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Developing Improved Civil Aircraft Arresting Systems (2009)

Chapter: Chapter 7 - Identification and Initial Assessment of Alternatives

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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
×
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Suggested Citation:"Chapter 7 - Identification and Initial Assessment of Alternatives." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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46 7.1. General Approach The identification and ideation of different arrestor alter- natives was approached through several avenues: • Vendors with design ideas were contacted, • Internal brainstorming meetings were undertaken, • Technical literature was reviewed, and • General web searches were undertaken. 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 selection of several promising candidates for detailed research in the experimentation phase of the effort. Please note that the TRB and ACRP do not endorse specific products. Research results are provided to assist in the evaluation of options by others. 7.2. Vendor-Developed Alternatives 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. 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 current cellular cement design, but with potentially improved durability. Other companies had more well-developed ideas, including patents (Appendix A), design drawings, and calculations for energy absorption. The broad range of maturity required an initial assessment of the alternatives (Section 7.4) in order to determine the most promising concepts for inclusion in the experimentation stage of the research. 7.3. Classification of Alternatives When trying to sort through the myriad of ideas devel- oped and identified, it became apparent that some of the concepts did not differ as greatly as initially assumed. For instance, within the domain of crushable foams, there is minimal difference in the dynamic arrestment process expected for different materials. A crushable polymer foam, a crushable cellular cement, and a crushable metal foam will function in a fairly similar manner where their compressive (crush) strengths and other material properties are the same. There are many foams available, and delving into the nuances that separate them has minimal benefit. This is especially true when compared with the differences, for example, between crushable foam arrestors and gravel arrestors. The dynamic responses of these arrestors are quite different. The classification structure that was developed divided the alternatives by dynamic behavior. Table 7-1 shows the classifica- tion for the different concepts in abbreviated form. Section 7.4 expands upon this table and indicates those alternatives that were selected for the detailed research. Subsequent chapters dis- cuss the detailed research findings. The systems were grouped into two broad classifications: passive systems, with no moving parts, and active systems, with moving mechanical components. Thereafter, they were grouped into four categories based on the energy absorption approach used. 1. Crushable Materials. Crushable materials absorb energy through permanent deformation of the material, either through brittle fracture or plastic deformation. • The aircraft is engaged at the tire/ground interface. • Current EMAS and phenolic foam beds are examples of crushable material systems. 2. Displaceable Materials. Displaceable materials do not undergo a physical breakdown of the material itself. Instead, the material is moved, and energy is absorbed through momentum transfer or the internal friction of the material. The mechanical behavior of these materials is fundamentally dissimilar from the crushable material and results in a dif- ferent dynamic system response. • The aircraft is engaged at the tire/ground interface. C H A P T E R 7 Identification and Initial Assessment of Alternatives

