All passengers and crew died in the United crash on approach to the Colorado Springs airport. After an exhaustive investigation, the National Transportation Safety Board could not identify the cause of the accident but concluded that severe turbulence was one of two factors most likely to have brought the plane down. There was a strong downslope windstorm the day of the crash.
Located just east of Pike's Peak, Colorado Springs lies in the path of powerful winds rushing down the eastern slope of the Rocky Mountains. These winds and related turbulence can disrupt the operation of aircraft attempting to land or take off at the Colorado Springs airport. The FAA requested that researchers from NOAA and NCAR study terrain-induced turbulence at the airport as part of an effort to reduce mountain flying risks. NCAR's portion of the work was funded by NSF through an interagency agreement in response to requirements and funding by the FAA's Aviation Weather Research Program.
Atmospheric turbulence can extend over meters or kilometers. "It's like a cloud," says NCAR's Peter Neilley. "It can completely change its shape within a few minutes." Turbulence is responsible for 43% of weather-related aviation incidents. Pilots face some of their greatest challenges when flying through turbulence over or near mountainous terrain. According to a U.S. General Accounting Office report issued in 1993, the accident rate in 11 western mountain states was nearly 40% higher than in the other 37 continental states. One of the key risk factors was weather.
To track turbulence in Colorado Springs, the researchers are deploying a wide array of surface and airborne instruments. The University of Wyoming's King Air aircraft is flying through the airport's approach and departure corridors, taking 25 atmospheric measurements per second. Balloons bearing atmospheric measuring devices are launched from an NCAR weather van and by staff at Fort Carson Army Base.
A Doppler lidar measures radial wind speeds to distances of over 19 kilometers. Due to its narrow beam, the lidar is able to scan near mountains while avoiding ground clutter and aircraft effects. Four wind profilers and radio-acoustic sounding systems are continuously recording winds, temperatures, and turbulence above the airport and at other sites. A new infrasonic observing system, previously used to detect tornadoes, is being used to detect low-frequency sounds that may be associated with severe winds. Automated weather stations and a dense network of anemometers measure surface airflow. NOAA is providing daily weather briefings at its Forecast Systems Laboratory in Boulder.
In a cooperative effort, various departments of the city of Colorado Springs are providing data from their own weather stations. The airlines--including the airport's two major commercial air carriers, United and Western Pacific--are cooperating with an FAA request that pilots make enhanced reports of turbulence as they fly in and out of the airport.
"We want to find out more about the physical nature of the hazards, test the reliability of the experimental forecasts, examine whether ground-based sensors and algorithms for detection and warning are feasible, and gather data sets to use in numerical models," says NOAA's Al Bedard.
According to Neilley, the Colorado Springs study may lead to the development of a real-time turbulence detection and warning system for pilots and air traffic controllers at that airport. The cutting-edge system would be similar to one recently developed by NCAR for Hong Kong's new Chek Lap Kok Airport, expected to open in 1998. FAA/NCAR turbulence studies under way in Juneau, Alaska, and considered for Anchorage may ultimately lead to warning systems for those airports as well.