Air Traffic Services, Airspace Classification and ATS Procedures
This domain tests Annex 11 and SERA: which services exist, what each airspace class guarantees, and what a clearance does and does not mean. The recurring question is 'in this class of airspace, what separation and what service does an IFR or VFR flight receive?'
Services and units
Air traffic services comprise the air traffic control service, the flight information service, the alerting service and the air traffic advisory service. ATC delivers area control, approach control and aerodrome control from an ACC, APP or TWR; flight information and alerting are provided by any ATS unit. Annex 11 sets three objectives: prevent collisions between aircraft and between aircraft and obstructions on the manoeuvring area, expedite and maintain an orderly flow of traffic, and provide advice and information useful for the safe and efficient conduct of flights.
Airspace classes A–G
Class A: IFR only, all aircraft separated. Class B: IFR and VFR, all separated. Class C: IFR separated from IFR and VFR; VFR separated from IFR but not from other VFR. Class D: IFR separated from IFR only, with traffic information about VFR; VFR not separated from any aircraft. Class E: IFR separated from IFR, traffic information about VFR as far as practicable, no separation for VFR. Class F: advisory service, separation as far as practicable. Class G: flight information and alerting only, no separation.
Clearances and surveillance
A clearance does not relieve the pilot-in-command of responsibility for compliance with the rules, nor of the duty to advise ATC when a clearance cannot be accepted. Read-back is mandatory for safety-related items such as route, level, heading, speed and frequency. Secondary surveillance radar codes: 7500 unlawful interference, 7600 radio failure, 7700 emergency.
Worked example
A VFR flight transits Class D airspace while an IFR arrival is sequenced. The tower separates the IFR aircraft from other IFR traffic and passes traffic information on the VFR aircraft, but is not required to separate the IFR aircraft from the VFR one, nor the VFR aircraft from anything. If the airspace were Class C, the IFR arrival would be separated from the VFR aircraft as well.
Traps
Class C separates VFR from IFR but never VFR from VFR. Class E does not separate VFR from IFR. Class F is advisory, not a control service. A clearance never overrides the rules of the air or terrain clearance. Do not confuse the alerting service with emergency response coordination by the operator.