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OCR for page 108
108 Compared with crushable materials, there are no material will be exposed to such conditions even where the aggregate breakdown concerns regarding freezethaw exposure. How- layer of the system is waterproofed. ever, it is presently unknown how substantially ice may affect the inter-particle friction of the aggregate; changes to the fric- 10.7.2. Bottoming and Braking Dynamics tion could alter the dynamic arresting response. Additionally, dirt entrainment over time could solidify the bed if the drainage The research performed has identified that tire bottoming approach is used for handling precipitation. Therefore, the and braking in the aggregate material require further study. waterproofed option is recommended to minimize the mainte- The current modeling method of the APC makes simplifying nance and performance impacts of water, soil, and ice. assumptions regarding both phenomena sufficient for a concept-level evaluation. However, to ensure accuracy of design predictions, some additional tests would be required. 10.6.4. Replacement One method could involve using a one-wheel bogy apparatus fitted with brakes and a load measurement system that is All materials included in the engineered aggregate concept towed through the material. are expected to last for the 20-year life cycle prescribed in the EMAS Advisory Circular, if not longer. It is not anticipated that replacement of the bed would be required after 10 years, 10.7.3. Aggregate Ingestion as anticipated for an EMAS in FAA Order 5200.9. While the presence of a cover layer appears to offer mitiga- tion of aggregate spray, it is unclear as to the level of residual 10.6.5. Repair hazard remaining. The spraying gravel from the nose-wheel may or may not pose an ingestion hazard for the aircraft After an overrun event, the rut areas would require repair. engines. If an ingestion hazard is present, it is unclear what prac- The aggregate remaining in the ruts would be reusable, and tical risk this presents with regard to engine damage, potential fresh aggregate would likely need to be added to bring the beds fire, and so on. A limited test series using a one-wheel bogy back to level condition. The damaged cover layer, if used, apparatus may be able to answer the issue of ingestion likeli- would need to be removed from the rut areas and replaced with hood. Discussions with aircraft manufacturers or other FAA fresh material. If the cover layer is turf, it is possible that safety personnel should provide insight into the practical risks replacement turf could be planted at another area of the facil- in the event that ingestion occurred. ity on initial installation of the system. When replacement needs arise, this turf could be harvested for immediate use to prevent delays that would accompany growing fresh turf. 10.7.4. Requirements and Standards Due to the speed dependence issues, development of a prac- 10.7. Transition to Fielded System tical design criterion for the design exit speed may be advisable In order to transition the engineered aggregate concept to to ensure a factor of safety for the landing gear. a fielded system, the following additional development steps may be advisable. 10.7.5. Full-Scale Testing A full-scale aircraft overrun test of an engineered aggregate 10.7.1. Cover-Layer Design arrestor bed is advisable because this concept represents a Several cover-layer design concepts are possible: substantial departure from the current EMAS design in terms of mechanical loading and the materials used. 1. Thin sealant, 2. Asphalt skim coat, 10.8. Summary 3. Reinforced turf layer, 4. Reinforced turf with geo-textile fabric, and Of the candidate systems evaluated, the engineered aggregate 5. Turf/geo-textile/plastic composite cover layer. arrestor concept is most dissimilar to the current EMAS tech- nology. It uses a hard spherical aggregate that primarily under- Only concept three was modeled during the research effort. goes displacement rather than the compaction that occurs with Additional testing and modeling would be required for crushable foams like cellular cement. The arresting bed would whichever method is selected since the membrane behavior be constructed with a shallow basin of the material topped with of a cover layer will impact the dynamic mechanical perfor- a reinforced turf cover layer. mance of the arrestor bed. The cover layer performance should Since the engineered aggregate material is not a porous foam, be further characterized under frozen conditions because it it is durable against moisture and other environmental factors,

OCR for page 108
109 including water immersion. However, while the material itself arrestor designs are still feasible. Arrestor bed lengths for does not degrade with such exposures, the mechanical response individual aircraft were nominally 15% longer than for the changes, exhibiting an increase in arresting loads when wet. To current EMAS technology due to the speed-dependent prop- maintain predictable arresting performance, the material would erties of the arrestor. Additionally, the multi-aircraft design require the use of protective plastic geo-membranes and typical case for the concept showed the weakest performance among drainage provisions. The material characteristics indicate that the three candidate systems. Bed designs that were safe for the long service life is possible, potentially eliminating the standard smallest aircraft did not exert a strong deceleration load on 10-year replacement interval assumed in FAA Order 5200.9. the largest aircraft. Installation of the system would likely be simpler and less The speed dependence of the engineered aggregate bed expensive than for the current EMAS since placement of would require development of a new design criterion. Over- blocks and sealing joints is unnecessary. Heavy equipment runs exceeding the rated exit speed or short landings into the would place the material in the bed basin and top it with pre- bed could result in overloads to the landing gear if the speed grown reinforced turf segments. Geo-membrane and geo- dependence is not considered in the design process. Arrestor textile layers, as applicable, would be placed and joined man- designs that include provisions for these events would neces- ually. The arrestor basin could be constructed with or without sarily have gentler decelerations and longer arrestment dis- paving, which could result in preparatory cost reduction. tances than the design cases cited in this research. The cost to establish such a system would be nominally Transition to a fielded system would require finalizing a 57% to 70% of the survey cost of the existing EMAS. Life- composite turf cover-layer design and calibrating a predictive cycle costs could be reduced due to longer bed life. Mainte- model to match the response. Characterization would be nance needs appear to be simplified, requiring standard advisable for the soil layer under various freezing conditions grounds-keeping measures, but no block or joint repairs. to assess the impact on arresting dynamics. Additionally, The APC predictions for the engineered aggregate arrestor investigation should be made regarding the basin geometry show that the deceleration varies throughout the arrestment to determine whether above- or below-grade construction is process, exhibiting a strong dependence on aircraft speed. preferable. Full-scale testing is advisable for evaluation of the This is not a preferable characteristic; however, functional complete system.