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

48 The Subcategory and Technology columns show progres- sively more detail on the concepts in each category. The Covering Layer Applicable column indicates whether a given concept can be combined with a covering layer. Cover- ing layers are often used to preserve the crushable or displace- able materials from the environment, jet blast, and so on. The current EMAS design uses such a “covering layer.” The latest generation uses thin plastic tops, while the prior generation used cement board tops. Water ponds historically had differ- ent covering concepts suggested to keep the water clean and prevent evaporation or animal intrusion. For new arrestor can- didates, a variety of creative covering layer concepts have been suggested by different vendors, all with similar purposes—to provide a durable top layer that protects the medium while not interfering with its arresting function. The Detailed Research column indicates whether a given technology underwent further evaluation in the experimen- tation phase of the research. It should be noted that combined approaches could be used by coupling a passive arresting bed with an active net or cable-based arresting system. 7.4. Initial Assessment of Alternatives The preliminary vetting process involved two components: (1) an initial physics analysis to determine if a concept had the energy capacity and physical feasibility to arrest and air- craft, and (2) analysis of other factors, such as flammability, environmental considerations, life-cycle performance, and so on. Table 7-2 gives the assessment criteria that were used to evaluate different concepts. In the analysis findings that follow, criteria of minimal relevance to different technologies are often excluded from the discussion. As previously shown in Table 7-1, the array of alternatives is broad. Table 7-3 expands upon Table 7-1 with evaluation comments. Arresting alternatives selected as candidates for the experimental investigation are denoted with a “C.” For alternatives not selected, the evaluation comments provide rationales for exclusion. 7.5. Crushable Material Systems 7.5.1. Drag Loading Dynamics In general, all of the crushable material alternatives func- tion on the same physical premise. The aircraft tires compact the material as they roll forward through it. The compaction of the material can take several forms at the microscale: brit- tle fracture, plastic deformation, reduction in void space, and so on. However, at the macroscale, the effect is similar: the aircraft is slowed by the drag load of the material and the energy absorbed is proportional to the volume of material compacted (Figure 7-1). Some materials, such as phenolic foam, exhibit a rebound after compaction, where the material rises back up to a small extent after the tire rolls over it. This rebound is inherently inefficient and undesirable. The currently used cellular cement is advantageous because it has nearly zero rebound. 7.5.2. Essential Material Properties To select a crushable material that is suitable for an arrestor bed, several properties must be considered. Figure 7-2 illus- trates a typical compression stress-strain curve for a crushable foam. The load increases up to a plateau value, typically desig- nated as the compressive strength of the foam (σu). During the plateau stage, the foam is compacted, and the void spaces of the foam microstructure collapse. When the void spaces have been fully compacted, the plateau ends and the material hardens. The degree of compression that has occurred at the end of this plateau will be referred to as the maximum com- pressive strain of the material (εmax). In actuality, further compression can take place, but not without dramatically higher compression forces. For the purposes of the arrestor applications, the concept of maximum compressive strain is useful. The maximum compression helps determine the overall energy absorption potential for the material because energy absorption relates to the area under the stress-strain curve. Greater maximum compression strain values correspond to long plateau regions in the load curve and high energy absorption. 7.5.3. Effect of Landing Gear Configuration The configuration of the tires on the aircraft’s struts directly affects the efficiency of the crushable material sys- tems. Figure 7-3 shows several landing gear tire configu- rations; in each case, only the leading tires participate in crushing the material and generate drag load. However, the vertical load on the strut is divided among all the tires. The dual configuration, and analogs to it, forms the most efficient coupling, since all tires participate in generating drag load. The dual tandem and tridem configurations are less efficient, regardless of the particular crushable material used. These efficiency reductions would typically only apply to the main struts of the aircraft because inline tires are uncommon for the nose gear. The pneumatic pressure of the tires has an effect on the tire-material interface. Often referred to as “flotation,” the ground pressure affects how much the tires sink into a soft

49 Criteria/Requirements Comments Arresting Performance Energy absorption The system must have the energy absorption capacity to arrest the aircraft. Mechanical engagement The system must effectively engage the aircraft for the energy capacity to be employed. Scalability The system must either actively or inherently scale its stopping force appropriate to the aircraft size/type to prevent overly abrupt deceleration of smaller aircraft while not sacrificing capabilities with larger aircraft. Reliability Device complexity in general will increase maintenance- related concerns and reliability. However, passive systems also have associated reliability despite the absence of moving parts. Post-Arrest Factors Emergency personnel access to aircraft System must permit emergency personnel and vehicles to access the aircraft after an arrest. Emergency egress of passengers System must permit emergency exits to open and allow evacuation of passengers. Costs Purchase/installation cost May be resolvable with refinement of production process and volume production. Some facilities may support higher cost than others. Repair cost Some alternatives will have little to no repair costs following an overrun, depending on the materials used. Maintenance cost Both active and passive systems have been shown to require maintenance. Life-Cycle Factors Pollution hazard Material hazards can probably be mitigated with proper packaging. Temperature performance Likely not resolvable with packaging or treatments of the system. This includes coupling with the moisture criteria in the form of freeze–thaw cycle performance. UV sensitivity Arrestor beds that use covering layers (cement board, plastic, other) inherently protect the arresting materials from UV exposure. Paint and dye for the covering layers must be selected for resistance. Moisture performance May be resolvable with proper packaging of materials used. Jet Blast resistance Cover layers tend to resolve jet blast issues. Chemical resistance May be resolvable with proper packaging of materials used. Non-flammable The materials involved, or their location, must be inherently non-flammable. Table 7-2. Assessment criteria for concept evaluation.

