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lateral deviations of aircraft. However, when standards can- not be met, there is no process by which to evaluate the level of risk of the smaller airfield separations. Many aircraft collisions occur during taxiing operations. Over 20 percent of Part 121 accidents in 2005 were character- ized as on-ground collisions with objects during taxi or stand- ing (NTSB, 2009). These are collisions between two aircraft, between an aircraft and ground equipment, or between an air- craft and a stationary structure. Some of these accidents may be associated with airfield separations, and it is necessary to evaluate how these separations and lateral aircraft deviations interact to provide an assessment of the risk of collision. Over 34 percent of fatal accidents with worldwide commer- cial jets occur on the ground (Boeing, 2009). Runway veer-offs and overruns represent 24 percent of all incidents and accidents in air transport operations (IFALPA, 2008). These types of events happen at an approximate rate of one per week, empha- sizing the challenge that airport operators face, particularly when considering substandard airfield separation distances. The Australian Transport Safety Bureau (ATSB) analyzed 141 accidents with 550 fatalities for commercial aircraft world- wide from 1998 to 2007. All fatal accidents were catastrophic runway excursions, and 120 of these occurred during landings (ATSB, 2009). Airfield separations and runway and taxiway safety areas have been established and regulated to help reduce the risk of collisions and to mitigate the consequences of runway and taxi- way excursions. Airfield separations are determined on the basis of the location, aircraft wingspan, random lateral and ver- tical deviations, and a separation margin of safety to account for extreme deviations. Over the years, aircraft wingspans have been increasing gradually, and the FAA has developed new sep- aration standards to accommodate these larger aircraft. The introduction of new large aircraft (NLA) is still in process and will continue bringing challenges to the aviation industry. NLA will have a significant impact predominantly on existing airports, particularly large hubs, due to the aircraft passenger capacity, weight, wingspan, length, tail height, and wheelbase. Some of the current airport separations between runways versus runways, taxiways, taxilanes, moveable and fixed objects, and taxiways versus taxiways, taxilanes, moveable and fixed objects, etc., may not be adequate to accommodate the introduction of NLA. Most airports with separations inad- equate to accommodate NLA do not have enough space for construction of new facilities or for relocation of existing facil- ities to comply with current FAA standards. It is important to emphasize that it is not only NLAs or exist- ing aircraft like Lockheed C5 and Antonov AN124 that pose challenges to existing airports; recent and new aircraft, such as the Airbus A340-600 and B777-300 ER, require changes in some aspects of airport infrastructure due to their long fuselage length and associated long wheelbase. FAA Modification of Standards The FAA established the Airport Reference Code (ARC) sys- tem to aid in the geometric design of runways, taxiways, and other airport facilities. The system and the airfield separations associated with each code are described in AC 150/5300-13 (FAA, 1989). The ARC is based on aircraft dimensions and approach speeds to define several physical characteristics of air- fields, including airfield separations. Standard distances were established for each aircraft category; although in certain cases it is possible to request a modification of standards. According to AC 150/5300-13 (FAA, 1989): Modification to standards means any change to FAA design standards other than dimensional standards for runway safety areas. Unique local conditions may require modification to airport design standards for a specific airport. A modification to an air- port design standard related to new construction, reconstruction, expansion, or upgrade on an airport which received Federal aid requires FAA approval. The request for modification should show that the modification will provide an acceptable level of safety, economy, durability, and workmanship. (CHG 10, Chapter 1, p. 5) A survey conducted by the FAA in 2008 identified 142 air- ports that can accommodate Cat II and Cat III approaches. Of these 142 airports, 63 airports have less than a 500-ft sep- aration between the runway and parallel taxiway, and three have less than a 400-ft separation, measured within the first 3,000 ft of the runway. What is the risk if larger aircraft are allowed to operate at these airports with non-standard separations? Currently, there are no risk-based methodologies for assessing such risks, and each situation is treated as a unique case. The FAA may allow operation at airports that do not comply with minimum sep- aration