Detecting Turbulence That No One Can See

As United Airlines jet liners roll out of hangars after routine maintenance checks, they are carrying something new: instructions in their flight computers that turn the airliners' sophisticated navigation systems into flying turbulence detectors.

The software, which United began installing a few months ago, is part of a federal effort aimed at helping commercial pilots and their passengers avoid potentially fatal encounters with rough air.

Sponsored largely by the Federal Aviation Administration (FAA) Aviation Weather research program, the effort could accelerate following a jumbo jet's encounter with turbulence over the Pacific Ocean Sunday. Two hours after leaving Narita, Japan, on a flight to Honolulu, United Airlines Flight 826 plunged 1,000 feet when it flew into what appeared to be clear-air turbulence. One passenger was killed, and 102 other people were injured.

"This is a silent problem of the industry," says Larry Cornman, an atmospheric scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and one of the researchers involved in the federal effort.

Airliner encounters with turbulence seldom generate much publicity, he explains, yet they represent one of the industry's most costly safety concerns. The problem is persistent and leads to lawsuits, workers' compensation claims, and sometimes-lengthy absences as crew members recover from injuries.

What is turbulence?

Turbulence often is linked to severe weather, such as thunderstorms, which can generate violent updrafts and downdrafts, or to passing weather fronts. But the toughest turbulence to detect, measure, and forecast involves clear air.

At high altitudes, clear-air turbulence is a byproduct of the jet stream, a river of air moving as fast as 170 m.p.h. As the air flow speeds by the slower air around it, friction at the boundaries can generate waves of air that act much like ocean waves - they build until they become unstable and break, generating turbulence.

Closer to the ground, the wind's encounter with mountains can set up turbulence as it tries to flow around and through them. When the wind flows over the top, it can deliver a series of "punches" to the layers of air above it, setting up undulations of air known as gravity waves. These can spread out from their points of origin like ripples in a pond. And like the waves the jet stream spawns, gravity waves can grow as they travel, until they, too, break.

To help pilots avoid the bumps, airlines provide turbulence outlooks for each leg of a flight. But these are only broad forecasts.

"You might get something that says: expect moderate turbulence between 4,000 and 15,000 feet over Utah, Colorado, and Wyoming between noon and midnight," says Peter Neilly, another NCAR scientist working on the turbulence problem. "But the atmosphere over any specific location can change in periods of minutes."

When they won't serve peanuts

Continental Airlines, for example, provides its pilots with "shear codes," says Tim Sullivan, who flies Boeing 737s for the airline. "If we see a 4 or 5" on a scale of 0 to 9, "we'll start sitting people down. If we see a 6 or 7, we won't serve food or we'll look for a different altitude."

Such forecasts are based on weather and terrain conditions, as well as reports from pilots already in the air. Yet turbulence can be short-lived and pilot reports can be laced with uncertainties, researchers say. A pilot buckled to a seat may feel a mild jolt, while the same bump may feel like a whip cracking to a flight attendant near the tail. If the turbulence is severe, pilots are more concerned about controlling the plane than taking notes.

Moreover, transcontinental flights have the benefit of a dense network of weather stations on the ground, as well as heavily traveled air lanes to provide information. Transoceanic flights, by contrast, are notoriously data poor.

To deal with these issues, research sponsored by the FAA is focusing on a few broad areas, and perhaps the most critical is gaining accurate information on in-flight turbulence. The new software that United is testing provides an objective measure of turbulence, which will be beamed once a minute from each aircraft to a data collection center and made available on the Web to a limited number of users. For now, only five jets have it, but Dr. Cornman says that number should increase to 210 by next summer. And once the program takes hold, other major carriers are expected to follow.

The in-flight information not only will be of interest to researchers, but it also will begin to feed into the computer programs the National Weather Service uses to forecast weather. The goal is to feed the data into forecast models designed to give pilots estimates of where turbulence will be one to three hours in advance.