Publications

Knowledge of present and future winds and temperature is important for air traffic operations in general, but is crucial for Decision Support Tools (DSTs) that rely heavily on accurately predicting trajectories of aircraft. One such tool is the Center-TRACON Automation System (CTAS) developed by NASA Ames Research Center. The Rapid Update Cycle (RUC) system is presently the principal source of weather information for CTAS. RUC provides weather updates on an hourly basis on a nationwide grid with horizontal resolution of 40 km and vertical resolution of 25 mb in pressure. However, a recent study of RUC data availability showed that the NWS and NOAA servers are subject to frequent service interruptions. Over a 210 day period (4/19/00-11/11/00), the availability of two NOAA and one NWS RUC server was monitored automatically. It was found that 60 days (29%) had periods of one hour or more where at least one server was out, with the longest outage lasting 13 hours on 9/21/00. In addition, there were 9 days (4%) for which all three servers were simultaneously unavailable, with the longest outage lasting 6 hours on 5/7/00. Moreover, even longer outages have been experienced with the RUC servers over the past several years. RUC forecasts are provided for up to 12 hours, but these are not currently used in CTAS as back up sources (except that the 1 or 2 hour forecasts are used for the current winds to compensate for transmission delays in obtaining the RUC data). Since RUC outages have been experienced for longer than 12 hours, it is therefore necessary to back RUC up with another weather source providing long-range forecasts. This paper examines the use of the Eta model forecasts as a back-up weather sources for CTAS. A specific output of the Eta km model, namely Grid 104, was selected for evaluation because its horizontal and vertical resolution, spatial extent and output parameters match most closely those of RUC. While RUC forecasts for a maximum of 12 hours into the future, Eta does so for up to 60 hours. In the event that a RUC outage would occur, Eta data could be substituted. If Eta data also became unavailable, the last issued forecasts could allow CTAS to continue to function properly for up to 60 hours. The approach used for evaluating the suitability of the Eta model and RUC forecasts was to compare them with the RUC analysis output or 0 hour forecast file, at the forecast time. Not surprisingly, it was found that the RUC model forecasts had lower wind magnitude errors out to 12 hours (the limit of the RUC forecasts) than the Eta model had. Hosever, the wind magnitude error for the Eta model grew only from 9 ft/s at 12 hours (comparable with RUC) to 11 ft/s at 48 hours. We therefore conclude that RUC forecasts should be used for outages up to 12 hours and Eta model forecasts should be used for outages up to 60 hours.

Many major airports in the U.S. rely on simultaneous approaches to closely-spaced parallel (CSP) runways to maintain a high airport acceptance rate. During Visual Meteorological Conditions (VMC), aircraft are able to utilize both runways by making side-by-side landings and are able to meet the demands of heavy volume. However, when conditions deteriorate to marginal-VMC or Instrument Meteorological Conditions (IMC), side-by-side approaches are not possible due to the inherent safety concerns associated with lowered ceilings and visibilities. This situation is severely limiting to an airport's capacity and can create large delays and increased costs. Various ideas have been suggested that would facilitate the simultaneous use of CSP runways during low ceiling and visibility (LCV) conditions at capacity-restricted airports. This report addresses the specific scenario of a pair of approaching aircraft being staggered by some longitudinal distance. This situation alleviates the collision hazard presented by LCV conditions, but also introduces the hazard of a wake vortex encounter, particularly if the following aircraft is downwind of the leading aircraft.

