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