Airborne Collision and Avoidance System — Page 484, Lesson 576

Let’s start with the big picture. The Airborne Collision and Avoidance System — TCAS — is the equipment that watches for other aircraft and, when needed, tells you how to avoid them. But TCAS can only see aircraft that are equipped to be seen. So the very first thing I want you to understand is the transponder requirement, because it drives everything else. For an aircraft to be visible to a TCAS-equipped aircraft, it must carry at least a Mode “A” transponder. That’s the minimum. Now, if that transponder is switched off, or if it’s unserviceable — meaning it’s broken — then that intruding aircraft is invisible to your TCAS. And that’s a real collision risk, because your system simply has no data on it. So the first rule: no working transponder, no protection. Now, what does a Mode A transponder actually give you? It transmits no height information at all. So the TCAS equipment receives only two-dimensional information — range and bearing, essentially a flat picture. Because it’s two-dimensional, the system can only give you what we call TAs — Traffic Advisories. A TA is an alert that says, “there’s traffic nearby, look out,” but it’s not a directive to manoeuvre. Now step up to a Mode “C” transponder. A Mode C intruder broadcasts height information to your TCAS. That makes the system three-dimensional — you now have range, bearing, and altitude. And with three dimensions, the system can give you both TAs and RAs. An RA is a Resolution Advisory — that’s the actual avoidance command, telling you to climb or descend. So the key contrast: Mode A gives you only TAs because it’s two-dimensional; Mode C gives you TAs and RAs because it’s three-dimensional. Then there’s Mode “S”. A Mode S transponder on an intruder, in addition to broadcasting height information, allows a discrete data link to be established between your aircraft and that intruder. “Discrete” means it’s a dedicated, point-to-point link between the two of you. And that data link is what allows avoidance manoeuvres to be mutually resolved — meaning your TCAS and theirs can coordinate, so you don’t both climb into each other. That mutual resolution is the real value of Mode S. Now let’s talk about how TCAS actually works — the operation. The range of an intruder is determined by measuring the time lapse between transmission of an interrogation and receiving the response. That’s the radar principle — you send a signal, you time how long until the reply comes back, and that time tells you the distance. An interrogation signal is sent out approximately once a second. But here’s the catch: in densely populated airspace, where many aircraft are being monitored, a system of reduced surveillance is employed. In that mode, the interrogation period is extended to every 5 seconds, and priority is given to the closest aircraft. So you’re checking less often, but you’re making sure the nearest threat is still watched most carefully. Now the numbers. Normal detection range is approximately 30 nautical miles. But when you’re in reduced surveillance, that detection range reduces to 5 nautical miles. So you trade range for the ability to handle many targets — you see less far, but you can track more aircraft. Now, the bearing of an intruder is determined by a directional antenna. And here’s the important limitation: because of the wavelengths involved and the necessarily small size of the antennas, bearing resolution is the least accurate parameter. In plain terms, TCAS is very good at range and altitude, but its weakest measurement is the direction — the bearing — because the antenna is physically small relative to the wavelengths it’s working with. And that leads to a fundamental design rule: TCAS II never offers collision avoidance commands in the horizontal plane. It only gives commands in the form of climb or descend. So all RAs are vertical — you will never get a “turn left” or “turn right” from TCAS II. The avoidance is always climb or descend, because the bearing information is just not accurate enough to trust for horizontal manoeuvring. Let me show you the bearing determination in Figure 35.2 — that’s the directional antenna concept I just described. So to pull it all together: you need at least a Mode A transponder on the intruder to be seen at all, Mode C to get height and therefore RAs, and Mode S to get the data link for mutual resolution. Range comes from timing the interrogation-response cycle, with normal detection at about 30 NM and reduced surveillance at 5 NM with a 5-second interrogation period. And bearing, measured by the directional antenna, is the least accurate parameter — which is exactly why TCAS II only ever commands climb or descend, never horizontal turns.