50 Subcategory Technology Evaluation Comments Cellular cement Current EMAS material. Baseline for passive system comparison. Moisture and freeze–thaw related issues have been qualitatively observed, but not quantified. Phenolic foam Previously evaluated by FAA and since superseded by cellular cement (5). Similar material properties as cellular cement, but with more material rebound (disadvantageous). Flammability concerns. Metal foam Excellent mechanical properties for energy absorption. Cost prohibitive. Corrosion vulnerability. Glass foam Glass-based material that is chemically inert and has moisture- resistant properties. Material generally stronger than required for an arrestor application, but can be modified. Density for desired mechanical properties will require determination through experimentation. C Crushable Foams Depth-varying foam Concept proposed by research team. Depth-varying foam idea involves hardening material throughout the arrestor bed depth. This variant would be broadly applicable and not tied to a particular foam material. (Depth-varying foam concept evaluated in a parallel graduate study research effort (33). Summary findings included in this report.) C Loose Crushable Fill Crushable aggregate foam Glass-based material that is chemically inert and has moisture- resistant properties. Made of recycled glass, it offers potential cost savings over cellular glass foam. Density for desired mechanical properties will require determination through experimentation. C Pumice aggregate Styrofoam aggregate Novel concepts developed by research team, but deemed less viable than hollow microsphere concept. Hollow microspheres Novel concept developed by research team. Hollow plastic or glass microspheres connected with a cementitious binder. Offers potential immunity to moisture and freeze–thaw degradation and has pour-in-place simplicity. Brief experiments indicated that obtaining the necessary 80% void ratios was infeasible. Crushable Aggregates with Binder Ceramic glass aggregate Crushable ceramic glass aggregate connected with a phosphate cement binder. This solution offered the possibility of a pour-in-place crushable material, which would reduce cost compared with prefabricated block installation. Concerns regarding moisture penetration and subsequent freeze–thaw performance. Eliminated from consideration due to lack of manufacturer interest. ClaySoil Sand Previously evaluated by FAA (2). Mechanical properties too dependent on moisture and temperature. Loose Aggregates Gravel Previously evaluated by FAA (2). Gravel composed of angular pieces settles over time, altering bed response. Loose gravel poses ingestion hazard for aircraft. Table 7-3. Arrestor alternatives with evaluation comments.

51 Subcategory Technology Evaluation Comments Loose Aggregates Engineered aggregate Aggregate used in the UK without covering layer, this poses a possible ingestion hazard. Concept allows for one of several covering layers, potentially eliminating this problem. Unlike normal gravel, engineered aggregate’s spherical shape prevents settling of the bed over time. Moist freezing conditions could potentially freeze particulates together. Issue: deceleration loading expected to be speed-dependent, which is not a desired trait. C Fluids Water or glycol pond Previously evaluated by FAA (2). Freezing in winter, contamination, and attracting animals made ponds impractical. Emergency vehicle access and passenger evacuation hindered. Loadings only suitable up to certain velocity thresholds (<50 knots). Hydraulic brake Water impeller Textile All three of these arresting brakes are commercially available at present and are typically used for military aircraft. The hydraulic braking systems, such as those used for BAK-12 arrestors, are applicable and offer the feature of active servo- controlled loading, making them capable of the most idealized loading of the various arrestor options. Coupling with an appropriate engagement device for civil applications remains the problem to be solved. CBraking Devices Eddy-current brake Typically used in roller-coaster applications. While viable, no inherent advantage has been found when compared with the time-proven hydraulic friction brakes. Barrier nets Long-standing design option, commercially available. Must be erected to a height that interferes with normal flight path, requiring active deployment in an overrun event. Imparts loading to the leading edge of the wings, with potential for entanglement with leading edge flaps. Can wrap over emergency exits, hindering egress of passengers and crew. Engagement Devices Landing gear strut engagement Long-standing design option. Low-slung engine design on some current civil aircraft creates timing issues regarding deployment. Range of landing gear dimensions and interfering landing gear features (gear doors, electrical/hydraulic features, etc.) creates further deployment complexities. C Mechanical Surface Surface of spring- supported panels Unique design approach that offers fully reversible system, which would not require repair after arresting events (32). Pulsed loading on landing gear due to surface segmentation would reduce performance compared with monolithic foam beds. Reducing pulse strengths to avoid landing gear overloads would require discretization of surface into many small panels. Number of mechanical components and degrees-of-freedom could be substantial, which creates concerns regarding maintenance, reliability, and cost. Table 7-3. (Continued).

