Top

Air Traffic Control Tower

Introduction:

  • Airport Traffic Control Towers (ATCTs) are established to provide for a safe, orderly and expeditious flow of traffic
    • When the responsibility has been so delegated, towers also provide for the separation of IFR aircraft in the terminal areas
  • ATCTs control traffic flow on and in the vicinity of an airport
  • They exist when traffic requirements demand and subsequently designated the airspace as either class B, class C, or class D depending on level of congestion and services provided
  • Additionally, ATCTs also provide for separation of Instrument Flight Rule (IFR) aircraft in the terminal areas
  • Air Traffic Control towers consist of three main components:
Encyclopedia Britannica, Air Traffic Control Tower
Figure 1: Air Traffic Control Tower

Clearance Delivery:

  • Clearance delivery is established at busy airports to relieve clutter on the ground frequency in order to facilitate instructions
    • Where clearance delivery is not a dedicated frequency, ground control absorbs this function
  • Clearance delivery has no control function and is used solely as the name implies, for the delivery of clearances
  • What is a clearance?

    • An ATC clearance is an authorization, for the purpose of preventing collision between known aircraft, for an aircraft to proceed under specified conditions within controlled airspace
    • Clearances are issued based on a prediction of known traffic and known physical airport conditions
      • A clearance is not an authorization to deviate from any rule, regulation, or minimum altitude nor to conduct unsafe operations of an aircraft
    • Reading back a clearance implies acceptance unless you reject and give a reason
  • Deviating from a clearance:

    • If ATC issues a clearance that would cause a pilot to deviate from a rule or regulation, or in the pilot's opinion, would place the aircraft in jeopardy, IT IS THE PILOT'S RESPONSIBILITY TO REQUEST CLARIFICATION AND/OR AN AMENDED CLEARANCE
      • Similarly, if a pilot prefers to follow a different course of action, such as make a 360° turn for spacing to follow traffic when established in a landing or approach sequence, land on a different runway, takeoff from a different intersection, takeoff from the threshold instead of an intersection, or delay operation, THE PILOT IS EXPECTED TO INFORM ATC ACCORDINGLY
      • When the pilot requests a different course of action, however, the pilot is expected to cooperate so as to preclude disruption of traffic flow or creation of conflicting patterns
      • The pilot is also expected to use the appropriate aircraft call sign to acknowledge all ATC clearances, frequency changes, or advisory information
    • Each pilot who deviates from an ATC clearance in response to a Traffic Alert and Collision Avoidance System resolution advisory must notify ATC of that deviation as soon as possible
  • Clearance Format:

    • At the most basic level you will receive a clearance in a standard format which goes by the acronym "CRAFT"
      • This stands for "Clearance, Route, Altitude, Frequency and, Transponder"
    • An example of this would be:
      • Pilot: "[Agency] clearance, IFR to [Destination] on request"
      • ATC: "[Callsign], I have your clearance, advise when ready to copy"
      • Pilot: "[Callsign], ready to copy"
      • ATC: "Bobcat 123, is cleared to Navy Pensacola via direct Montgomery, Pecan, Valdosta, Tallahassee, then as filed. Maintain runway heading for vectors on course. Climb to and maintain one zero thousand. Expect flight level two seven zero ten minutes after takeoff. Contact Meridian Departure Control on two seven six point four. Squawk four seven one zero. Reason for change of flight plan is a line of severe thunderstorm in vicinity of Jacksonville"
    • If operating at an airfield that does not have an organic clearance delivery then the clearance, control information, or a response to a request for information originated by an ATC facility and relayed to the pilot through an air−to−ground communication station will be prefixed with: "ATC clears," "ATC advises," or "ATC requests"
  • Clearance Items:

    • Clearance limit:

