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 the pilot to climb or descend to avoid a collision. I want you to think of it in two layers: first, what the other aircraft must carry so that TCAS can even see it, and second, how TCAS actually measures where that other aircraft is. So, aircraft equipment first. For an intruding aircraft to be visible to a TCAS-equipped aircraft, it must have at least a Mode A transponder. That’s the minimum. If that transponder is switched off, or it’s unserviceable, the intruder is invisible to TCAS — and that’s a real collision risk, because TCAS simply cannot see it. Now, here’s the key limitation of Mode A: it transmits no height information. So the picture TCAS gets is two-dimensional only — it knows the intruder is out there and roughly where, but not its altitude. Because of that, with a Mode A intruder, TCAS can only give a TA — a Traffic Advisory. That’s an alert that says “traffic is near,” but it doesn’t tell you what to do. Now step up to a Mode C transponder. A Mode C intruder broadcasts height information to the TCAS equipment. That makes the system three-dimensional — it now knows range, bearing, and altitude. And with that third dimension, TCAS can give both TAs and RAs. An RA is a Resolution Advisory — that’s the actual avoidance command, the climb or descend instruction. So the difference between TA and RA is fundamental: TA is a warning, RA is the instruction to manoeuvre. Then there’s Mode S. A Mode S transponder on the intruder does everything Mode C does — it broadcasts height — but it also allows a discrete data link to be established between the two aircraft. That data link is what lets avoidance manoeuvres be mutually resolved. In other words, if two TCAS aircraft are both equipped with Mode S, they can talk to each other and coordinate so that one climbs while the other descends — they don’t both pick the same direction. That mutual resolution is the whole point of the data link. Now let’s move to operation — how TCAS actually measures the intruder. The range 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 clever part: 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 the normal detection range is approximately 30 nautical miles, and that reduces to 5 nautical miles when operating in reduced surveillance. That’s a big drop — from 30 down to 5 — because you’re spreading your interrogations over more aircraft, so you can’t update each one as often. Now, bearing. The bearing of an intruder is determined by a directional antenna. And here’s the honest limitation: because of the wavelengths involved and the necessarily small size of the antennas, bearing resolution is the least accurate parameter. Range is good, altitude is good, but bearing — the direction — is the weakest measurement. That’s why TCAS II never offers collision avoidance commands in the horizontal plane. It only ever gives commands in the form of climb or descend. It won’t tell you to turn left or right, because the bearing information just isn’t accurate enough to trust for a horizontal manoeuvre. So all RAs are vertical — climb or descend — and that’s a direct consequence of that bearing inaccuracy. Let me tie it together. The equipment hierarchy is Mode A, then Mode C, then Mode S — each adds capability. Mode A gives you two-dimensional data and TAs only. Mode C adds height, making it three-dimensional, and unlocks RAs. Mode S adds the data link for mutual resolution between TCAS aircraft. And operationally, range comes from timing the interrogation-response round trip, with a normal range of about 30 NM that drops to 5 NM in reduced surveillance when you’re monitoring many aircraft. Bearing comes from a directional antenna but is the least accurate parameter, which is exactly why TCAS II only ever commands vertical manoeuvres — climb or descend — never horizontal ones.