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OCR for page 25
25 RSA Combining this approach with the longitudinal distribution y2 approach and the possibility of multiple obstacles, the risk for x y1 accidents with severe consequences can be estimated using the following model: x (dist. to obstacle) - e fi ) y N (e -byci m - bym Psc = e - a( xi + i ) n Figure 31. Modeling consequences. 2 i =1 where shown in Figure 32b, and the speed is high, severe consequences N = the number of existing obstacles; are expected. If the obstacle is off the orange and yellow zones, a, n = regression coefficients for the x-model; and no consequences related to that obstacle are expected. = the location parameter for obstacle i. In Figure 33, Obstacle 1 is located at a distance x1, y1 from the threshold and has dimensions W1 L1. When evaluating the Multiple obstacles may be evaluated using the same prin- possibility of severe consequences, it is possible to assume this ciple. A shadowing effect also is taken into account when part will be the case if the aircraft fuselage or a section of the wing of obstacle i+1 is behind obstacle i. Because it is assumed that close to the fuselage strikes the obstacle at high speed. Thus, aircraft will travel in paths parallel to the runway centerline, it is possible to assume the accident will have severe conse- any portion of the obstacle located behind at a distance greater quences if the y location is between yc and yf, as shown in the than i is disregarded from the analysis. figure. Based on the location models for lateral distance, the A quantitative assessment of risk likelihood will be obtained probability the aircraft axis is within this range can be calcu- as a function of operating conditions (aircraft, weather, runway lated as follows: distances available) and RSA dimensions and conditions (pres- ence of EMAS, presence, location, size, and type of obstacles, - bym etc.). For the analysis, the user may select the alternative to m e - byc -e f Psc = evaluate the probability that an aircraft will go off the RSA or 2 that severe consequences will take place. where Psc = the probability of high consequences; Implementation of Approach b, m = regression coefficients for the y-location model; yc = the critical aircraft location, relative to the obstacle, The implementation of the proposed approach is best ex- closest to the extended runway axis; and plained using one example. Figure 34 depicts an area adjacent yf = the critical aircraft location, relative to the obstacle, to the runway end with two obstacles. The area isn't necessar- farther from the extended runway axis. ily the official airport RSA but any available area that can be Wingspan (WS) 1/3 WS a) Obstacle Wingspan (WS) 1/3 WS b) Obstacle Figure 32. Lateral location versus consequences.

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26 Lateral Location Probability Distribution Psc sc Obstacle y w1 yc yf Figure 33. Modeling likelihood of striking an obstacle. Cliff Obstacle 1 - Building x1 Category 2 x x Obstacle 2 - Tree Category 4 y x2 Figure 34. RSA scenario with obstacles. used by an aircraft overrunning the runway end. The example combined in a manner similar to that for the analysis without shows the safety area surrounded by a cliff limiting its bound- obstacles; however, the safety area is transformed to account aries. Obstacle 1 is not frangible and is classified as a Category 2 for the presence of the obstacles, as shown in Figure 35. obstacle (e.g., building), maximum collision speed of 5 knots, The area used to calculate the probability as a function of located at distance x1 from the runway end. For this obstacle, the aircraft stopping location is shown in green. It should be the maximum speed without severe consequences is estimated noted that the safety area in the shadow of Obstacle 1 is much to be 5 knots. A second obstacle is a small size tree classified as larger than that for Obstacle 2 for three reasons: Category 4, maximum speed of 40 knots, and located at dis- tance x2 from the runway end. The remaining safety area is de- 1. Obstacle 1 is wider than Obstacle 2. fined by the cliff surrounding the RSA and such boundary is 2. The maximum speed for striking Obstacle 1 (Category 2) classified as Category 1, maximum speed of 0 knots. is lower than that for Obstacle 2 (Category 4). The typical aircraft deceleration in unpaved surfaces is 0.22g, where g is the acceleration due to gravity (32.2 ft/s2). Using Table 6. Obstacle categories. the relationship between acceleration, velocity, and distance, can be calculated as shown in Table 6. Obstacle Max Speed (ft) The values presented will be used to reduce the safety Category (knots) (See Figure 30) 1 0 0 area so that only the effective portion where the aircraft may 2 5 20 stop without severe damage is considered in the analysis. To 3 20 80 perform the analysis, the frequency and location models are 4 40 320