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First Test Flights Are A Hit: Onboard Sensor Reveals Invisible Turbulence Ahead of Aircraft in Time to Issue Warnings

April 14, 1998
National Center For Atmospheric Research (NCAR)
A clear-air turbulence sensor tested by NASA aboard an Electra aircraft owned by the National Science Foundation and operated by the National Center for Atmospheric Research successfully detected turbulence in Colorado skies.

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

An Electra research aircraft owned by the National ScienceFoundation (NSF) and operated by the National Center forAtmospheric Research (NCAR) tested the sensor on two flights overthe Rocky Mountains last week out of Jefferson County Airport nearDenver. More flights were planned through April 10.

The experiment is a collaboration among the National Aeronauticsand 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-flightMeasurement). CTI built the Doppler lidar sensor, which useslaser beams to track the motions of natural aerosol particles,some as small as a millionth of a meter, as they swirl inturbulent air several kilometers ahead of the plane.

On the first shakedown flight on March 23, "the system clearlyestablished the viability of this technique by detecting evenmild turbulence ahead,". said NCAR project manager Allen Schanot,who was aboard the plane. On the second flight the lidarsuccessfully detected moderate turbulence eight to ten seconds inadvance of the plane as it flew 300 miles per hour several timesinto disturbed air along the mountain ridges near Pueblo,Colorado. The Electra cruised at altitudes between 20,000 and25,000 feet during the first test flights. Commercial aircraftnormally fly between 30,000 and 40,000 feet.

NCAR atmospheric scientist Larry Cornman will work with CTIscientists to analyze the data and assess the ability ofthe lidar to quantitatively measure turbulence. Says Cornman,"The early results are an encouraging first step. Now that weknow the device can 'see' clear-air turbulence just ahead, there'sincentive to make it powerful enough to look farther and extendwarning times."

Cornman has studied turbulence for over a decade. He devised ameans for calculating atmospheric turbulence using measurements ofaircraft response motions. United Airlines is currently testinghis method on commercial aircraft.

Because clear-air turbulence is short-lived, chaotic, andinvisible to both the eye and radar, it is difficult to detectand forecast. During the experimental flights the Electra's pilotseeks turbulent areas predicted by meteorologists at NCAR and NASADryden. Unlike radar, which uses radio waves, CTI's lidar sensorshoots an infrared laser beam forward into expected turbulence inthe aircraft's flight path. Dust particles and aerosols reflectthe laser beam back to the plane, characterizing the turbulent airmotions ahead. When the plane encounters the choppy air someseconds later, its response (bouncing, falling) is measured andthe atmospheric turbulence inferred is later compared to thatdetected in the forward-looking lidar data. If the two mesh, thelidar could prove useful on commercial aircraft for detectingclear-air turbulence in time for pilots to instruct passengers andcrew to be seated and fasten their seat belts before injuriesoccur.

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

Conventional radar already installed on commercial aircraft mayplay 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 suchdevice for this purpose. Since the lidar works only in clearskies, the two systems would complement each other in detectingboth clear-air and convective turbulence.

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

"In the past, turbulence research related to aviation problems hasbeen intermittent," says Cornman. "Now, NASA and the FederalAviation Administration are conducting cohesive programs such asACLAIM, focused directly on turbulence problems. With thesenational resources, researchers have a better shot at developingaccurate detection devices and reliable warnings for safer andmore comfortable air travel in the future."

NCAR is managed by the University Corporation for AtmosphericResearch (UCAR) under sponsorship by the National ScienceFoundation. UCAR is a consortium of more than 60 universitiesoffering Ph.D.s in the atmospheric or related sciences.

Note to Editors: Beta video footage and photos of the Electraare available through NCAR Media Relations. The aircraft may befilmed throughout the experiment with prior arrangement by callingAnatta, NCAR Media Relations, 303-497-8604. Video animationsof the lidar are available from Kirsten Williams at Dryden.

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National Center For Atmospheric Research (NCAR). "First Test Flights Are A Hit: Onboard Sensor Reveals Invisible Turbulence Ahead of Aircraft in Time to Issue Warnings." ScienceDaily. ScienceDaily, 14 April 1998. <>.
National Center For Atmospheric Research (NCAR). (1998, April 14). First Test Flights Are A Hit: Onboard Sensor Reveals Invisible Turbulence Ahead of Aircraft in Time to Issue Warnings. ScienceDaily. Retrieved May 22, 2017 from
National Center For Atmospheric Research (NCAR). "First Test Flights Are A Hit: Onboard Sensor Reveals Invisible Turbulence Ahead of Aircraft in Time to Issue Warnings." ScienceDaily. (accessed May 22, 2017).