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OCR for page 135
135 The installation process will likely affect the performance substantial departure from the current EMAS design in terms properties of the material because compaction by construc- of mechanical loading and the materials used. tion equipment and foot traffic will take place. Since this process is inevitable, it would be advisable to create several 11.8. Summary larger-scale testbeds using the anticipated installation process and equipment. Tests could then be conducted on these small The aggregate foam arrestor concept was found to have a beds using larger test apparatuses than those of the current mechanical response similar to a crushable foam material, research. From the results, a revised material model could be except with depth-varying properties. It would absorb energy calibrated, and the arresting performance reassessed. In this from the aircraft primarily though material compaction rather manner, a calibrated prediction capability would exist for the than through displacement. An aggregate foam arresting bed as-installed material. Thereafter, installation of actual arrestor would be constructed using a shallow basin of the material beds could proceed per the developed methodology with high topped with a reinforced turf cover layer. confidence of the final system performance. The material is closed-cell glass foam that provides inher- Such tests could be conducted in conjunction with the ent moisture and chemical resistance, and improved handling cover layer design (next section). durability as compared with cellular cement. Manufacturer information indicates that long service life is possible, poten- tially eliminating the standard 10-year replacement assumed 11.7.2. Cover Layer Design in FAA Order 5200.9. Water immersion must still be avoided, The cover layer options that have been discussed include so the preferred design approach would use a sealed plastic reinforced turf with or without additional geo-textile/ geo-membrane envelope coupled with standard drainage geo-plastic layers. Additional testing and modeling would provisions. be required for whichever method is selected since the Installation of the system would likely be simpler and less membrane behavior of a cover layer will affect the dynamic expensive than the current EMAS since placement of blocks mechanical performance of the arrestor bed. The cover and sealing joints are both unnecessary. Heavy equipment layer performance should be further characterized under would place the material in the bed basin and top it with frozen conditions since it will be exposed to such condi- reinforced turf. Geo-membrane and geo-textile layers, as tions even where the foam aggregate layer of the system is applicable, would be placed and joined manually. The arrestor waterproofed. basin could be constructed with or without paving, which could provide for preparatory cost reduction. In order to pre- serve material gradation and prevent over-compaction dur- 11.7.3. Braking Dynamics ing installation, an appropriate installation process would be The research performed has identified that braking in the required. aggregate foam material may require further study. Of the The cost to establish such a system would be nominally three major concepts evaluated, braking dynamics appear 40% to 53% of the survey cost of the existing EMAS, which likely to affect the aggregate foam concept the most. Because provides the most substantial estimated cost reduction of the the depth-varying properties of the material have demon- candidates evaluated; much of the cost reduction is due to the strated tire "flotation" at mid-depth bed penetrations, there markedly less expensive aggregate foam material. Life-cycle is room for potential depth shifts of the tires due to braking. costs could be further reduced due to longer bed life. Main- If they occurred, such shifts could lead to excessive landing tenance needs appear to be simplified, requiring standard gear loads. grounds-keeping measures, but no block or joint repairs. The current modeling method of the APC makes simplify- The APC predictions for the aggregate foam arrestor show ing assumptions regarding this phenomenon that are suffi- fairly constant deceleration throughout the arrestment with cient for a concept-level evaluation. However, to ensure little speed dependence, which are desirable characteristics. accuracy of design predictions, some additional tests would The depth-varying material characteristic produced a unique be beneficial. One method would involve using a one-wheel "floating" rut depth that was not observed for the other mat- bogy apparatus fitted with brakes and a load measurement erials evaluated. This characteristic allowed each tire to settle system that is towed through the material. to its own natural depth in the material, creating more even load distribution among tires of different sizes. Bed lengths on a per-plane basis were nominally 15% longer 11.7.4. Full-Scale Testing than for the current EMAS. However, the multi-aircraft A full-scale aircraft overrun test of a foam aggregate design case demonstrated the best one-size-fits-all perfor- arrestor bed is advisable because this concept represents a mance from among the three candidate systems. A 400-ft
OCR for page 135
136 arrestor bed demonstrated 70+ knot exit speeds for the B737- tive model to match the response. Characterization would 400 and CRJ-200 and a 56-knot exit speed for the B747. The be advisable for the soil layer under various freezing condi- reason for the superior performance appears to be the depth- tions to assess the impact on arresting dynamics. Additionally, varying nature of the material. investigation should be made regarding the basin geometry to The floating rut characteristic also gave rise to oscillating determine whether above- or below-grade construction is tendencies in which the plane could exhibit porpoising behav- preferable. Because the material performance can be affected ior. This was mitigated through appropriate bed geometry by size gradation and compaction, development of a suit- design. However, additional investigation would be required able installation process would be required. This installa- to establish the effects during short landings and in cases tion process would be coupled with a predictive model where the pilot applies intermittent braking. matching the as-installed material characteristics. Finally, Transition to a fielded system would require finalizing a full-scale testing is advisable for evaluation of the complete composite turf cover-layer design and calibrating a predic- system.