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Avoiding Trouble Ahead

By Douglas Page | February 1, 2008
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A ground-based reporting system using information from the nation’s network of Next-Generation (NEXRAD) Doppler radars can help pilots avoid turbulence by transmitting real-time data on clouds and storms.

The system, designed by the National Center for Atmospheric Research (NCAR), has been successfully tested by United Airlines on dozens of scheduled commercial flights. It uses a mathematical scheme developed by NCAR scientists called the NEXRAD Turbulence Detection Algorithm, or NTDA.

NCAR, based in Boulder, Colo., is managed by the University Corporation for Atmospheric Research, a nonprofit consortium of research universities, on behalf of the National Science Foundation and universities.

The goal of the NTDA system is to give airline pilots enough advanced notice of approaching turbulence to take defensive action, or at least turn on the "Fasten Seat Belt" sign before passengers or flight attendants are thrown about the cabin.

Reliable detection of in-flight air turbulence has remained historically elusive.

"While there have been many advances in the attempt to detect turbulence in flight, experience and pilot reports from crews who have flown the same route ahead of you remain the most reliable method," said aviation safety consultant Bill Moyle.

Due to the high false alarm rate of current turbulence warning announcements and seat-belt advisories, passengers and flight attendants often place little confidence in them. Quite often, passengers and attendants are not in their seats, or are in their seats with belts unfastened, when turbulence occurs.

The FAA Joint Safety Analysis Team estimates there are more than 1,000 minor injuries caused by air turbulence on commercial flights every year.

Ninety-eight percent of those injuries happen because people don’t have their seat belts fastened. Sometimes it’s worse; according to a review of National Transportation Safety Board data between 1992 and 2001 by the National Aviation Safety Analysis Center, air turbulence was a factor in at least 509 domestic air accidents, including 251 fatalities, although most of the deaths occurred in the general aviation sector.

Beyond the air safety concerns, air turbulence has a major financial impact on commercial aviation. Annual dollar losses to airlines run into the millions from turbulence-related injury claims, late arrivals, scheduling delays, additional aircraft inspection and maintenance, and extra fuel costs expended in rerouting.

FAA guidelines suggest aircraft avoid thunderstorms by at least 20 miles when possible, even though large sections of that area may contain relatively calm air.

Doppler Data

The NTDA analyzes data obtained from the National Weather Service’s network of NEXRAD Doppler radars to get a real-time snapshot of turbulence, which is then immediately transmitted to flight decks. The data also is made available to airline meteorologists and dispatchers.

"This work uses Doppler radar measurements to create a three-dimensional mosaic showing turbulence across the country that can help pilots avoid hazardous areas," said physicist John Williams, co-manager of the Turbulence Remote Sensing program with NCAR’s Research Applications Laboratory.

The 3-D turbulence grids produced by the NTDA system are used to generate text-based maps for each aircraft enrolled in the system. The maps show a plan view of turbulence at the current flight level 160 km ahead and 60 km to either side, as well as a vertical cross section of turbulence 3,000 meters above and below the planned route. According to Williams, the system could be configured to cover even larger volumes.

NTDA uses Aircraft Situation Display to Industry (ASDI) data to determine aircraft position and route in order to accurately produce each map. Operational since 1998, ASDI provides for the dissemination of real-time air traffic data to registered aviation clients. The system is based on a client-server architecture, with the server hosted by the U.S. Department of Transportation’s Volpe Transportation Center in Cambridge, Mass.

NTDA output is presently in the primitive stage. Turbulence intensity is represented by character graphics displayed via the Aircraft Communication Addressing and Reporting System (ACARS) printer. Williams said the hope is to eventually make information available to pilots as a color graphics display.

Still, the experimental NTDA system has provided a low-cost way to demonstrate the utility of timely in-cloud turbulence information to en-route pilots. NTDA does not measure clear-air turbulence, such as that caused by the jet stream or wind behavior over mountainous terrain. "The focus of NTDA is on clouds and storms, which are associated with about two of every three turbulence encounters," Williams said.

NTDA testing began in the summer of 2005 when a few air turbulence messages were uplinked to registered routes flown by United Airlines east of the Rocky Mountains. In the late summer of 2006, NCAR created a Web site that lets pilots register their flights to receive uplinks, allowing turbulence information to be made available to all of United’s Line Check Airmen.

"This Web site also allows pilots to review turbulence messages generated for their flights and to provide feedback on message accuracy and utility," Williams said.

Most of the feedback has been positive, such as the following example provided by NCAR: "When the report printed I was rather surprised to see one. But the accuracy was right on for all four reports. At 2345Z we had about 30 sec of mod chop. Lt/mod chop started at 2346Z as noted on the 2343Z report. The noted "M" at 2354Z seemed right on. There was lightning well below us but no radar returns."

The latter assessment underscores the fact that NTDA is able to detect turbulence even in light clouds that may not show up on existing radar displays.

"I’ve tested NTDA numerous times and this system supplies turbulence information that is currently not available from any other source," said Capt. Joe Burns, United Airline’s managing director of Standards and Technology.

Burns said the messages he received on flights provided "a very accurate picture of turbulence location and intensity."

United tested the NTDA system on 300 to 400 flights before stopping last December, due to funding issues, Burns said. But the airline planned to "resurrect" the program, he added.

