Risk Assessment Method to Support Modification of Airfield Separation Standards (2011)

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The taxiway/taxiway separations were based on taking the most critical aircraft that would be using the taxiways (gener- ally, the aircraft with the largest wingspan) and placing its main gear on the edge of the usable taxiway. The separation between the taxiway centerlines then could be calculated by adding half the width of each taxiway to twice the length of the wingspan that extended beyond the taxiway plus a safety factor. Likewise, the taxiway/object separations were based on taking the most critical aircraft that would be using the taxi- ways and placing its main gear on the edge of the usable taxi- way. The separation between the taxiway centerline and the object could then be calculated by adding half the width of the taxiway to the length of the wingspan that extended beyond the taxiway plus a safety factor. In the 1980s, in response to feedback from the aviation com- munity, the FAA undertook an effort to consolidate the numer- ous design Advisory Circulars. In 1983, AC 150/5300-12, Air- port Design Standards—Transport Airports, was published (FAA, 1983). This consolidated many of the design standards for transport aircraft into one document. In 1989, the FAA pub- lished AC 150/5300-13, Airport Design (the fourth document with this title), which consolidated the design standards for all airports except heliports and sea plane bases into one document (FAA, 1989). This publication grouped standards according to the ARC, consisting of a letter and a Roman numeral. The let- ter indicates the aircraft approach category and relates to the FAA Flight Standards approach speed group of the design air- craft (as used in terminal instrument procedures [TERP]). Gen- erally, runway standards are related to the approach speed. The Roman numeral relates to the airport design group and the air- craft wingspan of the design aircraft. It is possible to have the approach category based on one design aircraft and the aircraft design group based on a different design aircraft. Appendix 9 of AC 150/5300-13 provides the design rationale for separations associated with taxiways and taxilanes, except for those between a runway and its parallel taxiway (FAA, 1989). A number of parameters are contained in this appendix. To maintain airport operational capacity, the taxiway system should be designed so that aircraft can maintain an average speed of 20 mph. The parameters affecting taxiway separations for other than parallel taxiways are wingspan and wingtip clearance, with the need for wingtip clearance being driven by the fact that pilots of most modern jets cannot see their wingtips from the cockpit. Appendix 9 then provides the following information on separations: • Taxiway to taxiway centerline (see Figure 1): Separation is calculated based on 1.2 times the wingspan of the most demanding aircraft plus 10 ft (wingtip clearance). where STWY-TWY is taxiway to taxiway centerline separation and WS is wingspan of the most demanding aircraft. • Taxiway centerline to object (see Figure 2): Separation is calculated based on 0.7 times the wingspan of the most demanding airplane plus 10 ft (wingtip clearance). where STWY-OBJ is taxiway centerline to object separation. • Taxiway object free area (OFA): Width is equal to twice the taxiway centerline to object separation. S WS ftTWY-OBJ = +0 7 10.  S WS ftTWY-TWY = +1 2 10.  6 Figure 1. Wingtip clearance, parallel taxiways.

In August 1959, the FAA issued Airport Engineering Data Sheet Item 24 as a revision to the table that appeared in the 1949 publication (see Table 1). This data sheet reduced the number of airport classes from eight to five. The data sheet stated, “In order to assure maxi- mum safety and the economical and efficient use of the air- port site, careful consideration must be given to the clearance and separation between the various aircraft operating areas” (FAA, 1959, p. 1). It states that the distances established are recommendations and further states “increases to these dis- tances may be desirable in some cases, necessary in others” (FAA, 1959, p. 1). It appears that although some classes were consolidated and renamed, only minor changes were made to the actual distances. No specific information is provided as to how these separation standards were defined. This document was issued 2 months before the “jet” age in U.S. commercial aviation began. In October 1959, nonstop transatlantic flights with Boeing 707s were initiated between New York’s Idlewild Airport (now known as John F. Kennedy International Airport) and Europe. In 1961, the FAA published the third document titled Airport Design (Federal Aviation Agency, 1961). The revised standards (see Table 2) were no longer based upon the type of service, but rather on the runway length. In the early 1960s, the FAA initiated the Advisory Circular publication series. Although the information in these publica- tions was advisory, the airport standards contained in them became mandatory when federal aid was used for airport development. As documents were updated to conform to the new publication system, there was a tendency to develop Advi- sory Circulars containing design information for a specific type of airport. For example, AC 150/5300-1, VFR Airports, applied to airports that were intended to have operations by general aviation aircraft during visual meteorological conditions (FAA, 1963). AC 150/5300-4A, Utility Airports, was issued to provide guidance and standards for airports that intended to serve air- craft weighing 12,500 lb or less (FAA, 1968). During the late 1960s and through the 1970s, there were several Advisory Circulars published on specific aspects of airport design for airports intended to serve air carriers. Sub- jects of these Advisory Circulars included such things as run- way geometry, taxiways, surface gradient and line-of-sight, and jet blast. During this period, there were no funds allocated for the research and development of design standards. The runway/ runway and runway/taxiway separation standards contained in these publications were based on the experience gained during the post-World War II period, including experience with the precision of navigational aids such as instrument landing systems (ILSs), the ability of pilots to stay on center- line, and air traffic control considerations. In the 1960s, the FAA’s Flight Standards organization and the ICAO Obstacle Clearance Panel (OCP) developed the Collision Risk Model (CRM)3 for ILS operations. The CRM was based on actual observation of 2,500 aircraft on an ILS precision approach to a runway. Four observations were made for each aircraft’s approach. This model was used to define the area that needed to be protected on an airport when an aircraft was making an ILS approach. The runway/taxiway separation also took into account the possibility of an aircraft on landing rollout or takeoff roll veering off the runway. Additional information on the CRM is provided in Appendix B. 5 Type of Service Runway Centerline to Taxiway Centerline Centerline of Parallel Runways for Contact Operations Centerline of Parallel Taxiways Taxiway Centerline to Aircraft Parking Areas Taxiway Centerline to Obstacle Secondary 150 300 125 100 75 Local 250 500 200 175 100 Trunk 350 500 275 240 150 Continental 400 700 300 260 175 Intercontinental 450 700 325 280 200 Table 1. Minimum clearance standards of airports (ft) (FAA, 1959). Runway Length Runway Centerline to Taxiway Centerline Centerline of Parallel Taxiways Taxiway Centerline to Aircraft Parking Areas Taxiway Centerline to Obstacle 1,600–3,200 150 100 100 75 3,201–4,200 250 200 175 100 4,201–6,000 400 300 250 200 6,001–7,500 400 300 250 200 7,501–10,500 400 300 250 200 Table 2. Airport design and clearance recommendations (ft) (Federal Aviation Agency, 1961). 3 FAA developed the CRM (ICAO, 1980) approach with the University of Oklahoma and input from other countries represented on ICAO’s OCP.