Approach Navigation Aid Classifications PDF

Navigation Systems / 101 Approach Navigation Aid Classifications Many en route navigation aids can be used as nonprecision approach aids if their transmitted signal is of a high enough quality and can be safely used dur- ing the entire instrument app...

Navigation Systems / 101 Approach Navigation Aid Classifications Many en route navigation aids can be used as nonprecision approach aids if their transmitted signal is of a high enough quality and can be safely used dur- ing the entire instrument approach procedure. FAA !ight-check aircraft rou- tinely check the quality of these navigation aids to determine their suitability as approach navigation aids. An en route navigation aid used for an instrument approach is classified as a nonprecision aid since no vertical guidance is pro- vided to the pilot. The following en route navigation aids have been certified by the FAA for use as nonprecision approach aids: VOR RNAV VOR-DME VORTAC INS TACAN GPS NDB LNAV LNAV/VNAV LPV Since these are primarily en route navigation aids, they may not be properly positioned to serve the needs of each airport within their immediate vicinity. Responding to a need for additional approach aids, the FAA has devel- oped an entire series of radio navigation devices to serve solely as instrument approach aids. The main ones in use are precision approach aids since they provide vertical guidance to the runway. The nonprecision aids are designed to be used at airports that are served by en route navigation aids but that do not qualify for the installation of a more expensive precision approach navigation aid. The instrument approach aids currently being installed and used by the FAA include the following: Terminal VOR (TVOR) nonprecision Instrument landing system (ILS) precision Localizer directional aid (LDA) nonprecision Simplified directional facility (SDF) nonprecision Microwave landing system (MLS) precision Precision approach radar (PAR) precision Airport surveillance radar (ASR) nonprecision Terminal VOR The terminal VOR (TVOR) was designed to provide an inexpensive method of providing VOR guidance to an airport needing an instrument approach. A ter- minal VOR is a low-powered version of a standard VOR that is usable to a distance of 25 nautical miles. The TVOR does not provide distance information 102 / CHAPTER 2 unless a civilian DME is colocated at the facility. Terminal VORs are not nor- mally combined with TACANs. Instrument The instrument landing system (ILS) is designed to provide the pilot with an Landing approach path that is perfectly aligned with the runway centerline. An ILS System provides both lateral and vertical guidance to the pilot (see Figure 2–49). The ILS system is equipped with three different types of transmitters: the localizer, the glide slope, and two or three marker beacons. Localizer The localizer system consists of a transmitter building, localizer antenna, and monitoring equipment (see Figure 2–50). Typically, the localizer antenna is located about 1,000 feet beyond the departure end of the runway being served by the ILS. The transmitter building is about 300 feet to one side of the localizer antenna. On older installations, the monitoring equipment is mounted on wooden posts a short distance in front of the antenna array. In newer installations, it is an integral part of the antenna. The localizer operates within the VHF band between 108.10 and 111.95 mHz (see Figure 2–51). The localizer provides the pilot with lateral course guidance information only. The antenna radiates a signal that is aligned with the runway centerline and is modulated with two different tones: 90 and 150 Hz. The final approach course is produced as a result of the radiation patterns emanating from the antenna array. The array is situated such that the 150 Hz tone is predominant on the right side of the runway while the 90 Hz tone predominates on the left (see Figure 2–52). Along the centerline, both tones are of equal strength. An aircraft to the right of the centerline will receive predominantly the 150 Hz tone. The airborne receiver will detect this condition and move the vertical needle of the ILS indi- cator to the left of center, thereby advising the pilot to alter the aircraft’s course to the left. If the aircraft is to the left of course, the 90 Hz tone will dominate and the vertical needle of the ILS indicator will move to the right, advising the pilot to alter the aircraft’s course to the right. If the aircraft is established on course, the 90 and the 150 Hz tones will be of equal strength and the vertical needle on the ILS indicator will be centered (see Figure 2–53). The localizer signal is transmitted along a fairly narrow path extending 35° to the left and right of the runway centerline and out from the transmit- ter to a distance of 10 nautical miles. Between 10 and 25 nautical miles from the runway, the localizer is certified to be accurate within a range of only 10° on either side of the extended centerline. The localizer signal is approximately 7° high. The ILS receiver on the aircraft is designed such that when the vertical needle on the indicator is fully displaced on one side or the other (known as full-scale deflection), the aircraft is 3° off course. Since the localizer is an angular device, the on-course beam narrows as the aircraft approaches the antenna. Ten miles from the end of the runway, a full-scale de!ection indicates that the aircraft is about a half mile off course. When crossing the approach end of the runway, 3° off course translates to approximately 300 feet. SE-4, 12 FEB 2009 to 12 MAR 2009 SE-4, 12 FEB 2009 to 12 MAR 2009 Figure 2–49. ILS runway 2 approach at Eastman, Georgia. 103 104 / CHAPTER 2 Michael Nolan Figure 2–50. An ILS localizer antenna installation. The localizer is one of the most precise and sensitive navigation aids avail- able for instrument approaches. Unfortunately, the localizer signal can be easily re!ected off terrain, buildings, aircraft, vehicles, and power lines, thereby creat- ing course scalloping or false courses (see Figure 2–54). When an ILS is initially being installed, the localizer radiation pattern is carefully studied to ensure that nearby buildings and power lines will not unduly interfere with the accuracy Figure 2–51. Graphic depiction of an instrument landing system. 