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Suggested Citation:"Contents." 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 8
Page 9
Suggested Citation:"Contents." 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 9

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The issue was given to a multidisciplined group called the Airport Obstructions Standards Committee (AOSC) to resolve. After much deliberation, this group grandfathered runways and taxiways meeting existing separation standards into the airport design standards and determined further analysis was neces- sary. In the interim, separation standards for construction of new runways or taxiways were increased to 500 ft for Cat II/III operations involving Group V aircraft and for Cat I approaches involving Group VI aircraft. For Group VI aircraft making a Cat II/III approach, a separation of 550 ft would be required between new runways and taxiways. The AOSC work is docu- mented in AOSC Decision Document #04 (AOSC, 2005). ICAO Rationale To harmonize the development of aviation (including air- ports) globally, the ICAO was established toward the end of World War II with the signing of the Convention on Interna- tional Civil Aviation (also known as the Chicago Convention) on December 7, 1944. Since then, ICAO has developed and updated international specifications on all aspects of aviation. In Annex 14 to the Chicago Convention, first published in 1949, the ICAO promulgated specifications on airport design and operations (ICAO, 1949). ICAO has kept pace with technological developments in the aircraft industry and kept Annex 14 current to provide to its Contracting States (or Member Nations) the minimum safety specifications for designing new airports and upgrading exist- ing ones to handle succeeding generations of newer, larger, and heavier aircraft. Many of the current standards and recommended practices (SARPs) contained in Chapter 3 of Annex 14 were defined by the Aerodrome Reference Code Panel (ARCP) in 1981 (ICAO, 1981). As part of the process to define a new reference code for airports, ARCP also undertook a fundamental review of SARPs based on a more rational approach. In 1990, Annex 14 was separated into two volumes (ICAO, 1990). Volume I now contains international specifications on aerodrome design and operations only, and Volume II deals with the design of heliports. Statistics have shown that approach and landing is the most critical phase of a flight because the aircraft must follow a pre- cise and stable approach path despite the challenging circum- stances that characterize this flight phase—aircraft engine power is at its minimum, weather conditions on the ground may pose difficulties for landing, and habitation and land devel- opments surrounding airports can be significantly impacted by deviations from the approach path. Moreover, aircraft may sometimes touchdown before the runway arrival end or reject the takeoff and depart the runway if something goes wrong. These are the main reasons runways have a safety area. The FAA refers to this area as an RSA, whereas the ICAO defines an area with an equivalent function, con- sisting of the runway strip plus the runway end safety area (RESA). The objective of this area is to reduce the risk of dam- age to aircraft running off a runway and protect aircraft flying over a runway during takeoff or landing operations. The safety area applies not only to the airspace on or around an airport but also to the ground itself. Runway Strip Width ARCP’s basis for the specifications of the graded portion of the strip is the acceptable risk of occurrence of aircraft veer- offs. From the information available, there seems to be no defined basis for the development of the specification of the full strip width for the protection of over-flying aircraft using the CRM. ARCP identified the following factors in the definition of the strip width: • Aircraft approach speed • Wingspan • Aircraft mass • Type of approach (visual or instrument) Statistical data on aircraft veer-off events presented at the 8th Air Navigation Conference in 1974 were the basis for the 8 Figure 4. Wingtip clearance for taxilanes.

analysis. The frequency of veer-offs exceeding a given dis- tance from the runway centerline were determined according to the type of operation and class of aircraft. To maintain a safety area around the runway, a runway strip width of 984 ft (300 m) is specified for instrument runways, and a runway strip of 492 ft (150 m) is speci- fied for smaller, non-instrument runways. The strip is symmetrically located on either side of the runway center- line. The central portion is required to be graded to certain specified slopes so that it is less likely that an aircraft will suffer substantial damage during runway veer-offs. These requirements are used to establish a safety area around the runway in which only those objects that must be located there are permitted, subject to the applicable frangibility criteria being met. Airfield Separations Every aircraft landing or taking off on a runway must pro- ceed along a system of taxiways. During these movements, aircraft should be protected by wide, obstacle-free areas; thus, even the circulation areas (taxiways and aprons) must be located at suitable distances apart and at a specified distance from the runway. The early specifications in ICAO Annex 14 were based on the layouts of airfields that existed at the time—mostly military airfields—that were deemed to be examples of best practice. It is not uncommon to see older airports with runways from 98 to 197 ft (30 to 60 m) wide. Similarly, the circulation taxiways were separated from the runways so that aircraft on the taxi- way did not cause major risk to aircraft landing or taking off from the runway. Since then, numerous ICAO studies, undertaken with assis- tance from ICAO technical panels and study groups, have fine- tuned the specifications as actual aircraft performance results and airport experience have become available. Improvements in aircraft manufacturing technology, better training, and the availability of modern visual aids have also contributed to this fine-tuning. The broad principles governing airfield separations are explained in greater detail in the following sections. Separation Distance between a Runway and a Taxiway A parallel taxiway is located such that no part of the largest aircraft expected to operate on the parallel taxiway would penetrate into the adjacent runway strip. This is intended to accommodate any potential veer-off of a landing aircraft when the taxiway is being used and also to provide a sterile area, free of obstacles that may endanger an aircraft executing a missed approach or balked landing maneuver. The separation distance is expressed as follows: where SRWY-TWY is the distance between the centerlines of a runway and a parallel taxiway, SW is the runway strip width, and WS is the aircraft wingspan. Figure 5 depicts the main factors in the ICAO rationale. Although a link taxiway for entry into and exit from the runway is located within the runway strip, whenever the runway is in use, an aircraft on a link taxiway is required to stop and hold at a distance of 295 ft (90 m) from the run- way centerline (ICAO Code E) (ICAO, 2004). For aircraft designated ICAO Code F, where the wingspan is greater than 213 ft (65 m) but not more than 262 ft (80 m), the holding position location is at a distance of 350 ft (107 m) from the runway centerline (ICAO, 2004). This distance may need to be increased for certain operational conditions, and this minimum holding distance should be reviewed if it interferes with radio navigational aids provided for the runway. The separation distances between a runway and fixed objects other than visual aids required for air navigation purposes are the following (ICAO, 2006a): • Within 254 ft (77.5 m) of the runway center line of a pre- cision approach runway Cat I, II, or III where the code number is 3 or 4 and the code letter is F; S SW WS RWY-TWY = + 2 2 9 Figure 5. Runway/parallel taxiway separation distance.

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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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