FAA And NCAR Chart Juneau's Turbulent Skies

The UND Citation is a high-performance research jet that is taking to the air over Juneau in winds that keep other aircraft on the ground. The National Aeronautics and Space Administration (NASA) is providing collaborative funding, and Alaska Airlines and the city and borough of Juneau are providing in-kind support for the project. NCAR's primary sponsor is the National Science Foundation.

The mountains that tightly encircle Juneau Airport don't just restrict flight paths; they also set up a complex wind-flow pattern. Under certain wind conditions, the only way to fly out is by making a 180-degree turn while turbulent winds coming over the mountains buffet the aircraft.

Accidents caused by turbulence cost U.S. commercial airlines about $100 million a year, according to a recent analysis of airline and National Transportation Safety Board data by NCAR's Tenny Lindholm. According to the NTSB, encounters with turbulence account for the majority of nonfatal air passenger injuries. Last December, en route from Japan to Hawaii, a United Airlines Boeing 747 caught in turbulence dropped 30 meters (100 feet), resulting in one death and more than 100 injuries.

Turbulent winds at Juneau Airport were implicated in severe upsets at least three times in the early 1990s. To reduce this risk, the FAA required that two major departure routes be closed to commercial aircraft whenever three anemometers on nearby mountains measure wind speeds of over 35 knots (40 miles per hour). But these threshold standards don't account for wind direction, which is also important at Juneau. Nor do they provide a quantitative evaluation of turbulence at the airport.

In addition to instituting the current guidelines, the FAA asked NCAR to figure out ways to keep Juneau's turning-departure routes open to commercial aircraft and give pilots up-to-the- minute warnings of dangerous winds. The current research may lead to a new prototype system in Juneau to detect turbulence and warn pilots in time to prevent injuries to passengers and crew. "The goal is to quantify the hazard at any point in time and to base operational decisions on those data," says NCAR scientist Peter Neilley, who coordinates field operations for the project.

Testing, testing

For field testing, the NCAR team, led by Larry Cornman and Robert Barron, has set up three Doppler wind profilers, a forward-looking wind-shear radar, and a Doppler lidar. Six more anemometers are measuring winds in key locations closer to sea level.

A Doppler wind profiler is an upward-looking radar that can measure winds and turbulence both in clear air and when clouds are present. It gives researchers a profile of horizontal winds and turbulence at 60-meter increments up to about 2.5 kilometers (about 200-foot increments up to about 1.5 miles). NCAR researchers first tested the profilers in Juneau's convoluted terrain during a feasibility study in March of 1997.

Also being tested is a forward-looking wind-shear radar manufactured by AlliedSignal Inc. This instrument senses turbulence in the presence of raindrops or ice particles; it can't see in clear air. There are about 1,000 airborne wind-shear radars mounted on commercial airplanes to detect wind shear at low altitudes, although for this experiment the radar is mounted on a truck. The Juneau experiment will determine whether this instrument can be used to detect turbulence in the presence of precipitation at any altitude. Since the radar is already flight- tested and FAA-certified for commercial aviation, "This could be an opportunity to provide some near-term convective-turbulence detection capability for the airlines," says Cornman.

While the forward-looking wind-shear radar's radio waves need precipitation to reflect signals, Doppler lidars bounce their laser beams off of dust particles and aerosols in clear air to detect wind motion. The new generation of lidars provides both rapid scanning and very high spatial resolution as far away as ten kilometers (just over six miles). The lidar deployed at the Juneau Airport was manufactured by Coherent Technologies Inc., of Lafayette, Colorado. NCAR researchers are evaluating the lidar as a ground-based turbulence detector. In another part of the NASA- sponsored lidar evaluation program, researchers are testing airborne lidar aboard the National Science Foundation/NCAR Electra aircraft.

Bumpy flights

The UND Citation aircraft takes 25 high-precision wind and turbulence measurements per second as it flies along the airport's departure and approach routes in high winds. The researchers use the aircraft readings to verify and fine-tune the ground-based instrument measurements. They check the mathematical formulas they are developing to infer turbulence strength against the real-world data from the Citation. The researchers are also collecting local pilots' reports of hazardous conditions to calibrate the system to the capabilities of the aircraft flying in and out of Juneau.