Nolan Chapter 6.pdf

242 / CHAPTER 6 Control Towers Air traffic control towers are operated by both the FAA and non-federal agencies to provide separation to aircraft using an airport. The primary responsibility of the control tower is to ensure that sufficient runway separation exists between aircraft landing and depar...

242 / CHAPTER 6 Control Towers Air traffic control towers are operated by both the FAA and non-federal agencies to provide separation to aircraft using an airport. The primary responsibility of the control tower is to ensure that sufficient runway separation exists between aircraft landing and departing. Other responsibilities of the control tower include relaying IFR clearances, providing taxi instructions, and assisting airborne aircraft within the immediate vicinity of the airport. These tasks are accomplished using two-way radio equipment to instruct the pilot to land or take-off or to adjust the aircraft’s flight pattern. There are three general categories of control towers: VFR towers, nonradar-approach control towers, and radar-approach control towers. Both radar- and nonradar-approach control towers have been delegated IFR separation responsibility by a letter of agreement between the control tower and the ARTCC. Nonradar-approach controllers are usually located in the tower cab itself and separate IFR aircraft using the nonradar procedures described in detail in Chapter 7. Radar-approach controllers are usually housed in a separate room near the base of the tower. These controllers separate IFR aircraft using radar and the procedures described in Chapter 8. VFR towers are not delegated any significant separation responsibility by the ARTCC. Primary responsibility for IFR separation around VFR towers is retained by the ARTCC or has been delegated to another control tower. Controllers in a VFR tower may be delegated limited responsibility for initially separating IFR departures or separating IFR arrivals from IFR departures. These procedures are covered in Chapter 7. All three types of control towers are responsible for the separation of aircraft taking off or landing at the airport. Only the procedures and techniques used by controllers to separate aircraft operating within the airport traffic area or on the airport surface are discussed in this chapter. The duties of personnel assigned to a control tower have been subdivided into four categories: flight data, clearance delivery, ground control, and local control. In a busy tower, these responsibilities may be assigned to four or more individual controllers, whereas at less busy facilities these responsibilities may be combined into fewer positions. Flight Data Controller Duties The flight data controller assists the other controllers in the tower and performs the clerical duties inherent in the operation of any facility. As noted in Chapter 5, this position is typically the first one assigned to a new controller at the facility. The basic responsibilities of and duties performed by a flight data controller include the following: Receiving and relaying IFR departure clearances to the clearance delivery controller Control Tower Procedures / 243 Operating the flight data processing equipment Relaying weather and NOTAM information to other positions of operation Aiding other tower controllers by relaying any directed information Collecting, tabulating, and storing daily records Preparing the Automatic Terminal Information Service (ATIS) recordings Processing field condition reports Receiving and Relaying IFR Departure Clearances The flight data controller is responsible for obtaining IFR clearances from the ARTCC and relaying them to the clearance delivery controller. These clearances are received over the telephone or through automated procedures. IFR clearances obtained by telephone are handwritten, whereas those obtained automatically are printed mechanically on a flight data input/output (FDIO) device. Clearances are printed in a standard format on forms known as flight progress strips (or flight strips), as shown in Figure 6–1. After obtaining the IFR clearance, the flight data controller passes the strip to the clearance delivery controller. To facilitate accurate interpretation, flight strips are printed using standard markings and abbreviations, ensuring that specific information will always be found in the same place. These locations of flight strips are known as fields. The approved field contents and format can be found in the Air Traffic Control Handbook. Flight progress strips used in control towers are formatted differently from those used in the ARTCCs but contain essentially the same information. A sample flight progress strip used in a control tower is shown in Figure 6–2 with the appropriate field numbers notated. The format for flight strips differs somewhat depending on whether the aircraft involved is a departure, an arrival, or an over flight (an aircraft that + + SV Figure 6–1. Sample terminal flight progress strip. 1 2 3 4 2A 5 8 6 8A 7 8B 9 9A Figure 6–2. Fields on a terminal flight progress strip. 9B 10 11 12 13 14 15 16 17 18 9C 244 / CHAPTER 6 passes through the airspace delegated to the tower but is not planning to land). Since the flight data controller in the tower will primarily be concerned with departing aircraft, that type of flight strip is discussed here. A flight progress strip for a departing aircraft includes the following information, by field number: 1. Aircraft identification. The aircraft identification consists of the approved identification as discussed in Chapter 4. 2. Revision number (FDIO strip only). When the first flight progress strip has been printed for this aircraft, a number 1 appears in this location. If the pilot’s flight plan is changed, or if the ARTCC amends the pilot’s clearance, a new flight strip is printed with a number 2 in the field. The old strip should be destroyed. If by some chance it is not destroyed, the revision number will help establish which strip contains the most current flight plan information. 2A. Strip request originator. At FDIO-equipped locations, this indicates the sector or position that requested the strip. 3. Type of aircraft. The type of aircraft is indicated using the conventions covered in Chapter 4. If more than one aircraft is included in the clearance, the number of aircraft involved precedes the aircraft type, separated by a slash (multiple aircraft flying under the same IFR clearance are known as a flight). If the aircraft’s gross weight is over 255,000 pounds, it is considered a heavy aircraft and usually creates a phenomenon known as wake turbulence. This turbulence, which can be dangerous to following aircraft, is discussed later in this chapter. A heavy aircraft is identified with an H preceding the aircraft type on the flight strip. Examples of aircraft types include the following: 2/F16 Two F16 fighters H/B747 A heavy Boeing 747 To assist subsequent controllers, an equipment suffix is added to the aircraft type. The type of equipment onboard the aircraft is determined using the information provided by the pilot upon filing the IFR flight plan. The equipment suffix printed on the flight strip will usually be one of the following: No transponder Transponder without altitude encoding Transponder with altitude encoding No DME /X /T /U DME /D /B /A TACAN Only /M /N Flight Management Systems (FMS) /P /E GPS/GNSS /G Required Navigation Performance (RNP) /R Control Tower Procedures No transponder Transponder without altitude encoding / 245 Transponder with altitude encoding Reduced Vertical Separation Minima (RVSM) /W RNP and RVSM aircraft /Q 4. Computer identification number (FDIO only). If the flight progress strip has been computer generated and printed, a unique computer identification number will be printed in this field. This number is unique to the aircraft and can be used in place of the aircraft identification number when using FDIO equipment to obtain additional information about the aircraft. 5. Assigned transponder code. The computer located in the ARTCC will assign a transponder code to this flight. The transponder code is allocated automatically according to the National Beacon Code Allocation Plan (NBCAP). Since two aircraft cannot be assigned the same transponder code while within the boundaries of the same ARTCC, the NBCAP computer program attempts to assign each aircraft a transponder code that will not be the same as that assigned to another aircraft. The NBCAP plan reserves some codes that cannot be assigned to IFR flights. These transponder codes include the following: 1200 Reserved for VFR aircraft not in contact with an ATC facility. 7500 Reserved for aircraft being hijacked. 7600 Reserved for aircraft experiencing radio communications failure. 7700 Reserved for aircraft experiencing some type of emergency. 