      • A clearance limit will normally authorize flight to the airport of intended landing
        • "cleared to Navy Pensacola"
      • Many airports and associated NAVAIDs are collocated with the same name and/or identifier, so care should be exercised to ensure a clear understanding of the clearance limit
      • When the clearance limit is the airport of intended landing, the clearance should contain the airport name followed by the word "airport"
        • Under certain conditions, a clearance limit may be a NAVAID or other fix
        • When the clearance limit is a NAVAID, intersection, or waypoint and the type is known, the clearance should contain type
        • Under certain conditions, at some locations a short−range clearance procedure is utilized whereby a clearance is issued to a fix within or just outside of the terminal area and pilots are advised of the frequency on which they will receive the long−range clearance direct from the center controller
    • Departure instructions or Standard Instrument Departures (SIDs):

      • Headings to fly and altitude restrictions may be issued to separate a departure from other air traffic in the terminal area
        • "Maintain runway heading for vectors on course"
      • Where the volume of traffic warrants, departures/SIDs have been developed
    • Route of flight:

      • Clearances are normally issued for the altitude or flight level and route filed by the pilot
        • However, due to traffic conditions, it is frequently necessary for ATC to specify an altitude or flight level or route different from that requested by the pilot
        • In addition, flow patterns have been established in certain congested areas or between congested areas whereby traffic capacity is increased by routing all traffic on preferred routes
        • Information on these flow patterns is available in offices where preflight briefing is furnished or where flight plans are accepted
      • When required, air traffic clearances include data to assist pilots in identifying radio reporting points
        • "via direct Montgomery, Pecan, Valdosta, Tallahassee, then as filed"
      • It is the responsibility of pilots to notify ATC immediately if their radio equipment cannot receive the type of signals they must utilize to comply with their clearance
    • Altitude Assignment:

      • The altitude or flight level instructions in an ATC clearance normally require the pilot "maintain" the altitude or flight level at which the flight will operate when in controlled airspace
        • "Climb to and maintain one zero thousand"
      • Altitude or flight level changes while en route should be requested prior to the time the change is desired
      • When possible, if the altitude assigned is different from the altitude requested by the pilot, ATC will inform the pilot when to expect climb or descent clearance or to request altitude change from another facility
        • "Expect flight level two seven zero ten minutes after takeoff"
      • If this has not been received prior to crossing the boundary of the ATC facility’s area and assignment at a different altitude is still desired, the pilot should reinitiate the request with the next facility
      • Pilots may request different altitudes
      • The term cruise may be used instead of maintain to assign a block of airspace to a pilot from the minimum IFR altitude up to and including the altitude specified in the cruise clearance
        • The pilot may level off at any intermediate altitude within this block of airspace
        • Climb/descent within the block is to be made at the discretion of the pilot
        • However, once the pilot starts descent and verbally reports leaving an altitude in the block, the pilot may not return to that altitude without additional ATC clearance
    • Departure Frequency:

      • Pilots are advised of the frequency on which they will receive the long-range clearance direct form the center controller
        • "Contact Meridian Departure Control on two seven six point four"
    • IFF Code:

      • IFF Codes will be assigned to link your aircraft to your assigned route/flight plan
        • "Squawk four seven one zero"
      • Learn more about the transponder and IFF codes here
    • Any holding instructions:

