Publications

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.

On June 1, 1999, American Airlines flight 1420 , arriving at Little Rock, AR from Dallas-Fort Worth, TX, was involved in a fatal accident upon landing, on runway 4R at Adams Field (LIT). There were eleven casualties, including the pilot, and numerous injuries among the 145 passengers and crew on board. At the time of the accident, 0451 UTC (11:51 PM CDT), severe thunderstorms existed in the vicinity of the airport. These storms were initiated by an approaching cold front and pre-frontal trough and were developmentally aided by veering low-level wind and warm air advection, which helped to further destabilize the atmosphere. This report will focus on the meteorological conditions preceding and immediately following the accident that could have played a contributing role in the crash. However, no theories on the actual cause will be put forth.

Automated air traffic decision support tools must compute the time it takes an aircraft to fly along a path. The estimation of Time-To-Fly (TTF) requires accurate knowledge of the wind. Two proposed sources of wind data for the Center-TRACON Automation System (CTAS) developed by NASA are the Rapid Update Cycle (RUC) and the Integrated Terminal Weather System (ITWS). The RUC is a mesoscale numerical weather prediction model run by the National Centers for Environmental Prediction. The ITWS was developed by MIT Lincoln Laboratory for the FAA. The ITWS winds product, Terminal Winds takes in RUC forecasts and refines them using recent local measurements of the wind from Doppler radars, aircraft, and ground stations. This report examines the question: does the use of RUC and ITWS wind fields lead to different Time-To-Fly estimates?

Adverse terminal weather is a key factor in the safety and efficiency of airline operations. Weather has been directly related to many of the air carrier accidents with fatalities in the 1990's, and the cost to airlines per year for weather delays is estimated to exceed one billion dollars, with at least half of this arising from convective weather. This paper discusses the airline operations center (AOC) use of information from the Federal Aviation Administration (FAA) terminal weather systems to improve safety and operational efficiency (e.g., reduce delays and diversions, improve predictability, and airline schedule integrity) during severe or rapidly changing conditions. Historically (e.g., prior to 1992), the FAA terminal weather information capability was fairly rudimentary, and airlines had no access to the information. However, with deployment of the ITWS, the ASR-9 Weather Systems Processor (WSP) production systems, and CDMnet (and perhaps Internet) product servers for ITWS and WSP airlines will have access to the products. Thus, it is important now to consider how these products could be used operationally and what refinements should be made to the ITWS/WSP products to better meet the needs of airline users.

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.

Gust front detection is an important Initial Operational Capability (IOC) of the Integrated Terminal Weather System (ITWS). The Machine Intelligent Gust Front Algorithm (MIGFA) being deployed for ITWS uses multi-dimensional, knowledge-based signal processing techniques to detect and track gust fronts in Terminal Doppler Weather Radar (TDWR) data. Versions of MIGFA have also been developed for the ASR-9 Weather Systems Processor (WSP) and NEXRAD, and within the past year MIGFA was installed as the primary gust front detection algorithm for operational TDWRs throughout the United States. (Not complete.)

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.

Air traffic delay due to convective weather reached historically high levels in 1999, as passengers blamed airlines and airlines blamed the FAA for the massive inconveniences. While coordination between the FAA's System Command Center and the regional centers and terminals can be expected to improve with the FAA's new initiatives, it is clear that air traffic management and planning during convective weather will ultimately require accurate convective weather forecasts. In addition to improving system capacity and reducing delay, convective forecasts can help provide safer flight routes as well. The crash of a commercial airliner at Little Rock, AR in June 1999 after a one-hour flight from Dallas/Ft. Worth illustrates the dangers and potential tactical advantage that could be gained with frequently updated one-hour forecasts of convective storms. The Terminal Convective Weather Forecast (TCWF) product has been developed by MIT Lincoln Laboratory as part of the FAA Aviation Weather' Research Convective Weather Product Development Team (PDT). Lincoln began by consulting with air traffic personnel and commercial airline dispatchers to determine the needs of aviation users (Forman, et. al., 1999). Users indicated that convective weather, particularly line storms, caused the most consistent problems for managing air traffic. The "Growth and Decay Storm Tracker" developed by Wolfson et al. (1999) allows the generation of up to 1-hour forecasts of large scale, organized precipitation features with operationally useful accuracy. This patented technology. represents a breakthrough in short-term forecasting capability, providing quantitative envelope tracking as opposed to the usual cell tracking. This tracking technology is now being utilized in NCAR's AutoNowcaster (Mueller, et al., 2000), the National Convective Weather Forecast running at the Aviation Weather Center (Megenhardt, et al., 2000) and by private sector meteorological data vendors. The TCWF has been tested in Dallas/Ft. Worth (DFW) since 1998, in Orlando (MCO) since 1999, and in New York (NYC) since fiscal year 2000 began. These have been informal demonstrations, with the FAA William J. Hughes Technical Center (WJHTC) assessing utility to the users, and with MIT LL modifying the system based on user feedback and performance analyses. TCWF has undergone major revisions, and the latest build has now been deployed at all sites. The TCWF is now in a formal assessment phase at the Memphis international Airport as a prerequisite to an FAA operational requirement. The FAA Technical Center will make a recommendation on whether TCWF is suitable for inclusion in the FAA's operational integrated Terminal Weather System (ITWS), which has an unmet requirement for 30+ minute forecasts of convective weather. Memphis was selected for the TCWF Assessment since it has not been exposed to the forecast product during prior demonstrations. Operations began on March 24, 2000 and operational feedback is being assessed by the FAA Technical Center (McGettigan, et al., 2000) and MCR Corporation is performing a quantitative benefits assessment (Sunderlin and Paull, 2000). This paper details the refined TCWF algorithm and display concept, gives examples of the operational impact of terminal forecasts, and analyzes the technical performance of the TCWF during the early stages of the Memphis Assessment.

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.