Airspace — Page 256, Lesson 335

Let's start with the big picture. Air Traffic Control is only provided inside what we call controlled airspace, or CAS. And there's a generally accepted ceiling to that controlled airspace: the upper limit of CAS is defined as flight level 660, FL660. So everything we talk about today lives below that. Controlled airspace is made up of two building blocks: Control Areas, CTAs, and Control Zones, CTRs. Their primary function is to let ATC serve IFR traffic — instrument flight rules traffic. That's the whole reason they exist. Let's take CTAs first. A CTA is airspace within which ATC is provided to controlled flights that are "en route" — meaning not in the departure or approach phases of a flight. The service provided inside a CTA is called Area Control, and that's covered in detail in Chapter 17. CTAs are usually established in the vicinity of major aerodromes and at the confluence of airways — where airways come together. And remember, the linking corridors of airspace, the airways themselves, are also CTAs. Now, how is a CTA defined vertically? It exists from a level above ground level, AGL, up to an altitude or a flight level. If the upper limit is defined as a flight level, it will be a VFR flight level. And there's a hard rule on the bottom: the lowest level of a CTA must be at least 200 metres, which is 700 feet, above ground level. The airspace inside a CTA can be sub-divided to allow different types of operation. The upper portion is predominantly for traffic in transit — just passing through. The lower levels are dedicated to aircraft entering the "manoeuvring" airspace from above, or leaving it. The lower part of the CTA is used to route arriving aircraft to the individual IAFs — initial approach fixes — for instrument approaches, and to separate departing aircraft from one aerodrome from arriving traffic to another. So the lower CTA is the busy mixing zone. Now CTRs. Control zones are established in the vicinity of aerodromes to provide ATC to arriving and departing controlled traffic. Inside a CTR, the ATC service is provided by Approach controllers, and that service is mandatory. A CTR may serve more than one aerodrome. Because a CTR is a zone, its base is defined as ground level or mean sea level, MSL. If the upper limit is defined as a flight level, it would always be a VFR flight level — same rule as the CTA. A CTR should be big enough to encompass the airspace required to provide the service, and it will usually extend at least 5 nautical miles from the aerodrome reference position in the direction from which approaches are made. And here's an important interaction: if a CTR is established within the lateral limits of a CTA, the CTR must extend upwards to the lower limit of that CTA. So the zone plugs right up into the base of the control area. Let me show you how these sit together. So to tie it together: CAS is capped at FL660, made of CTAs and CTRs, both serving IFR traffic. CTAs handle en-route traffic with Area Control, bottom at least 200 metres AGL. CTRs handle arrivals and departures with mandatory Approach control, base at ground or MSL, usually reaching out at least 5 NM toward the approach direction, and if inside a CTA, they rise to meet its lower limit.