Airspace Structure and ATS Units
Layers of the sky
Airspace is classified (ICAO classes A–G) by the services provided and the flight rules permitted. In Brazil, FIRs (Flight Information Regions) are subdivided into CTA, CTR, TMA, ATZ and airways. The ATS units are the aerodrome control tower, approach control unit, area control centre, and flight information centre. Each has a defined area of responsibility and a defined coordination interface.
Coordination and handover
Questions frequently test the transfer of control versus transfer of communication: control may be transferred before or after radio contact, depending on the letter of agreement. The accepting unit must be able to identify the aircraft and have coordinated the level, route, and any restrictions. Traps: assuming the tower owns the aircraft to the ground (approach owns the intermediate segment), and forgetting that the area control centre provides service in the CTA/FIR.
Rule of thumb
Ask who has control now? - the unit that has accepted the aircraft and can provide separation. Communication transfer alone does not transfer control unless agreed.
Worked example
An aircraft is cleared to descend by the ACC, then handed to approach. Approach may only issue a lower level after coordination if the aircraft has not yet entered its area of responsibility.