6. Proposed departure time. This is the proposed UTC departure time that the pilot filed in the original flight plan. 7. Requested altitude. This is the altitude requested in the pilot’s original flight plan. To conserve space on the flight progress strip, the last two zeros in the altitude are dropped. For example: Printed altitude Actual altitude 50 5,000 feet 100 10,000 feet 240 Flight level 240 (24,000 feet) 8. Departure airport. This is the airport from which the aircraft will depart. It is printed as a three-character identifier. Every airport that has a published instrument approach has been issued an identifier. Some of the more common identifiers include the following: ORD O’Hare International, Chicago, Illinois JFK John F. Kennedy International, New York ATL Hartsfield International, Atlanta, Georgia 246 / CHAPTER 6 A more complete list is included in Appendix C. 8A and 8B. Optional use. 9. Route of flight and destination airport. The clearance limit is either the destination airport or an intermediate en route fix. The route to be flown includes any airways or VORs that the pilot will be using. If the route is to be flown using area navigation (RNAV), either the waypoint names or their latitude-longitude coordinates will be included. If no airway is designated between two VORs, it is assumed that the pilot will fly directly from one VOR to the next. This field may also include any preferential routes that have been assigned by the ARTCC computer. A preferential route may be a departure, an en route, or an arrival route. Whenever the computer places a preferential route on the flight strip, it should replace the route of flight filed by the pilot. Preferential routes can be identified on the flight progress strip since they are bracketed with  symbols. This field may also contain the abbreviation FRC, which stands for “full route clearance.” This abbreviation is added to the flight plan whenever a controller has changed the pilot's requested route of flight, without the knowledge of the pilot. This information will be used by the clearance delivery controller. 9A, 9B, and 9C. Optional use. 10–18. These fields include any items that may be specified in the facility directives, including actual departure time, departure runway, or any other pertinent information. Standard symbols have been developed for use in these situations. These symbols may be found in the FAA handbook. A sampling is provided in Table 6–1. The flight data controller should check each flight progress strip to ensure that all the appropriate information has been obtained. It is the flight data controller’s responsibility to obtain a corrected flight progress strip, if necessary. Operating the Flight Data Processing Equipment In 1961, when President John F. Kennedy created the Project Beacon task force, controllers were still hand printing flight progress strips and passing along flight information to other controllers using teletypes and party line telephone equipment. A significant portion of a controller’s time was spent communicating with other controllers, requesting and passing along this essential flight information. The Project Beacon task force recommended that the FAA develop a computerized flight information system to automatically update and print out flight progress strips. Such a system was developed and finally installed by IBM in the early 1970s. By the mid-1980s this system had become outdated, and the FAA replaced it with a new computer system called the flight data processing (FDP) system. The flight data processing system uses computers located at each of the ARTCCs to store and update aircraft flight plan information. Whenever a pilot files an IFR flight plan with any air traffic control facility, the information contained in the flight plan is transmitted to and stored in the computer. A half hour prior to the pilot’s proposed departure time, the computer assigns the aircraft a transponder code and causes a flight progress strip to be printed on an FDIO printer at the departure airport. At facilities not equipped with FDIO, the flight progress strip is printed at the appropriate ARTCC sector, and the Control Tower Procedures Table 6–1. / 247 Symbols Used in Flight Strip Fields Symbol TS Meaning Depart (direction, if specified) c Climb and maintain T Descend and maintain S Cruise @ At  Cross ¡ M –    R  Q Q R  nW  ne S e Maintain Join or intercept airway While in controlled airspace While in control area Enter control area Out of control area Cleared to enter, depart, or pass through a control zone (direction of flight is indicated by an arrow and the appropriate compass letter) 250K Assigned airspeed  Before  After or past / –T –c  cL C RV Until At or below At or above Clearance void time Pilot cancelled flight plan Contact (facility) on the appropriate frequency Radar vector flight data controller in the tower must telephone the ARTCC and request the appropriate flight information. This information must then be handwritten by the flight data controller onto a flight progress strip. Departure Message When the aircraft departs, the FDIO is used to send a departure message to the computer at the center. A departure message may be sent either manually or automatically. To manually transmit a departure message, the flight data controller types the departure aircraft’s identification and time of departure into the FDIO. This information is then sent to the computer. The controller may use the aircraft’s call sign, transponder code, or computer identification number to identify any 248 / CHAPTER 6 particular aircraft. The departure time is always entered as UTC time. If no time is entered in the departure message, the current time is assumed by the computer. A departure message is preceded by the characters “DM” when being entered into the FDIO. For example: DM UA611 0313 United Airlines Flight 611 departed at 0313 UTC. DM 561 The aircraft assigned computer identification number 561 departing at current UTC time. If the control tower is equipped with the automated radar terminal system (ARTS) or STARS, the departure message will be automatically sent to the ARTCC computer whenever the secondary radar receiver detects the transmission from the aircraft’s transponder (ARTS is discussed in detail in Chapter 8). Upon receipt of the departure message, the ARTCC computer begins to automatically calculate the aircraft’s future position and prints a flight progress strip for every controller who will eventually be responsible for separating the aircraft. The computer transmits the flight progress strip to each sector approximately 20 to 30 minutes before the aircraft is scheduled to enter that sector. Amending Flight Progress Strips Using FDIO The flight progress strip is typically printed in the control tower 30 minutes before the pilot’s proposed departure time. If, for any reason, the flight strip has not been printed when the pilot is ready to depart, the flight data controller may be asked to obtain a flight strip using the FDIO. This is accomplished through the use of a strip request (SR) message. To request a flight strip through the FDIO, the controller must type the letters SR followed by the aircraft’s call sign (for e.g., SR UA611). If one of the fields on the flight strip contains incorrect information or if the pilot requests a change to the flight plan, the flight data controller may be asked to amend the strip to incorporate the new information. The controller does so by using the FDIO to send an amendment (AM) message. The proper procedure is to type the letters AM followed by the aircraft’s identification, the number of the field that needs to be changed, and the new information for that field. For example, AM UA611 7 120 changes the pilot’s requested altitude (field 7) to 12,000 feet. If the aircraft’s route of flight or altitude is amended, a new flight progress strip is automatically sent to every subsequent sector. If the aircraft’s route of flight will cause it to cross into another ARTCC’s area of responsibility, the appropriate flight information is automatically transmitted to the computer within that ARTCC. When the aircraft leaves the ARTCCs area or lands at the arrival airport, the flight information is erased from the computer’s memory, permitting that aircraft’s transponder code to be allocated to another aircraft. Relaying Weather and NOTAM Information The flight data controller is required to acquire and disseminate appropriate weather information to other controllers or to the National Weather Service (NWS). If the NWS office is located at the airport, its personnel are usually responsible for taking routine weather observations. The controllers in the Control Tower Procedures / 249 tower are required to make only tower visibility observations. If no NWS office is located at the airport, the tower controllers are likely to be responsible for performing all of the necessary weather observations, which they forward to the nearest NWS facility. The controllers in the tower are also responsible for soliciting pilot reports (PIREPs) from pilots operating within the vicinity of the control tower. PIREPs are an essential means of passing along actual flight conditions to other pilots and the NWS. The flight data controller is also responsible for disseminating this weather information to pilots through the use of Automatic Terminal Information Service (ATIS) equipment. ATIS is a continuous-loop digital recording usually made by the flight data controller and transmitted on a VHF frequency for pilot reception. ATIS recordings inform both arriving and departing pilots of weather conditions and other pertinent information at the airport. Pilot reception of ATIS information relieves the ground or approach controller of repeating weather conditions and non-control information to every aircraft. Recordings are made at least once every hour but may be made more often if weather conditions change rapidly. The following information should be included in an ATIS recording: 1. The name of the airport. 