106 / CHAPTER 2 Approach direction Runway centerline Predominately 150 Hz Back Runway Front course course Localizer antenna Predominately 90 Hz On course Figure 2–52. ILS approach procedure and the localizer radiation pattern. Bendix/King Division of Allied Signal Aerospace Company Figure 2–53. A typical light aircraft ILS receiver. of the transmission. However, a strong signal re!ected off a nearby object will create a change in the radiation pattern of the localizer and artificially “move” the localizer centerline to the left or right. If this situation is encountered, FAA technicians attempt to solve the problem by installing a different type of localizer antenna. In some cases, even this remedy will not solve the re!ection problem and the localizer must be relocated. It may be possible to move the antenna off the runway centerline or  Navigation Systems / 107 150 Hz Localizer Equal signal antenna strength Runway Reflection 90 Hz Building Figure 2–54. Example of localizer reflections that result in course scalloping. redirect the final approach course somewhat. If either of these modifications is necessary, the localizer is no longer considered to be part of an ILS and is called a localizer directional aid (LDA) (see Figure 2–55). This name change is necessary to alert pilots that the localizer is not aligned with the runway centerline. When a localizer is offset in this manner, vertical guidance is not normally provided, making an LDA-based instrument approach a nonprecision approach. The conversion of a localizer to an LDA is done only as a last resort, since an LDA procedure requires pilots to make a low-altitude turn to line up with the runway just prior to landing. Airway facility technicians employed by the FAA are responsible for ensuring that re!ections from terrain, buildings, and power lines do not disturb the localizer transmission. It is the air traffic controller’s responsibility to ensure that aircraft and vehicles do not interfere with the localizer transmission when- ever ILS approaches are in progress. To prevent any inadvertent re!ections, localizer critical areas have been established for every localizer antenna. Each localizer installation is unique and may not have the same critical area, but in general the standard localizer critical area is shaped as in Figure 2–56. Other than aircraft landing and exiting the runway, aircraft conducting the missed approach procedure, or aircraft using the runway for departure and !ying over the localizer antenna, no vehicles or aircraft are allowed within the localizer critical area when ILS approaches are in progress. When weather con- ditions are extremely poor (such as visibility below 1/2 mile or ceilings below 200 feet), no aircraft or vehicles are allowed in the critical area for any reason when an aircraft is inside the final approach fix during an ILS approach. The exact criteria to be followed concerning localizer critical areas are covered in Chapter 6 and can be found in the Air Traffic Control Handbook. The localizer transmission is radiated in a pattern that can be received from both the approach and the departure ends of the runway. The front course of the localizer is the transmission that serves as the primary instru- ment approach. The localizer back course is a mirror image of the front course, 108 / CHAPTER 2 N E-3, 12 F E B 2009 to 12 MA R 2009 N E-3, 12 F E B 2009 to 12 MA R 2009 Figure 2–55. LDA/DME runway 19 approach at Ronald Reagan Washington National.  Navigation Systems / 109 2,000 ft. 20 200 ft. 0 ft. 200 ft. Localizer antenna Hold line Figure 2–56. Localizer critical area. serving the opposite runway, with the 90 and 150 Hz areas reversed. At cer- tain ILS installations where the back course transmission meets TERPS crite- ria, the FAA has been able to establish localizer back course approaches (see Figure 2–57). When a pilot conducts a back course approach, the 90 Hz signal domi- nates the right side of the final approach course, whereas the 150 Hz signal dominates the left. Because this is the exact opposite of the front course, a pilot conducting a back course approach can become disoriented. If an aircraft on an ILS back course is to the right of the runway centerline, the localizer indicator will advise the pilot to “!y right.” If the aircraft is to the left of the centerline, the indicator will advise the pilot to “!y left.” This is the opposite to what the pilot should do if the aircraft is to remain on the back course centerline. This condition is known as reverse sensing. The pilot must remember to do the opposite of what the localizer indicator advises. Certain ILS indicators are equipped with a back course switch that reverses the localizer needle operation during back course approaches. Glide Slope The glide slope radiates a signal pattern that provides an elec- tronic glide path to be !own by the pilot. The glide slope system provides both above and below glide path indications to the pilot, using a horizontal needle on the ILS indicator (see Figure 2–58). The glide slope transmitting system con- sists of a transmitter building, the glide slope antenna, monitor antennas, and a clear zone. The glide slope antenna and the transmitter building are about 500 feet from the runway centerline and about 1,000 feet from the approach end of the runway. 110 / CHAPTER 2 E C-3, 12 F E B 2009 to 12 MA R 2009 E C-3, 12 F E B 2009 to 12 MA R 2009 Figure 2–57. Localizer back course runway 14L approach at Champaign, Illinois.  Navigation Systems / 111 One dot deflection 3°–10° 5°–10° 2°–5° Airport normally 2–1/2° Height to 10 times exaggerated Figure 2–58. ILS glide slope indications. The glide slope operates in the UHF band between 329 and 335 mHz and is paired to the localizer frequency. When a pilot selects the proper local- izer frequency, the glide slope frequency is also selected by the ILS receiver. The glide slope transmits a UHF signal modulated at 90 and 150 Hz just like the localizer. If an aircraft is above the glide path, the 90 Hz signal will predom- inate and the horizontal needle on the aircraft’s ILS indicator will move down, advising the pilot to “!y down.” If the aircraft is below the desired glide path, the 150 Hz signal will predominate and the ILS needle will move up, advising the pilot to “!y up.” If the aircraft is on the correct glide path, the horizontal needle will be in the middle, signalling ON GLIDE PATH. To properly transmit the glide slope with a single antenna would require an antenna 50 to 100 feet tall. Since an obstruction at this height next to an active runway is completely unacceptable during periods of low visibility, a number of methods have been tried in an attempt to decrease the anten- na’s height. The method currently used requires that the glide path signal be re!ected off the ground directly in front of the antenna. This area is known as the glide slope reflecting area. This solution reduces the height of the antenna 112 / CHAPTER 2 mast to about 30 feet (see Figure 2–59). Since the glide slope antenna is highly directional, there is no back course to a glide slope. A number of other problems are inherent in the current glide slope method of transmission. One such problem is the creation of false glide paths. At most glide slope installations, the desired glide path angle is about 3° above horizontal. However, when the glide