Aviation weather refers to any type of weather that can affect the operation of an aircraft – anything from a brief delay in departure to a catastrophic accident during flight. Wind shear and events associated with convective weather were recognized as an aviation hazard long before Dr. Theodore Fujita began publishing his now-famous treatises. On July 28, 1943, American Airlines Flight 63 from Cleveland, Ohio, USA to Nashville, Tennessee crashed after the pilot lost control of the Douglas DC3. The pilots and numerous passengers were fatally injured. The aircraft was destroyed by impact and post crash fire. The weather report at the time included warnings for storms, heavy rain, lightning and severe turbulence. The Civil Aeronautics Board found that the probable cause was a loss of control of the aircraft due to unusually severe turbulence and violent downdraft caused by a thunderstorm. In the ten-year period from 1987 through 1996, 24% of all U.S. accidents were judged to be "weather related". For the twenty-year period 1976 to 1996 fully 43% of U.S. accidents were judged to have involved wind or wind shear, and 2.3 % thunderstorm, although the two data elements are not mutually exclusive. In the U.S., approximately 82% of accidents are general aviation; the rest are air carriers and commuters of various types. When general aviation accidents are negated, and only air carriers are considered, wind and wind shear issues account for 9.5% of accidents. The Weather Systems Processor (WSP) has been developed to reduce the impact of severe weather conditions on air traffic by providing information concerning weather conditions in the airport terminal environment. WSP provides warnings to air traffic controllers and supervisors of hazardous wind shear and microburst events in the terminal area, forecasts the arrival of gust fronts, and tracks thunderstorms, providing a complete picture of current and future terminal area hazardous weather conditions that may impact runway and airport usage. Common weather situation awareness allows Terminal Approach, Tower Controllers and other traffic management personnel to jointly plan with confidence and safely manage more arrivals and departures with less delay. Knowledge of the location, severity and movement of hazardous weather allows dynamic adjustments to be made in routing aircraft to runways, approach and departure corridors, terminal arrival and departure transition areas (i.e. gate-posts) and other air routes.

Thunderstorms are dynamic obstacles to the flow of air traffic. Aircraft routing in the presence of thunderstorms is as dynamic as the position and intensity of the storms. The question of where pilots will and will not fly is relevant to the decisions made by human air traffic managers as well as to the development of automated decision aid tools. In order to accurately anticipate which routes will be useable one needs to be able to 1) forecast the relevant weather variables, and 2) convert those weather variables into a quantitative probability that pilots will request deviations from the nominal route. The Convective Weather Integrated Product Team at the FAA is improving the accuracy and lead time of forecasts of thunderstorm products. This paper provides an update on our examination of the issue of probability of deviation. In our recent examination of 63 hours of weather and flight track data from the DFW airspace (Rhoda and Pawlak, 1999a,b) we combined several weather variables (measurements, not forecasts) to correctly predict pilot deviation and penetration behavior for 70-85% of the encounters between thunderstorms and aircraft arriving at DW and Dallas Love (DAL) airports. We also found that pilots were more likely to penetrate strong precipitation when they: 1) were near the arrival airport, 2) were following another aircraft, 3) were flying after dark, 4) had been delayed in the air by 15+ minutes upstream of the DFW airspace. We did not find any statistically significant difference between the percentages of thunderstorm penetrations by various airlines. We also found that persistent penetration of storms near the airport is sometimes abruptly interrupted presumably by wind shear alerts from air traffic controllers or cautionary pilot reports from the penetrating aircraft. When the arrivals cease, aircraft on the final approach course may turn suddenly to the left or right to avoid the weather that caused the interruption. Aircraft that abort the approach sometimes fly through very intense precipitation-sometimes through downdrafts that are causing microburst outflows at the surface. The work described in this paper applies the methodology from the DFW study to data collected in the Memphis Terminal Radar Approach Control (TRACON). The methodology is described briefly here and in more detail in (Rhoda and Pawlak, 1999b). We developed several probability of deviation classifiers using a portion of the Memphis data and tested them on the remaining data to determine if it is possible to predict whether pilots will penetrate or deviate around the storms. We also tested the classifiers that were developed in the DNV study on the MEM data and vice versa. We repeated the DFW hypothesis tests for various dichotomies of encounters: near/far, leading/following, light/dark, delayed/undelayed.

In this paper we present results from a recently completed study of weather sensing and data fusion to improve safety and reduce delays at major west coast airports. With the exception of a summer stratus burn-off prediction project at San Francisco, these airports have received much less attention in terms of advanced FAA terminal weather decision support systems than major airports east of Los Angeles. This is because the principal concern for terminal weather decision support to date has been microburst-induced wind shear, which is very infrequent at the west coast airports. However, three factors warrant a reexamination of weather decision support provided to these major west coast airports: 1. The increased emphasis on significantly improving aviation safety while reducing delays at major airports in the face of expected increases in operations rates within the National Airspace System (NAS), 2. New air traffic management technology such as terminal automation, collaborative decision making (CDM), and weather adaptive wake vortex spacing systems, and 3. Advances in terminal weather decision support technology represented by the Integrated Terminal Weather System (ITWS) [including various P31 enhancements to ITWS (Evans and Wolfson, 2000)] The airports considered in this study were the Los Angeles (LAX), San Francisco (SFO), Portland (PDX) and Seattle (SEA) International Airports. It should be noted that because these airports did not receive a Terminal Doppler Weather Radar, there currently is no plan to provide them with an ITWS. LAX, SF0 and PDX are scheduled to receive an ASR-9 Weather System Processor (WSP). The paper proceeds as follows. Section 2 discusses the study's methodology and provides background information on delays and weather phenomena for these airports in the context of other major US airports as well as applicable air traffic management (ATM) and terminal weather system technology. Section 3 summarizes the principal findings for the four airports. We conclude with a summary of the potential benefits of improved weather sensing and data fusion that might be provided at these west coast airports by an augmented ITWS as well as recommendations for further studies.

