Airspace Classification, ATS and Flight Plan Procedures
ICAO airspace classes
Classes A-G describe services and clearance requirements, not physical shapes. Class A is IFR-only. Classes B and C allow IFR and VFR; in B all flights are separated, and in C VFR is separated from IFR. Class D allows IFR and VFR; both need clearance, IFR is separated from IFR, and VFR receives traffic information. Class E allows VFR without a clearance; IFR is separated from IFR. Class F is advisory; Class G is uncontrolled. Controlled airspace includes CTRs, CTAs, TMAs and airways. An ATZ is not automatically controlled airspace.
Flight plans and ATS
A flight plan is required for IFR and for VFR when the rules require it, for example for controlled airspace or crossing a FIR boundary as published. It contains aircraft identification, type, wake category, route, level, speed and endurance. An ATC clearance is permission to proceed under specified conditions; it does not relieve the pilot of responsibility for terrain and weather. ATS comprises air traffic control, flight information and alerting services.
Worked example
A VFR pilot wants to cross a Class D control zone. The pilot files a flight plan and calls the tower. The tower replies 'stand by', not 'cleared'. The pilot must remain outside the zone. A flight plan does not grant a clearance, and 'stand by' is not permission to enter. Once cleared to enter the control zone, the pilot complies with the clearance limit and reports as instructed.
Traps
Do not equate a flight information service with air traffic control. A clearance is not a guarantee of separation from VFR traffic in Class D. Class E does not require a VFR clearance, but national radio rules may still apply. Do not treat an ATZ as a CTR.