slope bounces off the ground, a number of additional glide paths are also created. Fortunately, none of these false glide paths is lower than 3°, but many exist above this level. The first false glide path usually occurs at an angle of about 9°. To ensure that the correct glide path is used by pilots conducting an ILS approach, it is imperative that the aircraft be allowed to transition from the airway structure to the ILS at an altitude that will place the aircraft below any of the false glide paths. Any obstruction directly in front of or to the side of the glide slope trans- mitter might re!ect some of the signal and cause glide slope scalloping. To prevent this, an area directly in front of the glide slope antenna has been des- ignated the glide slope critical area (see Figure 2–60). When aircraft are con- ducting ILS approaches, this area must be kept clear of aircraft, vehicles, deep snow, or any objects that may interfere with the correct operation of the glide slope transmitter. Another of the problems inherent in bouncing the glide slope is that extensive site preparation is needed to ensure that the ground in the re!ecting area will properly re!ect the glide slope at the desired approach angle. If the area in front of the antenna does not offer the proper re!ectivity, it may have to be resurfaced, which is usually expensive. Many factors can temporarily change the re!ectivity of this zone. Water-soaked ground, excessive snow, or extremely long grass can all cause the glide slope to re!ect at the wrong angle. To ensure proper glide slope operation, receivers called glide slope monitors are located within the clear zone. If these monitors detect that the glide slope radiation pattern is no longer within established tolerances, the glide slope transmitter is automatically shut down. Marker Beacons Marker beacons are located at known distances along the final approach course of the ILS to provide position information to pilots con- ducting the approach. Marker beacons transmit a cone-shaped signal on a fre- quency of 75 mHz, uniquely coded to identify each type of beacon. Outer marker (OM) beacons are usually located on the ground about 5 miles from the approach end of the runway (see Figure 2–61). When a prop- erly equipped aircraft !ies over an outer marker, a blue light !ashes and a 400 Hz series of continuous dashes is emitted from the marker beacon receiver on board the aircraft (see Figure 2–62). The middle marker (MM) is usually about 3,000 feet (or half a mile) from the approach end of the runway and causes an amber light to !ash and a series of 1,300 Hz dots and dashes to be heard in the cockpit. The middle marker is usually located such that an aircraft properly positioned on the glide slope will  Navigation Systems / 113 Michael Nolan Figure 2–59. An ILS glide slope antenna installation. 114 / CHAPTER 2 1,200 ft. or end of runway, whichever is greater Hold line 200 ft. Glide slope antenna 250 ft. to 600 ft. 27 R Figure 2–60. Glide slope critical area. over!y it at approximately 200 feet. This is the normal decision height for a Category I ILS approach. If a Category II ILS has been installed, an inner marker (IM) is placed approximately 1,000 feet from the end of the runway. The inner marker is located at the point where an aircraft on the glide slope passes through an alti- tude of 100 feet. This is the decision height for a Category II ILS approach. The inner marker causes a white light to !ash and a 3,000 Hz series of continuous dots to be heard in the cockpit. Compass Locators At many ILS installations, a low-powered nondirectional beacon (NDB) may be colocated with either the outer or the middle marker. Such nondirectional beacons assist the pilot when transitioning from the air- way structure to the ILS. An NDB used for this purpose has a transmitter power of less than 15 watts and is known as a compass locator. Combining a compass locator with an outer marker (OM) creates a facility known as a locator outer marker (LOM). When colocated with a middle marker, the facility is known as a locator middle marker (LMM). Since the increased use of radar in the ter- minal environment has diminished the need for compass locators, the FAA has begun to decommission the few existing locator middle markers and will install locator outer markers only where operationally necessary.  Navigation Systems / 115 Image not available due to copyright restrictions Figure 2–62. A typical light aircraft marker beacon receiver. ILS Distance Measuring Equipment In rare instances, distance measuring equipment may be installed at the localizer site to provide distance informa- tion to an aircraft conducting an ILS approach. DME is usually used when the local terrain precludes the installation of 75 mHz outer or middle markers. The proper DME frequency is automatically selected when the pilot tunes in the appropriate localizer frequency. DME-equipped aircraft can then use this distance information in place of the marker beacons. ILS Categories ILS systems are currently classified into one of three catego- ries, each category being defined in terms of minimum visibility and decision 116 / CHAPTER 2 Table 2–2. ILS Categories ILS Category Decision Height Visibility or RVR I 200 feet 1⁄2 mile or 2,400 feet † II 100 feet 1,200 feet † IIIa * 700 feet † IIIb * 150 feet † ‡ IIIc * *No decision height specified. Visibility is the only limiting factor. † No fractions of miles authorized when determining visibility. The runway served by the ILS must have operable RVR equipment. ‡ No ceiling or visibility specified. Aircraft must be equipped with automatic landing equipment. height altitudes (see Table 2–2). Minimum visibility is measured in fractions of a mile when measured by human observers or in hundreds of feet when mea- sured by a runway visual range. The standard ILS is a Category I, which provides accurate guidance infor- mation in visibilities as low as 1/2 mile and ceilings as low as 200 feet. These minima are representative of a standard ILS installation. With a slight change in the ground equipment, an ILS installation may be certified as a Category II, which permits a properly rated pilot to use the ILS in visibilities as low as 1,200 feet or ceilings as low as 100 feet (see Figure 2–63). The additional equipment required for a Category II installa- tion includes more stringent localizer and glide slope monitoring equipment, an inner marker, and additional approach lighting. Pilots and aircraft must be certified to use a Category II ILS and its associated minima. Those pilots not certified to Category II minima may still use a Category II ILS down to Category I minima. In those locations that qualify, a Category III ILS may be installed (see Figure 2–64). A Category III ILS installation is much more expensive since it requires completely redesigned localizer and glide