Air Traffic Services and Airspace: Classes, Clearances and Services
Service types
ATS comprises three services: air traffic control (preventing collisions and expediting flow), flight information (advice useful for safe conduct) and alerting (notifying rescue coordination). Traps arise from assuming flight information service provides separation - it does not. Similarly, advisory service does not clear traffic.
Airspace classification
Annex 11 classes A to G define what is provided and what the pilot must do. Learn the pattern: Class A permits IFR only with full separation; Class B permits both with separation; Class C separates IFR from all and VFR from IFR; Class D separates IFR from IFR and provides traffic information to VFR; Class E separates IFR but not VFR; Class F is advisory; Class G is uncontrolled. A very common question asks what separation is provided between two VFR flights in Class D - the answer is none, only traffic information. Another asks whether a VFR flight needs a clearance in Class E - it does not, though it must comply with VMC and, where required, obtain a clearance to enter controlled airspace.
Clearances
A clearance is an authorisation to proceed under specified conditions. It does not relieve the pilot of responsibility for terrain clearance, wake turbulence avoidance or compliance with regulations. Read-back is required for clearances involving runway, level, heading or frequency. Traps include treating a clearance limit as an amended clearance - reaching the limit without further clearance requires holding.
Worked example
A VFR flight in Class C wants to cross a control zone. It needs a clearance, must remain VMC, and is separated from IFR traffic but not from other VFR. Rule of thumb: know the class, then know who is separated from whom.
Communication failure
Know the transponder codes and the standard actions for communication failure in IMC and VMC.