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Suggested Citation:"Project Goals." National Academies of Sciences, Engineering, and Medicine. 2011. Risk Assessment Method to Support Modification of Airfield Separation Standards. Washington, DC: The National Academies Press. doi: 10.17226/14501.
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Page 11
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Suggested Citation:"Project Goals." National Academies of Sciences, Engineering, and Medicine. 2011. Risk Assessment Method to Support Modification of Airfield Separation Standards. Washington, DC: The National Academies Press. doi: 10.17226/14501.
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Page 10

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C is the clearance between the outer main gear wheel and the taxiway edge (maximum allowable lateral deviation), and Z = safety margin distance (for example, 10.5 m and 13 m for aircraft Codes E and F, respectively). Separation Distance between a Taxilane and an Object For the clearance distance required on an aircraft taxilane, the safety margins are reduced due to the slow taxiing speed of aircraft and the availability of a visual docking guidance system or a marshaller to accurately guide the aircraft. The condition is illustrated in Figure 8. Therefore, for Code E aircraft opera- tions, the C value was reduced to 8 ft (2.5 m) and the safety buffer (Z) was reduced to 25 ft (7.5 m). The formula is the same as the formula for the distance between a taxiway and an object. where STXL-OBJ is the separation distance between the taxilane centerline and an object. In the case of Code F aircraft, again the 1.6-ft (0.5-m) increase in the Z value was engineering judgment to account for the larger wingspan aircraft. For taxiways and taxilanes, the same document establishes that the minimum separation distance is equal to the wingspan plus max lateral deviation plus increment. ICAO Annex 14— Aerodromes (2006b) contains the standards and recommended practices on airport separations. S WS C ZTXL-OBJ = + + 2 11 Figure 7. Taxiway/apron taxiway-to-object geometry. Figure 8. Aircraft stand taxilane-to-object geometry.

• Within 197 ft (60 m) of the runway centerline of a precision approach runway Cat I, II, or III where the code number is 3 or 4; and • Within 148 ft (45 m) of the runway centerline of a precision approach runway Cat I where the code number is 1 or 2. In addition, no mobile object shall be permitted on this part of the runway strip during the use of the runway for landing or takeoff. Separation Distance between a Taxiway and Another Taxiway Taxiways are vital facilities of an airport on and around which certain safety areas must be provided at all times to ensure that a taxiing aircraft does not collide with another air- craft or an object. A primary assumption here is that an air- craft taxiing on a taxiway may deviate from its centerline; thus, using a “permissible” deviation, the taxiway strip width and taxiway width are determined. The taxiway strip, like the run- way strip, should be clear of objects that may endanger taxiing aircraft. Furthermore, all other taxiways and objects that need to be on the operational areas of an airport are built to meet these criteria. For two aircraft traveling in opposite directions on two parallel taxiways, it initially was deemed appropriate to pro- vide for deviations of both aircraft from their respective cen- terlines toward each other. It was considered necessary to ensure that in such deviations there was still an adequate safety margin between the wingtips. While the permissible value of the deviation of an aircraft from the taxiway center- line is one factor, abnormal conditions like steering malfunc- tions, very slippery pavement conditions, low visibility, and poor markings could not be ruled out. Thus, the concept of a safety buffer was introduced to provide an additional safety margin to the separation distance. The safety buffer, Z, is equal to the difference between the half width of the taxiway strip and the semi-wingspan of the largest aircraft in a given category (whose outer wheel is located at the edge of the paved taxiway, in other words, full deviation as permitted and added to the semi-wingspan). The safety buffer illustrated in Figure 6 is determined using the follow- ing equation: where Z is the safety buffer, TCS is the taxiway centerline separation, WS is the aircraft wingspan, and C is the clearance between the outer main gear wheel and the taxiway edge (maximum allowable lateral deviation). When deriving this specific dimension for Code F aircraft, it was considered that the steering mechanism of the Code F air- craft would not be worse than that of the Code E aircraft and, thus, the same deviation value of 15 ft (4.5 m) was retained. However, because it was felt that the wider, swept-back wings of modern aircraft might not permit a pilot in the cockpit to see where the wingtip would be, the safety buffer was increased in proportion to the wingspan increase vis-à-vis that for a Code E aircraft. Separation Distance between Taxiway and Object The separation distance between the taxiway and a station- ary object is specified in order to ensure that a taxiing aircraft’s wingtip does not collide with any stationary object. Because the object is not moving, only one deviation of the aircraft itself is taken into account along with the safety buffer, which includes all other factors that may cause further deviation of the taxiing aircraft. The relationship illustrated in Figure 7 is expressed as follows: where STWY-OBJ is the separation, WS is the wingspan, S WS C ZTWY-OBJ = + + 2 Z TCS WS C= − − 10 Figure 6. Parallel taxiway separation geometry.

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TRB’s Airport Cooperative Research Program (ACRP) Report 51: Risk Assessment Method to Support Modification of Airfield Separation Standards is intended to be used to support requests for modification of standards in those circumstances where the design criteria for separations between taxiways/taxilanes and other taxiways/taxilanes and fixed or movable objects as well as separations between taxiways and runways cannot be met.

The following appendices, included in the pdf and print version of the report, will be helpful in understanding the methodology.

  • Appendix A: Risk Assessment Methodology presents a methodology for five different types of circumstances: taxiway/taxilane to taxiway, taxiway to object, taxilane to taxilane, taxilane to an object, and runway to taxiway/taxilane or object;
  • Appendix F: Aircraft Database Summary presents a summary of aircraft characteristics by model; and
  • Appendix H: Analysis of MOS Cases summarizes information collected in the modification of standards survey and presents results of application of the methodology described in Appendix A to each modification of standards case.

Other report appendices, which are available online only, provide detail and information on the development of the methodology.

In addition, the project developed a

PowerPoint presentation

that may be useful for introducing and explaining the methodology to stakeholders.

In July 2021, an errata was posted for this publication: In Table 7 on page 25, the LDVO coefficient was changed from -3.088 to -13.088. The online version of the report has been corrected.

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