slope equipment. Category III ILS approaches are of three types: IIIa, IIIb, or IIIc. Category IIIc approaches may be conducted when the ceiling or visibility is zero! Aircraft conducting Category III approaches must be equipped with autoland devices that auto- matically land the aircraft. Category III installations are rarely justified for use in this country. Few airports need this type of approach and few aircraft are equipped to utilize them. Runway Visual Range Runway visual range (RVR) equipment measures the visibility along the runway being used for instrument approaches. The RVR system consists of a transmissometer projector, a transmissometer detector, a data converter, and a remote digital display. In a typical RVR installation, the transmissometer projector and the transmissometer detector are located to the side of the runway, approximately  Navigation Systems / 117 E C-2, 12 F E B 2009 to 12 MA R 2009 E C-2, 12 F E B 2009 to 12 MA R 2009 Figure 2–63. ILS runway 5R (CAT II) approach at Indianapolis, Indiana. 118 / CHAPTER 2 E C-3, 12 F E B 2009 to 12 MA R 2009 E C-3, 12 F E B 2009 to 12 MA R 2009 Figure 2–64. ILS runway 14R (CAT III) approach at Chicago O’Hare International.  Navigation Systems / 119 Detector Projector Figure 2–65. Runway visual range operation. 500 feet apart (see Figure 2–65). The projector emits a known intensity of light, which is measured by the detector. Any obscuring phenomenon, such as rain, fog, smoke, or haze, will reduce the light intensity received by the detec- tor. The light intensity measurement is transformed by the data converter into a visibility value measured in feet. This resultant value is then presented to the controllers using the remote digital display. The data converter adjusts the visibility value to approximate the visibil- ity that will be observed by a pilot conducting an approach to the runway. The data converter must take into consideration such variables as time of day and the runway light intensity. The RVR equipment is normally located at about the midpoint of the runway in order to provide service for pilots approaching the runway from either direction. At busier runways, two or even three RVR systems may be installed to provide accurate visibility measurement throughout the runway’s length. These three RVR installations are called the touchdown, midpoint, and rollout RVRs. Simplified At some locations where the installation of an expensive ILS cannot be justified Directional but where the existing navigation aids are unsuitable for the development of Facility an instrument approach, a simplified directional facility (SDF) may be installed. 120 / CHAPTER 2 An SDF provides course guidance similar to but less accurate than the localizer component of an ILS. An SDF transmitter broadcasts in the same frequency range as the ILS (108.10–111.95 mHz), with a signal modulated at 90 and 150 Hz. An SDF approach does not provide glide path information. Marker beacons and compass locators may be used as part of an SDF approach. The SDF final approach course may not be aligned with the runway and is wider than an ILS localizer. SDFs are usually much cheaper and easier to install and maintain than an ILS and are well suited for smaller airports or for use as a secondary approach at an airport already equipped with an ILS. GPS-Based Instrument Approaches The GPS approach overlay program permits pilots to use GPS to fly certain designated nonprecision instrument approach procedures. These procedures are identified by name, and the phrase “or GPS” is then added to the title. For example, “VOR/DME or GPS RWY 27L”. As the development of stand-alone GPS approaches has progressed, many of the original overlay approaches have been replaced with stand-alone procedures specifically designed for use by GPS systems. The title of these procedures will have only the GPS navigation system in the title. For example, “GPS RWY 24”. GPS GPS approaches make use of two types of navigational fixes or waypoints. Approach These are called either fly-by or fly-over waypoints. Fly-by waypoints are used Waypoints when an aircraft should begin a turn to the next course prior to reaching the waypoint that separates the two route segments. In most cases, when properly flown, the aircraft will not actually cross over the waypoint but will instead start the turn early so as to smoothly intercept the next leg of the approach procedure. This is known as turn anticipation and is compensated for in the airspace and terrain clearance calculations. Many of the waypoints in a GPS approach, except the missed approach and the missed approach holding way- points, will typically be fly-by waypoints. Fly-over waypoints are used when the aircraft must fly over the point prior to starting a turn. These waypoints are used when it is imperative that the aircraft actually cross the point defined by the waypoint. Approach charts depict fly-over waypoints as a circled waypoint symbol. Since GPS receivers are designed to always fly to the next waypoint, in contrast to VOR receivers that are designed to navigate both to and from VORs, GPS procedures must be designed such that the aircraft is always navigating to a defined point. To facilitate these waypoint identifications, a new system of identifiers was created. With this new system, any point used for the purpose of defining the navigation track of an aircraft is called a Computer Navigation Fix (CNF). The FAA has begun to assign five-letter names to CNFs and to chart these on  Navigation Systems / 121 various aeronautical charts. CNFs are not to be used for any air traffic control application, such as holding or re-routing of aircraft, but are names assigned to waypoints that can be included in the aircraft’s internal navigational database. To distinguish them from conventional reporting points, CNF names on charts will be enclosed in parenthesis. In most cases, CNF names will be unique, with the exception of some way- points associated with the runway itself. For example, some runway threshold waypoints, which are generally the beginning of the missed approach segment, may be assigned a unique, five-letter identifier, but they may also be coded with a runway number such as “RW36L”. Approach and Landing Procedures There are three basic types of instrument approaches currently in use in the national airspace system. Nonprecision approaches (300–500 minimum altitude—1–mile visibility) Category I approaches (200’ minimum altitude—1/2–mile visibility) Category II/III approaches (100’ or less minimum altitude—zero to1/4–mile visibility) Nonprecision approaches provide only lateral (horizontal) electronic guidance and typically provide guidance to a point 300 to 500 feet above the runway with minimum required visibilities of 1 mile or greater. Category I approaches provide vertical guidance in addition to lateral