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48 The Subcategory and Technology columns show progres- aircraft is slowed by the drag load of the material and the sively more detail on the concepts in each category. energy absorbed is proportional to the volume of material The Covering Layer Applicable column indicates whether a compacted (Figure 7-1). given concept can be combined with a covering layer. Cover- Some materials, such as phenolic foam, exhibit a rebound ing layers are often used to preserve the crushable or displace- after compaction, where the material rises back up to a small able materials from the environment, jet blast, and so on. The extent after the tire rolls over it. This rebound is inherently current EMAS design uses such a "covering layer." The latest inefficient and undesirable. The currently used cellular cement generation uses thin plastic tops, while the prior generation is advantageous because it has nearly zero rebound. used cement board tops. Water ponds historically had differ- ent covering concepts suggested to keep the water clean and 7.5.2. Essential Material Properties prevent evaporation or animal intrusion. For new arrestor can- didates, a variety of creative covering layer concepts have been To select a crushable material that is suitable for an arrestor suggested by different vendors, all with similar purposes--to bed, several properties must be considered. Figure 7-2 illus- provide a durable top layer that protects the medium while not trates a typical compression stress-strain curve for a crushable interfering with its arresting function. foam. The Detailed Research column indicates whether a given The load increases up to a plateau value, typically desig- technology underwent further evaluation in the experimen- nated as the compressive strength of the foam (u). During tation phase of the research. the plateau stage, the foam is compacted, and the void spaces It should be noted that combined approaches could be of the foam microstructure collapse. When the void spaces used by coupling a passive arresting bed with an active net or have been fully compacted, the plateau ends and the material cable-based arresting system. 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 (max). In actuality, further 7.4. Initial Assessment compression can take place, but not without dramatically of Alternatives higher compression forces. For the purposes of the arrestor The preliminary vetting process involved two components: applications, the concept of maximum compressive strain is (1) an initial physics analysis to determine if a concept had useful. the energy capacity and physical feasibility to arrest and air- The maximum compression helps determine the overall craft, and (2) analysis of other factors, such as flammability, energy absorption potential for the material because energy environmental considerations, life-cycle performance, and absorption relates to the area under the stress-strain curve. so on. Greater maximum compression strain values correspond Table 7-2 gives the assessment criteria that were used to to long plateau regions in the load curve and high energy evaluate different concepts. In the analysis findings that absorption. follow, criteria of minimal relevance to different technologies are often excluded from the discussion. 7.5.3. Effect of Landing Gear Configuration As previously shown in Table 7-1, the array of alternatives is broad. Table 7-3 expands upon Table 7-1 with evaluation The configuration of the tires on the aircraft's struts comments. Arresting alternatives selected as candidates for directly affects the efficiency of the crushable material sys- the experimental investigation are denoted with a "C." For tems. Figure 7-3 shows several landing gear tire configu- alternatives not selected, the evaluation comments provide rations; in each case, only the leading tires participate in rationales for exclusion. 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 7.5. Crushable Material Systems coupling, since all tires participate in generating drag load. The dual tandem and tridem configurations are less efficient, 7.5.1. Drag Loading Dynamics regardless of the particular crushable material used. These In general, all of the crushable material alternatives func- efficiency reductions would typically only apply to the main tion on the same physical premise. The aircraft tires compact struts of the aircraft because inline tires are uncommon for the the material as they roll forward through it. The compaction nose gear. of the material can take several forms at the microscale: brit- The pneumatic pressure of the tires has an effect on the tle fracture, plastic deformation, reduction in void space, and tire-material interface. Often referred to as "flotation," the so on. However, at the macroscale, the effect is similar: the ground pressure affects how much the tires sink into a soft
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49 Table 7-2. Assessment criteria for concept evaluation. 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 System must permit emergency personnel and vehicles to to aircraft access the aircraft after an arrest. Emergency egress of System must permit emergency exits to open and allow passengers 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 freezethaw 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.