Operational experience with the Integrated Terminal Weather Systems (ITWS) and Airport Surveillance Radar, Model 9, (ASR-9) Weather System Processor (WSP) demonstration systems, studies of pilot weather avoidance decision making), and recent accidents have demonstrated the need to provide timely, accurate information on the location and movement of storms to air traffic controllers, pilots, and airline dispatch. At medium-intensity airports, generally those with too few flight operations to justify the presence of Doppler radar systems like the Terminal Doppler Weather Radar (TDWR) or the WSP, terminal air traffic surveillance is currently provided with the ASR-7 and ASR-8 radar systems. The ASR-7 and ASR-8 do not provide calibrated precipitation intensity products or any storm motion information. The Medium-Intensity Airport Weather System (MIAWS) program is intended to address these terminal weather information deficiencies. MIAWS-generated products would be displayed to tower and Terminal Radar Approach Control (TRACON) supervisors and delivered to aircraft cockpits and airline dispatchers to assist pilots during landings. Initially, the MIAWS will provide a real time display of storm positions and motion based on Next Generation Weather Radar (NEXRAD) product data using a product generation and display system derived from the WSP. Airport wind and wind shear information will be acquired from an FAA Low Level Wind Shear Alert System (LLWAS). A demonstration system will be installed and demonstrated at experimental sites in Memphis, TN and Jackson, MS in 2000 and potentially at a third site in 2001. This demonstration system will be used to assess technical and operational issues such as compensation for the relatively slow updates of the NEXRAD products and, Anomalous Propagation (AP) ground clutter. The ASR-11 is a replacement for the ASR-7/8 radars that feature a weather reflectivity processing channel. When it becomes available at MIAWS locations, the MIAWS processor will acquire and display precipitation and storm movement products derived from the ASR-11. Likewise, when an LLWAS Relocation/Sustainment (LLWAS-RS) (Nilsen, et al., 1999) becomes available at MIAWS locations, the MIAWS will acquire wind and wind shear information derived from the LLWAS-RS.

Currently, the prototype Integrated Terminal Weather System (ITWS) displays six-level precipitation data generated from the Airport Surveillance Radar (ASR-9) and the Next Generation Weather Radar (NEXRAD). The ASR-9 data are updated every 30 seconds and provide a 0.5 nm spatial resolution to a distance of 60 nm (Weber, 1986). Since the ASR-9 is a fan beam radar, the data represent the average precipitation within the vertical column. As reported by Isaminger, et al., (1999), this sensor can significantly underestimate the precipitation intensity and areal coverage due to precipitation processing limitations and hardware failures. In particular, storms located near the sensor can be underestimated or missed entirely (Crow& et al., 1999). The NEXRAD data are updated every 5-6 minutes with a spatial resolution of 0.5 nm (2.2 nm) and a coverage region of 100 nm (200 nm). The maximum reflectivity value in the vertical column at each grid point is used to create the product. This sensor can overestimate the precipitation intensity near the surface due to bright band contamination and the composite technique (Crowe and Miller, 1999). The update rate can also become an issue if the storms are moving rapidly or developing quickly. In order to confront these issues, the specified ITWS product suite will include six-level precipitation derived from the Terminal Doppler Weather Radar (TDWR). The data from this sensor will be depicted in a high-resolution window (5-nm) around the airport. The TDWR one-minute update rate will provide timely information on rapidly moving or developing storm cells. In many regards, the data will be complimentary to that provided by the ASR-9 and NEXRAD. In others, the weather levels could vary significantly. This report will focus on a discussion of the 5-nm product capabilities and limitations based on an analysis of data collected in Memphis (MEM) and New York City (NYC). A discussion of key product enhancements will serve to illustrate the modifications required to improve this product suite. Finally, a list of recommendations will be presented to assist in product development.

