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Page 17
Suggested Citation:"ICAO Rationale." 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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Suggested Citation:"ICAO Rationale." 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 19
Suggested Citation:"ICAO Rationale." 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 20
Suggested Citation:"ICAO Rationale." 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 20

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

17 obtain information on causal and contributing factors (visi- bility, taxiway surface conditions, etc.) to develop a frequency model to estimate the chance of taxiway veer-off incidents occurring under certain conditions. In addition, it was initially thought that lateral deviation information for those incidents should be collected to develop location models, particularly for aircraft that departed the paved taxiway area. With these two models, it would be possible to estimate the likelihood of an incident occurring with a lateral deviation greater than a certain distance from the taxiway edge. Although enough information was available from the data collection exercise to develop frequency models, information on incident location and deviations was not available. A couple of observations led to the conclusion that a two- part frequency/location model was not the best approach for modeling taxiway veer-off incidents and the possibility of air- craft collisions with other aircraft and fixed or movable objects. The first observation was that historical taxiway collision events were not related to taxiway deviations; in almost every case, the collision occurred because another aircraft or mov- able object was inside the wing path of the taxiing aircraft. The second observation was that when a taxiway veer-off occurred, the aircraft stopped immediately after it departed the paved taxiway area. Therefore, the combination of frequency and location mod- els will only help to estimate the probability that the aircraft will stop off the taxiway paved area, and it will not be possible to quantify the risk of wingtip collisions associated with large deviations to evaluate the taxiway separation. In addition, very few incidents were found to occur when the pilot had control of the aircraft. For taxilanes, even the frequency model could not be developed due to the lack of cases with which to build a database. The alternative selected was to use the taxiway deviation models developed by FAA/Boeing (Scholz, 2003a and 2003b). The approach and models proved to be in line with the goals for this project. Appendix G provides a summary of the methodology and results from the FAA/Boeing studies. The first report describes the study and deviation models for John F. Kennedy International Airport (JFK), the second describes the study for Ted Stevens International Airport (ANC), and the third combines the deviation models from both studies and describes the models for risk of wingtip collision between two aircraft and between an aircraft and an object. The colli- sion risk between two taxiing aircraft can be estimated using the mean wingtip distance between the two aircraft, in other words, the wingtip clearance when both aircraft are located on the taxiway centerlines. The deviations at each airport were extrapolated to more extreme deviations as they could happen for significantly higher numbers of event exposures, for example, 106 − 109 taxiway operations. Based on the extreme value limiting assumption, absolute deviations were extrapolated using the 700 most extreme deviations at ANC and 200 most extreme deviations at JFK. The model resulting from the analysis was in the fol- lowing general form: where y is the specified threshold of exceedence, p is the probability estimate of exceeding the threshold y distance from the centerline, and λ, δ, n, and c are extrapolation parameters for the model. Cohen-Nir and Marchi (2003) conducted another study using the data collected from JFK and ANC. In processing the JFK deviation data, they identified some problems with the data collected. Several large deviations that would have put the B-747 outside of the taxiway were recorded in a very short period of time. One of the lasers used in the FAA experiment had gone out of service, and all subsequent unusually large deviations occurred with only one laser in service and no abil- ity to measure speed, wheelbase, or direction of travel. Because of the anomalies associated with the data collected from JFK, only the models developed for ANC were used in the airfield separation analysis methodology for this study. To avoid collision between two taxiing aircrafts with WS1 and WS2 wingspans, the combined deviations need to satisfy the following equation: where TTWY-TWY is the required separation between the taxiway centerlines and d1 and d2 are the deviations of each aircraft, as shown in Figure 10. A collision between an aircraft wingtip and a fixed or mov- able object can be avoided if: where TTWY-OBJ is the separation between the object and the taxi- way centerline, WS is the wingspan, and d is the aircraft deviation from the taxiway centerline. This situation is presented in Figure 11. One important observation when using the models devel- oped with data gathered from ANC and JFK is that taxiway centerline lights were available in all taxiway sections that were monitored. Therefore, probability distributions characteriz- ing those deviations rely on the conspicuity of taxiway center- lines. An important risk control recommendation when using T WS dTWY-OBJ > +2 T WS WS d dTWY-TWY > +( ) + +1 2 1 22 p n c y c = − − + −⎛⎝⎜ ⎞⎠⎟⎡⎣⎢ ⎤ ⎦⎥ ⎛ ⎝⎜ ⎞ ⎠⎟ − 1 1 1 1 exp λ δ

