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.

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.

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.)

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.

Weather Hazard Information distribution is a necessary component for a successful system of weather hazard avoidance for aviation. It is a very important component, but not the only one. In order to be successful, a complete set of components must be included in the system: 1) Accurate Conceptual Model (Appropriate models of the physical process responsible for generating the hazard); 2) Production Infrastructure (System of tools [hardware, software and manpower]; the raw data feeds necessary for production of the hazard information and a standardized message format); 3) Quality Control Infrastructure (System of tools [hardware, software and manpower] & data feeds necessary for identifying and correcting erroneous information immediately); 4) Distribution Infrastructure (A method to relay, in a timely manner, only the information pertinent to the specific user); 5) Policies and Procedures (There must be clearly defined expectations of actions required of the users and recipients of the hazard information); 5) Training (The users and recipients as well as individuals responsible for production and quality control of the information must receive initial and recurrent training regarding actions required). ICAO in their Annex 3, Chapter 7 titled, SIGMET Information, Aerodrome Warnings and Wind Shear Warnings [ICAO 19981, describes in part one such system for weather hazard avoidance. ICAO does a good job defining the necessary production infrastructure. ICAO especially has been successful in defining the standardized message format. The format for SlGMETs is described in detail in Annex 3. But, an international organization Such as ICAO is limited in its scope of influence. Quality control of the SIGMET product and the distribution of the SIGMET is, in large part, beyond ICAO’s control. In addition, the actual weather hazard avoidance policies, procedures and training must be accomplished internally by each individual commercial aviation operator. Since each component listed above is directly dependent on the other five for a successful weather hazard avoidance system, Northwest Airlines (NWA) has chosen to attempt to address all six components of the system internally with use of the NWA Turbulence Plot System (TPS) [Fahey et. al. 2000].

The U.S. Federal Aviation Administration (FAA) operates over 400 C- to L-band surveillance radars-Airport Surveillance Radars (ASRs), Air Route Surveillance Radars (ARSRs) and Terminal Doppler Weather Radars (TDWRs). Current generation terminal and en route aircraft surveillance radars (ASR-9, ASR-11 and ARSR-4) feature dedicated digital processing channels that measure and display precipitation reflectivity. Some of these "weather channels" will be upgraded to measure Doppler velocity, supporting, for example, wind shear detection at air terminals. The Terminal Doppler Weather Radar is a high quality dedicated meteorological surveillance radar deployed near many of the larger airports in the U.S. In this paper we consider how these radars could complement the WSR-88D network in providing a variety of meteorological services to the U.S. public. Potential benefits from a combined radar network would accrue from significantly increased radar density and the more rapid temporal updates of the FAA radars. Convective weather monitoring and forecasting, hydrological measurements and services to aviation are examples of areas where significant improvements could be expected. Section 2 reviews the status of the FAA radars their parameters, locations and capabilities. We also note the progress of various upgrade programs that will increase their weather surveillance capabilities substantially. In Section 3, we discuss benefits that would result from their usage in conjunction with the WSR-88D network. Finally, we discuss technological developments that will facilitate realization of these benefits.

Major terminals and the surrounding en route airspace are critical elements of the US National Air System (NAS). A large fraction of the US population lives near these terminals, and the bulk of the hub connecting operations are at these airports as well. Adverse weather in these terminal areas and surrounding en route airspace is a major safety concern for the NAS as well as causing a large fraction of all US aviation delays. The principal weather decision support tool for these terminals is the Integrated Terminal Weather System (ITWS) which commenced full-scale development by the FAA in 1995, with first articles to be deployed shortly. In this paper, we discuss how the initial ITWS operational capability needs to be extended to address performance problems identified in operational use and to meet the many new user needs that have developed in the past five years. The paper proceeds as follows. In Section 2, we provide some necessary background on the ITWS operational capability, followed by a discussion of new capabilities to meet urgent user needs. Section 3 discusses refinements to the initial capability to address problems/issues that have been identified based on five years of operational use of ITWS products from ITWS demonstration systems at eight major airports. Next, we consider extending planned ITWS coverage to other major terminals. The final section summarizes the paper's results and suggests additional studies.

San Francisco International Airport (SFO) is unable to use independent parallel approaches to its closely-spaced parallel runways when marine stratus is present in the approach. Delay programs are imposed to regulate the flow of traffic to match the true arrival capacity of the airport. Failure to forecast accurately the times of onset and dissipation of stratus in the approach results in unnecessary delays, costly airborne holding and diversions, or in wasted capacity as the traffic management planners fail to match the arrival rate to the actual airport capacity. Previous studies have shown that accurate 1-2 hour forecasts of the times of clearing in the approach could provide substantial reductions in the delays and inefficiencies associated with the marine stratus impacts on air traffic at SFO. The San Francisco Marine Stratus Initiative has provided a four-year focus on this problem and has resulted in the development of several forecast algorithms that will aid, the operational forecasting of the dissipation of marine stratus in the approach to SFO (Clark and Wilson, 1997). These algorithms involve new techniques for the analysis of observational data and statistical and dynamical prognosis of the behavior of the marine stratus. This discussion of the design and the performance of these algorithms provides an overview of the status of this project.