Navaids: a Brief Overview

AirTAP Briefings A publication of the Airport Technical Assistance Program of the Center for Transportation Studies at the University of Minnesota Winter 2006 NAVAIDs: A brief overview This issue of Briefings focuses on one Following is a description of some of Simplified short approach lighting sys- essential element of all airports. Navigation the more regularly used NAVAIDs. Other tem with runway alignment (SSALR)— aids, or NAVAIDs as they are more com- NAVAIDs, which are not discussed in this Used the same as MALSR. Generally, monly known, are visual or electronic article, include distance-measuring equip- the SSALR is not to be installed as a new devices, located either on the ground or in ment (DME), centerline lighting, distance- system. It is used when Category I condi- the air, that provide point-to-point guidance to-go sign (DTG), and surface movement tions exist on Category II-designated run- information or position data to aircraft guidance signs. [Note: this information has ways with a dual-mode approach lighting in flight. As such, NAVAIDs help pilots been taken from FAA Order 6850.2A, dated system. plan and execute flights from one point to December 17, 1981.] another. High-intensity approach lighting system NAVAIDs include: Approach lighting system (ALS) with sequenced flashers (MALS)—used • Approach lighting systems (ALS) An approach lighting system (ALS) is a on Category II and III precision approach • Precision approach path indicator configuration of signal lights placed sym- runways. (PAPI) systems metrically about the extended runway • Visual approach slope indicator (VASI) centerline, starting at the landing threshold Omnidirectional approach lighting sys- systems and extending straight outward into the tem (ODALS)—a configuration of seven • Runway end identifier lights (REILs) approach zone. This system provides visual omnidirectional sequenced flashing lights • Lead-in lighting (LDIN) systems information on runway alignment, height located in the runway approach area. • Airport beacons perception, roll guidance, and horizon The ODALS provide circling, offset, and • Instrument landing systems (ILS) references. The system used for preci- straight-in visual guidance for nonprecision • Nondirectional beacons (NDB) sion approaches, in conjunction with an approach runways. electronic aid such as an ILS, is normally In addition to the above, automated 2,400 feet long when the glide slope is weather reporting equipment (AWOS) is 2.75 degrees or greater, and 3,000 feet usually included under the umbrella of long when the glide slope is less than 2.75 NAVAIDs. degrees. When installation of a 2,400-foot The primary navigation systems used system is not possible, a system 1,400 feet during flight are the very high frequency long may be installed on nonprecision run- omnirange (VOR) station and the satellite- ways. based Global Positioning System (GPS). The ALS is classified as a high-intensity NAVAIDs used during the approach and or medium-intensity system, depending on landing portions of flight are, in order of the type of lamps and equipment used: Precision approach path indicators (PAPIs) precision, the instrument landing system (ILS), localizer (LOC), GPS, VOR, and Medium-intensity approach lighting sys- Precision approach path indicators nondirectional beacon (NDB). tem (MALS)—an economy-type system (PAPIs) In addition to electronic or satellite for nonprecision approaches. A precision approach path indicator, or systems, a pilot can use several types of PAPI, is designed to furnish a pilot with approach lighting systems for assistance in Medium-intensity approach light- visual approach slope information to pro- the landing phase after he or she has locat- ing system with sequenced flashers vide guidance for safe descent and landing. ed the airport and is making the approach (MALSF)—same as MALS, but equipped The system, intended primarily for use dur- to the runway. with three sequenced flashers at locations ing visual flight rule (VFR) weather condi- The terms “precision” and “nonpreci- where approach-area identification prob- tions, works with remote on and off control sion” differentiate between navigational lems exist. (controlled by the pilot through the aircraft facilities that provide combined azimuth radio). Two light intensity settings—day and glide slope guidance to a runway (pre- Medium-intensity approach lighting sys- and night—are provided and are controlled cision) and those that do not (nonprecision). tem with runway alignment (MALSR)— photoelectrically. Nonprecision refers to facilities without a an economy-type system used as the FAA A PAPI system provides a definite white glide slope and does not imply an unaccept- standard for Category I precision runways. and red light projection pattern along the able quality of course guidance. desired descent path to the touchdown Continued on back For more information on AirTAP, including past issues of Briefings, visit www.AirTAP.umn.edu. point. The lamp housing assemblies are that uses the VOR indicator