Reduced Vertical Separation Minima or RVSM requires aircraft flying in the same airspace to be separated by 1000 feet. Precision altimeters and ground-based secondary surveillance radars have made it possible to fly aircraft in such close proximity safely. However, as increasing demand continues to congest the airspace, the risk of mid-air collision has never been greater. A simple miscommunication or a lapse in concentration is enough to send two aircraft on a collision course. To avert the calamity, one of the most advanced systems is installed on the aircraft: the Traffic Collision Avoidance System (TCAS).

The TCAS system has a computer in the aircraft avionics compartment and two antennas. The top antenna scans the area above the aircraft, and the bottom antenna scans the area below. The TCAS computer collects several data from the aircraft systems and creates a three-dimensional area of protected airspace around the aircraft. The system has a wide scanning range and can detect several aircraft in the vicinity, but only gives warning alerts when a traffic is a potential collision threat.

Let's see how the system determines that. The onboard TCAS computer sends an interrogation signal to the traffic aircraft. The TCAS computer of the traffic aircraft responds with an identification code and altitude. The onboard TCAS computer locates the position of the traffic aircraft by tracking the direction of the response signal and calculating the distance from the traffic by measuring the time taken between interrogation and response. Change in altitude and position of the traffic over time helps the TCAS computer to calculate the airspeed, vertical speed, and heading of the traffic aircraft.

Using data of both aircraft, the TCAS computer solves three important questions: Number one, what is the closest point of approach (CPA)? CPA is the point where the two aircraft will meet having the minimum distance between them. Number two, is the traffic an intruder? That is, will the traffic aircraft path interfere with the protected airspace at the time of CPA? If yes, then the most important question: How much time is left to meet at the closest point if both aircraft continue on the same flight path?

With 45 seconds to go, the TCAS issues a Traffic Advisory: "Traffic, traffic," and the traffic is displayed on the navigation display. An amber dot indicates the aircraft, and the "+05" indicates the traffic is 500 feet above. In Traffic Advisory, there are no maneuvering instructions; it just draws the attention of the flight crew to an incoming aircraft. With 25 seconds to go, TCAS warning changes to Resolution Advisory. The amber indications turn red. Instruction is given in the cockpit to maneuver the aircraft. Red lines on the flight display indicate the areas to avoid flying. The TCAS computer on the traffic aircraft is coordinated to give an opposite direction command, "Clear up, conflict."

When it comes to possible mid-air collision, TCAS may be the last line of defense, but a very effective one.