distances by evaluating an MOS submitted by the air- port operator. The objective is to keep the airport/aircraft operations at a level of safety equivalent to that achieved by standard separations. The FAA uses a computer program that considers the rela- tionship between airplane physical characteristics and the design of airport elements to show that an MOS provides an acceptable level of safety for the specified conditions, includ- ing the type of aircraft (FAA, 1989). AC 150/5300-13 also states that values obtained from the specific equations presented in the next chapter may be used to demonstrate that an MOS will provide an acceptable level of safety (FAA, 1989). The criteria are based on engineering judgment and can only be used to compare taxiway and tax- ilane separations. However, in the context of this study, it was necessary to address separations between runways and taxi- ways or taxilanes. There is no procedure in the FAA guidance material to evaluate runway separations for the risk of colli- sion between an aircraft landing or taking off and a taxiing aircraft or an object. 3
4A preliminary task in the study was the identification of approaches that could be used as a framework for the risk- assessment methodology. The ï¬rst step was the gathering of information on two basic rationalesâthat used by the FAA and that used by the International Civil Aviation Organization (ICAO)âto establish their airï¬eld separation standards.1 The bases for the development of both rationales were the ran- dom deviations of aircraft during operations. Such deviations are greater for runways and less for taxiways and taxilanes. In addition, some incidents may lead to very large deviations (e.g., runway excursions), and safety areas must be planned to mitigate the risk of these large deviations. FAA Rationale Most of the FAA documents reviewed for this study pres- ent the separation standards and sometimes identify design considerations, but they rarely provide detailed information on the design rationale. For this reason, two engineers were interviewed. They worked in the FAA airport organization that was responsible for developing design standards, includ- ing the separation standards. Also, an attempt was made to place several of the documents in the context of the historical time when they were issued. In 1940, the Civil Aeronautics Authority issued a document entitled Airport Design Information (1940). The manual was âprepared for the instruction and guidance of Airport Section Engineers in their ï¬eld consultation activitiesâ (Hathi Trust Digital Library). It provided standards for four airport classes that were based on runway lengths required by aircraft expected to use the facility. The Civil Aeronautics Administration (CAA) published an updated version of this manual in 1941. In April 1944, the CAA published the first of four manuals titled Air- port Design. This manual had standards for five classes of airports, with the classes based on runway lengths required by aircraft expected to use the facility.2 It is important to remember that these manuals were devel- oped just prior to and during World War II. At that time, the United States had a massive war mobilization effort under- way, and it is unknown if the manuals were the output of any intensive research and development effort. In all likelihood, the standards were based on the best engineering judgment of the era. In January 1949, the CAA published the second manual titled Airport Design, and it provided for eight different airport classes. These classes were based on the type of service rather than the expected type of aircraft. In relation to the historical context of this document, there are two points worthy of note: ⢠The standards contained in the document represent the knowledge gained from aircraft operations during World War II. ⢠In 1946, the Federal Aid Airport Program (FAAP) was enacted to provide federal funds to airport sponsors for cap- ital development at their airports, and the program required that such development be done in accordance with stan- dards issued by the CAA. The importance of the FAAP cannot be overemphasized. This carrot and stick approach was successful in achieving uniformity of design within the various airport classes; how- ever, there were so many classes that it was easy to incor- rectly predict an airportâs ultimate role in the national airport system. C H A P T E R 2 Airfield Separation Rationale 1 FAA and ICAO may use different terminology for their standards and recom- mended practices. Throughout the text, the original terms for each agency were kept. For example, the FAA ârunway safety areaâ function is equivalent to that of ICAOâs âgraded area of the runway stripâ plus its ârunway end safety area.â 2 The information on these manuals is taken from a paper prepared by Robert David in 1973 (David, 1973). At that time, the documents were obtained from the FAA library, but they are no longer available.