      • Whenever an aircraft has been cleared to a fix other than the destination airport (clearance limit) and delay is expected, it is the responsibility of the ATC controller to issue complete holding instructions (unless the pattern is charted), an EFC time, and a best estimate of any additional en route/terminal delay
        • If the holding pattern is charted and the controller doesn’t issue complete holding instructions, the pilot is expected to hold as depicted on the appropriate chart
        • When the pattern is charted, the controller may omit all holding instructions except the charted holding direction and the statement AS PUBLISHED, e.g., "HOLD EAST AS PUBLISHED"
        • Controllers must always issue complete holding instructions when pilots request them
      • If no holding pattern is charted and holding instructions have not been issued, the pilot should ask ATC for holding instructions prior to reaching the fix
        • This procedure will eliminate the possibility of an aircraft entering a holding pattern other than that desired by ATC
        • If unable to obtain holding instructions prior to reaching the fix (due to frequency congestion, stuck microphone, etc.), hold in a standard pattern on the course on which you approached the fix and request further clearance as soon as possible
        • In this event, the altitude/flight level of the aircraft at the clearance limit will be protected so that separation will be provided as required
      • When an aircraft is 3 minutes or less from a clearance limit and a clearance beyond the fix has not been received, the pilot is expected to start a speed reduction so that the aircraft will cross the fix, initially, at or below the maximum holding airspeed
      • When no delay is expected, the controller should issue a clearance beyond the fix as soon as possible and, whenever possible, at least 5 minutes before the aircraft reaches the clearance limit
      • Pilots should report to ATC the time and altitude/flight level at which the aircraft reaches the clearance limit and report leaving the clearance limit
      • In the event of two−way communications failure, pilots are required to comply with 14 CFR Section 91.185
      • At an uncontrolled field if a clearance is required the order of ease is as follows:
        1. Depart VFR and contact TRACON/ARTCC while VFR airborne
        2. GCO
        3. Frequency found in the A/FD
        4. RCO/VOR on the field
        5. Telephone FSS clearance/delivery number
  • Pilot Responsibility upon Clearance Issuance:

    • Record ATC clearance:

      • When conducting an IFR operation, make a written record of your clearance
      • The specified conditions which are a part of your air traffic clearance may be somewhat different from those included in your flight plan
      • Additionally, ATC may find it necessary to ADD conditions, such as particular departure route
      • The very fact that ATC specifies different or additional conditions means that other aircraft are involved in the traffic situation
    • ATC Clearance/Instruction Readback:

      • Pilots of airborne aircraft should read back those parts of ATC clearances and instructions containing altitude assignments, vectors, or runway assignments as a means of mutual verification
      • The read back of the "numbers" serves as a double check between pilots and controllers and reduces the kinds of communications errors that occur when a number is either "misheard" or is incorrect
        • Include the aircraft identification in all readbacks and acknowledgments. This aids controllers in determining that the correct aircraft received the clearance or instruction. The requirement to include aircraft identification in all readbacks and acknowledgements becomes more important as frequency congestion increases and when aircraft with similar call signs are on the same frequency
        • EXAMPLE−“Climbing to Flight Level three three zero, United Twelve” or “November Five Charlie Tango, roger, cleared to land runway nine left.”
        • Read back altitudes, altitude restrictions, and vectors in the same sequence as they are given in the clearance or instruction
        • Altitudes contained in charted procedures, such as DPs, instrument approaches, etc., should not be read back unless they are specifically stated by the controller
        • Initial read back of a taxi, departure or landing clearance should include the runway assignment, including left, right, center, etc. if applicable
    • It is the responsibility of the pilot to accept or refuse the clearance issued

    Types of Clearances:

    • Amended Clearances:

      • Amendments to the initial clearance will be issued at any time an air traffic controller deems such action necessary to avoid possible confliction between aircraft
      • Clearances will require that a flight "hold" or change altitude prior to reaching the point where standard separation from other IFR traffic would no longer exist
      • A pilot may wish an explanation of the handling of the flight at the time of occurrence; however, controllers are not able to take time from their immediate control duties nor can they afford to overload the ATC communications channels to furnish explanations.
        • Pilots may obtain an explanation by directing a letter or telephone call to the chief controller of the facility involved
      • Pilots have the privilege of requesting a different clearance from that which has been issued by ATC if they feel that they have information which would make another course of action more practicable or if aircraft equipment limitations or company procedures forbid compliance with the clearance issued
    • Coded Departure Route (CDR):