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50 Table 7-3. Arrestor alternatives with evaluation comments. Subcategory Technology Evaluation Comments Crushable Cellular cement Current EMAS material. Baseline for passive system comparison. Foams Moisture and freezethaw 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- C 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. Depth-varying foam Concept proposed by research team. C 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.) Loose Crushable aggregate Glass-based material that is chemically inert and has moisture- C Crushable foam resistant properties. Fill Made of recycled glass, it offers potential cost savings over cellular glass foam. Density for desired mechanical properties will require determination through experimentation. Crushable Pumice aggregate Novel concepts developed by research team, but deemed Aggregates less viable than hollow microsphere concept. Styrofoam aggregate with Binder Hollow microspheres Novel concept developed by research team. Hollow plastic or glass microspheres connected with a cementitious binder. Offers potential immunity to moisture and freezethaw degradation and has pour-in-place simplicity. Brief experiments indicated that obtaining the necessary 80% void ratios was infeasible. 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 freezethaw performance. Eliminated from consideration due to lack of manufacturer interest. Soil Clay Previously evaluated by FAA (2). Sand Mechanical properties too dependent on moisture and temperature. Loose Gravel Previously evaluated by FAA (2). Aggregates Gravel composed of angular pieces settles over time, altering bed response. Loose gravel poses ingestion hazard for aircraft.
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51 Table 7-3. (Continued). Subcategory Technology Evaluation Comments Loose Engineered aggregate Aggregate used in the UK without covering layer, this poses a C Aggregates 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. 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). Braking Hydraulic brake All three of these arresting brakes are commercially available C Devices at present and are typically used for military aircraft. Water impeller The hydraulic braking systems, such as those used for BAK-12 Textile 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. 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. Engagement Barrier nets Long-standing design option, commercially available. Devices 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. Landing gear strut Long-standing design option. C engagement 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. Mechanical Surface of spring- Unique design approach that offers fully reversible system, Surface supported panels 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.
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52 Vertical Load Imparted Drag Load Direction of Travel Imparted Material Initial Height Energy Consumed Material Proportional to Volume Compressed Height of Material Crushed Pavement Height 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. Compression Stress () 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- Plateau cating a poor matchup in which the truck may or may not effectively crush the material. A reduction in the drag load u Energy Absorbed in would be expected in this case. Finally, the car's 35-psi Compression ground pressure is lower than the compression strength of Compression Strain () the arresting material. Note that the EMAS material strengths cited in the table max are based on a punch test methodology developed by the Figure 7-2. Crushable foam stress-strain curve. 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. Dual Dual Tandem (Only Lead Tires Involved in Arresting) Dual Tridem (Only Lead Tires Involved in Arresting) Figure 7-3. Effect of inline tires on arresting loads.
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53 Table 7-4. Sample pneumatic tire pressures. Vehicle or Material Ground/Pneumatic EMAS Material Arresting Pressure Strength 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) 60 80 psi High 205 psi (main gear) The simple tire comparison above has implications for the us assume a case of a 0.5 g deceleration created by normal brak- current research. There can be a substantial variation in pneu- ing or created by an arrestor bed (Table 7-5). The loading on matic tire pressure for different types of planes. Aircraft with the aircraft is similar in most respects. The 0.5 g deceleration in lower tire pressures will tend to skim the top of an arrestor the braking case is entirely due to the drag load on the main- bed, and those with higher pressures will dig in more deeply. gear struts. The 0.5 g deceleration in the arrestor bed case is due Also, differences in pressure between the main- and nose-gear to drag loads on all three struts: part of the 0.5 g is achieved by tires can affect the relative loading that each strut experiences. the main gear and part by the nose gear. The exact breakdown The dependencies on landing gear configuration and in percent contributions from the nose and main gear cannot pneumatic tire pressure are limitations of the crushable mate- be determined without an overall aircraft dynamics model, rial approach. which will be developed in Phase 2 of this effort. 7.5.4. Overall Aircraft Dynamics 7.5.5. Summary of Mechanical Factors When overrunning the arrestor bed, the nose gear and To summarize the dynamics of crushable material arrestor main gear of the aircraft experience vertical and drag loads beds, the following factors have a direct impact on the over- (Figure 7-4). For the main gear, this is normal, as they have all performance: been designed for braking loads. In contrast, the nose gear is not normally fitted with brakes, and so is not typically designed · Material properties to be subjected to the same rearward loading as the main gear. Compressive strength (u) The drag load that the arrestor bed applies to the nose gear con- Shear strength () stitutes an unnatural way to slow the aircraft. Maximum compressive strain (max) In addition, the deceleration of the aircraft generates a for- Volumetric energy absorption ward pitching motion that increases the vertical load on the · Material depth nose gear. This in turn drives the nose gear deeper into the · Landing gear configuration arrestor bed, generating additional drag loads on it. · Pneumatic tire pressure A critical distinction must be made with regard to decel- · Limiting landing gear design loads, with special consider- eration of the aircraft created by (1) a normal braking stop, ation for the nose gear and (2) an arrestor-bed stop: it is insufficient to compare the Vertical limit load two stops using the gross aircraft g-loading. For example, let Longitudinal (drag-direction) limit load DMG DNG VMG VNG Figure 7-4. Overall aircraft loading dynamics.
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54 Table 7-5. Comparison of braking and arrestor aggregate combination of the listed mechanical factors pro- bed deceleration. duces the aircraft stopping efficiency on a case-by-case basis. A similar performance variation is expected to persist for Variable Braking Arrestor Bed Deceleration Deceleration all surface-based arrestor beds, regardless of the crushable material chosen. Gross Deceleration 0.5 g 0.5 g Braking Condition Typical Reduced (due to arrestor material) 7.5.6. Candidate 1: Glass Foam Thrust Reverser Not used Not used Cellular glass foam was proposed as a material and would Main-Gear Vertical Typical Typical Loading be made as a modification of an existing insulating product Typical (caused Typical/reduced (Figure 7-6). The material would be used in a large bed either Main-Gear Drag Loading by braking) (caused by arrestor composed of blocks akin to the current EMAS or constructed material) as a monolithic structure. Nose-Gear Vertical Typical Typical The chemical makeup and closed-cell properties of glass Loading foam suggested good chemical resistance, excellent durabil- Nose-Gear Drag Loading Near zero (rolling High (drag loading ity to the environment, and resistance to water absorption friction only) due to arrestor material) (36). The potential improvement in durability suggested the possibility of reduced maintenance and replacement needs for an arrestor using this material. When all of these factors are combined, the performance Chapter 9 discusses the glass foam concept evaluation in of an arrestor bed varies across a given fleet of aircraft. Fig- detail. ure 7-5 illustrates the spread in design speeds for a typical arrestor bed, where the dashed line shows the average exit 7.5.7. Candidate 2: Aggregate Foam speed. The spread in exit speeds (43 to 57 knots in this case) represents the non-ideal performance that naturally results An aggregate foam arrestor concept has been proposed. The from designing the bed for a mixture of aircraft. The illus- arrestor would use rough-broken foam aggregate made from trated bed is most efficient for a B737, which it can arrest at recycled glass (Figure 7-7) (37, 38). The foamed, or aerated, nearly 60-knots, and it is least efficient for the DC-10. The glass material has nominally 80% void space by volume. Its plot shows that the weight of the aircraft alone does not closed-cell microstructure makes it resistant to water absorp- drive the stopping efficiency of the bed: the most efficient tion and degradation. stops are achieved for aircraft between 150,000 and 200,000 lbs, An arrestor using this material would be created by filling a with efficiency decreasing to the left and right. Thus, the shallow basin with the loose material and covering it with a 60 Rated Exit Speed for Aircraft (knots) 50 40 30 20 10 0 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 Aircraft Weight (lbs) and Type Figure 7-5. Exit speeds for sample EMAS arrestor (35).