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1 SUMMARY Developing Improved Civil Aircraft Arresting Systems Introduction Passenger aircraft can overrun the available runway area during takeoff and landing, creating accidents involving aircraft damage and loss of life. The increasing speed and weight of modern passenger aircraft require longer runways, but many airports are land-locked by surrounding buildings, bodies of water, or geographic features that prevent runway extension. These facts, combined with the increasing volume of air traffic, make overrun risks more relevant today than when many U.S. airports were first constructed. To mitigate overruns that take place, the Federal Aviation Administration (FAA) now requires that all runways have a Runway Safety Area (RSA) beyond the normal runway surface, typically with a length of 1,000 ft. This area provides an additional deceleration zone for planes that cannot stop before reaching the runway end. However, some airports do not have sufficient land area to meet this requirement. In such cases, the airport can install an arrestor system that provides an equivalent to the standard RSA. This use of arrestor systems permits the airport to satisfy the FAA requirement within a smaller land space. Currently, the only type of civil aircraft arresting system that meets FAA standards is an Engi- neered Material Arresting System (EMAS). A number of airports have installed EMAS, and these arrestors have demonstrated the ability to bring aircraft to a stop in several overrun incidents. However, various issues and concerns regarding the current EMAS technology exist. At many airports, the land area at the end of a runway is inadequate to accommodate a full-sized EMAS system. The costs associated with acquiring and installing an EMAS are high due to the labor-intensive assembly process. The durability of the system over time is unknown, and no tests are currently available that can verify that an installed EMAS maintains its original design characteristics. The objective of this research was to advance the development of alternative civil aircraft arresting systems to safely decelerate aircraft that overshoot the runway. The research sought to evaluate alternatives to the current EMAS technology, with the goal of finding options that might offer better performance, lower cost, or higher durability. The research involved technical and non-technical aspects such as candidate system evaluation, cost estimation, standards-related investigation, and airport surveys. A number of candidate systems and materials were evaluated; some were similar in function to the current system, while some were categorically different. EMAS Nomenclature In general, the FAA advisory circular regarding EMAS provides latitude regarding the materials and construction methods that may be used; multiple EMAS designs could exist. At present, however, only one manufacturer has an approved EMAS design, which is the Engineered Arresting

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2 Systems Corporation, or ESCO. In general use, the term "EMAS" has largely become synonymous with this ESCO product. Nevertheless, "EMAS" as a system definition could be expanded in the future to include a number of other arrestors besides this current product. Many of the new arrestor system concepts discussed in this report, if eventually approved and fielded, would qualify as EMAS systems. As such, it is necessary to clarify the nomenclature that will be followed in this report. When technical comparisons are made regarding the design, construction, and performance of "EMAS," the term will refer to the systems presently deployed. When regulatory discussions are undertaken, the term will refer to the general requirements that pertain to the current and future passive arrestor systems. When clarification is required, qualifications such as "current EMAS," "current EMAS design," or "current EMAS technology" will be used to denote the ESCO product. EMAS Description An EMAS is a surface-based arrestor constructed as a large bed that resides in the RSA beyond the end of a runway (Figure S-1). EMAS dimensions can vary considerably, but typical dimen- sions are approximately 300 ft in length by 150 ft in width, with a nominal 75-ft setback from the runway end. Depending on the available space in the RSA, it can be more cost-effective to install shorter EMAS beds with longer setbacks. The current EMAS design features 4-ft by 4-ft blocks of cellular (foamed) cement, usually in one of two compressive strengths. The blocks have narrow gaps between them for venting and drainage, and the tops of these joints are sealed against rain. The depth of the blocks varies depending on the bed design. The sides of the bed stair-step for pedestrian and emergency vehicle access. These side step blocks are not considered in performance calculations for the arresting bed. Prior to installing an EMAS, the site must be prepared with a paved surface that provides a solid foundation for the bed and adequate drainage. Two generations of EMAS are currently installed at U.S. airports. The older JBR-501 design used painted cement board tops for the individual blocks and caulking to seal the joints in between. The newer JBR-502 design uses plastic tops, which do not require painting, and silicone tape to seal the joints. As the only current FAA-approved arresting system, the current EMAS design will serve as a baseline for the arrestor alternatives examined in this research. Figure S-1. EMAS arrestor, MinneapolisSt. Paul (MSP) airport.

