Purpose and divisions of Air Traffic Services
Annex 11 provides the international framework for Air Traffic Services. The service is designed to prevent collisions, organise traffic, supply useful information and connect an aircraft in difficulty with the organisations able to help.
Objectives of Air Traffic Services
| Objective | Practical meaning |
|---|---|
| Prevent collisions between aircraft | Apply clearances, separation, traffic information and warning services appropriate to the airspace. |
| Prevent collisions on the manoeuvring area | Keep aircraft clear of other aircraft, vehicles, persons and obstructions while operating on the manoeuvring area. |
| Expedite and maintain an orderly flow | Sequence and coordinate traffic so capacity is used safely and efficiently. |
| Provide advice and information | Give information useful for the safe and efficient conduct of flights. |
| Support search and rescue | Notify the appropriate organisations about aircraft needing aid and assist those organisations as required. |
The three services
| Service | Core purpose | Who receives it |
|---|---|---|
| Air traffic control service | Achieves collision-prevention and orderly-flow objectives by issuing clearances and instructions. | Controlled flights and controlled aerodrome traffic according to airspace class and operation. |
| Flight information service | Supplies advice and information useful for safe and efficient flight. | Aircraft receiving ATC and other aircraft known to the relevant ATS unit and likely to be affected. |
| Alerting service | Connects aircraft needing search and rescue aid with the appropriate organisations. | Controlled flights, filed or otherwise known flights as practicable, and aircraft suspected of unlawful interference. |
The three divisions of ATC
Area control service serves controlled flights outside the portions handled by approach and aerodrome control. Approach control service serves the controlled parts of arriving and departing flights. Aerodrome control service serves aerodrome traffic, except those portions assigned to approach control.
Units that provide the service
An area control centre normally provides area control. An approach control unit, an aerodrome control tower or an area control centre may provide approach control when functions are combined. An aerodrome control tower provides aerodrome control. A flight information centre provides flight information and alerting outside controlled airspace and coordinates information across its flight information region.
Indian communication provisions
Bali records that communication with the appropriate aerodrome or approach unit is established before taxiing for departure and when intending to operate in Class C or D airspace. Direct controller-to-pilot communication is used in those classes. An IFR or VFR aircraft intending to enter, cross or operate in a control zone or aerodrome traffic zone requests clearance, gives position, level, track and crossing estimate, maintains listening watch and complies with instructions.
FIR, UIR, control areas and aerodrome airspace
Airspace is organised by both geography and function. An FIR is the broad information and alerting container; controlled areas and zones are placed inside it where ATC is required for particular traffic.
Flight information region and upper flight information region
A flight information region is airspace of defined dimensions within which flight information and alerting services are provided. It contains all airspace within its lateral limits except any portion limited by an upper flight information region. A UIR combines the upper airspace above one or more lower FIRs so high-level transit traffic can use a simpler structure.
Control area, terminal control area and airway
A control area is controlled airspace extending upward from a specified limit above the Earth. It is shaped to contain the paths of IFR flights that require ATC, taking account of navigation-aid capability. Bali gives 200 m, or 700 ft, above ground or water as the lowest permissible lower limit. A terminal control area is a control area normally established where several ATS routes converge near one or more major aerodromes. An airway is a control area, or a portion of one, established as a corridor around an ATS route.
Control zone
A control zone is controlled airspace extending upward from the surface. Its lateral limits contain the portions of arriving and departing IFR paths that lie outside a control area. It extends at least 5 NM from the centre of the aerodrome in the directions from which approaches may be made and may include two or more nearby aerodromes. When a CTR lies beneath a CTA, it reaches upward to the CTA lower limit.
Aerodrome traffic zone
An aerodrome traffic zone protects traffic operating on or near an aerodrome. Where no ATZ or local-flying reservation has been established, Bali treats a 5 NM radius from the aerodrome reference point and airspace up to 3,000 ft AGL as the vicinity of the aerodrome for VFR local operations.
