ICAO Annex 11 Air Traffic Services: Objectives, Airspace Classes, and ATS Structure

010-07-01Pablo Asensio Martínez2026-03-272 min

Annex 11 defines how airspaces and services are established and managed to ensure a safe and orderly flow.

Objectives of Air Traffic Services (ATS)

  1. Prevent collisions between aircraft.
  2. Prevent collisions between aircraft and obstructions on the maneuvering area.
  3. Expedite and maintain an orderly flow of air traffic.
  4. Provide advice and information useful for the safe and efficient conduct of flights.
  5. Notify and assist appropriate organizations regarding aircraft in need of search and rescue (SAR).

Airspace Classification

7 classes (A to G) are defined according to services provided and flight requirements:

  • Class A: IFR only. ATC service to all. Separation to all. (The strictest).
  • Class B: IFR and VFR. ATC service to all. Separation to all.
  • Class C:
    • IFR: Separated from IFR and VFR.
    • VFR: Separated from IFR. Receive traffic information concerning other VFR.
  • Class D:
    • IFR: Separated from IFR. Information about VFR.
    • VFR: Receive information about all flights.
  • Class E:
    • IFR: Controlled and separated from other IFR.
    • VFR: Permitted without clearance (in many cases), receive traffic information as far as practical. (Controlled airspace for IFR, uncontrolled for VFR).
  • Class F: Advisory (participating IFR receive advisory service). Phasing out.
  • Class G: Uncontrolled. Flight Information Service (FIS) only if requested.

Airspace Structure

  • FIR (Flight Information Region): Region where FIS and alerting service are provided.
    • UIR (Upper FIR): Established in the upper airspace to reduce the number of FIRs crossed by high-altitude, high-speed flights.
  • CTA (Control Area): Controlled airspace extending upwards from a specified limit above the earth (e.g., 700 ft).
  • CTR (Control Zone): Controlled airspace extending upwards from the surface. Protects IFR paths on arrival/departure. Minimum lateral limits: 5 NM from the center of the aerodrome.

Services Provided

  1. Air Traffic Control (ATC) Service: In controlled airspace (A-E).
    • Area Control, Approach Control, and Aerodrome Control.
  2. Flight Information Service (FIS): Advise and inform (available in all FIR).
  3. Alerting Service: Notify SAR and assist in emergencies.

ATIS (Voice-ATIS)

  • Continuous and repetitive broadcast of routine arrival/departure information to reduce communication load.
  • Preferably on a discrete VHF frequency or VOR (never on ILS).
  • Recommended maximum duration: 30 seconds.