navigation and typically provide guidance to a point 200 to 300 feet above the runway with visibilities of a 1/2 mile to 1 mile required. Category II and III approaches provide vertical guidance as well, to less than feet above the ground with visibilities of less than a 1/2 mile required. All procedures based on satellite navigation will eventually replace these three dif- ferent categories. These new categories will similarly be defined by their navi- gational accuracy. These new approach categories include the following: Lateral Navigation (LNAV) approach—similar to the traditional nonprecision approach. Lateral Navigation/Vertical Navigation (LNAV/VNAV) approach—similar to the traditional non–precision approach with the addition of vertical guidance. LNAV/VNAV will provide pilots with minimums close to, but not quite as low as, the Category I ILS currently in use. Localizer Performance with Vertical Guidance (LPV)—provides highly accurate lateral and vertical guidance and includes appropriate runway and approach lighting. LPV approaches using WAAS will provide Category I ILS capability. Enhanced augmentation, such as LAAS, might provide Category II and/or Category III capability in the future. 122 / CHAPTER 2 Lateral LNAV is the new terminology for a GPS nonprecision approach. The approach Navigation minimums for LNAV are similar to other nonprecision approaches in that (LNAV) RNAV (either GPS or other compatible area navigation equipment) will be used to provide lateral guidance. Aircraft conducting an LNAV approach will still descend incrementally to a minimum altitude and level off rather than fol- low a fixed glidepath. The pilot will navigate using RNAV to a missed approach point where either a safe landing can be accomplished or a missed approach must be conducted. LNAV is considered a nonprecision approach (no vertical guidance) with a minimum altitude of about 250 feet above obstacles along the flight path. At many airports, LNAV approaches will provide procedures with similar or lower minimums than existing VOR or NDB approaches. Approaches With the development of a means of providing calculated vertical guidance with Vertical information, in comparison to an actual transmitted glideslope, a new class of Guidance approach procedures, which provide calculated vertical guidance, have been developed. These new procedures are called Approach with Vertical Guidance (APV) procedures and have been adopted internationally. Lateral LNAV/VNAV approaches provide the pilot with vertical guidance calcu- Navigation/ lated by the GPS receiver. Due to the increased accuracy of GPS, LNAV/ Vertical VNAV approaches typically have minima lower than LNAV-only Navigation approaches. LNAV/VNAV will provide the pilots with a vertically guided (LNAV/VNAV) approach with a decision altitude about 250 to 350 feet above the runway. Visibility requirements are generally 1 mile at airports without approach lighting systems. Localizer Approaches that combine the augmented navigation capabilities of WAAS, Performance EGNOS, or LAAS with appropriate runway and approach lighting will be with Vertical known as localizer performance with vertical guidance (LPV) approaches.This Guidance procedure should provide for approach criteria very similar to that provided (LPV) by Category I ILS systems. (Decision altitudes as low as 200’ above touchdown with visibility minimums as low as 1/2 mile). At this time, the implementation of LAAS is still being studied. If LAAS is fully implemented, it might be pos- sible to provide LPV approaches similar to that now provided by Category II and III ILS systems. In theory, obstacle problems notwithstanding, it is theo- retically possible to develop LPV approaches for many airports across the country. It is likely that many, if not all, ILS systems could be decommissioned at that point. It is being suggested that as a backup system, many busy airports might keep one ILS system operational even with the advent of LPV. If LAAS is not implemented nationwide, Category II and III ILS systems might need to be retained.  Navigation Systems / 123 Runway and Approach Lighting When night !ying was first introduced, most airports consisted of an open area covered with either turf or cinders. Pilots could land in whichever direction they chose. Rotating beacons provided the pilots with the general location of the airport but did not provide sufficient visual cues to permit them to actually locate the cinder area and land. This problem was solved through the introduc- tion of airport boundary lighting, previously described in this chapter. In the late 1930s, because of the increased weight of aircraft that were being introduced into service, most of the airports began to construct concrete runways to replace the cinder landing surfaces. These runways were usually about 1 mile long and about 100 feet wide. Since each airport had only two or three runways, airport boundary lighting no longer satisfactorily assisted the pilot in locating the runway at night. A different type of lighting needed to be developed. Runway Runway Edge Lights Many different types of runway lighting systems were Lighting examined, including runway !oodlights and neon lights. After numerous experiments by both civilian and military aviation authorities, it was eventu- ally agreed that runway edge lights should be the standard type of runway lighting. Runway edge lights are placed on either side of the runway, spaced approximately 200 feet apart, outlining the edges of the runway. These lights are usually placed on short metal poles to elevate them from any obstruction such as long grass or drifting snow. Runway lights are white and are usually covered with a Fresnel lens (see Figure 2–66). Fresnel lenses are designed to focus the emitted light, concentrating it along and slightly above the horizontal plane of the runway’s surface. The lights installed on the last 2,000 feet of runways used for instru- ment approaches use lenses that are half white and half amber. These lights appear amber to a landing pilot, warning that the far end of the runway is fast approaching. The ends of the runway are clearly designated through the use of runway threshold lights, which are similar to runway lights but use red and green split lenses. As the pilot approaches the runway to land, the threshold lights on the near end of the runway appear green, while those on the far end of the runway appear red. Runway light systems are normally operated from the control tower and are turned on during nighttime hours and during daylight whenever the vis- ibility is less than 2 miles or at the pilot’s request. Whenever the control tower is not in operation, the lights are either left on or are operated using pilot- controlled lighting (PCL) systems. PCL systems permit pilots to switch on the lights by pressing their microphone