How ATS units and airspaces are named
An area control centre or flight information centre is identified by the name of a nearby town, city or geographical feature. An aerodrome control tower or approach unit uses the aerodrome name. A control zone, control area or flight information region is identified by the name of the unit having jurisdiction over it. An aerodrome becomes a controlled aerodrome when aerodrome control service is provided to its traffic.
| Structure | Vertical or lateral idea | Main service purpose |
|---|---|---|
| FIR | Defined lateral region, normally from the surface through its assigned vertical extent | Flight information and alerting |
| UIR | Upper region above one or more FIRs | High-level information, alerting and control as designated |
| CTA | Starts above the surface, not lower than 700 ft AGL | Area control for controlled en-route flight |
| TMA | A CTA around route convergence near major aerodromes | Organises dense arriving, departing and overflying traffic |
| Airway | Corridor forming part of a CTA | Contains an ATS route and associated protected airspace |
| CTR | From the surface, at least 5 NM in approach directions | Protects arriving and departing controlled flights |
| ATZ | Immediate aerodrome vicinity | Protects aerodrome traffic |
Airspace classes A to G
The class letter tells a pilot which flight rules are admitted, which aircraft ATC separates, what service is received and whether clearance and two-way communication are compulsory.
| Class | Flights admitted and separation | Service | Speed below 10,000 ft | Radio and clearance | VMC basis |
|---|---|---|---|---|---|
| A | IFR only; all aircraft separated | ATC | Not applicable | Continuous two-way; clearance required | VFR not admitted |
| B | IFR and VFR; all aircraft separated | ATC | No class speed limit stated in the Bali table | Continuous two-way; clearance required | Standard VMC minima |
| C | IFR separated from IFR and VFR; VFR separated from IFR | ATC; VFR receives traffic information on other VFR and avoidance advice on request | 250 kt IAS for VFR | Continuous two-way; clearance required | Standard VMC minima |
| D | IFR separated from IFR; VFR receives no ATC separation from other traffic | ATC, traffic information and avoidance advice on request | 250 kt IAS for IFR and VFR | Continuous two-way; clearance required | Standard VMC minima |
| E | IFR separated from IFR; VFR receives no ATC separation | ATC to IFR; traffic information to all as far as practicable | 250 kt IAS for IFR and VFR | IFR continuous two-way and clearance; VFR neither compulsory | Standard VMC minima |
| F | IFR separation as far as practicable; no VFR separation | Advisory service to participating IFR; flight information to all on request | 250 kt IAS for IFR and VFR | Participating IFR maintains two-way communication; advice is not an ATC clearance | Standard VMC, with low-level F and G rule |
| G | IFR and VFR; no ATC separation | Flight information on request | 250 kt IAS for IFR and VFR | IFR communication as prescribed; no clearance. VFR normally neither | Standard VMC, with low-level F and G rule |
VMC minima carried into the class table
At or above 10,000 ft, VFR requires 8 km flight visibility and 1,500 m horizontal plus 1,000 ft vertical clearance from cloud. Below 10,000 ft, visibility is normally 5 km with the same cloud clearance. In Classes F and G at or below 3,000 ft AMSL or 1,000 ft above terrain, whichever is higher, the flight may remain clear of cloud with the surface in sight, subject to the applicable visibility rule and any authorised reduction.
Class C procedure in India
Bali describes designated high-capacity terminal airspace with primary and secondary radar as Class C. VFR aircraft obtain pre-departure clearance before entry and must be capable of meeting meteorological, communication and speed requirements. Aircraft follow published SID and STAR speed restrictions and any additional ATC speed instruction. Terrain clearance remains the VFR flight crew's responsibility. A VFR flight unable to comply requests an alternative clearance.
In the structure described by Bali, Class C extends from 4,000 ft to FL 285 in the designated terminal airspace, airspace within 30 NM around the relevant airports up to 4,000 ft is Class D, and airspace above FL 285 is intended for Class A designation. Current AIP boundaries and classifications remain controlling.
Where two classes adjoin vertically, an aircraft exactly at the common level complies with the requirements of the less restrictive class and receives the services of that class.
Indian FIRs, location indicators and special-use areas
India's airspace uses ICAO location indicators and nationally assigned area designators. Reading the letters in order tells the pilot the region, the Indian FIR family and the individual station or airspace.
Indian flight information regions
Bali lists Delhi, Mumbai, Kolkata and Chennai FIRs, plus the Guwahati sub-FIR. Older names in the book are Bombay, Calcutta and Madras. The modern operational names are Mumbai, Kolkata and Chennai. Each FIR has a flight information centre or area control centre responsible for information, alerting and control as designated.
| Operational region | Second indicator letter | Example |
|---|---|---|
| Delhi FIR | I | VIDP, Delhi |
| Mumbai FIR | A | VABB, Mumbai |
| Kolkata FIR | E | VECC, Kolkata |
| Guwahati sub-FIR | E | VEGT, Guwahati |
| Chennai FIR | O | VOMM, Chennai |
How the four-letter indicator is built
The first letter identifies the ICAO air-navigation region. Indian aerodromes begin with V. The second letter identifies the Indian FIR family, using I, A, E or O. The third and fourth letters identify the station. Navigation facilities use shorter chart identifiers: an NDB commonly uses two or three letters, a VOR three letters and an ILS four letters beginning with I. A significant point normally uses a unique five-letter pronounceable name.