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3 Research Approach The research was divided into two phases. The first, the study phase, included research, identification of alternatives, and an initial down-selection of the most promising candidate arrestor concepts. The second, the experimentation phase, focused on evaluating the candidate concepts through testing and modeling. Survey of U.S. Airport Operators To obtain information and opinions regarding the existing EMAS from end-users, a survey was taken of 14 U.S. airports, and site visits were made to three. The information obtained was a mixture of objective data and subjective opinions. The airport operators were asked questions regarding past and present experience with EMAS pertaining to installation, maintenance, overrun performance, and its related costs. Additionally, a smaller survey of pilots and pilot organizations was undertaken. Many suggestions, opinions, and concerns were voiced. However, this summary will focus on the cost components. For baseline cost information regarding EMAS, the study referred to FAA Order 5200.9, Financial Feasibility and Equivalency of Runway Safety Area Improvements and Engineered Material Arresting Systems. It provided guidelines for estimating the costs to establish an EMAS at an airport and for estimating the life-cycle costs of a system. Airport operators were asked to provide the preparatory paving cost and installation cost for each arrestor installed at their airports. Eight airports provided preparatory paving and installation costs for a total of 11 arrestors. By way of comparison, the cost design values according to FAA Order 5200.9 were based on five separate EMAS installations. The mean cost values reported by the airports are shown in Table S-1. They were normalized by the associated pavement and bed areas. The survey costs were corrected for inflation and are expressed in 2007 dollars. For comparison, the suggested values of preparatory paving cost and installation cost from FAA Order 5200.9 are included in the table, in 2007 dollars. As shown, the mean reported paving and installation costs were significantly higher than the design costs estimated in FAA Order 5200.9. Actual costs also appeared to exceed the expectations of FAA Order 5200.9 with regard to annual maintenance. While the survey included more airports than the original data set used to create Order 5200.9, it did not include all EMAS systems installed at U.S. airports. It is possible that the average costs could shift if the remaining airports were included. Although essentially based on anecdotal experiences, the researchers have noted a general lack of awareness regarding the existence, usage, and function of EMAS among airline pilots. This lack of awareness has been observed in both newer and seasoned pilots. It is unclear whether the Table S-1. Normalized mean costs from survey compared with FAA Order 5200.9 (2007 dollars). FAA Order Survey 5200.9 Normalized Preparatory Paving $15 $48 Cost per Square Foot Normalized Installation Cost per $85 $134 Square Foot Cost to Establish EMAS (CTEE) $100 $182 per Square Foot Cost for 150 x 300-ft Bed $4.5M $8.2M

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4 awareness is higher for pilots that typically land at airports with EMAS. Unfortunately, response from the pilot community was limited during the survey process. Review of FAA Parameters The design parameters for an EMAS are based on FAA requirements contained in Advisory Circular 150/5220-22A. These parameters were reviewed to determine their relative impact on the cost, size, and performance of installed arrestors. Exit Speed Exit speed is how fast an aircraft is travelling when it rolls past the end of the runway. The advi- sory circular requires that an EMAS have a standard design exit speed of 70 knots (standard case) and a minimum of 40 knots (non-standard case). In designing an arrestor, the bed construction would be specified to ensure that the aircraft serviced by the runway could be stopped with these exit speeds. The 70-knot standard is intended to enable arrest of 90% of overruns, which historical data indicated was statistically the case. The 40-knot minimum is provided as an exception for airports with highly constrained RSAs. A review of the overrun data collected in a recent ACRP study on aircraft overruns and under- shoots indicated that the 70-knot requirement may no longer be sufficient to reach the intended 90% arrest rate (ACRP Report 3). Analysis of the new data conducted during this research effort suggests that the 90% threshold may have shifted to just above 80 knots. This could affect the design speeds for aircraft arrestors if a 90% criterion is to be maintained. Additional investigation may be warranted regarding the accuracy of the reported data that was used in the assessment. Damage to Aircraft The advisory circular requires that the arrestor not cause structural damage to the aircraft. This mandate has a direct impact on the arrestor's cost and performance. The requirements could be altered to permit overloading of the aircraft landing gear if certain criteria are met. For example, if an aircraft continues to penetrate beyond 75% of the bed length, it could enter an "all or nothing" zone, where the resistance of the bed increases in a final attempt to arrest the air- craft. The landing gear may be damaged, but that may be preferable to the alternative of failing to stop the aircraft. A more robust approach could include an overall risk assessment for the facility and the aircraft serviced by the runway, since the consequences of an unimpeded overrun would not be equal for all facilities. Cost The sensitivity of EMAS cost was assessed in the vicinity of the 40-knot minimum and 70-knot standard exit speeds. The impact of shifting the exit speeds of the advisory circular was assessed. The estimated length and cost of an EMAS with a 50-knot design exit speed is about 60% greater than an EMAS with a 40-knot design exit speed. The estimated length and cost of an EMAS with an 80-knot design exit speed is about 30% greater than an EMAS with a 70-knot design speed. Thus, increasing the exit speed requirements would have a direct cost and size impact on arrestor bed installations.