      • Provides air traffic control a rapid means to reroute departing aircraft when the filed route is constrained by either weather or congestion
      • Consists of eight-character designator that represents a route of flight
        • The first 3 alphanumeric characters represent the departure airport
        • Characters 4 through 6 represent the arrival airport
        • The last 2 are chosen by the overlying ARTCC
          • Example: PITORDN1 is Pittsburgh to Chicago
      • CDRs are updated every 56 days
      • General aviation customers who wish to participate in the program may now enter "CDR Capable" in the remarks section of their flight plan
        • When "CDR Capable" is entered into the remarks section of the flight plan the general aviation customer communicates to ATC the ability to decode the current CDR into a flight plan route and the willingness to fly a different route than that which was filed
    • Special VFR Clearances:

Pre-taxi Clearance Procedures:

Adherence to Clearance:

  • When air traffic clearance has been obtained under either visual or instrument flight rules, the pilot−in−command of the aircraft must not deviate from the provisions thereof unless an amended clearance is obtained. When ATC issues a clearance or instruction, pilots are expected to execute its provisions upon receipt. ATC, in certain situations, will include the word “IMMEDIATELY” in a clearance or instruction to impress urgency of an imminent situation and expeditious compliance by the pilot is expected and necessary for safety. The addition of a VFR or other restriction; i.e., climb or descent point or time, crossing altitude, etc., does not authorize a pilot to deviate from the route of flight or any other provision of the ATC clearance
  • When a heading is assigned or a turn is requested by ATC, pilots are expected to promptly initiate the turn, to complete the turn, and maintain the new heading unless issued additional instructions
  • The term “AT PILOT’S DISCRETION” included in the altitude information of an ATC clearance means that ATC has offered the pilot the option to start climb or descent when the pilot wishes, is authorized to conduct the climb or descent at any rate, and to temporarily level off at any intermediate altitude as desired. However, once the aircraft has vacated an altitude, it may not return to that altitude
  • When ATC has not used the term “AT PILOT’S DISCRETION” nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC. If it is necessary to level off at an intermediate altitude during climb or descent, advise ATC, except when leveling off at 10,000 feet MSL on descent, or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area), when required for speed reduction (14 CFR Section 91.117)
    • Leveling off at 10,000 feet MSL on descent or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area) to comply with 14 CFR Section 91.117 airspeed restrictions is commonplace. Controllers anticipate this action and plan accordingly. Leveling off at any other time on climb or descent may seriously affect air traffic handling by ATC. Consequently, it is imperative that pilots make every effort to fulfill the above expected actions to aid ATC in safely handling and expediting traffic
  • If the altitude information of an ATC DESCENT clearance includes a provision to “CROSS (fix) AT” or “AT OR ABOVE/BELOW (altitude),” the manner in which the descent is executed to comply with the crossing altitude is at the pilot’s discretion. This authorization to descend at pilot’s discretion is only applicable to that portion of the flight to which the crossing altitude restriction applies, and the pilot is expected to comply with the crossing altitude as a provision of the clearance. Any other clearance in which pilot execution is optional will so state “AT PILOT’S DISCRETION"
    • "United Four Seventeen, descend and maintain six thousand"
      • The pilot is expected to commence descent upon receipt of the clearance and to descend at the suggested rates until reaching the assigned altitude of 6,000 feet
    • "United Four Seventeen, descend at pilot’s discretion, maintain six thousand"
      • The pilot is authorized to conduct descent within the context of the term at pilot’s discretion as described above
    • "United Four Seventeen, cross Lakeview V−O−R at or above Flight Level two zero zero, descend and maintain six thousand"
      • The pilot is authorized to conduct descent at pilot’s discretion until reaching Lakeview VOR and must comply with the clearance provision to cross the Lakeview VOR at or above FL 200. After passing Lakeview VOR, the pilot is expected to descend at the suggested rates until reaching the assigned altitude of 6,000 feet
    • "United Four Seventeen, cross Lakeview V−O−R at six thousand, maintain six thousand"
      • The pilot is authorized to conduct descent at pilot’s discretion, however, must comply with the clearance provision to cross the Lakeview VOR at 6,000 feet