Prohibited, restricted and danger areas
An Indian designator uses the location group, then P for prohibited, R for restricted or D for danger, followed by a unique number. A prohibited area excludes flight. A restricted area permits flight only under stated conditions. A danger area warns that activities hazardous to aircraft may occur during notified times.
Within a restricted area, a civilian ATS class applies only to the portion controlled by civil ATC under the flexible-use arrangement. The status and activation times of special-use airspace are checked in current aeronautical information and NOTAM.
Temporary segregated and reserved areas
| Area | Meaning | Number allocation in Bali |
|---|---|---|
| TSA | Temporarily segregated for the exclusive use of a named user; other traffic does not transit while active. | 501 to 999 series |
| TRA | Temporarily reserved for a named user; other traffic may be authorised to transit under ATC clearance. | 501 to 999 series |
| Mumbai allocation | 501 to 600 | |
| Kolkata allocation | 601 to 700 | |
| Delhi allocation | 701 to 800 | |
| Chennai allocation | 801 to 900 | |
| Future use | 901 to 999 | |
Air Defence Identification Zones
Bali identifies North, South, East, West, Central and Southeast sub-zones within India's ADIZ structure. Flight in or across an ADIZ follows the notified flight-plan, air-defence clearance, identification, communication and reporting procedure. An ADIZ supports early identification and is not itself an airspace class.
Local and training flights
Scheduled and non-scheduled operators submit a flight plan for local training or test flight. A flying-club training flight within an ATZ may submit the ICAO model flight plan by the permitted means. Where no ATS reporting office exists, cross-country flying-club and general-aviation plans are sent to the FIC and include FIR-boundary elapsed times and a contact number.
ATS routes, designators and route-entry rules
An ATS route designator is compact operational data. It identifies a route family and number, while optional letters identify special use, vertical placement or service.
Basic route designator
The basic designator contains one letter followed by a number from 1 to 999. The complete designator should normally use no more than five characters and never more than six. Bali notes that domestic routes in India begin with W.
| Position | Letter | Meaning |
|---|---|---|
| Basic letter | A, B, G, R | Regional route that is not an area-navigation route |
| Basic letter | L, M, N, P | Regional area-navigation route |
| Basic letter | H, J, V, W | Non-regional route that is not an area-navigation route |
| Basic letter | Q, T, Y, Z | Non-regional area-navigation route |
| Prefix | K | Low-level route established primarily for helicopters |
| Prefix | U | Route or portion established in upper airspace |
| Prefix | S | Route established exclusively for supersonic operation during acceleration, deceleration and supersonic flight |
| Suffix | F | Advisory service only on the route or portion |
| Suffix | G | Flight information service only on the route or portion |
| Suffix | Y | Required turn performance on an RNP 1 route at and above FL 200, using the published 22.5 NM radius basis |
| Suffix | Z | Required turn performance on an RNP 1 route at and below FL 190, using the published 15 NM radius basis |
The basic letters A, B, G and R are spoken as Amber, Blue, Green and Red when a route designator is transmitted by voice. Bali records the AIP description of conventional navigation routes separately from air-navigation routes.
SIDs and STARs use the associated navigation aid or significant-point name, followed by a number and, when required, a letter suffix. The result identifies a published procedure unambiguously in a clearance.
Operating on Indian ATS routes
- Obtain ATC clearance at least 10 minutes before entering controlled airspace.
- Before joining or crossing an ATS route, obtain approval at least 10 minutes before entry when in direct VHF contact, or at least 20 minutes before entry when contact is relayed through an en-route frequency.
- Join or cross at or close to a designated reporting point. Cross approximately at right angles and at a level appropriate to magnetic track.
- Report position as soon as possible after passing each designated reporting point. On a route without designated points, report after the first 30 minutes and then hourly, unless ATS prescribes another interval.
- For transition from a foreign FIR into an Indian FIR, send the boundary estimate and flight level to the receiving FIR at least 10 minutes before entry, unless a published exception applies.
Route protection and border rule
The minimum flight altitude published for an Indian ATS route provides at least 1,000 ft vertical clearance above the highest obstacle within 10 km on either side of the route centre line. Flight within 15 NM of India's international border is prohibited unless specifically permitted, operated on a notified ATS route, or arriving at or departing from an aerodrome within that band under the applicable procedure. Flight outside ATS routes within Indian FIRs requires ATC authorisation.
