Annex 11 is the air traffic services Annex, and this lesson takes its skeleton: what the services are for, the vocabulary they are defined in, the airspace structure they inhabit, from the flight information region down to the control zone around a single aerodrome, and the class letters, A through G, that tell a pilot at a glance what service and what separation to expect. The lesson closes with the naming system of the routes that thread through it all.
What air traffic services are for
The objectives of the air traffic services are a five-item list the exam quotes, and everything else in the Annex serves one of them:
Prevent collisions between aircraft — the first and defining objective.
Prevent collisions on the manoeuvring area — between aircraft and obstructions on the runways and taxiways.
Expedite and maintain an orderly flow of air traffic — safety first, but throughput is an objective in its own right.
Provide advice and information useful for the safe and efficient conduct of flights — the information half of the system.
Notify appropriate organisations regarding aircraft in need of search and rescue, and assist as required: the alerting half.
To meet those objectives, Annex 11 defines three basic types of air traffic service:
Air traffic control service — separates and sequences.
Flight information service — advises and informs.
Alerting service — notifies the rescue organisations when a flight is in trouble.
The control service itself splits into three basic ATC services, one per phase of flight:
Area control service — for the en-route phase.
Approach control service — for arriving and departing flights.
Aerodrome control service — for aerodrome traffic itself.
Two practical rules complete the section: in an emergency a pilot can expect ATC to make contact on the frequency in use and on the emergency frequency 121.5 MHz; and an aircraft is transferred from one ATC unit to the next at agreed points or times, by coordination between the two units: the pilot experiences it as a simple frequency change, but behind the "contact" instruction the transferring unit has already passed the flight's details and obtained the accepting unit's agreement.
FIRs: the sky with no gaps
The base layer of the structure is the flight information region (FIR): airspace of defined dimensions within which flight information service and alerting service are provided. FIRs exist so that those two services cover the sky without gaps: every piece of airspace used by international civil aviation belongs to some FIR, and an FIR includes all airspace within its lateral limits, from the surface upward, unless an upper flight information region caps it. Where a UIR is established, the FIR runs up to the UIR's lower limit and the UIR takes the airspace above. The reason for the upper layer, and for delineating a UIR or upper control area to span the upper airspace above several smaller lower FIRs or CTAs, is the traffic itself: high-level flights cross the small lower regions in minutes, and one continuous upper region means fewer units, fewer transfers and a service matched to the speed of the traffic using it.
Controlled airspace: CTAs and CTRs
Air traffic control service needs its own designated portions of airspace, and Annex 11 names two:
Control area (CTA) — controlled airspace extending upwards from a specified limit above the earth.
Control zone (CTR) — controlled airspace extending upwards from the surface to a specified upper limit: the protection around an aerodrome, where arriving and departing IFR flights are low.
Both are carved out of an FIR and form part of the FIR they lie within: the FIR's information and alerting cover them; the control service is added on top. The numbers the exam wants:
CTA lower limit — at least 200 m (700 ft) above the ground or water, so that uncontrolled VFR flight beneath the airway remains possible; and the limit need not be established uniformly: it may step with the terrain and the traffic.
CTR lateral limits — they encompass at least those portions of the airspace, not inside a CTA, that contain the paths of IFR flights arriving at and departing from the aerodrome in instrument meteorological conditions.
CTR minimum size — the lateral limits extend at least 9.3 km (5 NM) from the centre of the aerodrome in the directions from which approaches may be made.
CTR upper limit under a CTA — a CTR lying within the lateral limits of a CTA extends upwards at least to the lower limit of the CTA: the two controlled volumes meet, leaving no uncontrolled slice between them.
The airspace classes
One aerodrome's CTR and another's may promise very different services, so ICAO grades all airspace into seven classes, A through G, each a fixed bundle of rules and services. Classes A to E are controlled airspace; F and G are uncontrolled. The figure stacks them; the bundles, class by class:
Class A — IFR flights only; all flights receive air traffic control service and are separated from each other.
Class B — IFR and VFR; all flights receive ATC service and all are separated from all others.
Class C — IFR and VFR; IFR separated from IFR and from VFR; VFR separated from IFR and given traffic information about other VFR.
Class D — IFR and VFR, all receiving ATC service; IFR separated from IFR, with traffic information about VFR; VFR receive traffic information about all other flights.
Class E — IFR and VFR; IFR separated from IFR and given traffic information about VFR as far as practical; VFR fly without a clearance and receive traffic information as far as practical.
Class F — IFR and VFR; participating IFR flights receive an air traffic advisory service (advice on collision hazards, short of full control), and all flights receive flight information service on request.
Class G — IFR and VFR; flight information service on request is the whole offer. No clearance, no separation: the see-and-avoid world of the rules of the air.