Compression Stress (σ) Compression Strain (ε) Energy Absorbed in Compression Plateau σu εmax Figure 7-2. Crushable foam stress-strain curve. Dual Tridem (Only Lead Tires Involved in Arresting) Dual Dual Tandem (Only Lead Tires Involved in Arresting) Figure 7-3. Effect of inline tires on arresting loads. 52 Material Initial Height Pavement Height Material Compressed Height Direction of Travel Vertical Load Imparted Drag Load Imparted Energy Consumed Proportional to Volume of Material Crushed Figure 7-1. Physical performance of crushable materials. surface. Tires with higher pneumatic pressures tend to penetrate more deeply; lower pressure tires tend to pene- trate less. Table 7-4 gives several sample pneumatic tire ground pres- sures, as well as the compressive strength of the current EMAS cellular cement. As may be seen, the internal pressure of the aircraft tire is greater than the strength of the EMAS material. A typical heavy truck with an 80-psi tire, however, has a pressure very close to that of the EMAS material, indi- cating a poor matchup in which the truck may or may not effectively crush the material. A reduction in the drag load would be expected in this case. Finally, the car’s 35-psi ground pressure is lower than the compression strength of the arresting material. Note that the EMAS material strengths cited in the table are based on a punch test methodology developed by the manufacturer that includes both compression and shear strength components together (34). The uniaxial compres- sion strength of the EMAS material would actually be less than these values.

53 The simple tire comparison above has implications for the current research. There can be a substantial variation in pneu- matic tire pressure for different types of planes. Aircraft with lower tire pressures will tend to skim the top of an arrestor bed, and those with higher pressures will dig in more deeply. Also, differences in pressure between the main- and nose-gear tires can affect the relative loading that each strut experiences. The dependencies on landing gear configuration and pneumatic tire pressure are limitations of the crushable mate- rial approach. 7.5.4. Overall Aircraft Dynamics When overrunning the arrestor bed, the nose gear and main gear of the aircraft experience vertical and drag loads (Figure 7-4). For the main gear, this is normal, as they have been designed for braking loads. In contrast, the nose gear is not normally fitted with brakes, and so is not typically designed to be subjected to the same rearward loading as the main gear. The drag load that the arrestor bed applies to the nose gear con- stitutes an unnatural way to slow the aircraft. In addition, the deceleration of the aircraft generates a for- ward pitching motion that increases the vertical load on the nose gear. This in turn drives the nose gear deeper into the arrestor bed, generating additional drag loads on it. A critical distinction must be made with regard to decel- eration of the aircraft created by (1) a normal braking stop, and (2) an arrestor-bed stop: it is insufficient to compare the two stops using the gross aircraft g-loading. For example, let us assume a case of a 0.5 g deceleration created by normal brak- ing or created by an arrestor bed (Table 7-5). The loading on the aircraft is similar in most respects. The 0.5 g deceleration in the braking case is entirely due to the drag load on the main- gear struts. The 0.5 g deceleration in the arrestor bed case is due to drag loads on all three struts: part of the 0.5 g is achieved by the main gear and part by the nose gear. The exact breakdown in percent contributions from the nose and main gear cannot be determined without an overall aircraft dynamics model, which will be developed in Phase 2 of this effort. 7.5.5. Summary of Mechanical Factors To summarize the dynamics of crushable material arrestor beds, the following factors have a direct impact on the over- all performance: • Material properties – Compressive strength (σu) – Shear strength (τ) – Maximum compressive strain (εmax) – Volumetric energy absorption • Material depth • Landing gear configuration • Pneumatic tire pressure • Limiting landing gear design loads, with special consider- ation for the nose gear – Vertical limit load – Longitudinal (drag-direction) limit load DNG VNGVMG DMG Figure 7-4. Overall aircraft loading dynamics. Vehicle or Material Ground/Pneumatic Pressure EMAS Material Strength Arresting Efficiency Typical Car 35 psi 60 – 80 psi Negligible Typical Work Truck 80 – 100 psi 60 – 80 psi Questionable B737-800 185 psi (nose gear) 205 psi (main gear) 60 – 80 psi High Table 7-4. Sample pneumatic tire pressures.

