General (ICAO)
ICAO Annex 11 §2.2 (Objectives of the Air Traffic Services) establishes the five main objectives of ATS:
(a) Prevent collisions between aircraft — primary safety objective.
(b) Prevent collisions between aircraft on the manoeuvring area and obstructions thereon — prevent runway and taxiway collisions (runway incursion / ground collision).
(c) Expedite and maintain an orderly flow of air traffic — efficiency; ATC not only prevents conflicts but also optimises sequencing, routing and altitudes.
(d) Provide advice and information useful for the safe and efficient conduct of flights — proactive information (FIS, ATIS, traffic information, weather).
(e) Alerting service — notify search-and-rescue authorities of aircraft in need of assistance and assist them (see SAR phases INCERFA/ALERFA/DETRESFA).
Hierarchy: Safety (a, b) always takes precedence over efficiency (c) and information (d). When requirements conflict, ATC decides in favour of safety.
ATS subdivision (Annex 11 §2.3)
Air traffic services comprise:
1. Flight Information Service (FIS) — advice and information for all flights in defined airspace; non-controlling.
2. Alerting Service — alerting SAR for aircraft in distress.
3. Air Traffic Advisory Service — advisory service for IFR flights in areas without ATC control (Class F, rarely used in Europe).
4. Air Traffic Control Service (ATC) — controlling, in three levels:
- Area Control Service (ACC) — controlled en-route flight (Classes A/B/C/D/E in upper airspace),
- Approach Control Service (APP) — approach and departure within TMA,
- Aerodrome Control Service (TWR) — at and in the vicinity of the aerodrome (CTR, ATZ).
Pilot responsibility: Even in controlled airspace the PIC remains responsible for see-and-avoid collision avoidance when VMC prevails (Annex 11 §3.2 — "See and avoid"); ATC provides primary active separation only under IFR and in controlled airspace.
Europe (EASA / EU)
Regulation (EU) 2017/373 (ATM/ANS Common Requirements) Annex IV (Part-ATS) adopts the Annex 11 objectives verbatim and requires ATS providers to meet them. The Single European Sky and Network Manager EUROCONTROL flank ATS provision.
Germany (national)
DFS Deutsche Flugsicherung GmbH (§27c LuftVG) provides civil ATS in Germany. 3 FIRs (Bremen, Langen, Munich) for ACC; approach and tower control at the commercial airports. In MUAC Maastricht the upper airspace > FL245 in the north-west is controlled directly by EUROCONTROL.