18 this methodology is the assurance that taxiway centerlines will be evident under any conditions when using the taxiways. Taxiway/Taxiway Separation The analysis procedure for taxiway/taxiway separation is based on the models presented in the FAA/Boeing study to assess taxiway/taxiway collision probabilities, which are based on the wingtip separation of two aircraft located at the center- lines of two parallel taxiways (Scholz, 2005). In this approach, lateral deviations were split into two halves, and the devia- tions from the first half were randomly paired with deviations from the second half. In all, 6,157 pairs of d1 + d2 were obtained. For each set of such pairs, the extreme value extrap- olation method was applied to the absolute value of⎟ d1 + d2⎟ to obtain estimates of deviation probabilities. To correct for random splitting and pairing effects, the process was repeated 500 times, and a combined estimate with confidence bounds was obtained. Table 4 presents the probability results, which were applied to the procedure to evaluate taxiway/taxiway separations described in this report. Table 4 presents the probability of wingtip collision based on the separation between taxiway cen- terlines and the wingspan distance. To facilitate the analysis, plots were developed for each Aircraft Design Group (ADG) based on the maximum wingspan for the specific ADG. Although the data used to develop the lateral deviation mod- els were collected for the B-747 only, the same models were used to develop risk plots for aircraft belonging to other ADGs. This is a conservative assumption because smaller deviations are expected for smaller aircraft. The data presented in Table 4 were converted to a plot based on the wingtip separation, which is presented in Figure 12. Figure 10. Taxiway/taxiway separation. Figure 11. Taxiway/object separation. Table 4. Required separation between taxiway centerlines.

The same taxiway/taxiway separation models are used to evaluate taxiway/taxilane separations. The estimates of risk for this situation are considered conservative since the speed of aircraft is usually slower in taxilanes and deviations are expected to be smaller than they are for taxiways. The extrapolation of risk for wingtip separations larger than 25 ft may lead to inaccurate results and very low risks. The standard separation for ADG V is 53 ft; based on the models, the risk for such a condition is lower than 1.0E-15, or one event in one quadrillion operations. This is no sur- prise, as there are no reports of wingtip collisions between two aircraft on parallel taxiways. In the accident and incident data collected, no record was found for wingtip collisions between two aircraft in parallel taxiways, and therefore the level of protection provided by the standards may be considered very high. From the point of view of risk and based on the records of incidents and accidents, the worst credible consequence expected for wingtip collisions of two taxiing aircraft is air- craft damage. In this case, according to the risk matrix recom- mended by the FAA, the risk is acceptable if it is less than one in 10 million operations (1.0E-07), the same collision risk probability level used by ICAO in CRM analysis. Taxiway/Object Separation Using the models based on ANC data developed by FAA/ Boeing (Scholz, 2003a), the wingtip collision risk can be derived based on the wingtip separation from an object when the air- craft is located at the taxiway centerline. Using this approach provides more flexibility to the analysis for specific aircraft wingspans, rather than considering the largest wingspan in the ADG. As shown in Figure 11, wingtip separation is the dis- tance of the object from the centerline of the taxiway less half of the operating aircraft wingspan. Collision happens only if the deviation of the aircraft exceeds this distance to the side of the taxiway where the object is present. One simplifying assumption is necessary: the wingtip deviation distribution is the same as the aircraft lateral deviation distribution from the taxiway centerline. Using data from the FAA study, it is possible to estimate the probability of taxiway-object collision based on the mean wingtip separation; in other words, the separation when both aircraft are located at the respective taxiway centerlines (Scholz, 2003a). The basic assumption in this case is that lateral devia- tions are similar, independent of the type of aircraft. Although this cannot be proved using existing data, the assumption is conservative because the data used to model risk were gathered for large aircraft. The resulting model based on taxiway deviation data col- lected at ANC is presented in Figure 13. Taxilane/Taxilane Separation Aircraft deviation data or studies were not available to develop probability models for taxilane deviations. The data collection exercise carried out for this research identified a few taxilane veer-off incidents, and insufficient information was available to develop any lateral deviation models. Records of taxilane accidents demonstrate that, in most cases, another air- craft or movable object was parked or located inside the taxi- lane OFA. Therefore, the occurrence could not be considered a “taxilane deviation” case, and it could occur independently of the existing taxilane separation. 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 5 10 15 20 25 Taxiway/Taxiway Wingtip to Wingtip Separation (ft) R is k pe r O pe ra tio n Figure 12. Taxiway/taxiway collision risk. 19

