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OCR for page 35
35 Inputs/Requirements Outputs/Consequences Initial Conditions Arrestor Performance Aircraft Type Exit Speed Deceleration Runway Arrestor Type Landing Gear Conditions Loads Etc. Current EMAS Aircraft Active System Controllability Other Passive Required Distance Performance System Multi-Aircraft/Fleet Requirements Performance Aircraft Condition Occupant Survival Cost Occupant Egress Installed Arrestor Reliability Site Preparation Exit Speed Rating Type Installation Size/Dimensions Maintenance Other Repair After Airport Constraints Specifications Overrun RSA Dimensions Replacement Runway Dimensions Time Installation Other Repair Requirements Service Life Life-Cycle Maintenance Access Emergency Vehicle Access Reliability Figure 4-1. Diagram of FAA parameters. 4.4. Critical Parameters long as the main gear remained intact. While this certainly results in a monetary impact with regard to aircraft damage, 4.4.1. Exit Speed prior EMAS testing suggests that it may pose minimal hazards FAA AC 150/5220-22A requires that an EMAS have a stan- to aircraft occupants in some cases. In the event of a nose gear dard design exit speed of 70 knots and a minimum of 40 knots collapse, some aircraft would be more adversely affected than (1). The 70-knot standard is intended to enable arrest of 90% others would. Aircraft with low-slung engines could poten- of overruns. The 40-knot minimum is provided as an excep- tially undergo engine damage and/or ingest arrestor material; tion for airports with highly constrained RSAs. A review of the risks of these effects have not been quantified. Additional the overrun data collected in subsequent ACRP research (24) concerns would apply to turbo-prop aircraft, where propeller indicated that the 70-knot requirement may no longer be damage could present additional hazards. sufficient to reach the intended 90% arrest rate (Chapter 5). As an example of potential implementation, if a plane continues to penetrate beyond 75% of an arrestor bed's length, it could enter an "all or nothing" zone, in which the resistance 4.4.2. Damage to Aircraft of the bed increases in a final attempt to arrest the aircraft. FAA AC 150/5220-22A requires that the arrestor not struc- Nose gear collapse could be preferable to the consequences of turally damage the aircraft. However, this requirement could failing to stop the aircraft, depending on what hazards lie potentially be revised to permit a degree of flexibility in the beyond the arrestor bed. interests of reducing overall risk. A robust approach could consider an overall risk assess- More aggressive decelerations would be possible if the ment, as the consequences of an un-averted overrun will not designs were permitted to overload/collapse the nose gear, as be equal for all facilities. It is suggested that the requirements

OCR for page 35
36 Inputs/Requirements Outputs/Consequences Initial Conditions Arrestor Performance Aircraft Type Exit Speed Deceleration Runway Arrestor Type Landing Gear Conditions Loads Etc. Current EMAS Aircraft Active System Controllability Other Passive Required Distance Performance System Multi-Aircraft/Fleet Requirements Performance Aircraft Condition Occupant Survival Cost Occupant Egress Installed Arrestor Reliability Site Preparation Exit Speed Rating Type Installation Size/Dimensions Maintenance Other Repair After Airport Constraints Specifications Overrun RSA Dimensions Replacement Runway Dimensions Time Installation Other Repair Requirements Service Life Life-Cycle Maintenance Access Emergency Vehicle Access Reliability Figure 4-2. Diagram of FAA parameters: detail for system design and installation. concerning landing gear loading be revisited in order to deter- occupant injury criteria (31). Several individuals from the FAA mine if case-by-case exceptions may be permissible. stated that a 1 g limit might stem from FAR landing gear load- ing criteria, but that they were not aware of any such injury criterion. This topic was investigated further by enlisting a 4.4.3. Collateral Damage human injury expert. The subsequent research indicated that To date, there has been minimal discussion in FAA liter- the deceleration injury threshold is substantially higher than ature about assessing collateral damage in the event of an 1 g. The conclusion, in the absence of other evidence, is that the aircraft overrun. As a resource for future consideration, 1 g deceleration limit mentioned in the reference previously aircraft damage and occupant injury have been included in the cited was likely errant. Details regarding the human injury risk analysis software developed in recent ACRP research analysis are given in Appendix E. (24). For airports with highly constrained RSAs, it may be advisable to include a metric for collateral damage. FAA AC 4.4.5. Deceleration Thresholds 150/5200-37 (25) provides a basic framework for risk-oriented planning. Table 4-1 summarizes deceleration thresholds for aircraft damage and occupant injury. For the nose gear criterion, the 0.23 g deceleration assumes 4.4.4. Occupant Injury that no rearward loading is applied to any other part of the A document found during the literature review indicated aircraft (main gear, thrust reversers, etc.). For the main-gear that arrestments should be limited to under 1 g, based on deceleration of 1.13 g, a similar assumption has been made. Both

OCR for page 35
37 Inputs/Requirements Outputs/Consequences Initial Conditions Arrestor Performance Aircraft Type Exit Speed Deceleration Runway Arrestor Type Landing Gear Conditions Loads Etc. Current EMAS Aircraft Active System Controllability Other Passive Required Distance Performance System Multi-Aircraft/Fleet Requirements Performance Aircraft Condition Occupant Survival Cost Occupant Egress Installed Arrestor Reliability Site Preparation Exit Speed Rating Type Installation Size/Dimensions Maintenance Other Repair After Airport Constraints Specifications Overrun RSA Dimensions Replacement Runway Dimensions Time Installation Other Repair Requirements Service Life Life-Cycle Maintenance Access Emergency Vehicle Access Reliability Figure 4-3. Diagram of FAA parameters: detail for arresting event performance. Table 4-1. Deceleration thresholds for aircraft damage and occupant injury. Event/Criteria Deceleration Nose Gear Rearward Failure 0.23 g Main-Gear Rearward Failure 1.13 g Combined Main- and Nose-Gear Rearward Failure 1.36 g Traumatic Brain Injury Long duration (typical arrest) 4.00 g (TBI) Threshold Short duration (<1 sec) 9.00 g Typical EMAS Deceleration (Section 5.2.1) ~ 0.35 to 0.85 g

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38 of these values represent cases that, in practice, are probably Given the deceleration thresholds indicated, the aircraft unattainable. Surface-based arrestors will apply a deceleration limit loads would constrain the arrestment rate, rather than load to the main and nose gear struts simultaneously. occupant injury criteria. The landing gear loads have been the As such, the combined main and nose gear deceleration of historical limitation for EMAS designs. The nose and main 1.36 g is the most useful landing gear metric. It represents gear are not loaded in an ideal fashion, with the nose gear load the theoretical limit for aircraft deceleration that could be generally limiting the obtainable decelerations to 0.35 to 0.85 g. obtained by perfectly distributed loading of the main and nose Perhaps the only realistic way in which to obtain higher gear struts, such that all struts were at their maximal loading decelerations for an aircraft is via an active arrestor system, thresholds simultaneously. While not realistically attainable, using either cables or barrier nets to engage the aircraft. Such the 1.36 g deceleration would represent the ideal case. systems would open the possibility of exceeding the 1.36 g The long-duration traumatic brain injury (TBI) thresh- limitation, at which point the 4 g TBI criterion would become old is most appropriate for application to typical aircraft the performance constraint. Obviously, arrestment distances arrests, which would last for several seconds. For short- could be reduced significantly. Offsetting this advantage, air- duration pulsed loads, the 9 g limit could be assumed instead. frame damage could result from such loads on the aircraft. Details regarding the human injury analysis are given in Assessing the potential impact to the aircraft was beyond the Appendix E. research scope.