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OCR for page 46
46 CHAPTER 7 Identification and Initial Assessment of Alternatives 7.1. General Approach instance, within the domain of crushable foams, there is minimal difference in the dynamic arrestment process The identification and ideation of different arrestor alter- expected for different materials. A crushable polymer foam, natives was approached through several avenues: a crushable cellular cement, and a crushable metal foam will Vendors with design ideas were contacted, function in a fairly similar manner where their compressive Internal brainstorming meetings were undertaken, (crush) strengths and other material properties are the Technical literature was reviewed, and same. There are many foams available, and delving into the General web searches were undertaken. nuances that separate them has minimal benefit. This is especially true when compared with the differences, for Following the identification of alternatives, taxonomy was example, between crushable foam arrestors and gravel developed to classify the different options and provide clarity of arrestors. The dynamic responses of these arrestors are quite comparison. An initial assessment of the alternatives led to the different. selection of several promising candidates for detailed research in The classification structure that was developed divided the the experimentation phase of the effort. Please note that the TRB alternatives by dynamic behavior. Table 7-1 shows the classifica- and ACRP do not endorse specific products. Research results tion for the different concepts in abbreviated form. Section 7.4 are provided to assist in the evaluation of options by others. expands upon this table and indicates those alternatives that were selected for the detailed research. Subsequent chapters dis- 7.2. Vendor-Developed Alternatives cuss the detailed research findings. The FAA William J. Hughes Technical Center supplied a The systems were grouped into two broad classifications: list of manufacturers that had previously contacted them with passive systems, with no moving parts, and active systems, different proposed arrestor concepts, materials, or designs. with moving mechanical components. Thereafter, they were Those companies were subsequently contacted to determine grouped into four categories based on the energy absorption if there was still active interest in arrestor development. approach used. The maturity of the different concepts varied widely. Some companies simply had an alternative crushable material that 1. Crushable Materials. Crushable materials absorb energy could be used in a similar manner as the current cellular through permanent deformation of the material, either cement design, but with potentially improved durability. through brittle fracture or plastic deformation. Other companies had more well-developed ideas, including The aircraft is engaged at the tire/ground interface. patents (Appendix A), design drawings, and calculations for Current EMAS and phenolic foam beds are examples of energy absorption. The broad range of maturity required an crushable material systems. initial assessment of the alternatives (Section 7.4) in order to 2. Displaceable Materials. Displaceable materials do not determine the most promising concepts for inclusion in the undergo a physical breakdown of the material itself. Instead, experimentation stage of the research. the material is moved, and energy is absorbed through momentum transfer or the internal friction of the material. 7.3. Classification of Alternatives The mechanical behavior of these materials is fundamentally When trying to sort through the myriad of ideas devel- dissimilar from the crushable material and results in a dif- oped and identified, it became apparent that some of the ferent dynamic system response. concepts did not differ as greatly as initially assumed. For The aircraft is engaged at the tire/ground interface.

OCR for page 46
47 Table 7-1. Classification of alternatives. Category Subcategory Technology Covering Current Detailed Layer Civil Use Research Applicable Crushable Crushable foams Cellular cement Yes Yes Materials (current EMAS) Phenolic foam Yes Metal foam Yes Glass foam Yes Yes Depth-varying Yes Yes foam Loose crushable Glass aggregate Yes Yes fill foam PASSIVE SYSTEMS Crushable Pumice Yes aggregates with aggregate binder Styrofoam Yes aggregate Hollow Yes microspheres Ceramic glass Yes aggregate Displaceable Soil Clay Materials Sand Yes Loose Gravel Yes aggregates Engineered Yes Yes Yes aggregate Fluids Water or Glycol Yes pond Cable/Net Braking devices Hydraulic brake Yes Systems Water impeller Textile ACTIVE SYSTEMS Eddy-current brake Engagement Over-wing barrier devices nets Landing gear Yes strut engagement Mechanical Surface of spring- Yes Surface supported panels Systems Water ponds are included in this category, as are beds Military arrestors, such as the BAK-12, are included in of loose aggregate (gravel, etc.) where the individual this category. pieces are compacted and moved, but not generally 4. Mechanical Surface Systems. Mechanical surface systems broken. use mechanical components that absorb energy as the 3. Cable and Netting Systems. Cable and netting systems aircraft rolls across different movable panels at ground utilize two components: (1) braking devices to absorb level. energy (water, hydraulic, electro-magnetic, etc.), and The aircraft is engaged at the tire/ground interface. (2) an engagement system that connects the aircraft and The concepts in this category are few, encapsulated the braking device (cables, nets, etc.). in the referenced U.S. Patent 6,969,213 (32); neverthe- Engagement can be done with an aircraft tail hook, the less, this approach represents a categorically different main gear, or the wings. arresting method.