Ever since the Wright Brothers flew the first powered airplane, low-altitude encounters with atmospheric turbulence have posed daunting challenges to the in-flight safety of aircraft of all types, including commercial airliners. Weather is typically a cause, direct or indirect, of about a third of all commercial aircraft accidents and over a fourth of all general aviation accidents.
One form of turbulence, wind shear, has been particularly catastrophic. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries.
Aircraft often experience a shift or drop in winds while in flight. But they usually have enough speed and altitude to compensate for the resulting loss of airflow over their wings. During takeoffs and landings, wind shear is a more serious threat because the aircraft is close to the ground and its speed is slower (Fig. 1).
Due largely to the development of ground-based and airborne wind-shear detection and alert systems, airplane flying is safer than ever. This safety record can be traced to the work performed by the National Aeronautics and Space Administration's Langley Research Center in Virginia. NASA's Airborne Wind-Shear Detection and Avoidance Program (AWDAP), a seven-year research and development effort, built the foundation for present commercially available wind-shear radar systems.
Today, nearly 4000 commercial airliners worldwide use wind-shear detection and alert systems based on NASA's AWDAP. "Statistically, the rate of wind-shear-related accidents today can be calculated as one incident in every 10 years, versus one each year before the AWDAP system was developed," says the program's director Roland L. Bowles.
Development actually began in the 1980s when alarm over commercial airliner crashes related to wind shear moved the U.S. government into action. On July 24, 1986, the Federal Aviation Administration (FAA) and NASA signed a memorandum of agreement formally authorizing the start of the AWDAP, and a wind-shear program was created in the Flight Systems Directorate of NASA's Langley Research Center.
The agreement mandated NASA to work with industry to develop a test-bed AWDAP for eventual technology transfer. In September 1988, the FAA ordered all commercial aircraft to have forward-looking airborne wind-shear detection and alert systems by 1993. Three airlines—American, Northwest, and Continental— received exemptions until the end of 1995. However, solving the problem was far more complex than originally estimated, and nearly another decade passed before commercial aircraft began to adopt the technology in earnest.
Many of the commercially available wind-shear radar systems continue to evolve with better features like more advanced warning time, greater detection precision, and fewer false alerts. Ever since wind-shear detection systems have seen commercial service, the record has been spotless. There have been no reported incidents related to wind shear.
The FAA had developed the ground-based Low-Level Wind-Shear Alert System (LLWS) in 1976 as well as a ground-based technology called Terminal Doppler Weather Radar (TDWR). These systems detected and automatically reported low-altitude wind shears to pilots. They also let air-traffic controllers change landing and takeoff runways to avoid the wind shears. But they weren't sensitive enough to detect all wind shears. It wasn't until an airborne system was developed that the technology of wind-shear detection and alert significantly contributed to aircraft flight safety.
A THREE-PRONGED APPROACH
To get a good handle on the wind-shear problem, NASA researchers working on the AWDAP divided the project into three main elements: hazard characterization, sensor technology, and flight management. Five years into the program and after intensely studying various weather phenomena and sensor technologies, the researchers needed to validate their findings in actual flight conditions.
They chose an extensively modified Boeing 737 flying laboratory, dubbed the Transport Systems Research Vehicle (TSRV), for the task. The aircraft came equipped with a rear research cockpit in what would have been the forward section of the passenger cabin (Fig. 2). Many flight tests were performed primarily at two locations: Orlando, Fla., where "wet" microbursts containing rain droplets were common, and Denver, Colo., where "dry" microbursts were observed.
In trying to understand the wind-shear phenomenon, Langley researchers used supercomputers to develop meso-scale numerical weather models. They also employed extremely high-fidelity fluid-dynamics models.
In time, researcher Fred Proctor came up with a detailed model known as the Terminal Area Simulation System (TASS). This 3D, time-dependent model included representations of liquid and ice microphysics with the effects of condensation, evaporation, freezing, and sublimation in the atmosphere. Their impact on atmospheric winds could then be used to simulate the time-dependent life cycle of a convective storm, including microbursts. Data sets generated from the TASS model were eventually used by the FAA to certify onboard wind-shear sensors.
After studying the work of meteorological researcher Tetsuya "Ted" Fujita at the University of Chicago, Bowles developed an F-Factor that was key to understanding the wind-shear phenomenon. Fujita developed a severity scale for tornadoes that bears his name. He subsequently coined the term microbursts after extensively studying weather-related aircraft crashes. His work played a key role in understanding the effects of wind-shear microbursts.
Fujita theorized that a concentrated strong 3D air outflow associated with a wind downdraft was the real hazard in airplane encounters with wind shears, particularly during takeoffs and landings. Radar meteorologists have since refined his explanation, defining a microburst as a divergent low-level wind field with a velocity change of at least 5 knots over the wind's surface for distances between 1 and 4 km.
Bowles' F-Factor quantifies the loss in aircraft performance during a specific wind-shear condition. This nondimensional metric considers the aircraft's weight, thrust, and drag as well as the effects of wind-shear velocities on a specific excess thrust (thrust minus drag divided by weight) required to maintain steady flight conditions due to wind variations in a microburst. Bowles, Michael S. Lewis (who was the AWDAP deputy program director), and David A. Hinton later refined this metric to include the length of time over which the aircraft is exposed to the wind shear averaged over a distance of 1 km.
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