switch a number of times in rapid succes- sion, producing an audible click on the control tower frequency. The number of clicks controls both the operation and the intensity of the runway lighting 124 / CHAPTER 2 Michael Nolan Figure 2–66. A typical runway Fresnel light installation. system. For more information about pilot-controlled lighting systems, consult the Aeronautical Information Manual. Runway Light Intensity Runway light systems are classified according to the brightness they are capable of producing. Low-intensity runway lighting (LIRL) is the least expensive to install and is typically equipped with 15-watt bulbs that operate on one intensity level. This intensity level is known as step one. The standard type of lighting for a runway used for instrument approaches is medium-intensity runway lighting (MIRL). Medium-inten- sity lights are similar in construction to low-intensity lights but are usually equipped with 40-watt bulbs. MIRL can be operated on three intensity levels: step one, step two, and step three. When operated on step one, medium- intensity lights produce the same light level as low-intensity lights (15 watts). When functioning on step two, they operate at about 25 watts, and on step three they operate at the maximum-allowable 40-watt level. During normal operation, medium-intensity lights are usually set to step one. This intensity is increased whenever the pilot requests or when the visibility drops below 3 miles. Runways that are heavily used during periods of low visibility may be equipped with high-intensity runway lighting (HIRL). High-intensity runway lights operate on five steps ranging from 15 watts to 200 watts. High-intensity  Navigation Systems / 125 lights are operated on step one until the visibility begins to decrease below 5 miles. At that point, higher intensities are used, with step five being reserved for periods when the visibility is less than 1 mile. Embedded-in-Runway Lighting Runways that are used extensively during periods of low visibility may be equipped with an assortment of embedded runway lights that provide the pilot additional visual cues when landing. These systems include touchdown zone lighting, runway centerline lighting, and taxi- way turnoff lighting. During periods of very low visibility, the runway edge lighting does not provide the pilot with sufficient visual cues to properly land the aircraft. In the 1950s, when ILS was initially being installed, various pilot groups com- plained that landing in these conditions was like landing in a black hole. They reported that during the last few seconds of the approach, as they were rais- ing the aircraft’s nose for landing, the runway edge lights were too far apart to provide an accurate altitude reference. In an attempt to provide additional visual cues during this critical phase of landing, a new supplemental lighting system was developed, known as touchdown zone lighting. Touchdown zone lights are embedded in the runway and extend from the landing threshold to a point 3,000 feet down the runway. Touchdown zone lights use 100- to 200-watt bulbs and are placed in sets of three, on both sides of the runway centerline. Touchdown zone light intensities are stepped up in conjunction with the runway edge lights. In conditions of reduced visibility, runway edge lights do not provide sufficient directional guidance information to enable pilots to accurately steer their aircraft along the center of the runway. To assist the pilot, many airports have installed runway centerline lights. Centerline lights are similar to touch- down zone lights but are placed along the entire centerline, at 75-foot intervals. Runway centerline lights are bidirectional: in the first part of the runway, the lights are white, while the last 1,000 feet of centerline lights are red; in the 2,000 feet preceding the red lights, the centerline lights alternate red and white to warn pilots that the runway end is approaching. Runway centerline lights are also varied in intensity in proportion to the setting chosen for the runway edge lights. When visibility is reduced, many pilots find it difficult to identify the intersecting taxiways for exiting the runway. Runway utilization rates are reduced as pilots taxi slowly, trying to find the proper turnoff. To reduce this taxi time, some airports have installed taxiway turnoff lights, which are simi- lar to centerline lights but are used to delineate the path that the pilot should use for exiting the runway. Taxiway turnoff lights are inset into the runway’s surface and are spaced at 50-foot intervals. These lights are colored green and extend from the runway centerline to the proper intersecting taxiway. Large airports may have a myriad of taxiways, runways, and vehicu- lar paths that all look similar to a pilot unfamiliar with the airport. To assist these pilots, taxiway edge lighting systems have been developed. Taxiway edge lights are similar to runway edge lights but operate at reduced wattage and 126 / CHAPTER 2 are equipped with blue lenses. Taxiway centerline lights are green. Taxiway lights may operate at different intensity levels and are usually operated from the control tower. Approach One of the most complex tasks facing pilots occurs near the end of an instru- Lighting ment approach, when they make the transition from instrument to visual !ying. Systems During this transition, they must locate the runway and properly maneuver the aircraft for landing within seconds. In conditions of low visibility, a pilot may be able to see only about 2,000 feet ahead of the aircraft. In today’s modern jets, this distance can be covered in less than 20 seconds. Within this short time, the pilot must locate the runway, determine the aircraft’s position, make any necessary adjustments in !ight attitude, and then land the aircraft. Without some form of visual assistance, this task is virtually impossible to perform safely in so short a time. These problems were noted as early as 1932 by officials from the airlines and the Bureau of Air Commerce. Experiments were conducted as early as 1935 in an attempt to simplify the transition from instrument to visual !ight during an approach. These experiments led to the construction of a number of different types of approach lighting systems. Approach lights are placed along the extended centerline of the runway and usually extend from the runway threshold out to a point where the pilot might make the transition from instru- ment to visual !ying. Approach lighting systems are designed to provide the pilot with visual cues that will permit accurate aircraft control during the final approach and landing phase of the !ight. Experimental Systems The first experimental approach lighting system con- sisted of three high-intensity incandescent lights placed approximately 500 feet apart along the extended centerline