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22 CHAPTER 2 Literature Review 2.1. General by the aircraft tire was identified as a potential engine inges- tion hazard. Additional, limited documentation was reviewed The literature review began with research into past arrestor for gravel arresting system research undertaken in the United system development and current arrestor requirements by Kingdom (UK) (4). the FAA. This research identified a number of technical areas The emphasis on development shifted, taking a direction of interest, which then turned into second- and third-stage that focused on crushable materials such as phenolic foam literature research. Altogether, over 130 references were and cellular cement (sometimes referred to as aerated/foamed collected and reviewed on a spectrum of topics pertinent to cement/concrete). White and Agrawal (5) outline that the arrestor systems. A summary of important information gleaned advantage of the crushable foam materials is predictability of the from this research is given in this section; the annotated drag load imparted on the landing gear and constant mechani- bibliography (Appendix A) contains a more complete listing cal properties over a broad temperature range. Cellular cement of documents. Table 2-1 summarizes the scope of the literature eventually became the material of choice due to its near-zero review. rebound after crushing and chemically inert composition. Computer modeling and simulation have played a substan- 2.2. Historical Aircraft Arrestor tial role throughout the development of these passive arresting Research and Development technologies. Initially, predictive codes had been developed Substantial historical work has been done in the area of for the modeling of taxi, takeoff, and landing on soil landing arrestor system development. FAA and industry documenta- strips (6). These methods were adapted to the soft-ground tion was reviewed, sometimes requiring retrieval in hard-copy arresting concept to evaluate different materials. Landing gear form due to the age of the material. loads for the main and nose gear struts have been a historical Civil arrestor research in the 1980s examined the usage of area of scrutiny (2). Loadings to the nose gear in particular materials such as clay, sand, gravel, water, and foam to develop were noted, since materials like gravel could jeopardize the a "soft ground" arrestor, as discussed by Cook (2). These integrity of the gear and cause potential collapse. materials are passive in nature, with no moving parts. The One military code, FITER1, was adapted for civil arresting arrestment of the aircraft is accomplished by these materials applications and called "ARRESTOR," documented by Cook as they impart a drag load on the landing gear, absorbing et al. (7). It featured three aircraft (B707, B727, and B747) and kinetic energy and bringing the aircraft to a stop. Cook dis- could be used to model different foam arresting bed geome- cusses how materials such as clay, sand, and water were not tries. Heymsfield et al. (8) have recently used the ARRESTOR appropriate for various reasons. For sand and clay materials, code to perform sensitivity analyses on various parameters. the mechanical behavior could not be consistently predicted This program has presently been superseded in capability by due to sensitivity to moisture content. Water could be shown a proprietary predictive code used by the EMAS manufac- to perform well mechanically, but only at speeds less than turer, ESCO. 50 knots, with the additional disadvantages of attracting Active arresting systems that use military-type friction brakes waterfowl and freezing in cold climates. Subsequent research (BAK-12, etc.) have been adapted to civilian aircraft in the past. was undertaken to examine different variants of foam arrestor Documentation in our research has been sparse on this subject, (3), from which it was determined that a cementitious foam however, and the majority of knowledge gained on the sub- provided advantages over polymer foams. Gravel spray caused ject has been through discussions with ESCO and by reviewing