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51 be expected for a certain period until the cause is identified by satisfying the high targets set for these service availability and rectified, but it would likely be a measure that would not performance measures. have otherwise been implemented nor permanently instituted Similarly, certain airport APM systems have the capabil had the spike in failures not have occurred. ity to automatically couple APM vehicles into larger train consists and immediately insert these trains into service to rapidly respond to either increased demand in the system or 6.2Measures for Planning and to degraded operational modes brought on by one or more Designing Airport APM Systems mainline failures. This automated coupling feature, along with APMs play a critical role in transporting passengers effi the crossover quantity/placement issue mentioned previously, ciently and safely at airports and therefore need to be highly can allow a system to be rapidly reconfigured and maintain dependable and safe. The measures that can be used for plan the same line capacity in a degraded operational mode as in ning and designing airport APM systems largely include the nominal operational modes. The consideration of this feature availability-, reliability-, and maintainability-type measures in the planning and design phases of an airport APM is again similar to those discussed in previous sections of the guidebook. driven, in part, by satisfying the high targets set for the avail For example, during the planning and design phases of an ability performance measures. airport APM system, APM system suppliers determine the Safety is also extensively considered in the planning and dependability, availability, reliability, and/or maintainability design of airport APM systems, and performance measures are values of subcomponents, equipment, and subsystems, based used to quantify and verify during the design phase that hazards on certain anticipated failure rates, to verify that the overall sys are limited to essentially negligible levels. For example, the tem will meet the target of the particular performance measure mean time between hazard element (MTBHE) performance once service is established. Often, these targets are stringent measure is used, in part, to verify and validate that the duration due to the critical role APM systems play at airports. of time between hazard elements for the ATC system meets or When planning APMs, such high performance standards can exceeds the required level. be considered in developing alignments and configurations that Other measures that can be considered during the planning will allow APM system suppliers to provide the equipment and and design of an airport APM system include the walk distance equipment redundancies necessary to ensure that the system to airport APM stations, number of level changes to and from will perform at these levels. For example, there could be several the APM station, the APM platform size and configuration crossovers in an airport APM system that allow APM vehicles to [i.e., center platform, side platforms, triple (flow-through) operate around a failed section of track and still move passen platforms, etc.], ultimate design line capacity of the system, gers among airport facilities fairly efficiently. The consideration travel time, and passenger vehicle space allocations. These of crossover quantities and placement during the planning are discussed in ACRP Report 37: Guidebook for Planning and and design phases of an airport APM system is driven, in part, Implementing Automated People Mover Systems at Airports.