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46 ramp closures than can large airports. This means that when may enhance safety, but at the cost of excessive downtime. On lightning is near, they may be able to shut down earlier and the other hand, standards designed to minimize disruptions wait a bit longer to declare an "all clear" than major airports may put airport workers at risk. System providers will nor- with higher traffic volumes and tighter schedules. Safety mally recommend an initial set of warning criteria, but allow issues become more critical and require closer, more expen- users to set their own criteria for alerts and warnings based on sive, monitoring of the situation when airport users are trying their collective experience with typical weather patterns at to push the envelope and keep operations going as long as their airport. As a practical matter, this means that the spe- possible without interruption. cific warning criteria used at different airports can vary Warnings based on NLDN monitoring the approach of greatly. active thunderstorms can identify perhaps 90% of the light- One approach to improving this situation and helping ning events that affect an airport, with the remainder coming individual airports and airlines refine their warning criteria from new storms that develop in the immediate vicinity of the would be to make use of intelligent, self-monitoring warning airport (28). To respond to this developing storms hazard, systems. A lightning detection and warning system with this high-end lightning detection systems typically augment the sort of capability would be able to monitor its own perform- NLDN observations with locally installed EFMs, which can ance and evaluate the adequacy of the specific warning criteria detect the buildup of the local electric field that normally pre- being used. Any unanticipated lightning strikes in the imme- cedes lightning. diate vicinity of the airport, or strikes that follow the declara- EFMs do, however, add significantly to the cost of a warning tion of an alert too closely for the ramp to be cleared, would system. An EFM can cost as much as $16,000, and they would be evaluated to see if reasonable changes to the warning need to be installed at several locations around the airport to criteria would have provided a better warning. Such a sys- provide a useful indication of the developing potential for tem could also keep track of excessive warnings or lengthy lightning strikes. Each EFM would require its own set of com- ramp closures and evaluate to what extent safety would have munication cables and regular maintenance to ensure reliable been compromised with slightly more relaxed criteria. The performance. While EFMs are routinely used at lightning- system would be, in effect, self-training and would provide an sensitive locations, such as the Kennedy Space Center in objective approach for making gradual adjustments to the Florida and weapons testing sites, because they can provide specific warning criteria used at an airport in response to early warning of developing storms, they are subject to false the actual lightning events it experiences over time. This alarms since not all developing storms actually produce light- approach could also be used to refine warning criteria to ning. In most cases, the buildup of the electric field should reflect the local storm climatology, and permit seasonable be considered a necessary, but not sufficient, criterion for adjustments to optimize performance. lightning activity (29). For example, consider an airport with a lightning warning From an airport operational perspective, the most impor- system that recommends that outdoor operations be stopped tant improvements that could be made in current lightning whenever a lightning strike is detected within 6 mi of the detection and warning systems would be to develop more airport and declares an "all clear" when there have been no precise and better defined warnings that still give operators additional lightning strikes within this distance for 15 min. As time to effectively clear the ramp and suspend operations, a routine matter, the lightning system could be designed to and then get back to work as quickly as possible with less keep track of the number of recommended alerts and alarms, downtime, but without compromising safety. the duration of the work stoppages, the number of lightning There are a number of promising ways to refine and im- strikes over the immediate airport area (or other designated prove lightning detection and warning systems for airports "area of concern"), and related statistics. Lightning strikes in by making better use of all the currently available weather the area of concern without adequate prior warning would observations, through the development of smarter software be of particular importance and would be identified and and analysis algorithms, and by incorporating new technolo- recorded. In parallel with the statistics for the operational set gies. These options are highlighted in the following sections. of warning criteria, system software could also generate com- parable statistics for other possible combinations of warning criteria. For example, there could be separate statistics gener- Smart Algorithms and Software ated for all distance thresholds from 3 mi to 10 mi, and for The performance of any lightning warning system is criti- "all clear" times from 5 min to 30 min. These statistics would cally dependent on the specific warning criteria that are used be collected and reviewed, perhaps once a year, identifying to stop work and clear the ramp, as well as the guidelines that possible changes to the warning criteria that could improve are subsequently used to decide when to resume work. These airport efficiency, while preserving safety. Any changes of this criteria affect both safety and efficiency. Conservative criteria sort would need to be done gradually and incrementally, but