Wind shear and lightning are classically associated with summertime convective weather events at airports east of Reno, NV. However, a recent study concluded that severe wind shear and lightning strike events occasionally occur during winter storms at west coast airports. One of the most surprising findings was that the Portland Oregon International Airport (PDX) has operationally significant vertical wind shear and a surprisingly high number of lightning strikes to aircraft within the terminal area during winter storms. The FAA has for a number of years planned to install an ASR-9 Weather System Processor (WSP) at PDX to provide protection against wind shear from microbursts and gust fronts. However, in view of the findings of the west coast weather study (conducted after the FAA's wind shear deployment study was completed, a research program was undertaken to: Better understand the phenomenology associated with the Portland winter storms; Determine whether the baseline ASR-9 Weather System Processor planned for PDX would adequately address operationally significant wind shear and other safety-related weather phenomena; and Identify alternative sensing/data fusion approaches to providing PDX terminal weather decision support if the WSP alone could not adequately provide safety warnings.

On December 6th, 1998, a fatal accident involving a twin engine Beech Baron occurred near the Max-Westheimer Airport at Norman Oklahoma (OUN). Although the National Transportation Safety Board (NTSB) conducted an extensive investigation into this accident, the probable cause for the accident has yet to be determined. Since the accident occurred outside of weather echoes that might be considered hazardous, it seems difficult to deduce a meteorological explanation for this accident. However, Doppler radar data suggested the presence of wave formations near the site of the accident. This report reflects examination of the data provided by the NTSB.

The Integrated Terminal Weather System (ITWS) is a capital investment of the Federal Aviation Administration (FAA) to provide a fully-automated, integrated terminal aviation weather information system that will improve the safety, efficiency, and capacity of major terminals. The ITWS acquires data from FAA and National Weather Service sensors as well as from aircraft in flight within the terminal area. Demonstration systems are being operated by the Massachusetts Institute of Technology's Lincoln Laboratory (MIT/LL) Weather Sensing Group at four airport terminal areas: New York, NY; Orlando, FL; Memphis, TN; and Dallas/Ft. Worth, TX. Real-time graphical weather information from the ITWS demonstration systems is relayed to primary users (airport towers, en route centers, TRACONS, the Command Center, and major airlines, etc.) via a situation display (SD) that consists of a Sun workstation and, a dedicated data line to the ITWS site. For users who do not have access to a fully operational SD or who want additional flexibility for accessing the ITWS information, MIT/LL operates a demonstration ITWS web server that provides the information for viewing with commercial-off-the-shelf (COTS) web browsers over the Internet and via the Collaborative Decision Making Network (CDMnet). This distribution of ITWS products has provided shared situational awareness between widely separated users. By sharing a common view of the same operational environment, controllers, dispatchers and other aviation decision makers and stakeholders have been better able to understand and coordinate the decisions that affect air traffic in the terminal area and surrounding en route airspace. In particular, by having up-to-the-minute weather information readily available to airline dispatch, safety during hazardous weather in the terminal area has been improved on a number of occasions at the ITWS demonstration sites (Evans, 2000). With the upcoming deployment of the ITWS as an operational FAA system to 44 major airports, a priority for the FAA is the distribution of the ITWS information from the production systems to airline dispatch and other non-FAA users. The operational ITWS is not designed to support SDS at the major airlines. Hence, distribution of ITWS information via a mechanism such as the Internet and the CDMnet is essential if the safety and coordination benefits achieved with the ITWS demonstration systems are to be obtained with the production ITWS. Because many airlines do not allow Internet access at all locations within the dispatch office, the current plan is to use CDMnet as the primary vehicle for ITWS data distribution to non-FAA users. However, to increase the availability of ITWS information to the broader ITWS user community, efforts are underway to make the data available on the Internet as well. Use of the Internet and CDMnet could also facilitate low-cost distribution of the ITWS information to additional FAA and non-FAA users alike. This paper describes the evolution of the ITWS demonstration web server, discusses the design of the web server and data processing, details how to access the web page and what products are currently available, presents some access statistics and current airline users, and discusses some future work which will allow for wide distribution of the production ITWS information.