2. The ATIS phonetic alphabet code. Every ATIS recording is assigned a code letter that identifies it. The code begins with the letter A and is incremented as new ATIS recordings are made. When pilots make initial contact with a controller, they advise that they have received “Information (code letter).” Whenever a new ATIS recording is made, it is the flight data controller’s responsibility to inform the other controllers in the facility of the new ATIS code letter. Because pilots may listen to the ATIS 10 to 20 minutes prior to contacting a controller, this procedure identifies whether the pilot has received the latest ATIS information. 3. The UTC time of weather observation. This may not be the actual time that the ATIS is recorded, as there is usually a delay between the weather observation and the recording. 4. Wind direction and speed. 5. The visibility in miles and/or fractions of a mile.* 6. The cloud ceiling. The ceiling is measured in feet above the ground and is either measured or estimated. Measured ceilings are determined using a ceilometer. 7. The current temperature in degrees Celsius. 8. The current dew point temperature in degrees Celsius. 9. The altimeter setting. 10. The instrument approach procedure(s) currently in use. 11. The runways(s) used for arrivals. 12. The runway(s) used for departures. *(Items 5 and 6 may be replaced by the phrase “better than five thousand and five”, if the ceiling is higher than 5,000 feet and the visibility is greater than 5 miles.) 250 / CHAPTER 6 13. Pertinent NOTAMS or weather advisories. These include any taxiway closures, severe weather advisories, navigation aid disruptions, unlit obstacles in the vicinity of the airport, or any other problems that could affect the safety of flight. 14. Braking action reports (if appropriate). 15. Low-level wind-shear advisories (if appropriate). 16. Remarks or other information. This may include VFR arrival frequencies, radio frequencies that have been temporarily changed, runway friction measurement values, bird activity advisories, and part-time tower operation. 17. Some towers are required to include a statement advising the pilot to read back instructions to hold short of a runway. The air traffic manager may elect to remove this requirement provided that it does not result in increased requests from aircraft for read back of hold short instructions. 18. Instructions for the pilot to advise the controller that the ATIS recording has been received. A typical ATIS recording is taped in the following sequence: Lansing Airport information charlie, one five five zero zulu weather, wind one six zero at one zero, visibility five, light snow, measured ceiling six hundred overcast. Temperature seven, dew point two, altimeter two niner five five. ILS runway two eight left approach in use, landing and departing runways two eight left and two eight right. Notice to airmen, taxiway bravo is closed. VFR arrivals contact Lansing approach control on one two five point niner. Advise the controller on initial contact that you have information charlie. Clearance Delivery Controller Duties The clearance delivery controller is responsible for obtaining and relaying departure clearances to pilots. The clearance should include the following: Aircraft identification Clearance limit Departure procedure Route of flight Altitude Departure frequency Transponder code The clearance delivery controller is also responsible for amending clearances as necessary. Aircraft clearances may need to be amended to conform to any of the procedures spelled out in letters of agreement or facility directives. Typical amendments might include temporary altitude restrictions or temporary changes in the aircraft’s route of flight. Temporary altitude restrictions may be placed on a pilot to ease the coordination required between the local and the departure controller. At most radar-equipped facilities, the local controller has been delegated the responsibility for initially separating departing aircraft. To ensure that these departures Control Tower Procedures / 251 120∞ Departure area 30∞ 30∞ 5,000 ft. 5 n mi Runway Figure 6–3. Example of a local controller’s delegated departure area. are properly separated from other aircraft within the approach controller’s airspace, facility directives usually describe a specific area that may be used only for departures. Arriving aircraft may not enter this airspace without approval from the local controller. Every facility has its own unique requirements that affect the shape of this departure area, but it is usually an area 40° to 180° wide, extending from the Earth’s surface up to about 5,000 feet AGL (see Figure 6–3). The facility directives usually state that the local controller may depart aircraft into this area without prior coordination with the departure controller. The departure controller must keep arriving aircraft out of this area unless the local controller grants approval. This procedure automatically provides for initial separation of aircraft. Once an aircraft departs, the local controller advises the pilot to contact the departure controller, who has the authority to amend the aircraft's clearance as necessary. It is the clearance delivery controller’s responsibility to temporarily amend the pilot’s clearance to comply with these departure restrictions, which usually consist of restricting the pilot’s altitude to the upper limit of the departure area. To reassure the pilot that this restriction is temporary and that the requested altitude will probably be granted at a later time, and to conform with the FARs, the clearance delivery controller must advise the pilot to expect his or her final altitude at some later time. This interval is specified in the facility directives and is usually 5 or 10 minutes. For example, if a pilot requests a cruising altitude of 15,000 feet MSL but the upper limit of the departure area is 5,000 feet MSL, the clearance delivery controller would advise the pilot to “maintain five thousand, expect one five thousand one zero minutes after departure.” The clearance delivery controller must also ensure that the pilot’s route of flight is accurate and conforms to any preferential routes that may have been established. If the route must be changed, the controller must issue the new route to the pilot and amend the route of flight using the FDIO equipment. 252 / CHAPTER 6 Ground Controller Duties The ground controller is responsible for the safety of aircraft that are taxiing on taxiways or inactive runways. The ground controller issues instructions to aircraft taxiing to or from runways or to vehicles operating around the airport. The ground controller is permitted to exercise this control authority only in areas where traffic can be observed and controlled. The controller is not responsible for aircraft taxiing where they cannot be observed from the control tower, such as aircraft parking areas, hangars, and terminal boarding. Aircraft operating within these nonmovement areas cannot be offered any ground control services. Aircraft and vehicles operating within these areas may proceed without contacting the ground controller. To ensure that the ground controller is always communicating with the correct pilot, the aircraft’s position must be positively determined before issuing any instructions. This position determination can be made through the use of visual observation, a pilot report, or airport surface radar. After determining the aircraft’s location, the ground controller should issue positive instructions to the pilot. These instructions should include the aircraft identification, the name of the ground controller’s facility, the route to be used while taxiing, and any restrictions applicable to the pilot. Here are some examples of phraseology: United six eleven, Lafayette ground, taxi to runway one zero. Cherokee two one four papa alpha, taxi to runway three five via taxiway bravo and charlie. American niner twenty-one, taxi to the terminal via the new scenic taxiway. To avoid confusion when issuing taxi instructions to pilots, the ground controller should never use the word “cleared.” The only person who should use this word is the clearance delivery controller when issuing a clearance or the local controller when clearing aircraft for takeoff or landing. Because the fidelity of aircraft communications equipment is low and the noise level in the tower and cockpit is fairly high, it is possible for the pilot to misinterpret “Cleared to taxi to runway three one” to mean “Cleared for takeoff runway three one.” Obviously these are two very different clearances. However, if the weather conditions are such that neither the ground nor the local controller can see the aircraft, this misinterpretation might prove to be very dangerous. Preventing Runway Incursions One of the primary responsibilities of the ground controller is to ensure that vehicles and taxiing aircraft remain clear of the active runways. If an aircraft or vehicle must cross an active runway, the ground controller must receive permission for that operation from the local controller. If an aircraft should inadvertently taxi onto an active runway without the