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5 Commercialization Study A commercialization study was undertaken to assess the requirements for a manufacturer to bring a new arrestor system to market. The study examined necessary steps to obtain FAA approval and identified barriers to market entry. One barrier to entry that was identified is a lack of predictive tools for manufacturers to assess aircraft landing gear loads. Without this predictive capability, it would prove difficult to satisfy the safe-design requirements specified in Advisory Circular 150/5220-22A. The FAA computer program, "ARRESTOR," though an old code at present, can predict arresting distances for various bed geometries. It models a general crushable foam material and permits the user to specify different compression strengths to be modeled. As such, ARRESTOR could serve the up-and-coming manufacturers who do not have in-house predictive codes. ARRESTOR currently contains a limited library of aircraft: the B707, B727, and B747. Only one of these aircraft is still in broad service, the B747. To serve as a modern planning tool, a broader range of aircraft should be considered for ARRESTOR. Alternately, a new design program could be developed. The Arrestor Prediction Code (APC) developed in the current effort could serve as the basis for such a replacement. Identification and Selection of Alternatives The identification of different arrestor alternatives to EMAS was approached through several means. The FAA William J. Hughes Technical Center supplied a list of manufacturers that had previously contacted them with different proposed arrestor concepts, materials, or designs. Those companies were subsequently contacted to determine if there was still active interest in arrestor development. Additionally, the research team reviewed historical arrestor designs and brainstormed to develop new concepts. Following the identification of alternatives, taxonomy was developed to classify the different options and provide clarity of comparison. An initial assessment of the alternatives led to the selec- tion of several promising candidates for detailed research in the experimentation phase of the effort. The maturity of the different concepts varied widely. Some companies simply had an alternative crushable material that could be used in a similar manner as the cellular cement of current EMAS, but with potentially improved durability. Other companies had more well-developed ideas, including patents, design drawings, and calculations for energy absorption. The broad range of maturity required an initial assessment of the alternatives in order to determine the most prom- ising concepts for inclusion in the experimentation stage of the research. Based on the initial screening and down-selection process, four arrestor candidates were selected for the experimentation phase of the effort: 1. Glass foam arrestor concept (passive), 2. Aggregate foam arrestor concept (passive), 3. Engineered aggregate arrestor (passive), and 4. Main-gear engagement active arrestor concept (active). Please note that the TRB and ACRP do not endorse specific products. Research results are pro- vided to assist in the evaluation of options by others. Candidate 1: Glass Foam Cellular glass foam was proposed as a material and would be made as a modification of an existing insulating product (Figure S-2). The material would be used in a large bed either com- posed of blocks akin to the current EMAS design or constructed as a monolithic structure. The

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6 Figure S-2. Cellular glass foam material. chemical makeup and closed-cell properties of glass foam suggested good chemical resistance, excellent durability to the environment, and resistance to water absorption. The potential improvement in durability suggested the possibility of reduced maintenance and replacement needs for an arrestor using this material. The glass foam material offered relatively similar mechanical properties to that of cellular cement because both materials are low-density crushable foams. However, glass foam in general appeared to be less fragile, easier to handle, and potentially more durable than the cellular cement. Additionally, adhesives and moisture sealants are available for glass foams that permit joining and weatherproofing. Because the glass foam material is generally manufactured in blocks sized at approximately 24 18 6 inches, two variants are possible (Figure S-3): 1. Block Method. The block method would use 4-ft square blocks of the foam, analogous to the current EMAS construction approach. The blocks would be constructed by adhering multiple smaller blocks together, followed by the potential addition of top and/or bottom cap materials. The sides of the block could potentially be sprayed with a sealant to further weatherproof the blocks. These blocks would be transported to and installed at a runway in essentially the same manner as the current EMAS beds. 2. Monolithic Method. The monolithic method would be assembled on-site at the runway by stacking and gluing blocks into a single large structure. The final assembly would then be fitted with a continuous top cover layer composed of a roll/spray-on polymer. This layup would preclude the need for joint seams, sealants, and maintenance, which are required for the current EMAS design. Moisture sealing of the vertical sides of individual blocks would be unnecessary. Monolithic layups such as this have been used in building roof applications. Candidate 2: Aggregate Foam An aggregate foam arrestor concept was proposed. The arrestor would use rough-broken foam aggregate made from recycled glass (Figure S-4). The foamed, or aerated, glass material has