"All of our aircraft east of the Rockies are able to receive this information," Burns said. "… It works well. Essentially, any aircraft with an analog ACARS link can take advantage of this program."

Testing has shown the NTDA successfully detects moderate-to greater turbulence more than 80 percent of the time. Even greater accuracy can be expected in the future as NCAR scientists continue to fine-tune the system.

The current NTDA demonstration system continues to run at NCAR and to provide automated turbulence uplink messages to registered flights.

"We delivered the NTDA software itself to the National Weather Service in February 2007, to be deployed on all NEXRADs when their software is upgraded in the summer of 2008," Williams said.

Shortly thereafter, turbulence grids from each radar should be available to interested users along with the current reflectivity (the amount of transmitted power returned to the radar receiver), velocity, and other NEXRAD Level III radar products.

NCAR was working on plans to have a 3-D, in-cloud turbulence mosaic product generated routinely by the National Centers for Environmental Prediction using these data. Under other FAA Aviation Weather Research Program funding, NCAR is developing a comprehensive turbulence "nowcast" product that will provide probabilistic assessments of turbulence from clear-air and mountain-wave sources every 15 minutes, in addition to the in-cloud turbulence detected by the NTDA.

"We expect the ‘Graphical Turbulence Guidance Nowcast’ grids to be available to private weather service providers for use in their own products and displays by early 2011," Williams said.

Other Approaches

NTDA is one of a several turbulence detection systems to emerge in the past decade, most of them part of a NASA program called Turbulence Prediction and Warning Systems (TPAWS), an activity involving NASA, NCAR, RTI International, AeroTech Research, Honeywell, Rockwell Collins and others.

Williams said NTDA is meant to compliment, not replace, other detection technologies, one of which is the Enhanced Turbulence, or "E-Turb" radar, a modified X-band radar designed to detect turbulence associated with thunderstorms. E-Turb technology was used and evaluated by Delta Air Lines in 2005 on revenue flights in the United States and South America before being incorporated in production weather systems. It is currently an option on Rockwell Collins’ MultiScan Hazard Detection System.

E-Turb has limitations, however. "Onboard X-band radar may not penetrate severe storms well, so airborne and ground-based detection systems may be seen as complementary," Williams said. "Using both systems in concert would probably provide pilots the most useful picture of in-cloud turbulence ahead."

A second technology to emerge under TPAWS is the Turbulence Auto Pilot Reporting System (TAPS), a non-flight critical software application loaded in aircraft computer systems, including electronic flight bags, that automates the reporting of all significant aircraft encounters with a variety of turbulence, including convective, clear air, mountain wave and wake.

TAPS software is launched when on-board accelerometers detect an encounter with turbulence. If turbulence loads are above a designated threshold, the TAPS algorithm generates and broadcasts over ACARS, satcom or other data link a turbulence reporting packet. Ground-based computers then rebroadcast the information to aircraft flying similar routes, giving pilots a better idea of where turbulence exists. TAPS is useful in the estimated 10 to 20 percent of turbulence events that occur in the absence of moisture.

According to AeroTech, of Newport News, Va., which developed the algorithms, TAPS had been installed on 123 Delta Air Lines Boeing 737-800, 767-300ER and 767-400ER aircraft flown in the U.S., Canada, the Caribbean, South America, Europe, India and the North Atlantic.

On the ground, turbulence reports were displayed on an evaluation version of the ARINC Web Aircraft Situation Display, a Web-based tool providing controllers, dispatchers and ground services personnel with real-time graphical flight-following information. AeroTech was working with Delta on the design of a cockpit display for TAPS information.

Another turbulence detection scheme to emerge from TPAWS is called the Airborne Coherent Lidar for Advanced In-Flight Measurements (ACLAIM), based on light detection and ranging (lidar) technology. The purpose of ACLAIM was to establish the viability of airborne lidar as a forward-looking sensor to detect clear-air turbulence.

Unlike NTDA, which uses Doppler radar transmitting in the microwave S band (10-centimeter wavelength), lidar employs an onboard laser operating in the shorter, 2-micrometer wavelength. The longer wavelength of NTDA, combined with its high-powered, ground-based output, enables it to detect turbulence where lidar cannot, such as inside severe storms or in clouds beyond storms.

"Lidar lasers, designed specifically to detect clear-air turbulence, do not penetrate clouds or precipitation well, whereas radar systems are specific to in-cloud turbulence, making lidar and radar very complimentary in this sense," said Larry Cornman, a physicist with NCAR’s Research Applications Laboratory.

But what appears to have doomed ACLAIM as a viable turbulence detection solution is its relatively high cost. Pursuit of lidar technology as a means to detect turbulence ahead of commercial aircraft was eventually reduced to low priority when it became evident that such a system would be prohibitively expensive. According to a 2001 NASA Glenn Research Center document, "Market Assessment of Forward-Looking Turbulence Sensing Systems," lidar and other X-band/lidar combinations have an unfavorable business case.

Williams said low cost is one of the main attractions of NTDA.

"One advantage is that it is essentially a software upgrade to the existing national network of Doppler weather radars, making the cost of implementation very low," Williams said. "NTDA does not require the user to purchase new hardware."

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