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39 CHAPTER 5 Sensitivity Analysis ESCO generously supplied the research team with plots of installation cost, but not recurring maintenance costs. There are velocity versus distance for several EMAS design cases. The two possible premises for the cost estimates: FAA Order 5200.9 cases were best-performance arrestor designs for three aircraft: and the cost data from the Survey of U.S. Airport Operators the CRJ-200, B737-800, and B747-400. From these plots, a (Chapter 3) (30). Results presented in this section used normal- substantial amount of data was derived for the sensitivity ized cost values, and therefore did not depend on which cost analysis. Technical details for the following subsections are estimates were used. found in Appendix C. 5.2. Results and Discussion 5.1. Introduction 5.2.1. Design Cases The sensitivity analysis investigated the dependence of cost For the purposes of the sensitivity analysis, standard design and reliability on mechanical parameters. Life-cycle issues, conditions were assumed: a 75-ft setback, no reverse thrust, and such as the durability of the bed, were not considered as part a 0.25 braking friction coefficient. The decelerations of the of the sensitivity analysis. Only the parameters discussed in three design aircraft--CRJ-200, B737-800, and B747-400-- Chapter 4 that have bearing on the mechanics of arrestment were significantly different. Figure 5-2 shows the mean decel- are included in this sensitivity analysis (Table 5-1). erations and error bars representing one standard deviation, the data of which was extracted manually from ESCO-provided 5.1.1. Reliability deceleration plots. As a brief aside, we note here that the B747 has less than The reliability of an EMAS refers to the percentage of over- one-half of the deceleration of a CRJ-200, which underscores runs arrested for a given configuration. Reliability is inherently an inherent limitation of crushable material arrestors. Even a probabilistic measure, and there are two empirical studies in the best-case designs (shown here), they simply are not as that have been conducted to establish relationships between effective on some plane types as others. Section 7.5 gives cumulative percentage of overruns and exit speed. The first further explanation regarding the mechanical basis for the of these was discussed in DOT/FAA/CT-93/80 (5). A more gross difference in deceleration. recent study was conducted by the ACRP (24). Both studies For the sensitivity analysis, exit speed was varied in the were considered as part of this analysis. vicinity of the 40-knot minimum and 70-knot standard exit Data was extracted from Figure 1 in DOT/FAA/CT-93/80 speeds. This showed the impact on cost and reliability if the and from an overrun database compiled by the ACRP. The requirements of the EMAS advisory circular were shifted by two sets of data were used to create cumulative distribution 5 to 10 knots. All three design aircraft were used for the study, plots of overruns as a function of exit speed. As Figure 5-1 and the results associated with each aircraft type were normal- shows, the trends of the older and newer data differ. ized and averaged. 5.1.2. Cost 5.2.2. Reliability The cost parameter in Table 5-1 refers to the total initial To assess the sensitivity of arrestor bed reliability to changes cost for an EMAS, including the preparatory paving cost and in the critical values of exit speed, cumulative percentage of
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40 Table 5-1. Sensitivity analysis parameters. Input Parameters to Vary Output Parameters from Models Exit speed arbitrary Stopping distance Geometry variations Reliability · Width · DOT/FAA/CT-93/80 exit speed probability curve · Setback · Revised exit speed probability Aircraft type curve · CRJ200 ER Cost · B737-800 · FAA Order 5200.9 · B747-400 · Survey data Aircraft braking condition · Braking · Skidding · Free-rolling overrun occurrences were calculated for exit speeds 5 and systems are described in detail in Section 7.7. It is noteworthy 10 knots greater than the current minimum and standard exit that, despite that fact that active arrestors are mechanical speeds of 40 and 70 knots. The basis of the calculations was systems with a number of moving components, they are able to the revised overrun probability curve. The results are shown achieve significantly higher reliability than an EMAS designed in Figure 5-3 and Figure 5-4. for the standard 70-knot design exit speed. The passive nature From Figure 5-3, an EMAS with a 40-knot design exit of the EMAS concept does not assure inherently superior speed would likely arrest fewer than 50% of aircraft over- reliability over an active arrestor. This comparison comes with runs. If the design exit speed is increased to 50 knots, the a caveat that the reliability numbers for the active systems EMAS would likely arrest 60% of aircraft. As shown in presume solid engagement with the aircraft; with civil aircraft, Figure 5-4, an EMAS with a 70-knot design exit speed would engagement proves to be the most challenging aspect of using arrest 80% of all aircraft. Increasing that design speed to active arrestors (Chapter 14). 80 knots would likely result in 88% of overrunning aircraft Furthermore, the function of active arrestor systems can be being arrested. tested periodically as the system ages to confirm correct oper- By way of comparison, active arrestors used for military ation. As was mentioned in Section 3.6, the reliability of the aircraft achieve a minimum of 97.5% reliability. These active current EMAS design after installation is uncertain because Figure 5-1. Exit speed probability curves.
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41 1.2 Deceleration [g's] 1.0 0.8 0.6 0.4 0.2 0.0 CRJ200 ER B737-800 B747-400 Increasing MTOW Figure 5-5. Cost impact of Figure 5-2. Mean EMAS deceleration increasing the minimum exit of design aircraft. speed requirement. 5.2.3. Cost As shown in Figure 5-5 and Figure 5-6, cost sensitivity was assessed in the vicinity of the 40-knot minimum and 70-knot standard exit speeds. Sensitivity is expressed in terms of arrestor bed costs for the current design exit speeds. The estimated cost of an EMAS with a 50-knot design exit speed is 60% greater than the cost of an EMAS with a 40-knot design exit speed. In addition, the estimated cost of an EMAS with an 80-knot design exit speed is about 30% greater than the cost of an EMAS with a 70-knot design speed. It should be noted that these cost impacts would apply to any type of Figure 5-3. Reliability impact EMAS and would not be confined to the current EMAS of increasing the minimum exit design. speed requirement. Thus, if the 70-knot exit speed in the EMAS advisory circular were increased to 80 knots, in order to return to the the internal condition cannot presently be tested. In light of targeted reliability of 90%, future construction costs could be the survey data, it must be acknowledged that it is possible that expected to increase by 30%, on average. degradation of the cellular cement could decrease the per- As shown in Figure 5-7, cost is essentially proportional to formance--and hence, reliability--of EMAS without being bed length. From a physics standpoint, the distance that a tire detected. rolls through an EMAS is proportional to absorbed energy, Figure 5-6. Cost impact of Figure 5-4. Reliability impact of increasing the standard exit increasing the standard exit speed. speed.
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42 Cost Length Energy Absorbed (Speed)2 Therefore, Cost (Speed)2 Figure 5-7. Proportionality of cost and exit speed, per-plane basis. which is in turn proportional to the square of the exit speed. The actual ratios in Figure 5-5 and Figure 5-6 were based Thus, it was expected that the cost would be essentially propor- on the cost data from the Survey of U.S. Airport Operators tional to the square of the exit speed. For example, the ratio and the cost estimates in FAA Order 5200.9 (30). Therefore, of the square of 502 to 402 is 1.56, or 156%, which is consistent the cost increase proportionalities shown are supported by cost with the cost ratio in Figure 5-5. data and the physics of aircraft arrest.