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November 1997

Fasten your seat belts: RAP turns aircraft into turbulence sensors

Larry Cornman

Since mid-September, RAP scientist Larry Cornman and software engineer Cory Morse have been turning commercial aircraft into in-flight sensing platforms to measure and report turbulence. With funding from the Federal Aviation Administration (FAA), they've created software that works with an aircraft's existing equipment to measure and report in-situ (in-flight) turbulence once every minute. The data will be used to create turbulence forecasts to help pilots steer clear of bumpy air. They should also provide scientists with the first large-scale quantitative measurements of turbulence in U.S. skies.

RAP associate scientist Tenny Lindholm used his experience as a pilot to make connections with the avionics company incorporating the software and with United Airlines. United expects to deploy the new system on more than 200 aircraft over the next six months.

Until now, the only data on turbulence--the sudden, invisible gusts that buffet a plane and its passengers--have come from pilot reports of bouncy or choppy air. "If we'd tried to come up with a new sensor to load onto the aircraft, it would have been too costly," explains Larry. Part of that cost comes from testing new equipment to ensure that it doesn't affect flight operations. "So we looked for a way to use sensors, computers, and communications systems that were already on board, without interfering with their normal functions."

Instead of measuring turbulence directly, the researchers use the aircraft's response to turbulence to deduce its magnitude. "We're solving an inverse problem," says Larry. "If I measure what the aircraft's doing, I can infer what the turbulence must have been." The result is an in-situ turbulence algorithm, or mathematical problem-solving procedure, that uses measurements of how much the aircraft is bouncing up and down (taken from the plane's vertical accelerometer) while accounting for its weight, air speed, altitude, and whether the plane is on autopilot or not. Human or automated pilot response to turbulence--trying to damp out its effects--proved to be an important element in the equation.

In tandem with the in-situ turbulence sensing program, RAP is working on prototype displays for eventual use in helping pilots steer clear of turbulence. This image and the one below address the horizontal and vertical dimensions, respectively. The cockpit display above allows pilots to refine their original choice of flight path from Denver (DEN) through Salina, Kansas (SLN). The optimal wind route takes into account turbulence information in two ways: "Strategic Forecast," the cross-hatched area calculated and issued to the pilot pre-flight, and "Tactical Situation," a more specific, shorter-term forecast issued en route. The software uses these to calculate a path, the strategically planned route, that angles to the north, or left, of the turbulence-compromised route. (Illustration courtesy RAP.)

The algorithm is incorporated into software installed by Allied Signal, Inc., within Allied's onboard flight management system and aircraft condition monitoring system. The data are transmitted via the Aircraft Communication Addressing and Reporting System, which already carries temperature, wind speeds, and other aircraft data. The data's destination is the Meteorological Data Communication and Reporting System, an FAA/National Weather Service (NWS) data base.

Not just a bumpy ride

On average, a significant turbulence incident happens every other day on a commercial flight somewhere in the United States. The result can be anything from spilled food trays to broken bones for flight attendants and passengers not buckled into their seats. In 1991, severe turbulence tore the engine off a 747 cargo plane leaving the Anchorage, Alaska, airport. While a cause for the crash of United Flight 535 on final approach to the Colorado Springs airport in 1991 has never been conclusively determined, the National Transportation Safety Board's accident report cited turbulent winds and a rudder problem as the most likely explanations.

Larry and colleagues will use the data compiled on the FAA/NWS data base to create a turbulence detection product--a view of flight tracks showing what all the aircraft in a given region have measured in a 30-minute period. That flight track information will be provided to United Airlines (and to other airlines as they become participants), as well as to the NWS Aviation Weather Center in Kansas City, Missouri. Tenny expects Delta, Northwest, and American to join the project during the next two years.

This display shows the same scenario as above, but in a vertical cross section. The initial flight route from Denver to Kansas City, Missouri (MKC), which appears as the lower line, is chosen based on wind and temperature data alone. Turbulence is detected near Hill City, Kansas (HLC), as indicated by the light-shaded area. An optimal alternative route would take the aircraft above the strongest core of the jet stream (darkest shading). This would reduce its fuel efficiency but would bypass the turbulence. (Illustration courtesy RAP.)

In addition to interest from commercial aviation, Larry expects the scientific community to take advantage of the developing data base. The current system of pilot reports poses problems not only because it's qualitative, but because pilots rarely report the absence of turbulence. Under the present system, pilots often report turbulence only as "light," "moderate," or "severe," or with terminology such as "intermittent chop." "While this information is somewhat useful in an aviation safety context, it is of very little use to the atmospheric science community," says Larry. Some knowledge of turbulence in the upper atmosphere has been gained from short-term projects using research aircraft, but when such studies end, the data stream dries up.

Larry expects the new in-situ reporting system to make a real difference to researchers. "We're not just quantifying turbulence by labeling it 'level 3 on a scale of 1 to 5,' we're generating a turbulence measurement that's already well established in the scientific community."

With 200 United aircraft reporting once a minute from flights over the entire continental United States during the next several years, plus hundreds more possibly joining the system from other airlines, Larry says the potential is there to gather a lot of data. "I think people are excited about having something like this. It will really help us map the atmosphere in terms of turbulence."

As more aircraft are brought on line, Larry expects forecasting products to improve to the point that "nowcasting," or turbulence warnings in real time, will be possible. "Having such a comprehensive and accurate data base will really boost our development of new forecasting tools," Larry explains. One goal is a cockpit weather display (see prototype pictured on pages 1 and 2). He expects turbulence to join icing and convective weather on a cockpit weather menu within the next five years.

Before the RAP project, no one had tried to use an inverse problem-solving method incorporating the current list of variables in quite the same way. A different method has been in use in Australia for the last few years. The International Civil Aviation Organization (ICAO) will compare results from the U.S. and Australian detection programs with the goal of establishing an international standard for turbulence measuring and reporting. •Zhenya Gallon

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