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3 lateral deviations of aircraft. However, when standards can- FAA Modification of Standards not be met, there is no process by which to evaluate the level of risk of the smaller airfield separations. The FAA established the Airport Reference Code (ARC) sys- Many aircraft collisions occur during taxiing operations. tem to aid in the geometric design of runways, taxiways, and Over 20 percent of Part 121 accidents in 2005 were character- other airport facilities. The system and the airfield separations ized as on-ground collisions with objects during taxi or stand- associated with each code are described in AC 150/5300-13 ing (NTSB, 2009). These are collisions between two aircraft, (FAA, 1989). The ARC is based on aircraft dimensions and between an aircraft and ground equipment, or between an air- approach speeds to define several physical characteristics of air- craft and a stationary structure. Some of these accidents may fields, including airfield separations. Standard distances were be associated with airfield separations, and it is necessary to established for each aircraft category; although in certain cases evaluate how these separations and lateral aircraft deviations it is possible to request a modification of standards. interact to provide an assessment of the risk of collision. According to AC 150/5300-13 (FAA, 1989): Over 34 percent of fatal accidents with worldwide commer- Modification to standards means any change to FAA design cial jets occur on the ground (Boeing, 2009). Runway veer-offs standards other than dimensional standards for runway safety and overruns represent 24 percent of all incidents and accidents areas. Unique local conditions may require modification to airport in air transport operations (IFALPA, 2008). These types of design standards for a specific airport. A modification to an air- events happen at an approximate rate of one per week, empha- port design standard related to new construction, reconstruction, expansion, or upgrade on an airport which received Federal aid sizing the challenge that airport operators face, particularly requires FAA approval. The request for modification should show when considering substandard airfield separation distances. that the modification will provide an acceptable level of safety, The Australian Transport Safety Bureau (ATSB) analyzed economy, durability, and workmanship. (CHG 10, Chapter 1, p. 5) 141 accidents with 550 fatalities for commercial aircraft world- wide from 1998 to 2007. All fatal accidents were catastrophic A survey conducted by the FAA in 2008 identified 142 air- runway excursions, and 120 of these occurred during landings ports that can accommodate Cat II and Cat III approaches. (ATSB, 2009). Of these 142 airports, 63 airports have less than a 500-ft sep- Airfield separations and runway and taxiway safety areas aration between the runway and parallel taxiway, and three have been established and regulated to help reduce the risk of have less than a 400-ft separation, measured within the first collisions and to mitigate the consequences of runway and taxi- 3,000 ft of the runway. way excursions. Airfield separations are determined on the What is the risk if larger aircraft are allowed to operate at basis of the location, aircraft wingspan, random lateral and ver- these airports with non-standard separations? Currently, there tical deviations, and a separation margin of safety to account are no risk-based methodologies for assessing such risks, and for extreme deviations. Over the years, aircraft wingspans have each situation is treated as a unique case. The FAA may allow been increasing gradually, and the FAA has developed new sep- operation at airports that do not comply with minimum sep- aration standards to accommodate these larger aircraft. aration distances by evaluating an MOS submitted by the air- The introduction of new large aircraft (NLA) is still in port operator. The objective is to keep the airport/aircraft process and will continue bringing challenges to the aviation operations at a level of safety equivalent to that achieved by industry. NLA will have a significant impact predominantly on standard separations. existing airports, particularly large hubs, due to the aircraft The FAA uses a computer program that considers the rela- passenger capacity, weight, wingspan, length, tail height, and tionship between airplane physical characteristics and the wheelbase. Some of the current airport separations between design of airport elements to show that an MOS provides an runways versus runways, taxiways, taxilanes, moveable and acceptable level of safety for the specified conditions, includ- fixed objects, and taxiways versus taxiways, taxilanes, moveable ing the type of aircraft (FAA, 1989). and fixed objects, etc., may not be adequate to accommodate AC 150/5300-13 also states that values obtained from the the introduction of NLA. Most airports with separations inad- specific equations presented in the next chapter may be used equate to accommodate NLA do not have enough space for to demonstrate that an MOS will provide an acceptable level construction of new facilities or for relocation of existing facil- of safety (FAA, 1989). The criteria are based on engineering ities to comply with current FAA standards. judgment and can only be used to compare taxiway and tax- It is important to emphasize that it is not only NLAs or exist- ilane separations. However, in the context of this study, it was ing aircraft like Lockheed C5 and Antonov AN124 that pose necessary to address separations between runways and taxi- challenges to existing airports; recent and new aircraft, such as ways or taxilanes. There is no procedure in the FAA guidance the Airbus A340-600 and B777-300 ER, require changes in material to evaluate runway separations for the risk of colli- some aspects of airport infrastructure due to their long fuselage sion between an aircraft landing or taking off and a taxiing length and associated long wheelbase. aircraft or an object.