of a runway at the airport in Newark, New Jersey. Later experimental systems installed at the airport included neon bar lights and 1,500-foot rows of incandescent lights. Additional experiments were conducted at the airports in Indianapo- lis, Indiana, and Nantucket, Massachusetts. In 1945 the CAA, Army Air Corps, and Navy Department agreed to join efforts to establish the Landing Aids Experiment Station (LAES) at the Naval Air Station at Arcata, California, where most of the pioneering research in approach lighting was conducted. Opinions differed about the requirements and the configurations for approach lighting systems. The military services preferred a system that did not lie along the extended centerline of the runway. Military officials felt that the area directly below the aircraft should remain clear of obstructions during the final phase of the approach. They preferred approach lights to be placed to the left, to the right, or on both sides of the aircraft. The CAA, on the other hand, preferred to place the approach lights directly under the aircraft. Although this system created a slight obstruction problem, it did not require pilots to look out their side window to see the  Navigation Systems / 127 approach lights. An approach lighting system along the centerline of the run- way would permit pilots to concentrate directly ahead of the aircraft, which would simplify runway detection and make it easier to note any changes in aircraft altitude or attitude. By 1953, each organization had selected a different system as its stan- dard. The CAA selected a centerline system, known as Configuration A. The Air Force and the Navy chose systems known, respectively, as Configuration B and Configuration C. In 1958, after years of discussion, the Depart- ment of Defense agreed to cease building any additional Configuration B and C systems and to use Configuration A approach lighting in all new installations. The CAA configuration consisted of a series of high-intensity white lamps placed five abreast, extending from the runway threshold out to a distance of 2,400 to 3,000 feet (see Figure 2–67). These light bars were spaced 100 feet apart. At a point 1,000 feet from the end of the runway, a triple set of light bars provided the pilot with both roll guidance and a definite, unmistakable distance indication. The threshold of the runway was delineated with a series of four red light bars and a continuous line of green threshold lights. To provide for identification of the approach lighting system, a high- intensity strobe light was placed on each of the light bars that extended beyond Michael Nolan Figure 2–67. A typical approach light installation. 128 / CHAPTER 2 Michael Nolan Figure 2–68. A typical high-intensity approach light. the 1,000-foot mark (see Figure 2–68). These strobe lights !ashed in sequence, at a rate of two times per second, and appeared to the pilot as a moving ball of light leading to the runway. These sequenced flashing lights (SFL) are also referred to by the slang name “the rabbit.” This combination approach lighting system became the standard for runways equipped with Category I ILS and is known as approach lighting system type 1 or ALSF-1. When Category II and III ILSs were being developed, it was realized that an improved approach lighting system was necessary. During Category II approaches, the pilot may be required to transition to visual references dur- ing the last 15 seconds of !ight. Category III approaches permit the pilot even less time to make this transition. In response, the FAA developed an improved approach lighting system known as approach lighting system type 2, or ALSF-2 (see Figure 2–69). ALSF-2 is similar to ALSF-1 but includes additional lighting during the last 1,000 feet. A supplemental set of white light bars is located 500 feet from the runway threshold to provide the pilot with an additional distance indica- tion. Red light bars are also placed on both sides of the centerline, providing pilots with aircraft roll guidance during the last 1,000 feet. ALSF-2 approach lighting systems are wired such that the additional lights can be switched off whenever Category I ILS approaches are being conducted  Navigation Systems / 129 ALSF-1 ALSF-2 Landing threshold Landing threshold 1,000 ft. 1,000 ft. 2,400 ft./3,000 ft. 2,400 ft./3,000 ft. Steady-burning red lights Steady-burning white lights Sequenced flashing lights Threshold lights Figure 2–69. ALSF-1 and ALSF-2 installations. and appear as ALSF-1 systems. The system is operated in the ALSF-2 con- figuration only when the pilot requests or when the visibility decreases below ¾ mile. Both the ALSF-1 and ALSF-2 systems are similar to high-intensity runway lighting systems in that they can be set to one of five intensity steps. Step five, the brightest, is used only during periods of extremely low visibility. Simplified Approach Lighting Systems Both the ALSF-1 and the ALSF-2 systems are expensive to install, operate, and maintain. This expense can be justified only at airports that use this type of equipment routinely. At most air- ports, a smaller, less expensive system can provide pilots with the same benefits as these larger systems. Some runways are located such that identification of the extended run- way centerline is difficult. If extensive instrument approaches are not being conducted to that runway, a full approach lighting system may be economi- cally unfeasible. It is usually more practical to simply install the sequenced !ashing lights and let them guide pilots to the runway end. When installed in 130 / CHAPTER 2 FAA Figure 2–70. High-intensity approach lighting system. (FAA) this manner, SFLs are usually spaced 200 feet apart and are known as runway alignment indicator lights (RAILs). At some locations, a full-length (3,000-foot) approach lighting system is unnecessary. For many, the FAA has chosen to install a version of ALSF-1 that is only 1,200 feet long. This system utilizes the same high-intensity white approach lights as the ALSF-1 system, but they are spaced at 200-foot intervals. This is known as the simplified short approach lighting system (SSALS) (see Figure 2–70). In most of these locations, runway alignment indicator lights are also installed out to a distance of 2,400 feet. In this configuration, the system is known as the simplified short approach lighting system with RAIL (SSALR) (see Figure 2–71). Most ALSF-1 and ALSF-2 systems are wired such that they can operate as SSALR systems during periods when low-visibility approaches are not conducted. The FAA has begun to place a smaller approach lighting system at most airports that do not routinely conduct a large number of low-visibility approaches. This system is designed to include most of the important compo- nents available in the ALSF and SSALR systems but reduces the installation, operating, and