Risk Assessment Method to Support Modification of Airfield Separation Standards (2011)

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c. Using Figure AA-21, enter the wingtip separation and calculate the risk. 5. Using the risk level estimated, compare to 1 × 10−7, the upper probability for risk of major consequences accord- ing to the risk matrix used by the FAA. Section 4—Taxilane to Object Procedure to Estimate Risk of Collision 1. Identify the taxilane and the object separation to be evaluated. 2. Identify the ADG for analysis or the aircraft with the largest wingspan that will be using the taxilane. 3. If the assessment is for a specific ADG, the simplified colli- sion risk plots can be used based only on the taxilane center- line to object separation (Figures AA-22 to AA-27). 4. If the risk assessment involves a specific aircraft, the wingtip separation plot in Figure AA-28 should be used, and, in this case, the following steps will be required: a. Place theaircraftatthetaxiwaycenterline(seeFigure A-9). b. Calculate the wingtip clearance for this situation: where: WD is the distance between the wingtip and the object when the aircraft is positioned at the centerline of the taxilane, CS is the separation between the taxilane centerline and the object, WS is the aircraft wingspan. WD CS WS= − 2 4( ) c. Using Figure AA-28, enter the wingtip separation and calculate the collision risk. 5. Using the risk level estimated, compare to 1 × 10−7, the upper probability for risk of major consequences accord- ing to the risk matrix used by the FAA. Section 5—Runway to Taxiway, Taxilane, or Object The runway/taxiway, runway/taxilane, or runway/object separation has two scenarios: takeoff and landing. For land- ing operations, the analysis is divided into two parts: air- borne (approach) phase and ground (landing rollout) phase. For takeoff operations, the analysis considers only the ground (takeoff roll) phase. In most cases, the runways are used for both landing and takeoff operations, and the analy- sis for takeoff operations will not be necessary because the risk of major lateral deviations during takeoff is lower than the risk during landing. The airborne collision risk during the approach for land- ing is characterized using the FAA/ICAO CRM. A series of plots, one for each ADG, was developed to facilitate the use of this methodology. For the landing ground roll phase, risk plots were derived based on a two-part model: frequency and location. Each plot integrates historical runway veer-off accident/incident rates with veer-off location models to simplify the use of this methodology. Given that the aircraft veered off the runway, the chance that the aircraft deviates more than a certain dis- tance from the runway edge is given by the location model. A-8 Figure A-8. Example of taxilane/taxilane separation analysis for specific aircraft. Figure A-9. Example of taxiway/object separation analysis for specific aircraft.

The combination of the frequency and location models will provide the probability that an aircraft will veer off the run- way and deviate more than a given distance from the run- way edge. Table A-5 provides the average incident rates for landing and takeoff veer-offs. It is also possible to use an alternative approach that may be more accurate but will require intensive calculations and the need to use an electronic spreadsheet or computer software. Details of the second approach can be found in the attachment to this appendix. Subsection 5.1—Landing For landing, it is necessary to estimate two types of risk: the risk of collision during the approach phase before the touch- down and the risk of collision during the ground phase in case the aircraft veers off the runway during the landing rollout. These two risks may be combined to provide the total risk. The veer-off risk is estimated for every landing operation, whereas the airborne risk is computed only for missed approaches under instrument conditions, which are assumed to be the worst scenario. For the airborne phase, because this analysis is intended to evaluate the risk of collision between the approaching aircraft and an aircraft located in a parallel taxiway or an object, the analysis will focus only on the area within the immediate vicin- ity of the runway threshold and touchdown zone. The basis for the analysis is the FAA/ICAO CRM and ranges of −300, 0, 750, 1,500, 3,000, and 4,500 ft along the runway, which were evaluated to develop the curves presented in Fig- ures AA-29 to AA-34. The range corresponding to the nega- tive number represents a distance before the runway end for an approaching aircraft, and positive values are for distances after the runway arrival end. The plots provide the highest probability of collision dur- ing missed approaches under instrument (Cat I or Cat II) con- ditions. Although the CRM was developed in the 1970s and the FAA has made modifications to improve these models, the original CRM will serve as a screening tool to lead to further analysis by the FAA if the risk estimated is within a feasible range for additional analysis. The runs were made with obsta- cles located at various distances from the runway centerline and along the runway length. The following steps apply to the estimation of risk during landings: 1. Calculate the risk during the airborne phase. 2. Calculate the risk during the landing rollout phase. 3. Calculate the total risk during landing. Each of these steps is explained below. 5.1.1—Risk in Airborne Phase (Landing) For this phase, the obstacle to the approaching aircraft is assumed to be another aircraft located in any segment of the parallel taxiway. This is a conservative assumption because, in most cases, the obstacle will be an aircraft moving on a par- allel taxiway and the obstacle will have a small length com- pared to the total runway length. Figure A-10 presents a typical scenario for this type of analy- sis. The plots are based only on the horizontal separation between the runway and taxiway centerlines, and the vertical separation is already considered in the plots presented in this section. A-9 Table A-5. Average probability of occurrence by type of incident (U.S. data—1982 to 2009). Type of Incident Probability Landing veer-off (LDVO) 1 per 837,002 landings Takeoff veer-off (TOVO) 1 per 3,860,665 takeoffs Figure A-10. Example of runway/taxiway separation analysis.