Airspace structure and classes

A service boundary is not a clearance

The vertical and lateral shapes explain which unit and service apply.

FIR, UIR, CTA and CTR

A flight information region is airspace of defined dimensions where flight information and alerting services are provided. A UIR performs that function in upper airspace. Together the designated regions cover the airspace concerned, including controlled and uncontrolled portions. A CTA or CTR remains part of its FIR; it does not replace the FIR.

A control area extends upward from a specified lower limit. The ICAO ordinary lower limit is at least 200 m (700 ft) above the ground or water; it need not be uniform. A control zone extends upward from the surface and normally covers at least five nautical miles from the aerodrome centre in directions from which approaches may be made, with boundaries suited to instrument paths. Where under a CTA, it extends at least to the CTA’s lower limit.

Why upper boundaries differ

A UIR or upper CTA can cover several lower regions to simplify high-level service provision. Lateral boundaries should support safe and efficient traffic handling rather than merely reproduce political borders. Delegation of ATS does not change sovereignty.

Read an airspace chart in three dimensions. Being below a CTA’s lower limit does not mean being outside an overlying FIR or exempt from a nearby CTR or restricted area. The chart’s altitude/flight-level references and effective times are part of the boundary.

Which flights receive separation?

Compare flight rules before deciding whether two aircraft are separated.

Classes A to E

Class A permits IFR only and separates all flights. Class B permits IFR and VFR and separates all flights. Class C separates IFR from IFR and VFR; VFR is separated from IFR and receives traffic information about VFR, with avoidance advice on request. Class D separates IFR from IFR; IFR receives VFR traffic information and VFR receives information on all other flights, with avoidance advice on request.

Class E separates IFR from IFR; VFR does not require an ATC clearance solely by being in E, and traffic information is provided as far as practicable. Controlled airspace comprises A to E, but the ATC clearance obligation in E applies to IFR, not ordinary VFR. Controlled aerodrome or other local requirements can still apply.

Classes F and G

Class F provides advisory service to participating IFR flights and FIS as required; it is not the same as controlled separation. Class G provides FIS as required. IFR and VFR can coexist in G without ATC separation. The SERA table also specifies communication, clearance and speed conditions: the ordinary 250 kt IAS limit below 10,000 ft applies in C to G subject to authorised exceptions, and to the relevant flights as set out in the table.

In C, VFR remains responsible for avoiding other VFR even though it is separated from IFR. In D, a clearance and radio contact do not imply IFR–VFR separation. At a common boundary between classes, the less restrictive class’s requirements apply under the stated SERA rule. Use the published classification and activation time.

Can you apply the rules?

Four original questions. Choose from the facts, then review the explanation.