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7 Glass Foam Arrestor Concept 1: Block Method Lid Each Major Block is a Glued Assemblage of Seams Between Blocks, Smaller Blocks Sealed with Tape/Caulk Glass Foam Arrestor Concept 2: Monolithic Method Small Blocks Glued Together in Brick Pattern Monolithic Sealant Top Layer without Joints Figure S-3. Glass foam arrestor variants: block method (top) and monolithic method (bottom). Figure S-4. Aggregate foam material: microstructure (upper-left), as a pile (upper-right), and individual pieces (bottom).

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8 Cover Layer of Engineered Turf Aggregate Foam Bed Arrestor Basin Figure S-5. Aggregate foam arrestor concept. nominally 80% void space by volume. Its closed-cell microstructure makes it resistant to water absorption and degradation. An arrestor using the aggregate foam would be constructed by creating a basin and filling it with the material. An engineered turf would serve as a top cover layer for the bed (Figure S-5), which can serve several purposes: 1. Prevent material dispersion due to jet blast; 2. Mitigate material spraying during overrun by an aircraft tire, thus limiting engine ingestion hazard; 3. Regulate water drainage and potential ice crust formation in winter; and 4. Act as a structural component to prevent lightweight land vehicles from penetrating the arrestor bed. This simple fill-and-cover construction would likely produce lower manufacture and installation costs than the block construction methods used at present. This potential advantage is offset by the possibility that the material could settle over time and result in altered arresting performance. Candidate 3: Engineered Aggregate An engineered aggregate arrestor concept was proposed. Its primary material is a spherical engineered aggregate that has excellent flow properties and resists settling and compaction that are more typical for angular gravels (Figure S-6). This material would reside in a shallow bed and be covered with a reinforced turf layer. However, the engineered aggregate may also be used without a turf layer, which has been done at four airports in the UK. Other top layer materials are possible, such as a thin asphalt skim coat. Figure S-7 Figure S-6. Engineered aggregate.

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9 Cover Layer of Engineered Turf Aggregate Bed Figure S-7. Engineered aggregate arrestor concepts. illustrates the cover-layer variant of the engineered aggregate arrestor concept. The confining top layer would serve similar purposes as that of the aggregate foam concept. The engineered aggregate solution offered the advantage of construct-in-place simplicity, requiring no fabrication of blocks or cure time. Repairs after overruns would essentially involve shoveling or scraping the material back into place, with little material replacement required. Candidate 4: Main-Gear Engagement System Active arresting systems have long been used to arrest military aircraft. Prior attempts have been made to adapt them for use with civil transport aircraft, using over-wing barrier nets to engage the wings and fuselage in the absence of a tail hook. To surpass these predecessors, any new attempts to adapt the active systems would require innovation and new sensor approaches. However, the active systems offer elegance and a decided performance advantage over passive systems. As such, they were revisited in this effort and re-examined for feasibility. The active system concept evaluated was a main-gear engagement cable-based arrestor. For this concept, a cable-based arrestor would pop up from underneath to engage the main landing gear as the aircraft passes by. Military-style arrestor brakes would retain the cable at either end to decelerate the aircraft (Figure S-8). An aircraft identification system would determine the type of plane being engaged, and embedded runway sensors would detect the overrun event, compute aircraft speed, and determine the timing for deployment. The system would, therefore, be automated and not require manual activation. Pilot overrides could be included if desired. One of the main advantages of the cable-based arrestor is that the nose gear would not be engaged. As shown in Figure S-9, the nose gear would only be loaded vertically due to the weight of Braking Unit Braking Unit Figure S-8. Main-gear engagement active arrestor.