local controller’s knowledge, an accident could result. Such accidental entry, known as a runway incursion, should be avoided at all costs. Control Tower Procedures / 253 One of the best ways to prevent a runway incursion is to use and understand the appropriate phraseology for communicating with taxiing aircraft. When the clearance to the aircraft begins with “Taxi to (runway number),” the pilot is authorized to cross any and every taxiway and runway along the route. The pilot does not know which of these runways are active and assumes that any required coordination has been accomplished. If the aircraft is required to taxi across an active runway en route to the departure runway, the ground controller must coordinate with the local controller to receive permission to cross the active runway. If that permission is not received, the ground controller must advise the pilot to stop prior to the runway. This is known as holding short of the runway. To differentiate a “hold short” type of clearance from the others, the phraseology of this clearance has been somewhat modified. It includes the aircraft identification, the facility name, the departure runway number, the taxi route, the words “hold short,” the position to hold, and the reason for holding short. For example: Jetblue twenty-three eleven, Lafayette ground, runway five, hold short of runway one zero, traffic landing on one zero. Cactus four fourteen, runway three two right via the cargo and the old scenic taxiway, hold short of runway two seven left, traffic landing two seven left. Kingair four papa uniform, runway one zero, hold short of the parallel taxiway, traffic inbound on the taxiway. If the aircraft must cross an active runway, the ground controller must receive permission from the local controller and advise when the operation is complete (see Figure 6–4). For example: AMERICAN 810: O’Hare ground, American eight ten ready to taxi. GROUND CONTROLLER: American eight ten, O’Hare ground, runway two-seven right via taxiway hotel, hold short of runway three two right, traffic departing runway three two right. AMERICAN 810: American eight ten, roger, taxi to runway two-seven right, hold short of runway three two right. (as the aircraft approaches runway 32R) GROUND CONTROLLER: (to the local controller): Cross three two right at hotel? LOCAL CONTROLLER: Cross runway three two right at hotel. GROUND CONTROLLER: American eight ten, cross runway three two right. AMERICAN 810: American eight ten, roger. If the aircraft must taxi quickly across the runway, the ground controller should use either “taxi without delay” or “immediately.” “Taxi without delay” advises the pilot to cross the runway safely but using a minimum of time. “Immediately” should be used only in an imminent emergency. After the aircraft has crossed the active runway, the ground controller must advise the local controller that the crossing is complete, either verbally or through any visual means specified in the facility directives. 254 / CHAPTER 6 Figure 6–4. Airport taxi chart for Chicago O’Hare International Airport. Control Tower Procedures 255 Areas other than the active runway where the ground controller may want aircraft to hold short include the localizer, glide slope, and precision approach critical areas. The ground controller should not authorize any aircraft or vehicular operation within the confines of a localizer or glide slope critical area when both of the following conditions occur: The reported weather conditions at the airport include a lower than 800-foot ceiling or a reported visibility of less than 2 miles. An arriving aircraft is using the ILS and is located between the outer marker and the airport. Since Category II and Category III ILS approaches permit the pilot to land when visibilities are extremely low, it is necessary to provide additional obstacle clearance during these approaches. Therefore, precision approach critical areas have been defined and demarcated wherever a Category II or III ILS is in operation. Whenever the weather conditions are such that either the ceiling is less than 200 feet AGL or the reported RVR visibility for the runway is 2,000 feet or less, the ground controller is responsible for keeping aircraft and vehicles clear of the obstacle critical area as an aircraft is conducting an approach or a missed approach (see Figure 6–5). It is the airport management’s responsibility to determine whether ILS critical areas affect any runways or taxiways and to install appropriate signs and markings to delineate these areas. 200 ft. 3,000 ft. 400 ft. 27R Protecting Critical Areas / 400 ft. Taxiing aircraft Figure 6–5. Obstacle critical area. Category II hold line 256 / CHAPTER 6 Local Controller Duties It is the responsibility of the local controller to safely sequence arrivals and departures at the airport. The primary responsibility of the local controller is to ensure that proper runway separation exists between aircraft. The local controller issues appropriate instructions to arriving and departing aircraft to ensure this runway separation. It is not the local controller’s responsibility to separate VFR aircraft inbound to the airport, although the controller may offer assistance and issue traffic advisories. It is assumed that the pilots will apply the see and be seen rules of traffic avoidance. Runway Separation For the purpose of runway separation, every aircraft is classified by aircraft category. Aircraft categories are determined as follows: CATEGORY I Lightweight, single-engine, propeller-driven personal aircraft. This category includes the Cessna 152 and 172, Piper Cherokee, and Bellanca Viking. It does not include highperformance single-engine aircraft such as the T-28. CATEGORY II Lightweight, twin-engine, propeller-driven aircraft weighing 12,500 pounds or less. This category includes aircraft such as the Twin Comanche, Piper Seneca, and Cessna 320, but does not include larger aircraft such as the Lockheed Lodestar or Douglas DC-3. CATEGORY III All other aircraft not included in either Category I or II. This category includes high-performance single-engine, large twin-engine, four-engine propeller-driven, and turbojet aircraft. Category III includes aircraft such as the Douglas DC-3 and DC-6, Cessna Citation, and Boeing 757 and 777. Departing Aircraft Separation The local controller is required to separate departing aircraft using the same runway by ensuring that an aircraft does not begin its takeoff roll until at least one of the following conditions exists: 1. The preceding landing aircraft has taxied off of the runway. 2. The preceding departing aircraft is airborne and has crossed the departure end of the runway or has turned to avoid any conflict (see Figure 6–6). If the local controller can determine runway distance using landmarks or runway markings, the first aircraft need only be airborne before the second aircraft begins its takeoff roll if the following minimum distance exists between the aircraft involved (see Figure 6–7): a. If both aircraft are Category I, a 3,000-foot separation interval may be used. b. If a Category II aircraft precedes the Category I, a 3,000-foot separation interval may be used. Control Tower Procedures / 257 Figure 6–6. Preceding aircraft must have crossed the departure threshold or turned to avoid a conflict before the following aircraft can depart. 3,000 ft. 4,500 ft. 6,000 ft. Figure 6–7. The following aircraft must wait until the preceding aircraft is both airborne and a specified distance down the runway before it can begin its takeoff roll. c. If the succeeding or both of the aircraft are Category II, a 4,500-foot separation interval must be used. d. If either of the aircraft is a Category III aircraft, a 6,000-foot separation interval must be used. Thus, if a Piper Cherokee (Category I) departs and is followed by a Cessna 152 (Category I), the local controller must not permit the Cessna to begin its takeoff roll until the Piper has crossed the departure end of the runway, has turned to avoid a conflict, or is airborne and at least 3,000 feet down the runway. But if the Piper is followed by a Cessna 310 (Category II), the local controller must 258 / CHAPTER 6 not permit the Cessna 310 to begin its takeoff roll until the Piper has crossed the departure end of the runway, has turned to avoid a conflict, or is airborne and at least 4,500 feet down the runway. If the Cessna 310 precedes the Piper, however, only 3,000 feet of separation would be needed. To increase runway utilization, it may be advantageous to have the aircraft on the runway, in position to depart, waiting for the preceding aircraft to complete its departure. When this procedure is used, the pilot can be advised to “taxi into position and hold.” The controller should then state the reason that the departure clearance is being withheld: Bellanca six eight charlie, runway two three, taxi into position and hold, traffic landing runway one zero. Clipper one seventeen, runway two one center taxi into position and hold traffic crossing the runway at midfield. Controllers should be careful when using this clearance to ensure that the pilot does not misinterpret the instruction as a takeoff clearance. It is for this reason that the word “cleared” should never be included in a clearance to hold short or to taxi into position and hold. The instruction “cleared for takeoff” clears the pilot to perform a normal takeoff on the runway specified. If more than one runway is active, the runway number should precede the clearance. If additional departure instructions are necessary, they should also precede the takeoff clearance: United seven twenty-five, cleared for takeoff. Kingair six papa uniform, runway two three, cleared for takeoff. Cessna six niner eight, after departure fly heading one eight zero, runway one five, cleared for takeoff. When issuing a “cleared for immediate takeoff,” the controller is expecting the pilot to minimize any delay in departing. The pilot will do his or her best to comply with this clearance, but certain procedures may have to be performed by the pilot while the airplane is still on the runway. If there is any doubt about safe separation, one of the following alternative clearances should be used: “Cleared for immediate takeoff or hold short” The controller is advising the pilot that an immediate departure is required. A pilot who feels that a safe departure can be accomplished will proceed; otherwise, he or she will hold the aircraft short of the runway. “Cleared for immediate takeoff or taxi off the runway” This clearance is most often used when an aircraft has been taxied into position to hold and departure clearance has been delayed. Control Tower Procedures / 259 Here are some examples of phraseology: Piedmont two fifty, runway one seven, cleared for immediate takeoff or hold short, traffic one mile on final. TWA six ninety-one, runway one eight, cleared for immediate takeoff or taxi off the runway, traffic is a DC-9 landing runway two four. Intersecting Runway Separation If the departing aircraft is taking off on a runway that intersects another active runway, or if the flight path of the aircraft will intersect another runway, the local controller must ensure that the aircraft does not begin the takeoff roll until at least one of the following conditions exists: 1. A preceding, landing aircraft has: a. taxied off the landing runway, or b. completed the landing roll and has advised the local controller that it will stop prior to the runway intersection, or c. passed the intersection (see Figure 6–8). 2. A preceding, departing aircraft is airborne and has passed the intersection or is turning prior to the intersection to avert a conflict (see Figure 6–9). (a) (b) (c) Departing aircraft Figure 6–8. Before the departing aircraft on the intersecting runway can be cleared for takeoff, the arriving aircraft must have (a) landed and turned off the runway, (b) landed and advised that it will hold short of the intersection, or (c) passed through the intersection. 260 / CHAPTER 6 (b) (a) Departing aircraft Figure 6–9. An aircraft departing on an intersecting runway must wait until the preceding departure has either (a) passed through the intersection or (b) turned to avoid a conflict. Anticipated Separation The local controller need not actually wait for the appropriate separation interval to clear an aircraft for takeoff. If there is reasonable assurance that the correct separation will exist before the departing aircraft actually begins its takeoff roll, the clearance may be issued at that time. This is known as anticipated separation. In accordance with the FAA handbook, air traffic controllers are permitted to issue both anticipated arrival and departure clearances if proper separation can be expected when needed. Arriving Aircraft IFR pilots use a standard instrument approach when arriving at the airport, whereas VFR pilots approach the airport using all or a portion of a standardized traffic pattern. (A typical traffic pattern is shown in Figure 6–10.) It is the local controller’s responsibility to properly space these two types of inbound aircraft while also sequencing departures into the traffic flow. A VFR traffic pattern consists of five portions known as traffic pattern legs: UPWIND A flight path parallel to the landing runway in the direction of landings and departures. CROSSWIND A flight path at right angles to the landing runway on the departure end. DOWNWIND A flight path parallel to the landing runway in the direction opposite to landing. Control Tower Procedures / 261 Landing direction Runway Crosswind leg Base leg Downwind leg Final approach Upwind leg Figure 6–10. Traffic pattern legs. This example depicts an aircraft in left traffic. BASE A flight path at right angles to the landing runway off its approach end and extending from the downwind leg to the intersection of the extended runway center line. FINAL A flight path in the direction of landing along the runway centerline extending from the base leg to the runway. If the turns performed by the aircraft in the pattern are to the left, the traffic pattern is known as left traffic. If all turns are made to the right, it is known as right traffic. Unless specified in the facility directives, either left or right traffic can be used for any runway at a tower-controlled airport; left traffic is considered standard at uncontrolled airports. The local controller is required to apply runway separation standards to arriving aircraft just like departures. This requirement is accomplished by requiring the pilots to adjust their flight pattern as necessary to provide the following separation for single runways and intersecting runways. Single Runway Separation If only one runway is in use, the local controller must separate arriving aircraft from other aircraft by ensuring that the arriving aircraft does not cross the landing threshold until at least one of the following conditions exists: 1. If the preceding aircraft is an arrival, it has landed and taxied off of the runway (see Figure 6–11). Between sunrise and sunset, the preceding aircraft need not 262 / CHAPTER 6 Figure 6–11. An arriving aircraft may not cross the landing threshold until the preceding aircraft has landed and turned off of the runway. 3,000 ft. Figure 6–12. An arriving Category I aircraft can be cleared to land if the preceding aircraft has touched down, is a Category I aircraft, and is at least 3,000 feet down the runway. have taxied off of the runway if the distance between the two aircraft can be determined using landmarks or runway markings, and the following minimums can be maintained: a. A distance of 3,000 feet if a Category I aircraft is landing behind either a Category I or a Category II aircraft (see Figure 6–12). b. A distance of 4,500 feet if a Category II aircraft is landing behind either a Category I or a Category II aircraft (see Figure 6–13). 2. If the preceding aircraft is a departure, it must have already crossed the departure end of the runway. This minimum can be disregarded if the departing aircraft is airborne and is at least the following distance from the landing threshold: a. A distance of 3,000 feet if a Category I aircraft is landing behind either a Category I or a Category II aircraft. Control Tower Procedures / 263 4,500 ft. Figure 6–13. An arriving Category II aircraft can be cleared to land if the preceding aircraft has touched down, is a Category I or II aircraft, and is at least 4,500 feet down the runway. Cat III Cat II Cat I 3,000 ft. 4,500 ft. 6,000 ft. Figure 6–14. The following aircraft must wait until the preceding aircraft is airborne and has traveled a specified distance down the runway before it can cross the landing threshold. b. A distance of 4,500 feet if a Category II aircraft is landing behind either a Category I or a Category II aircraft. c. A distance of 6,000 feet if either of the aircraft is a Category III aircraft (see Figure 6–14). Intersecting Runway Separation If intersecting runways are in use, a landing aircraft must be sequenced so as not to cross the landing threshold until at least one of the following conditions exists: 1. A departing aircraft from an intersecting runway has either crossed the intersection or has turned to avert any conflict (see Figure 6–15). 2. An aircraft landing on the intersecting runway has taxied off the landing runway, has crossed the runway intersection, or has completed the landing 264 / CHAPTER 6 Figure 6–15. Example of an intersecting runway departure separation. roll and advised the local controller that the aircraft will hold short of the intersecting runway. 