A Tool for Estimating Airport Access Benefits from Gps-Based Instrument Approaches

AIAA 2001-4236 A TOOL FOR ESTIMATING AIRPORT ACCESS BENEFITS FROM GPS-BASED INSTRUMENT APPROACHES H. Leslie Crane Sebastian V. Massimini, DSc, AIAA Member The MITRE Corporation McLean VA USA ABSTRACT all the possible runways would be an impossibly long task. The MITRE Corporation developed a computer-based Application of the MITRE Corporation computer model tool that estimates the Height Above Touchdown enables a timely high-level evaluation of HAT (HAT) and visibility minima for Global Positioning estimates for a large number of runway ends, and will System (GPS) instrument approaches using electronic eventually include considerations for airport databases of terrain and obstacles. The model uses infrastructure. This paper provides a statistical criteria from the U.S. Federal Aviation Administration summary of preliminary results showing HAT and (FAA) Terminal Instrument Procedures (TERPS) visibility benefits for approaches to a large number of developed for GPS approaches, but can be modified to airports in the USA, and also provides a comparison of use different criteria. The design of the model allows approach minima between different classes of the evaluation of a large number of runways, with augmented GPS approaches. The intent of the analysis subsequent analysis of the benefits of a particular is to assist in the final design criteria for GPS approach procedure. approaches in the USA. The development of wide area systems to augment the BACKGROUND GPS has long promised the capability of providing or improving the instrument approach capability at many The U.S. Federal Aviation Administration (FAA) and airports. In particular, one of the objectives of the Wide many other countries have declared the Global Area Augmentation System (WAAS) is to provide, Positioning System (GPS) suitable as a supplemental where possible, straight-in instrument approaches with means of navigation and have published criteria for use vertical guidance to all instrument capable runways in of the system. GPS can currently be used for oceanic, the USA. en route, terminal, and non-precision instrument approach navigation. At present, there are There are over 5000 airports in CONUS that have at approximately 2500 GPS instrument approach least one runway over 3000 ft long. At present, the procedures to airports in the U.S. minima for approaches with vertical guidance that would result from the development of WAAS are Nonprecision approaches provide horizontal guidance known for only a few runways. Using the present FAA to the aircraft, but vertical profiles are flown using the approach design apparatus to estimate the minima for aircraft barometric altimeter. Typically, the aircraft flies level at HAT until visually acquiring the runway or approach lights, after which the aircraft begins a descent to land. Copyright © 2001 The MITRE Corporation. Published by the American Institute of Aeronautics and An FAA goal is to improve aircraft safety by providing Astronautics, Inc. with permission. instrument approaches with horizontal and vertical guidance. These approaches provide a stabilized This paper is based on system analysis studies descent path for the aircraft, in addition to the performed by the Center for Advanced Aviation System horizontal guidance. This allows a constant rate of Development (CAASD), the MITRE Corporation for descent down the approach path—there is no need to the FAA Satellite Program Office (AND-730). This level off, as in the nonprecision approach. Approaches paper reflects the views of the authors, who are with vertical guidance are currently performed by using responsible for the accuracy of the facts, analyses and ground-based Instrument Landing System (ILS). suggestions presented herein, and does not reflect the However, due to cost and other considerations, only official views or policy of the FAA. about 1100 of the 4000+ instrument approach runways in the U.S. have ILS. Also, many airports and runways 1 American Institute of Aeronautics and Astronautics do not have instrument approach capability at all. To The required visibility minimum is computed in address these concerns, the FAA is developing the different ways for nonprecision approaches (i.e., Wide Area Augmentation System (WAAS). WAAS LNAV) than for approaches with vertical guidance (i.e., will augment GPS, and allow primary means satellite- LNAV/VNAV and GLS). For an LNAV approach, a based navigation in the U.S., including approaches with table is used to determine the required visibility by vertical guidance to nearly all instrument capable using HAT and the aircraft category.