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10 DMG VMG VNG Figure S-9. Loads on aircraft subjected to active arrestor deceleration. the aircraft and pitching moment. This main-gear engagement approach circumvents the weakness of the crushable bed systems: it does not subject the nose gear to drag loads. Consequently, it would be possible to achieve higher decelerations and shorter stopping distances. Further, with automated servo control of the braking units, it could obtain uniform decelerations for a wide range of aircraft weights. Therefore, the deceleration performance could readily exceed that of passive arrestor beds. Experimentation Overview The research funding did not support full-scale testing on a broad enough basis to be meaningful for the effort. Because the experimentation phase was intended to compare the most promising alternatives to the current EMAS technology, and because there were several candidates to evaluate, a modeling-centric approach was adopted. Within this approach, various physical tests were performed in order to characterize the materials involved and to provide a basis for calibrating high-fidelity computer models of the systems. The models were then used to assess the performance of the arrestor concepts in different configurations and for different aircraft. The candidate systems and evaluation methods are given in Table S-2. As the table indicates, the passive systems shared similar evaluation approaches, but the active system differed. Crushable material technologies were emphasized most because they have a proven track record; finding a similar material solution with better life-cycle performance would provide a useful alternative in the near-term. Table S-2. Summary of evaluation methods for candidates. Category System Evaluation Approach Crushable Candidate 1: Glass foam Material testing material systems Candidate 2: Aggregate foam One-wheel bogey testing Numerical modeling to Displaceable Candidate 3: Engineered aggregate develop tire/material material systems response surfaces Overall aircraft response evaluation using an aircraft suspension model Active systems Candidate 4: Main-gear engagement Extended paper study active system Analytical spreadsheet model

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11 Figure S-10. Simplified diagram of modeling and testing approach. Aggregate systems have historically experienced a discrepancy of acceptance, seeing use in the UK but not in the U.S. The evaluation approach outlined herein offers the promise of resolving the question of predictable arresting performance. The evaluation method for the passive systems was composed of (1) modeling and (2) physical testing. Figure S-10 illustrates a simplified version of the modeling and testing approach. The active system approach required a feasibility study to determine its merit. As such, a small task was dedicated to completing a more detailed paper/analytical study. Early in the research effort, ESCO provided best-case EMAS arrestment prediction data for three aircraft from different size regimes: the CRJ-200, B737-800, and B747-400 (Figure S-11). In order to compare the candidate systems' performance with these EMAS baseline cases, aircraft computer models for each were developed. As such, these three aircraft are cited throughout the evaluation. B747-400 CRJ 200 MTOW = 875,000 MTOW = 51,000 88 ft B737-800 MTOW = 174,200 230 ft 130 ft Figure S-11. Three evaluation aircraft for the effort.

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12 Passive System Evaluation Results Overview The experimentation phase evaluated three passive arrestor candidate systems: 1. Glass foam arrestor, 2. Aggregate foam arrestor, and 3. Engineered aggregate arrestor. Each demonstrated relative strengths and weaknesses. All three options provide concepts with increased material durability over cellular cement, which would likely result in longer life cycles and decreased maintenance requirements. Performance Comparison The performance of the different candidates can be compared in two primary ways: based on (1) single-aircraft or (2) multi-aircraft bed designs. When comparing single-aircraft bed designs, the thickness of each bed and its material properties are optimized for the plane of interest. However, the bed designs for the different aircraft may not be compatible with one another. For example, a best-case design for the B747-400 was typically found to overload the landing gear of the CRJ-200. When comparing multi-aircraft bed designs, a single bed is designed for best-case perfor- mance with all three of the subject aircraft simultaneously. A single-bed thickness and material property are determined such that the overall performance is optimized for all three aircraft. The single-aircraft comparisons always produce the shortest feasible stopping distances. However, the multi-aircraft comparisons are more relevant to actual applications at airports, where arrestor beds are designed as a compromise between the different aircraft serviced. Figure S-12 compares the best single-aircraft bed designs for the three alternatives and compares them to the current EMAS technology. The four sets of bars show very similar trends in terms of relative stopping lengths for the different aircraft. The glass foam stopping distances are slightly shorter than those of the current EMAS (except for the B737-800, where it is slightly longer), while the other two concepts would require slightly longer beds. The performance similarity of 700 600 Stopping Distance (ft) 500 400 CRJ-200 300 B737-800 200 B747-400 100 0 Current EMAS Glass Foam Engineered Aggregate Aggregate Foam Figure S-12. Comparison of single-aircraft bed performance for all candidates: distance travelled in bed for full arrest assuming 70-knot exit speed.