When all of these factors are combined, the performance of an arrestor bed varies across a given fleet of aircraft. Fig- ure 7-5 illustrates the spread in design speeds for a typical arrestor bed, where the dashed line shows the average exit speed. The spread in exit speeds (43 to 57 knots in this case) represents the non-ideal performance that naturally results from designing the bed for a mixture of aircraft. The illus- trated bed is most efficient for a B737, which it can arrest at nearly 60-knots, and it is least efficient for the DC-10. The plot shows that the weight of the aircraft alone does not drive the stopping efficiency of the bed: the most efficient stops are achieved for aircraft between 150,000 and 200,000 lbs, with efficiency decreasing to the left and right. Thus, the aggregate combination of the listed mechanical factors pro- duces the aircraft stopping efficiency on a case-by-case basis. A similar performance variation is expected to persist for all surface-based arrestor beds, regardless of the crushable material chosen. 7.5.6. 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 7-6). The material would be used in a large bed either composed of blocks akin to the current EMAS or constructed as a monolithic structure. The chemical makeup and closed-cell properties of glass foam suggested good chemical resistance, excellent durabil- ity to the environment, and resistance to water absorption (36). The potential improvement in durability suggested the possibility of reduced maintenance and replacement needs for an arrestor using this material. Chapter 9 discusses the glass foam concept evaluation in detail. 7.5.7. Candidate 2: Aggregate Foam An aggregate foam arrestor concept has been proposed. The arrestor would use rough-broken foam aggregate made from recycled glass (Figure 7-7) (37, 38). The foamed, or aerated, glass material has nominally 80% void space by volume. Its closed-cell microstructure makes it resistant to water absorp- tion and degradation. An arrestor using this material would be created by filling a shallow basin with the loose material and covering it with a Variable Braking Deceleration Arrestor Bed Deceleration Gross Deceleration 0.5 g 0.5 g Braking Condition Typical Reduced (due to arrestor material) Thrust Reverser Not used Not used Main-Gear Vertical Loading Typical Typical Main-Gear Drag Loading Typical (caused by braking) Typical/reduced (caused by arrestor material) Nose-Gear Vertical Loading Typical Typical Nose-Gear Drag Loading Near zero (rolling friction only) High (drag loading due to arrestor material) Table 7-5. Comparison of braking and arrestor bed deceleration. 0 10 20 30 40 50 60 16,600 148,000 150,000 200,000 255,000 335,000 455,000 B1900 MD-82 B737-400 B727-200 B757B 767 DC-10 R at ed E xi t S pe ed fo r A irc ra ft (kn ot s) Aircraft Weight (lbs) and Type Figure 7-5. Exit speeds for sample EMAS arrestor (35). 54

55 become harder at deeper levels could achieve a degree of per- formance leveling between large and small aircraft (Figure 7-8). Further, this concept is fairly independent of the material cho- sen. Many crushable materials exist, and depth-varying layups for most could be achieved, including cellular cement. This concept was evaluated as a parallel graduate-level research study; Chapter 12 summarizes the relevant find- ings from the study. 7.6. Displaceable Material Systems Several displaceable material systems are listed in Table 7-3, but only one of these systems was selected for detailed evalu- ation: loose engineered aggregate. This material is much like normal gravel in that the individual aggregate pieces do not Figure 7-6. Cellular glass foam material. Figure 7-7. Aggregate foam: close-up of microstructure (left) and pile (right). reinforced turf layer. The loose fill approach offered the poten- tial advantage of reduced manufacturing and installation cost, as compared with pre-fabricated foam blocks. Chapter 11 discusses the aggregate foam concept evaluation in detail. 7.5.8. Additional Study: Depth-Varying Foam The depth-varying foam concept was developed by the research team. It involves the use of non-homogeneous crush- able foam that becomes firmer as the bed depth increases. The cellular cement used in existing EMAS arrestors is homo- geneous, having the same density and strength throughout each block of material. Changing the density and strength to

