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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 StandardsâTransport 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.