20 FAA established separation criteria for taxilanes and taxi- ways and made changes in 1989 (FAA, 1989). These stan- dards are based on aircraft design categories. There is no ref- erence describing the quantitative basis for the criteria, and it is likely that engineering judgment was used to define those standards. Both the FAA and ICAO recognize that aircraft deviations in taxilanes are usually smaller than those occurring in taxi- ways. Aircraft taxiing in taxilanes are moving at very low speeds, and pilots are usually very focused on parking operations in areas where movable objects are common. For the approach presented in this report, the ratios of wingtip separations of taxiway/taxiway to taxilane/taxilane for each ADG were calculated. As shown in Table 5, the ratios var- ied from 0.75 for ADG I to 0.58 for ADG V and VI. These ratios were used to adjust the models used for taxi- way/taxiway separations developed by the FAA. The rationale is that the risk for veering off a taxilane OFA should be simi- lar to or lower than the risk of veering off a taxiway. Therefore, the wingtip to wingtip separation distances associated with each level of risk for taxiways was adjusted using the ratio cor- responding to the ADG. For example, a ratio of 0.58 was applied to the model for lateral deviations on taxiways for ADG V. The resulting model is illustrated in Figure 14. The plot shows the risk trend for taxilane/taxilane separa- tion of ADG V aircraft based on the mean wingtip separation (i.e., based on the wingtip distance when both aircraft are located on the centerlines of parallel taxilanes). The standards for taxilane separations are considered conservative, given the lack of recorded taxilane incidents associated with lateral deviations. Figures similar to Figure 14, based on separation between taxilane centerlines, were developed for each ADG based on the ratios presented in Table 6 and the maximum wingspan for the ADG. Taxilane/Object Separation Similar to the approach for taxilane/taxilane separations, adjustment factors were applied to the FAA/Boeing models for taxiway/object wingtip separations (Scholz, 2003a). The ratios are the same as those used for the taxilane/taxilane model because the standard wingtip separations between two aircraft and between an aircraft and an object are similar according to AC 150/5200-13 (FAA, 1989). The standard minimum wingtip to object separation for ADG V in parallel taxilanes is 31 ft and, based on Figure 15, the risk of wingtip collision is lower than 1.0E-09. 1.E-16 1.E-08 1.E-14 1.E-06 1.E-10 1.E-04 1.E-12 1.E-02 0 10 403020 50 60 Taxiway/Object Wingtip Separation (ft) R is k pe r O pe ra tio n Figure 13. Taxiway/object collision probability based on wingtip separation. ADG Distances in ft Item I II III IV V VI Taxiway/Object Wi ngtip Separation 20 26 34 44 53 62 Taxilane/Object Wi ngtip Separation 15 18 22 27 31 36 Ratio 0.75 0.69 0.65 0.61 0.58 0.58 Table 5. Taxiway/object and taxilane/object mean wingtip separations for aircraft design groups.

Next: Chapter 3 - Data for Modeling Aircraft Deviations »
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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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