Pavement Height Material Compressed Height Vertical Load Imparted Drag Load Imparted Lower Region Compacted, Solid- Like Behavior Upper Surface Can Spray at High Speed Upper Region has Pseudo-Fluid-Like Behavior Rut Created Direction of Travel Figure 7-9. Physical performance of gravel/aggregate materials. 56 Material Becomes Harder Figure 7-8. Depth-varying material concept. (in general) break or compress. They can be compacted by removing the void spaces between them, or they can be displaced. However, after a tire overrun, they can simply be raked back into the ruts with little change in the effec- tiveness of the material. Gravel systems are most often used for arresting large trucks that lose effective braking on down-hill roads, but they have been used for aircraft in the U.K. (4, 13). The engineered aggregate differs from normal gravel in that the particulates are all roughly spherical, instead of angu- lar or elongated shapes. This has the principal advantage of preventing settling of the material over time, which is typical for normal gravels. Similarly, since the particulates are made of the same material, the friction between individual pieces is more consistent. 7.6.1. Drag Load Dynamics Although a gravel-type arrestor bed appears to load the landing gear in a similar fashion as the current EMAS design, the dynamics involved are quite different. The tire–gravel interaction is highly complex because aggregates have more than one mode of behavior. Consider a pile of sand or gravel, created by pouring the material from a spout above it. The pile forms in a cone shape whose sides are at an inclined “angle of repose.” At an angle steeper than the angle of repose, the material runs freely, act- ing similarly to a fluid (39). The loose sand or gravel that pours on top slides down the surface, returning the sides once again to the angle of repose. At slopes shallower than the angle of repose, the material does not behave like a fluid, but a solid: a fluid would eventually flatten out into a pancake shape of uniform thickness, as a puddle. Mechanically, the angle of repose is determined by the internal angle of friction for the material. This angle of friction divides the different regimes of behavior. For a tire rolling through an aggregate, the physical behav- ior follows the illustration of Figure 7-9. The material below a certain level behaves in a solid-like fashion and undergoes compaction beneath the weight of the tire. Above this level, fluid-like behavior dominates, and the material is pushed out of the way of the tire, similar to water being pushed by the hull of a boat. The uppermost part of the aggregate can spray to the sides at high speed when the tire cuts through the ma- terial quickly. In the “wake,” a rut is left in the gravel. Altogether, there are three major components to the energy dissipation: 1. Compaction of the lower layer, 2. Friction between pieces of aggregate moving around the tire, and 3. Momentum transfer in projecting the aggregate away from the tire. Unlike the crushable material systems, the drag load from the aggregate is highly rate dependent. The momentum components are proportional to the tire speed squared (2, p. 8). This means that the drag load decreases as the aircraft slows, progressively exerting less stopping force. It has also been found that at higher rates, the tires skim the surface, not penetrating as deeply and generating lower drag loads (2, p. 16). Low inter-particle friction allows for more fluid-like behav- ior and permits the tires to sink more deeply into the aggregate

57 (13). This can be either desirable or undesirable, depending on the vehicle to be stopped. For truck arresting, allowing the vehi- cle to sink to the axles is positive, as the gravel also engages the undercarriage of the truck. For an airplane, however, there is no undercarriage to halt the descent into the medium, and sinking too deeply could result in drag loads high enough to cause land- ing gear to fail. Another factor related to internal friction is the impact of environmental variables such as moisture, dust, or ice in the arrestor bed. Dry, clean gravel generally has lower inter-particle friction than damp, dusty gravel. Friction, or the resistance of sliding between the particles, is in contrast to cohesion, or the sticking of the particles together. Dust, moisture, and ice can all generate cohesion between the particles as well. In severe winter environments, testing has revealed that an open bed of gravel can form a deep ice crust layer, rendering it ineffective (13). A variant to be considered in this research effort involves applying a protective cover layer to the aggregate bed (Fig- ure 7-10). Open arrestor beds produce a spray from the uppermost layer of the aggregate, which could be ingested by an aircraft engine; this cover layer will protect against inges- tion. It could also mitigate or eliminate the development of ice crusts in cold environments. However, the layer could alter the dynamic response of the aggregate bed because the aggregate would be confined. Thin protective cover layers added to crushable systems do not induce a fundamental change in dynamic behavior, so this effect (if it exists) would be unique to covered aggregate beds. Altogether, the aggregate behavior is highly complex and must be treated as an entirely different dynamic response from that of the crushable material systems. 7.6.2. Effect of Landing Gear Configuration The effect of the landing gear configuration is essentially the same as for the crushable system (Section 7.5.3). 7.6.3. Overall Aircraft Dynamics The overall aircraft dynamics are the same as for the crush- able system (Section 7.5.4). 7.6.4. Summary of Mechanical Factors To summarize the dynamics of aggregate/gravel arrestor beds, the following factors have a direct impact on the over- all performance: • Material properties – Particle size/gradation – Particle shape – Particle density • Inter-particulate properties – Internal angle of friction – Inter-particle friction – Inter-particle cohesion • Speed of travel • Material depth • Landing gear configuration • Pneumatic tire pressure • Limiting landing gear design loads, with special consider- ation for the nose gear – Vertical limit load – Longitudinal (drag-direction) limit load 7.6.5. Candidate 3: Engineered Aggregate An engineered aggregate arrestor concept is called the Engineered Root-zone Arresting System (ERAS). It uses a manufactured aggregate composed of nearly spherical parti- cles (Figure 7-11) and is topped with a reinforced turf cover layer. Vertical Load Imparted Drag Load Imparted Top Cover Layer Results In Material Confinement Figure 7-10. Physical performance of gravel/ aggregate materials with confining top layer. Figure 7-11. Engineered aggregate.