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13 1000 900 800 Stopping Distance (ft) 700 DATA NOTAVAILABLE 600 500 CRJ-200 400 B737-800 300 B747-400 200 100 0 Current Glass Foam Engineered Aggregate EMAS Aggregate Foam Figure S-13. Comparison of multi-aircraft bed performance for all candidates: distance travelled in bed for full arrest assuming 70-knot exit speed. glass foam and the current EMAS is not surprising because both designs use crushable foam block material with similar mechanical behavior. However, this comparison of single-aircraft perfor- mance is ultimately less relevant to real applications than the multi-aircraft comparisons that follow. Figure S-13 compares the best multi-aircraft arrestor bed designs for the three alternatives. In each case, the B747-400 required the longest bed for arrestment from a 70-knot exit speed. It should be noted that performance predictions for the existing EMAS have not been included in the figure because the design cases provided did not apply to multi-aircraft bed designs. In general, the material could be assumed to follow a similar trend to glass foam, owing to the mechanical similarities just discussed. In comparing Figure S-12 with Figure S-13, the trend for leading and trailing concepts shifts considerably. The differences illustrate how substantially the multi-aircraft performance deviates from aircraft considered individually. For the multi-aircraft case, bars of similar height indicate improved equality in the treatment of the three aircraft. Of the three concepts, the aggregate foam shows the most consistent performance, with dramatic reduction in the stopping distance for the B747-400. As an example, for a bed with a practical 400-ft length, the exit speeds for each aircraft are as shown in Table S-3. The 400-ft bed would obtain a full 70-knot exit speed rating for the CRJ-200 with all arrestor concepts. Both the glass foam and aggregate foam beds would further obtain a 70-knot rating for the B737-800, while the engineered aggregate falls behind at only 63 knots. For the B747-400, none of the beds obtains a full 70-knot rating; the aggregate foam leads at 56 knots and the engineered aggregate falls to below the minimum allowable speed at 39 knots. Table S-3. Comparison of multi-aircraft bed performance: exit speeds for full arrest in 400-ft bed. Aircraft Glass Foam Engineered Aggregate Aggregate Foam CRJ-200 70+ 70+ 70+ B737-800 70+ 63 70+ B747-400 46 39 56

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14 Overall, the performance of the three concepts can be summarized as below: The aggregate foam concept provided the best overall mixed-fleet bed performance, showing the smallest spread in arrestor performance for the three aircraft. However, the bed design must be correctly specified to prevent oscillatory "porpoising" behavior of the aircraft. The engineered aggregate produced speed-dependent landing gear loads. This would typically require designs to hedge against overloading by under-designing them, resulting in longer arrests than illustrated above. The glass foam beds produced the most predictable and constant decelerations without speed dependence or porpoising effects. Environmental Performance Comparison From an environmental performance standpoint, all three alternatives appear likely to offer superior performance to the current EMAS technology. The environmental performance estimates are based on some test data, historical usage of the materials, and engineering judgment. An exhaustive environmental test program has not been undertaken as part of this program. Life-cycle performance has been assumed to result from a combination of the core materials used and the protective measures taken to shield those materials from the elements. From a materials standpoint, the glass foam and aggregate foam concepts both use closed-cell glass foams that inherently resist water penetration. The engineered aggregate is composed of hard spherical pellets. All three of these materials appear to offer superior inherent resistance to general handling and moisture/chemical exposure when compared to cellular cement. With regard to protective measures, methods for covering and sealing the three candidate materials against moisture, standing water, jet blast, and freezing conditions have been examined (see respective chapters). For the aggregate foam and engineered aggregate approaches, the use of geo-plastics and geo-textiles could render the beds essentially isolated from water entrainment and freezethaw damage. The glass foam material could be packaged in a manner similar to that of the current EMAS cellular cement or equipped with an alternative monolithic sealed top layer. Cost Comparison The relative costs for the current EMAS and the candidate systems are compared in Figure S-14 and Figure S-15, using survey cost assumptions and estimates from Order 5200.9, respectively. The general trends appear similar in either case, with the aggregate foam concept providing the least expensive alternative, and the glass foam providing the most expensive alternative. The costs in the figures denote the total cost to establish such a system and do not include life-cycle costs of bed replacement and maintenance. Summary Comparison Glass foam provided equivalent dynamic behavior to the currently approved EMAS system. Its performance, cost, and construction are also similar to the current EMAS. However, use of glass foam with a monolithic construction offers reduced maintenance and a longer service life. Additionally, glass foam could be constructed using a stratified depth-varying layup, which would likely improve multi-aircraft bed performance. Aggregate foam provided a novel approach that featured excellent multi-aircraft bed perfor- mance due to its depth-varying crushable material; this would effectively lead to shorter arrestor beds. Its cost was the lowest of the alternatives, combining an inexpensive material with a simple installation process.