3. When approved in the facility directives, the local controller may authorize an aircraft to land on a runway that intersects the departure runway when all of the following conditions can be met: a. VFR conditions exist at the airport. b. The aircraft has been instructed to hold short of the intersecting runway, has been informed of the traffic departing on the intersecting runway, and has acknowledged the instruction. c. The departing aircraft has been advised that the other aircraft will be holding short. d. Both runways are clear and dry with no reports that the braking action is less than “good.” e. The aircraft instructed to hold short has no tailwind. f. If requested by the pilot, the distance from the landing threshold to the intersection has been issued by the local controller. Facility directives specifically state which intersections may be used and which aircraft group is authorized to hold short. The aircraft group number can be found in Appendix B of the FAA handbook. Here is an example of the phraseology to be used when landing aircraft are holding short (see also Figure 6–16): Control Tower Procedures / 265 N18R is cleared to land using the full length of runway 12 N2PA is cleared to land on runway 9 but must hold short of runway 12 Figure 6–16. Example of one aircraft holding short of an intersecting runway. LOCAL CONTROLLER: CHEROKEE 2PA: LOCAL CONTROLLER: SPORT 18R: Land and Hold Short Operations Cherokee two papa alpha, cleared to land runway niner, hold short of runway one two, traffic landing runway one two. Cherokee two papa alpha, roger. Sport one eight romeo, cleared to land runway one two, traffic landing runway niner will hold short of the intersection. Sport one eight romeo, roger. At selected controlled airports where appropriate data have been published, air traffic controllers may use an expanded procedure whereby they may clear a pilot to land and hold short of an intersecting runway, an intersecting taxiway, or some other designated point on the runway. This operation is known as a land and hold short operation (LAHSO). Once procedures have been developed and approved and appropriate runway or taxiway signage has been installed, controllers may routinely issue LAHSO clearances. LAHSO procedures improve the efficiency of certain airports by essentially eliminating crossing runways. Although the runways (or runway and taxiway) in question may in reality cross each other, by requesting that a pilot hold short, the controller can move traffic as if the runways were physically disconnected from each other. Pilots may accept LAHSO clearances provided they determine that their aircraft can safely land and stop within the available landing distance (ALD). ALD data are published in the special notices section of the Airport Facility Directory (AFD) (see Table 6–2). Controllers can also provide ALD data to the pilot upon request. Although controllers may routinely issue LAHSO clearances, the pilot in command has the final authority to accept or decline any land and hold short clearance. The safety and operation of the aircraft remain the responsibility of the pilot. Pilots are expected to decline a LAHSO clearance if they determine it will compromise safety. 266 / CHAPTER 6 Table 6–2. LAHSO Runway Distance Information Landing Runway Hold Short Point Distance Available 09R 14L–32R 6,100 10 Taxiway S 12,156 14R 10–28 9,800 22R 09R–27L 6,050 27L 04L–22R 5,700 28 14R–32L 6,500 When LAHSO operations are conducted, that information is included on the taped ATIS broadcasts to pilots. The pilots should, as part of their pre-flight briefing, review any applicable LAHSO procedures and check to see whether their aircraft can meet the LAHSO requirements (see Figure 6–16). Here is an example of the phraseology to be used: AIR TRAFFIC CONTROLLER: N234PU: AIR TRAFFIC CONTROLLER: N252MN: Spacing Aircraft Cherokee two three four papa uniform, cleared to land runway one zero, hold short of taxiway bravo for crossing traffic, traffic is a Cessna one seventy-two. Cherokee two three four papa uniform, wilco, cleared to land runway one zero to hold short of taxiway bravo. Cherokee two five two mike november, cross runway one zero at taxiway bravo, landing aircraft will hold short. Cherokee two five two mike november, wilco, cross runway one zero at bravo, landing traffic to hold. The local controller may use any of the following phrases to achieve proper spacing of aircraft in the traffic pattern: 1. “Enter (pattern leg) runway (runway number).” The controller uses this phrase to direct the pilot to enter one of the five identified pattern legs. For example: Cessna niner papa uniform, enter left downwind runway two three. 2. “Report (position).” For the purposes of identifying and spacing aircraft, the pilot can be requested to make various position reports. The controller may request distance from the airport, distance from the runway, distance from a prominent landmark, or entry into the pattern. This request is usually combined with the previous instruction: Diamond eight delta mike, report three miles north of the airport. Cherokee two papa uniform, enter left downwind for runway five, report over the red and white water tower. Sport zero two romeo, report two mile final runway one zero. Control Tower Procedures / 267 3. “Number (sequence number, runway).” This phrase advises the pilot of the planned landing sequence for the aircraft. The pilot assumes that the preceding aircraft is landing on the same runway unless stated otherwise. If the local controller is using more than one runway for arrivals, the pilot should be advised of the sequence for the airport and for the arrival runway. This instruction is usually used in conjunction with a “follow” phrase. 4. “Follow (description and location).” Once the preceding aircraft has been located and identified, it is the pilot’s responsibility to provide the proper spacing in the traffic pattern. The local controller should advise the pilot of the location and type of the preceding aircraft to make it easier to locate and follow. 5. “Traffic is (description and location) landing (runway).” If the landing aircraft is sequenced behind an aircraft landing on a different runway, the pilot should be advised of the type and location of the preceding aircraft in order to provide proper spacing. The controller may refer to a local landmark or to the pilot’s aircraft when pointing out the preceding traffic: Cherokee niner alpha uniform, number two runway two three. Bellanca six alpha victor, follow the twin Cessna ahead and to your right. Twin Beech one seven three, enter and report left base for runway two three, number two for the airport, traffic is a Cessna on a quarter mile final runway one zero. Spacing Instructions The following instructions can be used to either increase or decrease the spacing between aircraft in the traffic pattern. “Extend Downwind/Upwind” The pilot can be requested to extend either the downwind or the upwind leg a specified distance or until over a prominent landmark. The pilot should never be requested to extend the crosswind leg unless it is absolutely necessary. Extending the crosswind leg will result in the downwind leg being flown far enough from the airport that the pilot may be unable to glide to the runway in case of engine failure. An extension to the base leg is impossible since the distance from the downwind leg to the final leg is fixed. Here are examples of the phraseology: United six sixteen, extend downwind one mile to give room for a departure. Cessna one niner foxtrot, extend upwind to the lake. “Short Approach” A short approach is a request for the pilot to shorten the downwind leg as much as possible, which results in an equivalent reduction in the length of the final approach leg (see Figure 6–17). Because the pilot is still required to fly a pattern within the capabilities of the aircraft, this request may not provide consistent results. Some pilots may be able to fly a very short pattern, whereas others are unable to do so. “Make Left (or Right)” In a normal traffic pattern, the pilot makes a 90° turn when transitioning from one leg to another. One method of increasing the spacing between two aircraft is to request that the pilot turn 270° in the 268 / CHAPTER 6 Downwind leg Normal pattern Runway Short approach Figure 6–17. Example of an aircraft conducting a short approach. Left 270∞ turn Left 360∞ turn Runway 27 Right 360∞ turn Normal pattern Figure 6–18. Example of a 360° turn and a 270° turn while in the traffic pattern. “wrong” direction when transitioning to the next leg (see Figure 6–18). For instance, if the pilot is on a right downwind, a request to “make a left two seventy to base” will result in a longer turn and increased separation. If the pilot is not transitioning from one leg to another and increased spacing is necessary, a 360° turn in either direction may be requested (“Sport one two romeo, make a right three sixty”). Caution should be used when issuing such instructions, since they can be potentially disorienting to pilots. Controllers should also refrain from using these methods when the aircraft has begun to descend from pattern altitude and is on either the base or final leg. It can be dangerous for a pilot to perform these maneuvers at low airspeeds while close to the ground. “Go Around” If it is apparent that proper runway separation cannot be achieved and neither aircraft’s traffic pattern can be adjusted, it will be necessary to cancel landing clearance for one of the aircraft. In this case, the local controller determines which aircraft’s landing clearance should be cancelled and instructs that aircraft to “go around.” Upon receipt of this instruction, the Control Tower Procedures / 269 pilot will immediately begin a climb to pattern altitude and will reenter the traffic pattern as instructed. Here are some examples of phraseology: American six eleven, go around, enter right downwind runway two seven left. Cessna niner eight delta, go around, enter left base for runway two five. “Cleared to Land” This clearance authorizes the pilot to make a full-stop landing. If the local controller is using anticipated separation and has cleared more than one aircraft to land, the preceding traffic should be included in the landing clearance. Any restrictions or requests should precede this clearance. These might include instructions to hold short of a runway or to plan