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ACRP study are based on data from veer-off accident/incident reports taken from several countries and for aircraft with max- imum takeoff weight (MTOW) larger than 5,600 lb. The collision risk during the approach phase of landing is modeled for missed approach during instrument approaches under Cat I and II. This is assumed to be the highest risk con- dition, and the phase when the pilot is under visual condi- tions is not modeled in the risk curves presented. CRM risk is estimated for an aircraft located on the cen- terline of a parallel taxiway. The taxiing aircraft is of the same ADG as the approaching aircraft, and the maximum tail height for the ADG is taken to characterize the obstacle located in the taxiway. The same plots may be used to assess risks associ- ated with other types of obstacles at a certain distance from the runway centerline; however, such obstacles must be lower than the maximum tail height of the ADG used to develop the charts. Risk Criteria The suggested risk criteria to use with this methodology are those used by the FAA and represented by the risk matrix shown in Figure A-1 (FAA, 2010). A risk classification (high, medium, or low) is provided based on the combination of severity and likelihood. Severity is the measure of how bad the results of an event are predicted to be and is defined as the worst credible conse- quence that may take place for risk associated with a given hazard. Likelihood should be considered only after determin- ing severity, and at the same time, likelihood should not be considered when determining severity. Definitions for each level of severity and consequence are presented in Tables A-3 and A-4. Two cases can serve as examples: (1) risk of collision between an aircraft landing and an aircraft located in a parallel taxiway and (2) risk of wingtip collision between aircraft taxiing in parallel taxiways. The first step is to determine the worst cred- ible consequence for each of these events. The worst credible consequence for runway veer-offs in most cases is hull loss and multiple fatalities, which is classified as catastrophic. Accord- ing to the FAA risk matrix, such a condition is acceptable only if it occurs less than once every 100 years or less than once in 25,000,000 departures. For the second case, based on historical data of accidents and incidents, the worst credible consequence may be classi- fied as major. In this case, the risk is acceptable if it is expected to occur about once every year or every 2.5 million departures (4 Ã 10â7), whichever occurs sooner. The ICAO Obstacle Clearance Panel (OCP) has set the acceptable risk of collision during the approach phase at a value of one in 10 million operations (1 Ã 10â7). Since this is the risk level used to establish most of the airfield design stan- dards defined by the FAA, and this methodology will serve as a screening tool, this criterion is used in this screening method- ology. However, the risk classification based on the risk matrix defined by the FAA must be highlighted when submitting MOS for FAA approval (FAA, 2010). A-3 Figure A-1. FAA risk matrix (FAA, 2010).
How to Use This Methodology This methodology is intended to serve as a screening tool for analysis associated with requests for modification of stan- dards (MOS) related to airfield separations. In no case should this methodology be used to justify changes to current FAA design standards for airfields. Conclusions drawn based on this methodology shall be subject to further analysis and approval by the FAA before the non-standard separation is adopted. Additional mitigating procedures and risk control measures may be required to achieve an acceptable level of safety for operations in the airfield. This appendix provides a step-by-step methodology to eval- uate the risk of aircraft collision associated with airfield sepa- rations in parallel segments. The methodology uses informa- tion on existing or planned conditions and provides estimates of risk. The level of risk should be compared to acceptable levels of risk recommended by the FAA. Different procedures are used depending on the type of analysis desired, as explained in ensuing sections. The method- ology is divided into five basic sections, and each section pres- ents the procedure to assess the risk for a specific scenario. The outcome of the analysis is the risk of collision between two aircraft or between an aircraft and an object, depending on the type of analysis required. To determine the appropriate sec- tion containing the methodology and step-by-step procedure for the desired type of analysis, the two types of structures must be selected from Table A-1. For example, to analyze the separation between a taxiway and an object, the user should use the procedure described in Section 2 and the risk plots presented in Figures AA-8 to AA-14 presented in the attach- ment to Appendix A. When describing the procedure, some acronyms are used to characterize specific parameters. Definitions of these acronyms can be found within the section in which they appear. When an equation is included in the procedure, a number located in parenthesis to the right of it is used to reference the equation in the text. Many of the risk plots presented in this methodology should be used for specific Aircraft Design Groups (ADGs) as defined in FAA Advisory Circular (AC) 150/5300-13 (FAA, 1989). Table A-2 presents a summary of tail height and wingspan ranges for each ADG. As mentioned earlier, the outcome of the analysis is the risk of collision. Both the FAA and the International Civil Avia- tion Organization (ICAO) have been using a collision risk value of one in 10 million operations (1 Ã 10â7) as the acceptable level during the approach phase under instrument conditions. This is also the level criterion suggested when applying this method- ology. Limitations This methodology should be used carefully, and the user must be aware of its limitations. This methodology can be applied to estimate the risk of collision between two aircraft or an aircraft and an object only on straight parallel segments of taxiways and taxilanes. Also, because the taxiway deviation models used in this study were developed from lateral devia- tion data collected on taxiways with centerline lights, the conspicuity of the taxiway/taxilane centerline is an added risk mitigation measure that should be used when justifying an MOS request for separations that do not include runways. Although lateral deviation data in taxiing operations used to develop the risk plots were measured only for the B-747 aircraft, it is assumed that smaller aircraft have lateral devia- tion distributions that have smaller ranges. Thus, the model applied can be considered conservative when applied to smaller aircraft. The FAA/ICAO Collision Risk Model (CRM) during missed approach was developed based on data for two- and three- engine jet airplanes. The veer-off models developed under this A-2 Taxiway Taxilane Runway Taxiway Section 1 (Figures AA-1 to AA-7) Section 1 (Figures AA-1 to AA-7) Section 5 (Figures AA-29 to AA-54) Taxilane Section 1 (Figures AA-1 to AA-7) Section 3 (Figures AA-15 to AA-21) Section 5 (Figures AA-29 to AA-54) Object Section 2 (Figures AA-8 to AA-14) Section 4 (Figures AA-22 to AA-28) Section 5 (Figures AA-29 to AA-54) Table A-1. Procedure selection. Table A-2. Airplane Design Groups (FAA, 1989). Group # Tail Height (ft) Wingspan (ft) I < 20 < 49 II 20 to < 30 49 to < 79 III 30 to < 45 79 to < 118 IV 45 to < 60 118 to < 171 V 60 to < 66 171 to < 214 VI 66 to < 80 214 to < 262