RNP and RVSM
Required navigation performance expresses the containment accuracy needed in a defined operation, stated as a nautical-mile value for at least 95 percent of flight time. Reduced vertical separation minimum airspace extends from FL 290 through FL 410 inclusive. Approved aircraft may use 1,000 ft vertical separation there. A non-approved aircraft or an aircraft that loses the required height-keeping capability follows the published contingency and advises ATC promptly.
ATC application, clearances and traffic management
ATC is applied where the airspace class or aerodrome operation requires control. Its clearances organise known traffic, but the pilot retains responsibility for safe aircraft operation and for requesting an alternative when a clearance cannot be accepted.
Where ATC is provided
ATC is provided to all IFR flights in Classes A, B, C, D and E; to all VFR flights in Classes B, C and D; to all Special VFR flights; and to all aerodrome traffic at controlled aerodromes.
Who ATC separates
| Airspace or operation | Separation provided |
|---|---|
| Classes A and B | Between all flights admitted to the class |
| Classes C, D and E | Between IFR flights |
| Class C | Between IFR and VFR flights |
| Special VFR | Between IFR and Special VFR, and between Special VFR flights when prescribed |
Clearance and mandatory read-back
An ATC clearance authorises an aircraft to proceed under specified conditions. The crew reads back safety-related voice instructions, including ATC route clearances; instructions to enter, land on, take off from, hold short of, cross or backtrack on a runway; runway in use; altimeter settings; SSR codes; level, heading and speed instructions; and transition levels. The controller listens to the read-back and immediately corrects any discrepancy. Voice read-back is not required for a CPDLC message.
Essential traffic
Essential traffic is controlled traffic to which separation should apply but which is not, or will not be, separated by the required minimum from another controlled flight. Essential traffic information is passed early enough for the crew to act and normally includes direction of flight, aircraft type and wake category when relevant, cruising level and estimated time over the reporting point or closest approach. Receiving information does not transfer separation responsibility to the pilot unless a specific procedure allows it.
Emergency and contingency priority
An aircraft known or believed to be in emergency, including unlawful interference, receives maximum consideration, assistance and priority as circumstances require. It may select Mode A code 7700 for general emergency, 7500 for unlawful interference, activate the appropriate ADS emergency or urgency function, or send the appropriate CPDLC emergency message. ATS protects other traffic, coordinates assistance and avoids disclosing an unlawful-interference situation if doing so could aggravate it.
Separation minima and Air Traffic Flow Management
The ATS authority selects separation minima from PANS-ATM and the applicable regional procedures. ATFM balances demand against airport, airspace and aircraft capacity. Bali describes India's central system with a Central Command Centre at Delhi supported by flow-management positions at Delhi, Mumbai, Bengaluru, Chennai, Kolkata and Hyderabad, and at other major ACCs and towers. Where demand exceeds capacity, traffic may receive a calculated take-off time or another restriction so delay is absorbed safely, preferably on the ground.
Flight information, advisory service and ATIS
Flight information is broader than weather. It warns pilots about hazards, changes to facilities and conditions that may affect a safe, efficient flight, whether or not the aircraft is receiving ATC.
Scope of flight information service
- SIGMET information and relevant special air-reports. Bali notes that AIRMET is not provided in India.
- Pre-eruption volcanic activity, eruptions and volcanic ash clouds.
- Release of radioactive material or toxic chemicals into the atmosphere.
- Changes in serviceability of navigation aids.
- Changes in aerodrome condition and associated facilities, including movement-area contamination.
- Unmanned free balloons and any other information likely to affect safety.
- Reported or forecast weather at departure, destination and alternate aerodromes.
- Collision hazards for aircraft in Classes C, D, E, F and G.
- For overwater flight, available surface-vessel information when requested.
For VFR flight, FIS includes available traffic and route-weather information likely to make VFR operation impracticable. ATS units pass relevant special air-reports promptly to affected aircraft, the meteorological office and other ATS units.
Air traffic advisory service
Class F advisory service assists participating IFR flights outside controlled airspace. The aircraft follows procedures similar to a controlled flight, but the flight plan and changes are not subject to ATC clearance. The unit provides advice about essential traffic and suggests a course of action. Air-ground contact is maintained with the designated advisory unit.
