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104 Table 10-6. Fleet design arrestor bed for 10.5.4. Braking Effects glass foam arrestor system. The assumed braking factor of 0.25 is applied to the main Nose-Gear Ultimate Load Criterion gear as a coefficient of friction, which helps to slow the air- Bed Dimensions 14.1-in. depth craft. However, this may not be a valid assumption for the 400 ft long engineered aggregate systems. Because the aggregate is loose Aircraft Exit Speed (knots) Stopping Distance (ft) and rolls easily over itself, it may not provide a good base beneath the tire for braking. This "bottoming" behavior, CRJ-200 70+ 335 when the material is compacted in a thin layer beneath the B737-800 63 400 tire, will require more experimentation to better understand B747-400 39 400 and accurately represent within the APC. However, in the absence of data to the contrary, a braking factor of 0.25 is likely still a reasonable, conservative assumption. would require 922 feet to decelerate from 70 knots. Per typical design practice for an EMAS, the bed length may be specified 10.5.5. Short Landings such that all aircraft satisfy the minimum 40-knot exit speed requirement. For comparison with the other alternatives, the Short landings involving an aircraft touch down inside the bed designs in the table assume a 400-ft length. At this length, arrestor bed were not simulated. However, the potential for the B737-800 and B747-400 would have maximum exit speeds short landings presents an additional issue for consideration. of approximately 63 and 39 knots, respectively. The former sat- The basin geometry of the arrestor concept would force the isfies the requirements of AC 150-5220-22a, while the latter is aircraft to roll up the decline slope in the reverse direction just below the 40-knot minimum. from normal, acting as a ramp that would cause a strong load to the landing gear. This issue could be eliminated by only partially recessing the bed, as in the ideal EMAS design cases. 10.5.3. General Observations Retaining walls or berms would be required for confining the Figure 10-24 and Figure 10-25 give sample output plots aggregate and allowing the bed to maintain its shape in an from the APC for the B737 arrestor bed case for the limit and above-grade orientation. With a nominal bed thickness of ultimate design cases, respectively. The setback for the bed is 20 in., it is unlikely that this would present a significant obsta- given in negative x-distance values, with the arrestor bed cle to implementation. beginning at x = 0. As shown, the deceleration and nose gear drag loading are highest just after entry into the bed, when 10.6. Estimated System Cost the nose wheel is located at 70 ft into the bed. Here the bed and Upkeep depth has increased to a maximum, but the forward velocity is still high. Due to the rate dependent nature of the loading, 10.6.1. Installation Process the drag load peaks here and tapers steadily as the aircraft The engineered aggregate concept would require excava- decelerates. tion of an arrestor bed basin with a depth nominally equiv- From a standpoint of mechanical efficiency, this behavior alent to that of an EMAS bed. This basin may or may not is less desirable than an ideal constant deceleration rate. As require paving before being filled with aggregate. However, the aircraft speed decreases, the arrestor becomes ever less the below-grade nature of the basin would require drainage efficient, stretching out the end of the arresting process. from the bed to be included in the design using standard From a standpoint of safety, this behavior requires a suit- roadway engineering practices. The basin would be filled able design criterion to be developed with regard to design with aggregate using earth-moving heavy equipment. If a exit speeds. If, for example, a bed has been designed for a par- reinforced turf layer is used, the turf would be grown ahead ticular aircraft at a 70-knot overrun speed, the landing gear of time, then cut into segments and placed atop the bed with loading would reach a maximum redline value at about 70 heavy equipment. knots. If that aircraft were to overrun the arrestor at 80-knots The loose aggregate solution offers the advantage of in an actual event, damage or failure of the landing gear could construct-in-place simplicity that could produce installa- occur. A second possible scenario would involve a short tion cost savings over a traditional EMAS. It reduces site landing, when the aircraft touches down in the arrestor bed. preparation and eliminates block manufacturing, place- A suitable criterion could require higher design speeds to ment, and joint sealing. Additionally, the durable nature of ensure a margin of safety; however, the current EMAS advi- the engineered aggregate means that the bed filling process sory circular does not specifically contain such a provision. will not require special care to be taken; this will further
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105 Aircraft Velocity and Bed Profile 80 Aircraft Speed Upper Surface of Arrestor Speed [knot] and Depth [in.] 60 Lower Surface of Arrestor 40 20 0 -20 -100 0 100 200 300 400 500 Nose-Wheel Location [ft] Aircraft Decleration 1 Aircraft Deceleration 0.8 Deceleration [g] 0.6 0.4 0.2 0 -100 0 100 200 300 400 500 Nose-Wheel Location [ft] Landing Gear Forces - NOSE STRUT 40 Nose Gear Drag Nose Gear Limit Load 30 Nose Gear Ultimate Load Force (kip) 20 10 0 -10 -100 0 100 200 300 400 500 Location [ft] Figure 10-24. Limit criterion engineered aggregate arrestor design plots for B737-800 showing speed (top), deceleration (middle), and nose-gear load (bottom).