maintenance expenses. This system, known as the medium-intensity approach lighting system with RAIL (MALSR), operates with only three steps of intensity, using medium-  Navigation Systems / 131 SSALR SSALS Landing threshold Landing threshold 200 ft. 200 ft. 1,000 ft. 1,000 ft. 1,400 ft. 1,400 ft. 2,400 ft./3,000 ft. Steady-burning white lights 3,000 ft. Sequenced RAIL flashing lights Threshold lights Figure 2–71. SSALR and SSALS installations. intensity white lamps. MALSR systems extend 2,400 feet from the runway threshold, with the light bars spaced at 200-foot intervals. MALSR systems operate on step one through step three, with step three being equivalent in intensity to step three on an ALSF system (see Figure 2–72). MALSR is now the U.S. standard for precision approach lighting. VFR At airports in densely populated areas, it may be extremely difficult for a pilot Approach !ying VFR to identify the location of the runways. Thus, it may be necessary to Lighting provide pilots with a positive means of locating the runways. If the area is par- Systems ticularly noise sensitive and pilots are required to !y a specific !ight path to the runway, it may also be necessary to delineate the approach !ight path. Two types of identifier lights have been developed for these purposes: runway end identifier lights and omnidirectional approach lights. Runway End Identifier Lights Runway end identifier lights (REILs) provide pilots with rapid and unmistakable identification of the end of the runway. REIL units are located on both sides of the approach end and are synchronized to !ash together two times per second. Each light is unidirectional, is pointed approxi- mately 15° away from the centerline, and !ashes with an intensity of 600 watts (see Figures 2–73 and 2–74). Some REIL units are single step, whereas others may be three step and connected to the runway light-intensity controller. 132 / CHAPTER 2 MALSR Landing threshold 200 ft. 1,000 ft. 1,400 ft. Medium steady-burning 2,400 ft. white lights Sequenced RAIL flashing lights Threshold lights Figure 2–72. MALSR installation. ODALS LDIN REIL Landing threshold Landing threshold 15° Landing threshold 15° 300 ft. 3,000 ft. to 5,000 ft. Typical distance 10° 10° Landing threshold 360° Flasher 1,500 ft. Sequenced flashing lights Figure 2–73. ODALS, LDIN, and REIL installations.  Navigation Systems / 133 Figure 2–74. A typical runway end identifier light installation. Omnidirectional Approach Lighting System Omnidirectional approach light- ing systems (ODALSs) are used to delineate the !ight path that should be used by a pilot approaching a specific runway. The lights are installed in groups, are omnidirectional, and !ash in sequence. ODALSs may be installed directly in front of the runway or may be placed miles from the airport, under the proper !ight path. ODALSs are also being used experimentally to delineate VFR !ight paths in congested areas. Vertical Guidance Systems The previously described systems were primarily designed to provide lateral guidance to the pilot, with vertical guidance being provided by an electronic glide path. At night, or during periods of reduced VFR visibility, pilots are deprived of many of the visual cues used to determine the proper glide path. Without these cues, pilots may be unable to correctly orient their aircraft during the final approach phase and may misjudge their distance, glide angle, or rate of descent. Any miscalculation of one of these fac- tors may cause the pilot to incorrectly approach the runway and collide with obstructions in the approach path or land at an excessive speed and roll off the end of the runway. Since it is financially impractical to install an electronic glide path at every runway across the United States, an inexpensive method of glide path indication was necessary. After extensive evaluation at the National Aviation Facilities Experimen- tal Center (NAFEC) in Atlantic City, New Jersey, in 1960 the FAA introduced the visual approach slope indicator (VASI) system. VASI lights are designed to 134 / CHAPTER 2 Michael Nolan Figure 2–75. A typical VASI light installation. be installed on runways with or without ILS approaches and can provide pilots with accurate glide path information as far as 20 miles from the runway. The VASI system uses either two or three light units arranged to provide the pilot with a visual glide path. These light units are next to the runway, with the first located approximately 700 feet and the second approximately 1,200 feet from the approach end (see Figure 2–75). Each VASI unit provides a narrow beam of light filtered such that the upper portion (above the glide path) of the beam is white and the lower por- tion (below the glide path) is red. Pilots looking at a VASI light know that the aircraft is too high if they see a white light and too low if they see a red light. The two VASI units are installed such that a pilot on the desired glide path is above the near VASI (the white beam) but below the far VASI (the red beam). A pilot who is too high will see the white light from both units, whereas the pilot who is too low will see the red beams from both (see Figure 2–76). The glide path provided by the standard two-light VASI system is of insufficient altitude for large aircraft (such as DC-10s and 747s) conducting approaches to the runway. At airports frequented by these types of aircraft, a third light bar is installed farther down the runway. Pilots of these aircraft use the middle and far VASI units, and pilots of small aircraft use the near and middle VASI units.  Navigation Systems / 135 Cockpit View The glide path area is approximately 97 feet deep at 4 nautical miles from threshold White White White White 27 wh ite 1/2∞ 1/2∞ Glide slope ite/ Wh e /whit Red Red 27 1/2∞ Pink it e 1/2∞ h White White Red/w k 27 e d /pin R slope Glide Light bars Red/red Too low Ground level Red Red Runway threshold Red 27 Red Downwind bar VASI reference point Upwind bar Figure 2–76. VASI operation. High Slightly high On glide path Slightly low Low (more than (3.2°) (3°) (2.8°) (less than 3.5°) White Red 2.5°) Figure 2–77. PAPI operation. Precision Approach Path Indicator The VASI system is highly effective but can be difficult to use since the pilot must constantly observe light units that are sep- arated by up to 1,000 feet. A similar system, the precision approach path indi- cator (PAPI), has been developed that remedies this situation (see Figure 2–77). PAPI units are similar to VASI units but are installed in a single row. Each light unit emits a white and a red beam but at progressively higher angles. If the pilot is more than half a degree above the desired !ight path, all the light units will appear to emit white light. But as the pilot descends to a lower angle, the system is designed so that the pilot will begin to see red light emitted from