and wondered how they didn’t collide? It’s not magic or chance, it’s TCAS.
Today, we are going to talk about one of the most important pieces of modern aviation safety: the Traffic Collision and Avoidance System, colloquially known as TCAS.
Join us on this flight to find out what it is, how it works and why mastering this system is an essential step on your way to becoming a commercial pilot.
Imagine you are flying an airliner at 35,000 feet. The sky seems immense, but in reality, air routes are like invisible highways that can become very busy.
TCAS, which stands for Traffic Collision Avoidance System, is the system that allows us to navigate these highways with maximum safety.
Simply put, TCAS is an on-board electronic system that operates independently of air traffic control (ATC) on the ground. Its sole mission is to detect the presence of other nearby aircraft and, if there is a risk of collision, to provide pilots with clear and precise collision avoidance instructions.
It is not just a detector; it is a proactive system. It doesn’t just say ‘look out, there’s another plane’, it coordinates an evasive manoeuvre in the air, making sure that if one plane goes up, the other goes down.
TCAS communicates with the transponders or Secondary Surveillance Radars (SSRs) of other nearby aircraft to obtain their altitude and trajectory.
When it detects a potential conflict, it acts in a tiered logic so as not to create unnecessary stress in the cockpit. These are the two levels of warning:
This is the first level of alert. It occurs when another aircraft is approaching and could become a threat in about 35-45 seconds.
It sounds: «Traffic, traffic».
At this point, you do not have to make any manoeuvres. The instruction is to look visually for conflicting traffic and be prepared to react if the situation worsens.
This is the most critical warning and requires immediate action.
It occurs when a collision could happen within 20-30 seconds. TCAS calculates a vertical avoidance manoeuvre and gives a clear and concise command to the pilot.
It sounds: «Climb, climb!», «Descend, descend now!»; or even «Adjust Vertical Speed, Adjust».
Thanks to the Mode S transponder, which allows communication and ‘data link’ between aircraft, the system is collaborative, i.e. if one aircraft is ordered to climb, the other will be ordered to descend, ensuring safe separation.
On the Navigation Display, TCAS represents nearby aircraft with specific symbols and colours.
◇ Hollow Rhombus: Represents other non-threatening traffic.
🔹 Solid Rhombus: Nearby traffic that has approached the TCAS observation radius.
🟠 Solid Circle: Traffic Advisory (TA)! The aircraft symbol turns into a yellow circle. This is the visual signal that accompanies the audible ‘Traffic, Traffic’ alert.
🟥 Solid Square: This is a Resolution Advisory (RA)! The conflicting aircraft is shown as a red square. It indicates that TCAS is giving an immediate evasive manoeuvre order.
In addition, on the vertical speed indicator (VSI), the TCAS will display green and red arcs. The green arc indicates the rate of climb or descent that the pilot must follow to comply with the RA, while the red arc shows the rates to avoid.
The FAA and other international organisations are already progressively implementing the successor to TCAS II: the ACAS X.
This new generation system is capable of coordinating more complex and adaptive RA, including horizontal manoeuvres (turns), resulting in a much more intelligent and flexible tool.
But how does it do this? ACAS X integrates information from multiple sources, not just the transponder, and is designed for safe drone (UAS) operation in shared airspace.
By using a probabilistic model and advanced algorithms to analyse the entire environment, it is able to offer even more optimised and safer solutions.