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106 Aircraft Velocity and Bed Profile 80 Aircraft Speed Upper Surface of Arrestor Speed [knot] and Depth [in.] 60 Lower Surface of Arrestor 40 20 0 -20 -50 0 50 100 150 200 250 300 350 400 Nose-Wheel Location [ft] Aircraft Decleration 1.2 Aircraft Deceleration 1 Deceleration [g] 0.8 0.6 0.4 0.2 -50 0 50 100 150 200 250 300 350 400 Nose-Wheel Location [ft] Landing Gear Forces - NOSE STRUT 40 Nose Gear Drag Nose Gear Limit Load 30 Nose Gear Ultimate Load Force (kip) 20 10 0 -10 -50 0 50 100 150 200 250 300 350 400 Location [ft] Figure 10-25. Ultimate criterion engineered aggregate arrestor design plots for B737-800 showing speed (top), deceleration (middle), and nose-gear load (bottom).
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107 speed the installation process. However, turf preparation Table 10-7. Estimated costs to establish and placement would be additional tasks not required for engineered aggregate arrestor, 150 x 300 ft, the current EMAS design. assuming survey average costs for current EMAS, units of millions USD. 10.6.2. Cost to Establish System Cost Category Engineered Current Aggregate System EMAS A preliminary estimate was made for the cost to establish an Lower Upper Bound Bound engineered aggregate arrestor system. It must be noted that Site Preparation $ 1.08 $ 2.16 $ 2.17 the cost estimate from this section is only a basic approxima- Installation $ 3.61 $ 3.61 $ 6.03 tion for the purposes of comparing the different arrestor alter- natives. The cost estimate is based on a mixture of information Cost to Establish $ 4.69 $ 5.77 $ 8.19 from the manufacturer, the airport survey, and FAA Order Percent of EMAS 57% 70% 5200.9. To develop a more accurate estimate of the costs to install such a system, it is recommended that a detailed cost quote be sought from a firm qualified to undertake an instal- In addition to the tables in this section, longer-term life-cycle lation effort. Where possible, the methodologies used were issues could also be considered. FAA Order 5200.9 includes a consistent with the prior survey information collected regard- standard 10-year replacement interval for an EMAS, which ing the existing EMAS (Section 3.5). translates into present-value life-cycle costs. Such a replace- The costs may be broken into two major categories: site ment could arguably be unnecessary for this arrestor concept preparation and installation. The site preparation costs were (Section 10.6.4). Eliminating the assumed 10-year replacement estimated for two cases. The engineered aggregate arrestor could effectively trim about $2.6M off present-value life-cycle would use a basin for the arresting materials rather than a flat costs (based on the EMAS replacement cost estimates of the runway-type surface as is used for the current EMAS. The bot- survey). tom of the basin could either be paved or earthen. Drainage, excavation, and leveling would be required for either option. Assuming that a full-paved surface is not provided under the 10.6.3. Maintenance bed, the cost for site preparation was assumed to be reduced Maintenance for the engineered aggregate concept would be by half; this value was used for the lower-bound cost estimate relatively simple, and should be limited to standard grounds- for the system. If a full-paved surface is provided, identical to keeping measures for the protective turf layer. Drainage of the that of an EMAS, then the preparatory costs were assumed to area to prevent standing water is required and periodic inspec- be the same as for an EMAS; this provided the upper-bound tions would be advisable to ensure that no issues arise due to cost estimate. seasonal weather changes. Due to the lack of joints, blocks, and The installation cost estimate was separated into specific degradable materials, many protective measures used in cur- materials and general installation labor needs. Because these rent EMAS construction would not be necessary. Therefore, costs were specific to the engineered aggregate arrestor con- the predicted maintenance needs are lower than for that of the cept, they do not have a direct connection to any prior EMAS existing EMAS. data. Discussions with the manufacturer produced cost esti- mates for the engineered aggregate, reinforced turf cover layer, and geo-textile/geo-plastic layers. Where applicable, Table 10-8. Estimated costs to establish materials included freight costs for trans-Atlantic shipping. engineered aggregate arrestor, 150 x 300 ft, The labor costs were based on estimates from the manufac- assuming Order 5200.9 costs for current turer established from similar installation efforts. EMAS, units of millions USD. Finally, the site preparation and estimated EMAS costs were computed in two ways: (1) assuming average survey Cost Category Engineered Current Aggregate System EMAS costs from this research, and (2) assuming FAA Order 5200.9 Lower Upper costs. The final cost estimates for both options are given in Bound Bound Table 10-7 and Table 10-8, respectively. Site Preparation $ 0.34 $ 0.68 $ 0.68 Using the survey cost assumptions of Table 10-7, a 300-ft Installation $ 3.61 $ 3.61 $ 3.83 arrestor bed would cost between 30% and 43% less than the Cost to Establish $ 3.95 $ 4.29 $ 4.50 current EMAS. If the Order 5200.9 costs are assumed, the cost Percent of EMAS 88% 95% advantage drops to between 5% and 12% (Table 10-8).