The previous lesson mapped the services and the airspace; this one works through the three services themselves. Air traffic control: who provides it, what a clearance is and what it is not, and the five parts every clearance is built from. Flight information: what the service must tell a flight, and the ATIS broadcast that delivers the aerodrome's share of it. And alerting: the three emergency phases (INCERFA, ALERFA, DETRESFA) by which the system decides, minute by minute, how worried to be about a flight that has gone quiet.
Air traffic control: units and clearances
Air traffic control service is provided in airspace classes A, B, C, D and E, the controlled classes, by three kinds of unit, one per service:
Area control centre (ACC) — provides area control service.
Approach control unit — provides approach control service.
Aerodrome control tower (TWR) — provides aerodrome control service.
Combinations are permitted: an ACC or a tower may be assigned the approach task where no separate unit exists. On the ground, specified services on the apron may be assigned to the aerodrome control tower or to a separate apron management service: the apron is not part of the manoeuvring area, so ICAO leaves its management flexible. The manoeuvring area itself is different: the movement of persons, vehicles and towed aircraft on the manoeuvring area is controlled by the aerodrome control tower as necessary, because everything moving among the runways and taxiways can conflict with aircraft taking off, landing and taxiing. Collision prevention on the manoeuvring area is one of the ATS objectives, and it only works if a single authority sequences all of it.
The instrument of control is the clearance, and its purpose is exact: an ATC clearance is an authorisation to proceed under conditions specified by a control unit, issued for the purpose of preventing collisions between known traffic and of expediting and maintaining an orderly flow. The two words that matter most are known traffic: a clearance separates the flight from the traffic the controller knows about — it is not an authorisation to violate any rule, and not a guarantee of terrain clearance. A controller may clear a flight to a level below the safe altitude; flying it is still the pilot's error. The clearance itself is built from five possible parts, in order:
Aircraft identification — who the clearance is for.
Clearance limit — the point to which the clearance is valid: an aerodrome, a fix, a boundary.
Route of flight — the path the clearance authorises.
Levels of flight — for the entire route or a part of it.
Necessary instructions and information — everything else the flight needs: the SSR code, approach or departure instructions, communications, the clearance expiry time.
Flight information service
Flight information service is owed to all aircraft likely to be affected by the information which are provided with air traffic control service or are otherwise known to the ATS units: a controlled flight gets it automatically; an uncontrolled flight gets it by being known, through its flight plan or its radio contact. What the service must carry is a defined list, and it includes SIGMET and AIRMET information: the significant-weather warnings head the list. The rest of it:
Volcanic activity — pre-eruption activity, eruptions, and volcanic ash clouds.
Radioactive materials and toxic chemicals — the release of either into the atmosphere.
Navigation aids — changes in the serviceability of the aids the traffic depends on.
Aerodrome conditions — changes in the condition of aerodromes and associated facilities, including snow, ice or significant water on the movement area.
Unmanned free balloons — and any other information likely to affect safety.
On top of the common list, each set of flight rules gets its supplement:
IFR flights — additionally receive weather conditions reported or forecast at the departure, destination and alternate aerodromes, and collision hazard information for flight in classes C through G; plus, for flight over water, in so far as practicable and on a pilot's request, available information on surface vessels.
VFR flights — additionally receive traffic and weather conditions along the route that are likely to make visual flight impracticable: the service watches the VMC (and the traffic) ahead of the flight that depends on them.
ATIS: the aerodrome on a loop
The aerodrome's share of the information service is broadcast continuously by the automatic terminal information service (ATIS). The operating rules the exam asks: the broadcast is continuous and repetitive, on a discrete frequency or on the voice channel of a navigation aid (though never on the voice channel of an ILS), and it relates to a single aerodrome. Each message is identified by a letter of the phonetic alphabet (information Alpha, then Bravo), issued in sequence, and a pilot acknowledges receipt of the identified message on first contact ("information Bravo received"), whereupon the controller need only confirm or correct it — though the unit always passes the current altimeter setting to the aircraft in any case. Where messages are offered in more than one language, separate channels carry them, and a message is kept short: brevity is a stated design aim.
The content of an ATIS message runs in a fixed order, and it is the arrival briefing in miniature:
The aerodrome name and message designator letter, and the time of observation.
The type of approach to expect and the runway in use, with significant runway surface conditions and braking action where relevant.
Any holding delay, and the transition level where applicable.
Other essential operational information.
Weather block — wind, visibility or RVR, present weather, cloud, temperature and dew point, and the QNH, with trend information where available.
The message is updated whenever a significant change occurs (a runway change, a jump in the wind, a new weather report), and the new message takes the next letter, so no crew can mistake old information for current.
Alerting: the three phases of worry
The alerting service is provided by the air traffic services units: the unit serving a flight alerts for it, with the flight information centre or area control centre serving as the central collecting point for the region; and it is that responsible ATS unit which initiates the appropriate emergency phase and notifies the rescue coordination centre (RCC) immediately when an aircraft is considered to be in a state of emergency. The service is owed to:
All aircraft provided with ATC service.
In so far as practicable, all other aircraft that have filed a flight plan or are otherwise known to the ATS.
Any aircraft known or believed to be subject to unlawful interference.
Worry is graded into three phases, each with defined triggers, and the figure ladders them:
Uncertainty phase (INCERFA) — doubt about the safety of the aircraft: no communication within 30 minutes of the time contact should have been made (or was first attempted, whichever is earlier), or the aircraft fails to arrive within 30 minutes of its estimated arrival last notified or estimated by the ATS units, whichever is later.
Alert phase (ALERFA) — apprehension: the uncertainty-phase enquiries have failed; or an aircraft cleared to land fails to land within 5 minutes of its estimated landing time and communication cannot be re-established; or the aircraft's operating efficiency is impaired, though not to the extent that a forced landing is likely; or the aircraft is known or believed to be subject to unlawful interference.
Distress phase (DETRESFA) — reasonable certainty of grave and imminent danger: further attempts and enquiries point to the probability that the aircraft is in distress; or the fuel is considered exhausted or insufficient to reach safety; or the operating efficiency is impaired to the point that a forced landing is likely; or the aircraft is about to make, or has made, a forced landing.