    • "United Four Seventeen, descend now to Flight Level two seven zero, cross Lakeview V−O−R at or below one zero thousand, descend and maintain six thousand"
      • The pilot is expected to promptly execute and complete descent to FL 270 upon receipt of the clearance. After reaching FL 270 the pilot is authorized to descend “at pilot’s discretion” until reaching Lakeview VOR. The pilot must comply with the clearance provision to cross Lakeview VOR at or below 10,000 feet. After Lakeview VOR the pilot is expected to descend at the suggested rates until reaching 6,000 feet
    • "United Three Ten, descend now and maintain Flight Level two four zero, pilot’s discretion after reaching Flight Level two eight zero"
      • The pilot is expected to commence descent upon receipt of the clearance and to descend at the suggested rates until reaching FL 280. At that point, the pilot is authorized to continue descent to FL 240 within the context of the term “at pilot’s discretion” as described above
  • In case emergency authority is used to deviate from provisions of an ATC clearance, the pilot−in−command must notify ATC as soon as possible and obtain an amended clearance. In an emergency situation which does not result in a deviation from the rules prescribed in 14 CFR Part 91 but which requires ATC to give priority to an aircraft, the pilot of such aircraft must, when requested by ATC, make a report within 48 hours of such emergency situation to the manager of that ATC facility
  • The guiding principle is that the last ATC clearance has precedence over the previous ATC clearance. When the route or altitude in a previously issued clearance is amended, the controller will restate applicable altitude restrictions. If altitude to maintain is changed or restated, whether prior to departure or while airborne, and previously issued altitude restrictions are omitted, those altitude restrictions are canceled, including departure procedures and STAR altitude restrictions. For example:
    • A departure flight receives a clearance to destination airport to maintain FL 290. The clearance incorporates a DP which has certain altitude crossing restrictions. Shortly after takeoff, the flight receives a new clearance changing the maintaining FL from 290 to 250. If the altitude restrictions are still applicable, the controller restates them
    • A departing aircraft is cleared to cross Fluky Intersection at or above 3,000 feet, Gordonville VOR at or above 12,000 feet, maintain FL 200. Shortly after departure, the altitude to be maintained is changed to FL 240. If the altitude restrictions are still applicable, the controller issues an amended clearance as follows: “cross Fluky Intersection at or above three thousand, cross Gordonville V−O−R at or above one two thousand, maintain Flight Level two four zero"
    • An arriving aircraft is cleared to the destination airport via V45 Delta VOR direct; the aircraft is cleared to cross Delta VOR at 10,000 feet, and then to maintain 6,000 feet. Prior to Delta VOR, the controller issues an amended clearance as follows: “turn right heading one eight zero for vector to runway three six I−L−S approach, maintain six thousand"
      • Because the altitude restriction “cross Delta V−O−R at 10,000 feet” was omitted from the amended clearance, it is no longer in effect
  • Pilots of turbojet aircraft equipped with afterburner engines should advise ATC prior to takeoff if they intend to use afterburning during their climb to the en route altitude. Often, the controller may be able to plan traffic to accommodate a high performance climb and allow the aircraft to climb to the planned altitude without restriction
  • If an "expedite" climb or descent clearance is issued by ATC, and the altitude to maintain is subsequently changed or restated without an expedite instruction, the expedite instruction is canceled. Expedite climb/descent normally indicates to the pilot that the approximate best rate of climb/descent should be used without requiring an exceptional change in aircraft handling characteristics. Normally controllers will inform pilots of the reason for an instruction to expedite

Ground Control:

  • Ground control provides control of movement aircraft or vehicles through taxi instructions on the airport surface area
  • Issues information on ramp service, hazardous ground conditions, fueling operations
  • Air Carrier Ops:
    • Should contact before starting engines, to receive engine start time, taxi and/or clearance information
  • Relays clearance:
    • If clearance delivery is not available, ground will obtain and relay as well as provide ATIS information is unavailable
  • Ground is responsible for the ATIS
  • Listen here (requires flash)
  • Issued with receipt of new official weather on the hour or when conditions change and warrant an update
  • When talking to FSS over a VOR broadcasting ATIS, ATIS will be interrupted
  • "Have the numbers" means you have only received wind, altimeter, and runway (WAR)
  • Important to check before takeoff and contracting approach inbound