to turn off of the runway at a designated taxiway. If the local controller should be able to see the landing aircraft but cannot do so either visually or using radar, the phrase “not in sight” should be added to the landing clearance. This phrase alerts the pilot to the fact that the controller is unsure of the aircraft’s position. It is not uncommon for a pilot to be in contact with the control tower at one airport while mistakenly attempting to land at another. Advising the pilot that the aircraft is not in sight will make the pilot aware that they might be approaching the wrong airport. Here are some examples of landing phraseology: Cessna two six mike, cleared to land runway two three. Tomahawk six four november, not in sight, cleared to land runway one zero. United one twenty-five, cleared to land runway two three, traffic landing runway one zero. Clipper four seventeen, cleared to land runway one four right, hold short of runway niner right, traffic landing runway niner right. After the aircraft has landed, the local controller should advise the pilot where to exit the runway and what frequency to use for contacting the ground controller. “Cleared for Touch and Go” A touch and go clearance permits an aircraft to land on the runway but to take off again before actually coming to a stop. This maneuver is usually used by students practicing takeoffs and landings. An aircraft performing a touch and go is considered an arriving aircraft until actually touching down and then is considered a departure. “Cleared for Stop and Go” A stop and go clearance is similar to a touch and go except that the aircraft comes to a full stop on the runway before beginning its takeoff run. A stop and go is also considered an arriving aircraft until coming to a complete stop, after which it is considered a departure. “Cleared for Low Approach” In a low approach, the pilot approaches to land on the runway but does not actually make contact with the runway surface. Upon reaching the desired altitude, the pilot begins a climb. Low approaches 270 / CHAPTER 6 are usually used by pilots practicing instrument approaches. In many cases, the pilot may wish to execute the published missed approach procedure. When it is desirable to determine the pilot’s intentions prior to issuing this clearance, the controller may ask the pilot, “State your intentions.” An aircraft conducting a low approach is considered an arriving aircraft until it crosses the landing threshold, after which it is considered a departure. “Cleared for the Option” An option clearance permits the pilot to perform a landing, touch and go, stop and go, or low approach. The pilot will not typically inform the controller which option he or she has chosen. This maneuver is generally used in flight training to permit a flight instructor to evaluate a student’s performance under changing conditions. If the controller is unable to approve all the options, the following phraseology should be used to restrict the pilot to the options that can be safely accommodated: Sport one three romeo, unable option, make a full-stop landing. Cessna three niner eight, unable stop and go, other options approved. Runway Selection Since aircraft landing into the wind touch down at lower ground speeds that shorten the landing roll, most pilots, when given a choice, prefer to land or depart on a runway as nearly aligned with the wind as possible. Unless otherwise specified by facility directives, it is usually the local controller’s responsibility to decide which runway becomes the active runway. Local controllers should comply with the following guidelines from the FAA handbook when selecting active runways: 1. Whenever the wind speed is greater than 5 knots, use the runway most nearly aligned with the wind. 2. The calm wind runway should be used whenever the wind is less than 5 knots. The calm wind runway will be specified by the airport management and is contained in the facility directives. This runway is chosen to maximize arrivals and departures while minimizing the noise impact on local dwellings. 3. The local controller can use any other runway when it is operationally advantageous to do so. 4. If a runway use program has been designated for the facility, the runways specified in the program should be used as the active runways. Runway Use Programs To minimize the noise impact of landing and departing aircraft, the FAA has implemented a nationwide Aviation Noise Abatement Policy. This policy places the primary responsibility for planning and implementing a noise abatement program on the operator of each airport. The runway use program put into place may be either informal or formal. Informal runway use programs primarily affect aircraft that weigh more than 12,500 pounds. At airports with informal runway use programs, the controllers will assign these aircraft to the runway chosen by airport management whenever all of the following conditions can be met: Control Tower Procedures / 271 The wind direction is within 90° of the runway heading. The wind does not exceed 15 knots. The runway is clear and dry, which means that there is no snow, ice, slush, or water on the runway. If pilots wish to use a different runway from that specified in the informal runway use program, they are expected to inform the controller. Air traffic controllers are required to honor these requests, but they will advise the pilot that the runway is “noise sensitive.” If airport management wishes to have aircraft use specific runways even when the runway conditions exceed those listed earlier, a formal runway use program must be initiated. A formal program requires that aircraft operators, airport management, and the FAA consummate a letter of agreement specifying the preferential runways and the weather conditions that must exist to use those runways. The establishment of a letter of agreement ensures that everyone concerned completely understands the conditions of the runway use program. The letter of agreement specifies that although pilots are expected to comply with these procedures, pilot requests for other runways will be honored. However, the pilot will be advised that the previously assigned runway is specified in the formal runway use program. Helicopter Operations Helicopters can taxi around the airport by ground taxiing, hover taxiing, or air taxiing. Ground taxiing of a helicopter is similar to that of a taxiing plane. Only those helicopters equipped with landing gear are able to ground taxi. In hover taxiing, the helicopter actually lifts off of the ground and remains airborne while maneuvering around the airport. A hover-taxiing helicopter usually remains within about 50 feet of the ground and proceeds at airspeeds less than 20 knots. Helicopters that are air taxiing operate below 100 feet and proceed at speeds in excess of 20 knots. Each type of taxiing has its advantages and disadvantages. Ground taxiing is the most fuel efficient of the three and creates less air turbulence around and behind the helicopter. Hover taxiing is much faster than ground taxiing but creates a high level of air turbulence both below and behind the helicopter. Air taxiing is the fastest method and actually creates less air turbulence since the helicopter is at a greater altitude and most of the air turbulence is directed backward. Whenever a helicopter is taxiing, aircraft in the vicinity should be advised that it could be creating wake turbulence. Helicopters are unique in that they may descend and climb with little or no forward movement. Nevertheless, helicopter pilots must be careful that they never depart from the safe flight envelope. For a pilot to properly control 272 / CHAPTER 6 the aircraft in case of an engine failure, a helicopter must have sufficient speed, altitude, or a combination of the two to safely perform a maneuver known as an autorotation. An autorotation is similar to a glide in a fixed-wing aircraft. During an autorotation, the helicopter descends at a rapid rate but is able to reduce that rate of descent just prior to touchdown. Typically, a hovering helicopter needs about 600 feet of altitude to safely perform an autorotation. A helicopter travelling forward at a speed of about 40 knots needs virtually no altitude to autorotate. Keeping these factors in mind, helicopter pilots normally prefer to approach for landing in a manner similar to fixed-wing pilots. The only difference is that the helicopter does not need to use the entire length of the runway to decelerate. Wake Turbulence Every aircraft in flight trails an area of unstable air behind it known as wake turbulence. This turbulence was originally attributed to “prop wash” but is now known to be caused in part by a pair of counter-rotating vortices trailing from the wing tips (see Figure 6–19). These vortices are a by-product of the lift produced by the wing, which is generated by the creation of a pressure differential between the lower and the upper wing surfaces. High pressure is created below the wing, while low pressure is created above. The resultant upward Vortex core Figure 6–19. Wake turbulence behind an aircraft. Control Tower Procedures / 273 Vortex flow Vortex core Figure 6–20. Example of vortex rotation and movement behind an aircraft. pressure on the wing, known as lift, causes whirling vortices of airflow to be created at the wing tip. The