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15 $9 $8 Cost to Establish ($M) $7 $6 $5 $4 $3 Installation $2 Site Preparation $1 $- AS am e am at M Fo eg Fo tE gr s e s n Ag at la re eg G ur ed gr C er Ag ne gi En Figure S-14. Relative estimated cost comparison assuming survey costs (150' x 300' bed). Engineered aggregate features the most durable candidate arrestor material, much of which could be reused after an arresting event. It has a cost that falls between the other concepts. Its speed-dependent nature produces weaker multi-aircraft performance, which would require longer arrestor beds to obtain the same exit speed ratings. Active System Evaluation Results The main-gear engagement active system concept offered multiple advantages not available in passive surface-based arrestor beds. Feedback control offered the potential for ideal deceleration of aircraft. The energy-absorbing brakes could adjust automatically to apply less load on a small $7 $6 Cost to Establish ($M) $5 $4 $3 $2 Installation $1 Site Preparation $- AS am te am ga M Fo Fo e tE gr s e s n Ag at la re eg G ur ed gr C er Ag ne gi En Figure S-15. Relative estimated cost comparison assuming Order 5200.9 costs (150' x 300' bed).

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16 plane than a large one, enabling one arrestor to treat all aircraft equally. The overall effect of these advantages would be shorter arresting distances for the entire design fleet of aircraft. Load calculations undertaken confirmed that the essential mechanics for the arrestor system would function as anticipated. Tension regulation in the cable would be essential to prevent overloading of the landing gear. Despite these promising features, a number of complicating issues remain for the active system concept: Aircraft identification and speed calculation would be required for the system to function correctly. Systems to accomplish both could likely be developed, although research of such facets has not yet been undertaken. Cable sizes are of concern because the thicker cables required for rapidly arresting large planes could be heavy enough to damage smaller ones. The cable engagement process would likely result in damage to landing gear doors, actuators, wiring, and hydraulic features on the front side of the main struts. The window of time for deploying the system could be narrow when the aircraft has low slung engines because the vertical path of the cable or net must miss the engine nacelles yet engage the main strut above the tires. The geometry of some aircraft would exclude the possibility of engaging the main gear. These aircraft could not be engaged and arrested by a main-gear cable/net system. For these reasons, the active system approach is feasible for stopping aircraft, but the main-gear engagement concept should be eliminated from consideration. Over-wing barrier net systems have been developed in the past, and they may still offer the best overall engagement approach. Past engagement issues for the nets could be resolved using automation concepts as discussed, thereby eliminating the need for direct triggering by airport personnel. Conclusions In the first phase, research was done into multiple studies of different topics relevant to devel- oping arrestor alternatives. After identifying promising alternatives, several candidate concepts were selected for detailed evaluation in the second, experimentation phase. In the second phase, testing and modeling were used to evaluate the performance of the candidates. This section summarizes important conclusions from both phases of research. Study Phase The survey of U.S. airport operators revealed that actual EMAS costs appear to exceed the pre- dicted values contained in FAA Order 5200.9 in terms of preparatory paving, installation, and maintenance. While the survey included more airports than the original data set used to create Order 5200.9, it did not include all EMAS systems installed at U.S. airports. It is possible that the average costs could shift once the remaining airports were included. However, since the survey data for the cost to establish an EMAS (CTEE) was 1.8 times higher than the predicted value, an update to the guidance document may be advisable. A review of aircraft overrun data led to a revised probability curve for aircraft overrun exit speeds. This revised curve indicated that 90% of aircraft overruns may no longer take place at or below an exit speed of 70 knots. The new curve suggests that the 90% threshold may have shifted to just above 80 knots. This could affect the design speeds for aircraft arrestors if a 90% criterion is to be maintained. Additional investigation may be warranted regarding the accuracy of the reported data that was used in the assessment. However, increasing the 70-knot standard exit