Tower Control:

  • Tower control is responsible for aircraft on the active runway departing or landing, and all aircraft airborne within the designated airspace
  • Line up and Wait (LUAW):
    • Formerly called "Position and Hold," LUAW is designed to place an aircraft in position for an imminent departure
    • Not an authorization to takeoff
    • Delay may be for traffic, wake turbulence, etc.
    • Contact ATC if you do not receive a clearance to take off within a reasonable amount of time
      • Could be due to a stuck mic or maybe a spacing requirement
    • When ATC issues intersection position and hold clearances the intersection designator will be used
    • ATC: "[Callsign], [Runway], at [Location], line up and wait"

Services:

  • ATCT control runway traffic, issuing take-off and landing clearances
    • They contain the "tower controller" who is responsible for aircraft on the active runway, and anything airborne with the designated airspace
    • Additionally "Ground" and "Clearance Delivery" personnel are physically located in the tower and are responsible for issuing clearances and controlling movement on the ground up to the switch to tower
  • Provides airport advisory information to arriving flights in absence of ATIS
  • Will keep the pilot informed of the latest reported weather and actual field conditions such as current ceiling, runway visibility, surface winds, and runway conditions

Communications:

  • On departure you are given a clearance, be sure to read back all altitudes, headings and frequencies
  • ATC: "[Callsign] climb and maintain [Heading], switch [Facility] departure"

Tower En Route Control:

  • Tower En Route Control (TEC) is an ATC program to provide a service to aircraft proceeding to and from metropolitan areas (sometimes called "tower to tower"
  • The program uses a series of networked identified routes within the approach control of multiple terminal facilities
  • These routes are made up of existing airway structure in the national system to provide an overflow resource in the low altitude system which would enhance ATC services
    • Essentially, it is for relatively short flights
    • Participating pilots are encouraged to use TEC for flights of two hours duration or less
    • If longer flights are planned, extensive coordination may be required within the multiple complex which could result in unanticipated delays
    • A few facilities have historically allowed turbojets to proceed between certain city pairs, such as Milwaukee and Chicago, via tower en route and these locations may continue this service
    • However, the expanded TEC program will be applied, generally, for non-turbojet aircraft operating at and below 10,000 feet
  • Pilots are subject to the same delay factor as other aircraft
    • In addition, departure and en route delays may occur depending upon individual facility workload
    • When a major metropolitan airport is incurring significant delays, pilots in the TEC program may want to consider an alternative airport experiencing no delay
  • There are no unique requirements to use TEC
  • If TEC is desired, list TEC in the remarks section of the flight plan
  • All approach controls in the system may not operate up to the maximum TEC altitude of 10,000'
  • IFR flight may be planned to any satellite airport in proximity to the major primary airport via the same routing
  • Some advantages include abbreviated filing procedures and reduced traffic separation requirements
  • TEC is dependent upon the ATC's workload, and the procedure varies among locales
  • For more information visit: PilotMag - Tower En Route Control

Conclusion:

  • All three positions may be manned by separate controllers or in the case of low traffic hours or non-busy airports, the same person
    • IFR flight plans should be filed at least 30 minutes prior to your estimated departure time to preclude possible delay on receiving a release time
    • Non-scheduled operations at and above FL 230 should be filed at least 4 hours prior to the estimated time of departure
  • Ground control and clearance delivery are often the same at average to small sized airports, often operated by the same person, in order to provide appropriate clearance and taxi instructions before handed off to the Air Traffic Control Tower
    • Unless otherwise advised by the tower, remain on that frequency during taxiing and run-up, then change to local control frequency when ready to request takeoff clearance

References: