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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 Figure 7-13. Deployed barrier net (41). advantage over passive systems. As such, they have been revis- ited in this effort and re-examined for feasibility. ity. Therefore, if civil aircraft could be engaged with hydraulic 7.7.1. Braking Devices brakes, the result would be a highly efficient and reliable arrest- A wide range of braking technologies has been developed ing system. to arrest military aircraft, including hydraulic brakes, water Figure 7-13 shows an active arrestor with a barrier net impellers, and textiles. Hydraulic brakes function much like engagement. During aircraft engagement, the nose of the air- automobile disc brakes, where a caliper compresses to a rotat- craft passes through the barrier net, and the net wraps over ing disc. Water impellers dissipate kinetic energy by generat- the aircraft wings. A braking device is then used to decelerate ing turbulence in a reservoir of water. Finally, textile devices the aircraft. As Figure 7-13 suggests, over-wing barrier nets absorb kinetic energy by tearing fibers. are a commercially available technology. Of the three types of braking devices, the hydraulic brake, There are three issues that complicate use of barrier nets for pictured in Figure 7-12, is the most precise. Because it features civil aircraft. First, because the arrestor constitutes a vertical servo-controlled loading, it is capable of applying a consistent obstruction, it cannot remain erected at the runway end under deceleration profile to a variety of aircraft. As a result, the normal conditions. Thus, in the event of an overrun, either the hydraulic brake could be used to apply the ideal deceleration pilot or an air traffic controller would activate the arrestor. As profile to a given aircraft, thus minimizing the stopping dis- discussed in Section 3.8, airport operators and pilots have tance. For example, hydraulic brakes could be used to stop a expressed hesitancy about use of active arrestors for civil air- B747-400 with a 1 g deceleration at 70 knots. Furthermore, the craft. Also, for most net-based arrestors, activation requires military hydraulic brakes have a minimum of 97.5% reliabil- 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- Figure 7-12. Hydraulic brake (servo controlled) (40). ment, a main-gear engagement approach was considered. For

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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- Braking Unit 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. Braking Chapter 14 discusses the active arrestor concept evaluation Unit in detail. Figure 7-14. Main-gear engagement active arrestor (42). DMG VMG VNG Figure 7-15. Loads on aircraft subjected to active arrestor deceleration (42).