Voice and data ATIS
Voice ATIS reduces load on aerodrome VHF channels. It uses a discrete frequency whenever practicable, may use a suitable navigation-aid voice channel other than ILS, and is continuous and repetitive. A message should normally not exceed 30 seconds. Data ATIS supplements the voice service with identical operational content and format. Each message concerns one aerodrome, is updated after a significant change and carries a sequential phonetic-letter designator. The aircraft acknowledges the current designator on first contact.
Combined arrival and departure ATIS contents
| Group | Items |
|---|---|
| Identity | Aerodrome name; arrival, departure or combined indicator; data-link contract type if applicable; designator; observation time |
| Approach and runway | Expected approach type; runway in use; hazardous arresting-system status; significant runway surface condition and braking action |
| Traffic management | Holding delay if applicable; transition level; other essential operational information |
| Wind and visibility | Surface wind and significant variation, including runway section when relevant; visibility and RVR |
| Weather and cloud | Present weather; cloud below 5,000 ft or the highest minimum sector altitude, whichever is higher; cumulonimbus; vertical visibility if obscured |
| Temperature and pressure | Air temperature; dew-point temperature; altimeter setting |
| Hazards and forecast | Significant approach and climb-out weather, including wind shear; recent weather of operational significance; trend forecast |
| Closing item | Specific ATIS instructions |
Runway surface terms used in information
Where surface condition may affect braking, Bali lists COMPACTED SNOW, DRY, DRY SNOW, DRY SNOW ON TOP OF COMPACTED SNOW, DRY SNOW ON TOP OF ICE, FROST, ICE, SLUSH, STANDING WATER, WATER ON TOP OF COMPACTED SNOW, WET, WET ICE, WET SNOW, WET SNOW ON TOP OF COMPACTED SNOW and WET SNOW ON TOP OF ICE. A friction device must not be presented as predicting aircraft braking performance unless its correlation is accepted under the applicable DGCA procedure.
When CAVOK applies, it replaces the visibility, present-weather and cloud elements. When rapidly changing weather makes an ATIS report unsuitable, the broadcast tells pilots that relevant weather will be passed on first contact. A directed transmission need not repeat acknowledged current ATIS information, except the current altimeter setting remains available as prescribed.
Alerting phases, strayed aircraft and ATS response
The alerting service escalates from uncertainty to alert and then distress as evidence strengthens. The phase is selected from communication, arrival, landing, fuel and aircraft-condition information, not merely from the passage of time.
Who receives alerting service
The service applies to every aircraft receiving ATC, as far as practicable to other aircraft that filed a flight plan or are otherwise known to ATS, and to an aircraft known or believed to be subject to unlawful interference. The FIC or ACC collects relevant information and forwards it to the appropriate rescue coordination centre. An approach or aerodrome unit may activate immediate local assistance before completing the normal coordination chain.
| Phase | Typical declaration conditions |
|---|---|
| INCERFA, uncertainty | No communication within 30 minutes after the expected call or first unsuccessful attempt, whichever is earlier; or arrival more than 30 minutes late compared with the last notified or ATS estimate, whichever is later, unless there is no doubt about safety. |
| ALERFA, alert | After INCERFA, further communication attempts and enquiries produce no news; an aircraft cleared to land fails to land within 5 minutes and communication is not restored; operating efficiency is impaired but forced landing is not likely; or unlawful interference is known or suspected. |
| DETRESFA, distress | After ALERFA, wider enquiries indicate probable distress; fuel is exhausted or insufficient to reach safety; operating efficiency is impaired enough to make a forced landing likely; or the aircraft is about to make or has made a forced landing, unless occupants are known not to face grave and imminent danger. |
Information to nearby aircraft
When ATS establishes that an aircraft is in emergency, other aircraft known to be nearby are informed of the nature of the emergency as soon as practicable. For suspected unlawful interference, ATS does not mention the nature of the emergency unless the affected aircraft has already referred to it and disclosure will not aggravate the situation.
Strayed and unidentified aircraft
A strayed aircraft has deviated significantly from intended track or reports that it is lost. An aircraft can be strayed to one ATS unit and unidentified to another. ATS identifies the aircraft, establishes its position, advises corrective action, coordinates with adjacent units and provides navigation assistance while safeguarding other traffic.
Contingency principle
When normal communication, navigation, surveillance or airspace capacity is degraded, ATS uses the published contingency plan, protects affected traffic, establishes alternate coordination and communication where available, and may restrict entry or reroute aircraft. The pilot reports the limitation promptly, follows the contingency clearance when able and states any inability clearly.