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17 speed or the 40-knot minimum exit speed requirements would have a notable impact on the size and cost of arrestor systems. The FAA EMAS design requirements currently prohibit damage to the aircraft, which typically results in arrestor bed designs constrained by the rearward nose-gear loads. More aggressive decelerations would be possible if the designs were permitted to collapse the nose gear as long as the main gear remained intact. Prior EMAS testing suggests that this may pose minimal hazards to aircraft occupants. However, aircraft with low-slung engines could potentially be damaged and/or ingest arrestor material in such cases, and the risks of these effects have not been quantified. Additional concerns would apply to turbo-prop aircraft, where propeller damage could present additional hazards. It may be advantageous to revisit the requirements regarding landing gear loading in order to determine if case-by-case exceptions may be permissible. In some circumstances, the benefits of aggressive arrestor performance may outweigh the risks of failing to stop an overrun. The approval and commercialization study determined that the current lack of a general pre- dictive software tool presents a barrier for new entrants to the arrestor system field. Development of such a tool, or an update of the older ARRESTOR code, should be considered. Although based on anecdotal experiences, the researchers have noted a seeming lack of aware- ness regarding the existence, usage, and function of EMAS among airline pilots. This lack of awareness has been observed in both newer and seasoned pilots. It is unclear whether awareness is greater among pilots that typically land at airports with EMAS. Unfortunately, response from the pilot community was limited during the survey process. Nevertheless, it may be beneficial to consider pursuit of an educational effort to increase awareness of the existence and function of EMAS within the pilot community. Experimentation Phase The experimentation phase of the effort involved an extensive evaluation of four arrestor candidates: 1. Glass foam arrestor (passive), 2. Aggregate foam arrestor (passive), 3. Engineered aggregate arrestor (passive), and 4. Main-gear engagement active arrestor (active). A combined modeling and simulation effort successfully replicated each candidate in order to evaluate its merit and compare it with the existing EMAS performance. Passive System Evaluation The findings of this research indicate that a fieldable system is feasible for all three passive system candidates. The aggregate foam concept offers superior multi-aircraft performance due to its depth-varying material properties. This multi-aircraft performance is arguably the most important factor for keeping arrestor beds short, reducing land requirements, and increasing the rated aircraft exit speeds. Additionally, it provides a substantially lower estimated cost per square foot. However, because the aggregate foam concept uses a novel crushable material and cover layer, the number of unknowns is greater. The arrestor materials require additional evaluation in order to produce high-confidence performance estimations. Overall, the combination of unknown factors and anticipated cost and performance improvements make it a moderate-risk/high-payoff concept. Conversely, the glass foam concept is the most conservative of the three alternatives in terms of development risk and payoff. Glass foam provides an alternative to the current EMAS technology

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18 with promising improvements regarding service life and maintenance, but at equivalent cost and performance. The mechanics of the material are predictable with relatively few unknown factors. Overall, the combination of unknown factors and anticipated cost and performance improve- ments make it a low-risk/moderate-payoff concept. Finally, the engineered aggregate concept provides a feasible alternative, but one without a particular distinguishing advantage. It provides cost savings and increased material durability. However, the speed-dependent arrestor performance generally requires longer arrestor beds and diminishes the multi-aircraft performance. As with the aggregate foam, the materials to be used will require additional characterization in order to make high-confidence performance estimations. Overall, the combination of unknown factors and anticipated cost and performance improvements make it a moderate-risk/low-payoff concept. The selection of one or more of these alternatives for fielding and approval is a task that requires consideration by the relevant stakeholders, which include the government, airport community, and manufacturers. Each candidate offers different advantages, risk levels, and payoff potential. Depending on the development track that is pursued, manufacturer investment may be required. The extensiveness of subsequent development plans and the associated costs may determine the feasibility of such participation. As a precursor to any such development, it is recommended that the concepts for pursuit be determined and that the relevant manufacturers be contacted for preliminary discussions of development scope and participation. Active System Evaluation The main-gear engagement active system candidate is not recommended for additional pursuit. While stopping the aircraft proved mechanically feasible, engaging the main-gear struts involved multiple complexities, including damage to landing gear features, damage to landing gear bay doors, and timing and deployment complexity. Additionally, some aircraft geometries made capture infeasible altogether. Nevertheless, active systems remain feasible if barrier nets are used for engagement. Using the suggested sensor and activation methods, an automated system could be developed that would eliminate the need for manual triggering, but permit pilot overrides. The active system approach offers the highest theoretical deceleration limits, which could produce substantially shorter arrestment distances than any passive system alternative. The survey findings indicate that endeavors to implement an active civil aircraft arresting system would meet with resistance by the aviation community. If pursued, it is recommended that an educational component be included to increase awareness and reduce misconceptions regarding such systems. Final Conclusions The research effort achieved its stated objective, which was to advance the development of alternative civil aircraft arresting systems. The research evaluated four alternatives to the current cellular cement EMAS technology and successfully identified options that can provide improved performance, lower cost, and/or higher durability. Pursuing full development and testing of one or more of these candidate systems would be the next step toward obtaining requisite FAA approval. Once fielded, the new arrestor system would provide additional options to airport operators for achieving RSA compliance. Increasing the choices available would allow decision makers to select the arrestor option that best fits with the budgetary, climate, and space constraints of the facility.