First Test Flights Are A Hit: Onboard Sensor Reveals Invisible Turbulence Ahead Of Aircraft In Time To Issue Warnings

April 06, 1998

BOULDER -- During early test flights aboard a research aircraft, a lidar sensor seeking clear-air turbulence over the Rocky Mountains signaled choppy air ahead, which aircraft occupants experienced seconds later as the plane flew through the turbulent patches. The lidar could eventually be part of a detection and warning system used on commercial planes to prevent injuries to passengers and crew.

An Electra research aircraft owned by the National Science Foundation (NSF) and operated by the National Center for Atmospheric Research (NCAR) tested the sensor on two flights over the Rocky Mountains last week out of Jefferson County Airport near Denver. More flights are planned through April 10.

The experiment is a collaboration among the National Aeronautics and Space Administration (NASA), NSF, and Coherent Technologies, Inc. (CTI), of Lafayette, Colorado.

The NASA Dryden Flight Research Center is leading the experiment, called ACLAIM (the Airborne Coherent Lidar Advanced In-flight Measurement). CTI built the Doppler lidar sensor, which uses laser beams to track the motions of natural aerosol particles, some as small as a millionth of a meter, as they swirl in turbulent air several kilometers ahead of the plane.

On the first shakedown flight on March 23, "the system clearly established the viability of this technique by detecting even mild turbulence ahead,". said NCAR project manager Allen Schanot, who was aboard the plane. On the second flight the lidar successfully detected moderate turbulence eight to ten seconds in advance of the plane as it flew 300 miles per hour several times into disturbed air along the mountain ridges near Pueblo, Colorado. The Electra cruised at altitudes between 20,000 and 25,000 feet during the first test flights. Commercial aircraft normally fly between 30,000 and 40,000 feet.

NCAR atmospheric scientist Larry Cornman will work with CTI scientists to analyze the data and assess the ability of the lidar to quantitatively measure turbulence. Says Cornman, "The early results are an encouraging first step. Now that we know the device can 'see' clear-air turbulence just ahead, there's incentive to make it powerful enough to look farther and extend warning times."

Cornman has studied turbulence for over a decade. He devised a means for calculating atmospheric turbulence using measurements of aircraft response motions. United Airlines is currently testing his method on commercial aircraft.

Because clear-air turbulence is short-lived, chaotic, and invisible to both the eye and radar, it is difficult to detect and forecast. During the experimental flights the Electra's pilot seeks turbulent areas predicted by meteorologists at NCAR and NASA Dryden. Unlike radar, which uses radio waves, CTI's lidar sensor shoots an infrared laser beam forward into expected turbulence in the aircraft's flight path. Dust particles and aerosols reflect the laser beam back to the plane, characterizing the turbulent air motions ahead. When the plane encounters the choppy air some seconds later, its response (bouncing, falling) is measured and the atmospheric turbulence inferred is later compared to that detected in the forward-looking lidar data. If the two mesh, the lidar could prove useful on commercial aircraft for detecting clear-air turbulence in time for pilots to instruct passengers and crew to be seated and fasten their seat belts before injuries occur.

According to Dryden project manager Rod Bogue, "During the tests, the system observed turbulent regions ahead of the aircraft, and the aircraft experienced disturbances as it penetrated the turbulence. If an alarm were sounded when the turbulence was first detected, passengers would have been able to return a short distance to their seats, seat themselves, and fasten seat belts prior to the encounter."

Conventional radar already installed on commercial aircraft may play a future role in detecting another type of disturbance-- convective turbulence--which occurs in or near clouds. In Juneau, Alaska, NASA and AlliedSignal Inc. are currently testing one such device for this purpose. Since the lidar works only in clear skies, the two systems would complement each other in detecting both clear-air and convective turbulence.

Turbulence plunged a United Airlines plane 300 meters (900 feet) last December, killing one passenger and injuring over 100 other people on a flight from Japan to Hawaii. In other incidents, turbulent air has ripped off airplane engines, snapped wings in two, hurled food carts to the ceiling, and broken passengers' and flight attendants' bones. Each year societal costs resulting from turbulence-related incidents reach almost $100 million for human injuries, aircraft damage, and government investigations. Turbulence is the primary cause of nonfatal injuries to airline passengers and crew.

"In the past, turbulence research related to aviation problems has been intermittent," says Cornman. "Now, NASA and the Federal Aviation Administration are conducting cohesive programs such as ACLAIM, focused directly on turbulence problems. With these national resources, researchers have a better shot at developing accurate detection devices and reliable warnings for safer and more comfortable air travel in the future."

NCAR is managed by the University Corporation for Atmospheric Research (UCAR) under sponsorship by the National Science Foundation. UCAR is a consortium of more than 60 universities offering Ph.D.s in the atmospheric or related sciences.
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National Center for Atmospheric Research/University Corporation for Atmospheric Research

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