SSR and ACAS — Page 249, Lesson 325

Let’s pick this up right where the transponder failure procedures leave off, because the next block is about how you actually change a code — and then we move into ACAS. Code selection. The preferred method of selecting or changing a transponder code is to first set the option switch to STBY — that’s standby — and then select the new code. Once you’ve dialled it in, you reset the option switch back to ON or ALT. The reason for doing it this way is that you don’t want the transponder transmitting a wrong or intermediate code while you’re spinning the dials. By going through standby, you suppress transmission until the new code is fully set. Now, modern SSR installations have two controllers, designated A and B, with a switch that selects either the A controller or the B controller. In that case, the technique is slightly different: you set the new code on the controller that is not in use, and then you flip the switch over to that controller. So you’re pre-loading the code on the spare controller, then switching to it — again, so you never transmit a half-set code. Now ACAS — the Airborne Collision Avoidance System. The technical details of ACAS are covered in detail in module 022 03, Warning and Recording. But the response to the alerts that ACAS generates is a matter for Air Law, so that’s what we focus on here. Basically, ACAS uses the redundant capacity of the SSR system to determine the proximity of another aircraft that is operating its SSR transponder. In other words, it interrogates the other aircraft’s transponder and uses that reply to work out how close it is and whether there’s a collision threat. Here’s a key regulatory point: in areas where RVSM is applied — that’s Reduced Vertical Separation Minima — the use of ACAS is mandatory, and operators are required to ensure compliance with that requirement. So if you’re operating in RVSM airspace, ACAS isn’t optional; the operator has to make sure the aircraft is equipped and compliant. Use of ACAS. ACAS is to be used by pilots for three specific purposes: the avoidance of potential collisions, the enhancement of situational awareness, and the active search for, and visual acquisition of, conflicting traffic. So it’s not just a warning device — it’s a tool you actively use to find traffic and build a picture of what’s around you. And there’s a critical legal point that follows: the use of ACAS alerts, and the procedures applicable to them, do not relieve the pilot of the basic responsibility for the safety of the aircraft. ACAS helps you, but it doesn’t take over your fundamental duty to see and avoid and to fly safely. That’s the core of ACAS from an Air Law perspective — the technical workings live in the other module, but your legal obligations around using it, and the fact that it doesn’t shift your basic responsibility, are what matter here.