with only one arranged on a single bar on a line perpen- radial aligned with the runway. The local- dicular to the runway centerline. izer beam’s width ranges from 3 degrees to 6 degrees. It also uses a second beam Visual approach slope indicator (VASI) called a glide slope beam that provides A visual approach slope indicator (VASI) vertical information to the pilot. The glide system provides visual approach slope slope is usually 3 degrees wide and 1.4 information by projecting a definite white degrees high. and red light beam along the desired A horizontal needle on the VOR/ILS descent path to the touchdown point. The head indicates the aircraft’s vertical posi- light units are arranged in bars, with the tion. Three marker beacons (outer, middle, light units in each bar located on a line per- Runway end identifier lights (REILs) and inner) are located in front of the land- pendicular to the runway centerline. Each ing runway and indicate their distances light unit projects a split beam of light; Typically PAPIs and VASIs are set for a 3- from the runway threshold. The outer the upper segment is white and the lower degree visual approach angle. Their place- marker (OM) is located four to seven miles is red. The transition from red to white, or ment along the runway edge is determined from the runway. The middle marker (MM) vice versa, occurs over a vertical angle of by the classification of aircraft using the is located about 3,000 feet from the landing approximately one-quarter degree, with the airport, runway length, and obstructions threshold, while the inner marker (IM) is light in this area pink. The system produces within the approach. VASIs and PAPIs are located between the middle marker and the a well-defined corridor or corridors of light sited to provide a minimum of 1-degree runway threshold where the landing aircraft consisting of red and white beams. clearance over obstructions in the approach. would be 100 feet above the runway. VASI systems can be two- or three-bar: The VOR indicator for an ILS system • The two-bar system uses light units Lead-in lighting system (LDIN) uses a horizontal needle in addition to the arranged into an upwind and downwind The lead-in lighting system, or LDIN, vertical needle. When the appropriate ILS bar. The downwind bar is the one nearest provides positive visual guidance along an frequency is entered into the navigation to the runway threshold and the upwind approach path, either curving or straight, radio, the horizontal needle indicates where bar is the farthest from the threshold. where special problems exist with hazard- the aircraft is in relation to the glide slope. When on the proper glide path, the ous terrain, obstructions, or noise abate- If the needle is above the center mark on downwind bar appears white and the ment procedures. An LDIN consists of one the dial, the aircraft is below the glide upwind bar appears red. If the approach or more series of flashing lights installed slope. If the needle is below the center mark is too low, both bars will appear red. along the approach path at or near ground on the dial, the aircraft is above the glide • The three-bar system was developed level. The series or groups of lights are slope. to provide dual glide paths to accom- positioned and aimed so as to be conve- modate aircraft of varying wheel-to- niently sighted and followed from the cockpit heights. The three-bar VASI is approaching aircraft under conditions at or installed at only those locations having above approach minimums. 747, C5A, or other similar long-bodied Each light group contains at least three aircraft operations. The three-bar system flashing lights in a linear or cluster configu- AirTAP was developed through the joint efforts of the Minnesota Department of Transportation (Mn/DOT), uses light units arranged in bars called ration and may be augmented by steady- the Minnesota Council of Airports (MCOA), and the the upwind, middle, and downwind bar. burning lights if required. Where practi- Center for Transportation Studies (CTS). The downwind bar is the one nearest to cal, lights in one group flash in sequence Briefings is published as a quarterly insert of the MCOA the runway threshold and the upwind toward the runway. The light groups are newsletter. Please direct comments to: bar is the farthest, with the middle bar in spaced close enough to provide continu- Amy Friebe, between. Two glide paths are projected ous lead-in guidance along each segment Briefings Editor Jim Grothaus, by the three-bar system. of the approach course (approximately one Technology Transfer and Training Director mile). The FAA recommends sequencing Cheri Marti, Runway end identifier lights (REILs) successive light groups. The LDIN may be AirTAP Program Director The primary function of the runway end terminated at any approved approach light- Center for Transportation Studies identifier lights (REILs) is to provide rapid ing system, or it may be terminated at a University of Minnesota 200 Transportation & Safety Bldg.

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