airflow along the wing pushes this upward flow backward, creating a whirling body of air that resembles a horizontal tornado (see Figure 6–20). Each wing produces its own vortex, resulting in two counter-rotating cylindrical vortices trailing from each aircraft. The strength of the vortex is governed by the weight, speed, and shape of the wing of the generating aircraft. In general, the maximum vortex generation occurs when the generating aircraft is heavy and slow—precisely the conditions found during takeoff and landing. Wing-tip vortices created by larger aircraft can completely encompass smaller aircraft. The rotational velocities in these vortices have been measured as high as 133 knots. A small aircraft encountering one of these vortices may become completely uncontrollable. Wing-tip vortices begin to be generated the moment an aircraft’s nose wheel lifts from the ground and are continually created until the aircraft lands (see Figure 6–21). These vortices tend to descend at 500 feet per minute until they level off at about 900 feet below the aircraft’s cruising altitude. They remain at this point until dissipating (see Figure 6–22). If while descending they make contact with the Earth’s surface, they tend to move outward at a speed of about 5 knots. Any surface wind will tend to dissipate and move these vortices. A crosswind will tend to increase the speed of the downwind vortex while 274 / CHAPTER 6 Rotation Wake begins Touchdown Wake ends Figure 6–21. Wake turbulence starts when a departing aircraft’s nose wheel leaves the ground. It stops when a landing aircraft’s nose wheel touches the ground. Sink rate 400/500 ft. per min. Residual chop remains Max. sink 800/900 ft. Breakup starts Figure 6–22. Wake turbulence behind an aircraft as it descends and dissipates. The wake turbulence will descend at 500 feet per minute until it begins to break up approximately 900 to 1,000 feet below the cruising altitude of the originating aircraft. Control Tower Procedures / 275 5-knot wind 10 knots (5 + 5) Upwind vortex stationary Vortex movement in ground effect with crosswind Figure 6–23. Wake turbulence near the ground will begin to move horizontally at approximately 5 knots. A 5-knot crosswind will effectively “stall” one of the vortices over the runway until it dissipates 2 minutes later. impeding the progress of the upwind vortex (see Figure 6–23). A cross wind between 3 and 7 knots may prevent the upwind vortex from moving. Although it is primarily the pilot’s responsibility to avoid wake turbulence, controllers are required to assist the pilots of smaller aircraft whenever they fly behind an aircraft that could be creating potentially dangerous wake turbulence. For the purposes of wake turbulence separation minima, the FAA has classified every aircraft as small, large, or heavy. Small aircraft are aircraft whose maximum certificated takeoff weight is less than or equal to 41,000 pounds. Large aircraft have maximum certificated takeoff weights greater than 41,000 pounds up to and including 255,000 pounds. Heavy aircraft have maximum certificated takeoff weights in excess of 255,000 pounds. The controller should be aware that pilots following large or heavy aircraft may wish to adjust their flight patterns to avoid their ensuing wake turbulence. Wake turbulence is generated from the moment a departing aircraft’s nose wheel leaves the ground until it lands and the nose wheel is lowered to the runway. For this reason, aircraft departing behind a large or heavy jet will usually plan to rotate their nose wheel before reaching the preceding aircraft’s rotation point and attempt to climb at a greater angle than that aircraft. If they are unable to climb at a greater angle, a slight turn will usually permit them to avoid the wake turbulence (see Figure 6–24). Pilots must also be aware of aircraft departing from parallel runways. If the parallel runways are less than 2,500 feet apart, it is quite possible that the wing-tip vortices may drift from one runway to the other (see Figure 6–25). In these cases, the pilot of the smaller aircraft will attempt to rotate prior to the point of rotation of the heavy aircraft. 276 / CHAPTER 6 Heavy transport Light transport Light airplane Figure 6–24. To avoid wake turbulence, departing aircraft should rotate prior to the point of rotation of the preceding aircraft and should climb at a steeper angle. Touchdown point Wind Figure 6–25. Aircraft landing on parallel runways could encounter wake turbulence “blown” over from the parallel runway. Since the wake turbulence caused by an arriving aircraft ceases when the heavy aircraft’s nose wheel settles to the ground upon landing, pilots of smaller aircraft following heavy aircraft will attempt to remain above the flight path of the heavy aircraft and land beyond the point where the heavy aircraft touched down (see Figure 6–26). Small aircraft following the same flight path as the heavy aircraft (such as on an ILS glide slope) rarely encounter wake turbulence since the wing-tip vortices will descend fairly rapidly. Control Tower Procedures / 277 60 Touchdown point 9,000 ft. 3,000 ft. Figure 6–26. To avoid wake turbulence, landing aircraft should plan to touch down beyond the point where the preceding aircraft’s nose wheel touched the ground. Aircraft following a heavy jet making a low approach, stop and go, or touch and go landing are in the most danger because there may not be any safe area of the runway on which to land. In this case, the best procedure is to delay the following aircraft’s arrival or departure for at least 2 minutes to let the wing-tip vortices dissipate. Tower controllers must apply the following procedures to small aircraft following larger aircraft creating potentially dangerous wake turbulence. (Controllers in an approach control or in an ARTCC have a different set of procedures with which they must comply. Those procedures are explained in detail in Chapter 9.) Since wake turbulence tends to dissipate in a matter of minutes, time is used as a means of ensuring that a following aircraft does not encounter any severe wake turbulence. In general, the following aircraft will usually be delayed by either a 2- or 3-minute interval wherever dangerous wake turbulence might exist. Two minutes of separation must be applied to any aircraft departing behind a heavy aircraft using the same runway or a parallel runway if the runways are separated by less than 2,500 feet. Two minutes of separation must also be applied to an aircraft whose flight path will cross that of a heavy jet departing from an intersecting runway. The pilot of the following aircraft may waive this wake turbulence separation by stating “Request waiver of the 278 / CHAPTER 6 2-minute interval” or by making a similar statement. This request means that the pilot has accepted responsibility for wake turbulence separation. Three minutes of separation must be provided to any small aircraft departing behind a large aircraft whenever the small aircraft is departing from an intersection or in the opposite direction on the same runway. This interval may be waived upon pilot request. A 3-minute interval will also be provided to any small aircraft departing behind a heavy aircraft whenever the small aircraft is departing from an intersection or in the opposite direction on the same runway. This 3-minute interval may not be waived by the pilot. A second type of wake turbulence, produced by turbine engines, propellers, and helicopter rotor blades, is fairly localized and not long lasting but can be just as dangerous to an unsuspecting pilot. The wake turbulence found behind a turbine engine can overturn or hurl a small aircraft hundreds of feet. Controllers must always remember that the cockpit of a small aircraft is fairly noisy and the pilot may not be able to hear the engine noise of a nearby jet. The pilots of small aircraft should thus be warned whenever they are taxiing behind jet aircraft. KEY TERMS air taxiing aircraft category aircraft group amendment (AM) anticipated separation automated radar terminal system (ARTS) Automatic Terminal Information Service (ATIS) autorotation Aviation Noise Abatement Policy calm wind runway ceilometer clearance delivery controller critical areas departure message fields flight data controller flight data input/output (FDIO) flight data processing (FDP) flight progress strips formal runway use program ground controller ground taxiing heavy aircraft holding short hover taxiing inactive runways informal runway use program land and hold short operation (LAHSO) large aircraft left traffic local controller low approach National Beacon Code Allocation Plan (NBCAP) National Weather Service (NWS) nonradar-approach control towers option clearance pilot reports (PIREPs) radar-approach control towers right traffic runway incursion runway separation runway use program short approach small aircraft stop and go clearance strip request (SR) taxiways touch and go clearance tower visibility traffic pattern traffic pattern legs VFR towers wake turbulence REVIEW QUESTIONS 1. What are the four operating positions in a control tower, and what are the duties assigned to each? 2. What are the separation minima for departing aircraft? 3. What are the separation minima for arriving aircraft? 4. What may the pilot be asked to hold short of during LAHSO procedures?