SSR and ACAS — Page 256, Lesson 328

I want to walk you through the end of the SSR and ACAS chapter, and then we're going to step into the new chapter on Airspace. Let's start with the last piece of the SSR material, because it's a precise regulatory point you'll need verbatim. This is about determining an aircraft's level using Mode C. Mode C is the altitude reporting function of the transponder — it tells ATC what pressure altitude the aircraft is at. ICAO defines the standards for level determination using Mode C as generally plus or minus 300 feet. That's the international baseline. However, most contracting states — and this is a key contrast — employ a higher standard of plus or minus 200 feet. So the ICAO norm is 300, but many states tighten it to 200. Now, the specific ICAO standards are defined for five distinct situations, and each one uses that 300-foot figure in a slightly different way. You need to know each definition exactly. First, level occupancy. This means Mode C indicates the allocated level plus or minus 300 feet. In other words, if the aircraft is within 300 feet of the level it's been given, it's considered to be occupying that level. Second, maintaining a level. Again, Mode C indicates the allocated level plus or minus 300 feet. So maintaining is the same tolerance as occupancy — you're holding the level within that 300-foot band. Third, vacating a level. Here the definition flips. Mode C indicates that the aircraft is more than 300 feet from the previously allocated level. So you've only truly vacated the old level once you're beyond that 300-foot margin from it. Fourth, passing a level. Mode C indicates that the aircraft is within 300 feet of a specified level in the climb or descent. So during a climb or descent, when you come within 300 feet of a level you're passing through, that's the definition of passing it. Fifth, reaching a level. Mode C indicates that the aircraft is within 300 feet of the allocated level at the completion of a climb or descent. So reaching is the same 300-foot proximity, but specifically at the end of the climb or descent, when you're arriving at your allocated level. There's also a note in the excerpt about a condition that applies only when directed by ATC — that's the phrase "only when directed by ATC" attached to one of these situations. And then there's a requirement that applies at all times during flight, regardless of whether or not the aircraft is within or outside airspace where SSR is used for ATC purposes. That's the universal obligation — the transponder operation isn't limited to SSR-controlled airspace. Now let's move into the new chapter — Airspace. This is where we define the structure of the sky itself. The foundational concept is the division of airspace. All the airspace within a state must be contained within one or more Flight Information Regions — FIR for short. The FIR is the basic unit of airspace. Within it, the most basic form of Air Traffic Service is available, and that's a Flight Information Service. The alerting service is also available in an FIR. So an FIR gives you two things: flight information and alerting. It's usual to give each FIR a name — for example, the London FIR — which geographically identifies the location of the FIR and its associated Area Control Centre, the ACC. Within the ACC, the Flight Information Centre is located. Now, where a state strictly enforces its sovereignty, the boundaries of that state's FIRs usually coincide with the national borders of the state. But — and this is an important contrast — it's not unusual, particularly in Europe, for FIR boundaries to be 'convenient'. That means they follow latitude/longitude lines or median lines rather than following often convoluted national borders. So the boundary is drawn for operational convenience, not political geography. Beyond FIRs, the airspace of a state will be divided into Control Areas — CTAs — and Control Zones — CTRs. And it may include restricted, prohibited, and danger areas. Those are the special-use airspaces. CTAs can exist in the form of corridors linking other CTAs, and these are known as airways. So an airway is essentially a CTA corridor connecting other CTAs. The airspace in the vicinity of an aerodrome is known as an Aerodrome Traffic Zone — ATZ. That's the protected volume around an airfield. That figure shows you how these pieces fit together — the FIR, the CTA, the CTR, the ATZ, and the special-use areas. Then we have Upper Information Regions — UIRs. Where a state applies a division of airspace vertically, the upper portion of the airspace is defined as an Upper Information Region. This division exists to facilitate the application of different rules and separation standards to those of the underlying airspace. The basic assumption is that traffic using the UIR will be essentially in transit en route — they're passing through. Whereas lower traffic will be arriving or departing, and therefore manoeuvring. So the UIR is for the cruise traffic, and the lower airspace is for the climbing, descending, and turning traffic. In Europe, the division between the FIR and UIR is at FL — and the excerpt cuts off right there at "FL1". That's the flight level where the split occurs, and we'll pick that up when we continue. So to tie it together: you've got the Mode C level-determination standards — five precise definitions all built around that 300-foot tolerance, with many states tightening to 200. And then you've got the airspace structure — FIR as the basic unit, CTAs and CTRs, airways as CTA corridors, the ATZ around aerodromes, and the UIR for the upper transit traffic.