* See Figure 1. The runways. For larger airports, the FAA is also limiting visibility varies with HAT, but Category A and developing the Local Area Augmentation System B aircraft can attain the lowest visibility (without (LAAS). LAAS will provide approaches at individual approach lights✝) of one mile if the HAT is 740 ft or airports with vertical guidance similar to WAAS, but less. Category C aircraft require an HAT of 400 ft or with the very high availability necessary for larger less to attain one-mile visibility. An LNAV approach airports, in addition to very low landing minima normally results in the aircraft descending to HAT and associated with Category II and III approaches. This flying level until the runway is in sight. For Category paper will not consider Category II or III approaches. A/B, the descent gradient to the runway could be quite steep if the visibility is near the minimum (e.g., as steep An important consideration for GPS approaches is as 740 ft per statute mile). However, these are generally airport access. The presence of an instrument approach small aircraft that have high maneuverability, and can can significantly benefit airport users and enhance also land farther down the runway if necessary. safety. Category C aircraft, which are generally less maneuverable than Category A/B aircraft, would have a Currently, the FAA has published design criteria for lower maximum descent gradient (e.g., 400 ft/statute three types of area navigation (RNAV) approaches mile), which is closer to the three-degree descent path using GPS. The LNAV approach provides horizontal normally flown on most approaches. guidance only, and is a nonprecision approach. Approximately 2000 LNAV approaches have currently been designed for U.S. airports. The LNAV/VNAV approach has a similar size area over which obstruction Approach Path HAT (visibility minimum clearance criteria are evaluated, but provides horizontal determined from table) and vertical guidance to the pilot during the approach. Only a few hundred LNAV/VNAV approaches have currently been developed. The GLS approach uses Runway vertical and horizontal guidance of higher quality, and therefore has reduced obstruction clearance criteria. Figure 1. LNAV Visibility Minimum GLS approach criteria are essentially identical to criteria currently used to design Category I ILS The visibility minimum for an approach with vertical approaches. Few GLS approaches have been developed. guidance (i.e., an LNAV/VNAV or GLS) is determined using a geometric method. In a vertically-guided The design of the approach dictates how low the pilot approach, when the aircraft arrives at the HAT, the pilot can legally descend without visual reference to the must be able to see the runway and continue on the runway or approach lights. The lowest height, called same descent path. Thus the horizontal distance from the Height Above Touchdown (HAT), is determined by the point at which the descent path reaches HAT to the applying the criteria for the approach to terrain and runway threshold determines the visibility minimum. obstacles that surround the airport. Different approach See Figure 2. Like a nonprecision approach, the criteria (e.g., LNAV, LNAV/VNAV, or GLS) can visibility minimum can be reduced with approach produce different HATs for approaches to the same runway. Although HAT is an important criterion for instrument * Aircraft category is based on the stalling speed of the approaches, current U.S. regulations use the flight aircraft. General aviation and commuter aircraft are visibility, expressed in statute miles, as the limiting usually Category A or B, while most airliners are factor for landing from an instrument approach. Category C. Large airliners are usually Category D. Typically the aircraft will arrive at HAT during the ✝ The visibility can also be affected by airport approach; if the pilot can see the runway or approach infrastructure. For example, approach lights can reduce lights, then s/he may continue the approach. If not, the required visibility, while runway length, pavement, then a missed approach must be initiated. and taxiway placement can increase the required visibility. 2 American Institute of Aeronautics and Astronautics lights, or may be increased by other airport (GAME). This computer model is based on a Unix infrastructure considerations. workstation and is written in the C computer language. The output is ported to a PC, where results are analyzed In general, we will measure airport access benefits in Microsoft Access. primarily by the improvements to the visibility minimum of the instrument approach, with The Model uses Digital Terrain Elevation Data (DTED) improvements to HAT as an important secondary from the National Imagery and Mapping Agency benefit. (NIMA) to provide information on terrain features. Obstacle and runway data are contained in Exchange Files provided by the National Geodetic Survey’s Aeronautic Survey

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