58 The engineered aggregate solution offers 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. Chapter 10 discusses the engineered aggregate concept eval- uation in detail. 7.7. Cable/Net Active Systems Active arresting systems have long been deployed to arrest military planes. Prior attempts have been made to adapt them for use with civil transport aircraft. To surpass these predeces- sors, 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 have been revis- ited in this effort and re-examined for feasibility. 7.7.1. Braking Devices A wide range of braking technologies has been developed to arrest military aircraft, including hydraulic brakes, water impellers, and textiles. Hydraulic brakes function much like automobile disc brakes, where a caliper compresses to a rotat- ing disc. Water impellers dissipate kinetic energy by generat- ing turbulence in a reservoir of water. Finally, textile devices absorb kinetic energy by tearing fibers. Of the three types of braking devices, the hydraulic brake, pictured in Figure 7-12, is the most precise. Because it features servo-controlled loading, it is capable of applying a consistent deceleration profile to a variety of aircraft. As a result, the hydraulic brake could be used to apply the ideal deceleration profile to a given aircraft, thus minimizing the stopping dis- tance. For example, hydraulic brakes could be used to stop a B747-400 with a 1 g deceleration at 70 knots. Furthermore, the military hydraulic brakes have a minimum of 97.5% reliabil- ity. Therefore, if civil aircraft could be engaged with hydraulic brakes, the result would be a highly efficient and reliable arrest- ing system. Figure 7-13 shows an active arrestor with a barrier net engagement. During aircraft engagement, the nose of the air- craft passes through the barrier net, and the net wraps over the aircraft wings. A braking device is then used to decelerate the aircraft. As Figure 7-13 suggests, over-wing barrier nets are a commercially available technology. There are three issues that complicate use of barrier nets for civil aircraft. First, because the arrestor constitutes a vertical obstruction, it cannot remain erected at the runway end under normal conditions. Thus, in the event of an overrun, either the pilot or an air traffic controller would activate the arrestor. As discussed in Section 3.8, airport operators and pilots have expressed hesitancy about use of active arrestors for civil air- craft. Also, for most net-based arrestors, activation requires approximately two seconds. Thus, detection of an overrun and deployment of the arrestor may require more time than is available to engage the aircraft in an emergency. Second, any net-based engagement of civil aircraft would subject the leading edge wing flaps to loads for which they were not designed. The flaps could either become entangled or suffer damage as a result. Third, nets tend to envelope the fuselage of the aircraft. This result is problematic for civil aircraft because it could hinder emergency egress of occupants. 7.7.2. Candidate 4: Main-Gear Engagement System Given the complications associated with barrier net engage- ment, a main-gear engagement approach was considered. ForFigure 7-12. Hydraulic brake (servo controlled) (40). Figure 7-13. Deployed barrier net (41).

59 this concept, a cable-based arrestor would pop up from underneath to engage the main landing gear and decelerate the aircraft. This approach to arresting the aircraft is illus- trated in Figure 7-14. One of the main advantages of the cable-based arrestor is that the nose gear would not be engaged. As shown in Fig- ure 7-15, the nose gear would only be loaded vertically, due to the weight of 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. Chapter 14 discusses the active arrestor concept evaluation in detail. Braking Unit Braking Unit Figure 7-14. Main-gear engagement active arrestor (42). VNGVMG DMG Figure 7-15. Loads on aircraft subjected to active arrestor deceleration (42).

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TRB’s Airport Cooperative Research Program (ACRP) Report 29: Developing Improved Civil Aircraft Arresting Systems explores alternative materials that could be used for an engineered material arresting system (EMAS), as well as potential active arrestor designs for civil aircraft applications. The report examines cellular glass foam, aggregate foam, engineered aggregate, and a main-gear engagement active arrestor system.

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