Monday, April 3, 2006
Efforts to Understand Dangers Of Mountain Flying Renewed
A research project in California's Sierra Nevada mountains is attempting a major leap forward in unraveling the mysteries surrounding one the biggest atmospheric hazards in modern aviation -- a wind phenomenon known as a "mountain rotor." The findings could have a huge impact on the safety of regional and general aviation flights over mountainous terrain.
Both mountain rotors and a related phenomenon, mountain waves, regularly form on a mountain's lee side, the side opposite the direction from which strong winds typically blow. Just such a site is Owens Valley, Calif., which is between the Sierra Nevada and the city of Fresno.
A series of airborne and ground-based readings are being taken for the "terrain-induced rotor experiment" (T-REX), says the lead researcher, Vanda Grubisic, of the Desert Research Institute (DRI) based in Las Vegas and Reno, Nev. The readings started on March 1 and are to be taken through April 30. Primary funding for the experiment is coming from the National Science Foundation (NSF) in Washington, D.C.
Around the world, there are 60 principle researchers and another 40 interns and technical staff who are starting to analyze the new data, Grubisic adds. For the scientific community, it's a chance to revisit these atmospheric phenomena whose existence have been known for a long time. But now, researchers are coming back to them with the latest technology and research methods, and the chance over the next three years to develop numerical models from the T-REX data.
"Despite numerous investigations during the last 70 years concerning lee waves, the structure and dynamics of [mountain] rotors remains largely unknown," Ren須eise, a meteorologist in Germany with the Mountain Wave Project (MWP), tells Regional Aviation News.
Mountain waves, which in form are something like the waves that break at a sea shore, result from oncoming air that confronts a mountain face and is then forced up and over the crest. On the other side, gravity suddenly pulls the air down and the waves form. Rotors develop right below the waves, and resemble a whirlwind or vortex tilted to the horizontal.
But little is known so far about the "whys and hows" of rotor formation, Grubisic tells Regional Aviation News. T-REX will be an important step in better understanding these phenomena, but much more will probably remain to be done after the current project is over.
In aviation, mountain waves and rotors have long been recognized as significant dangers. The Australian Transport Safety Bureau cites a 1968 incident when a BOAC Boeing 707 was ripped apart by a mountain wave as the craft flew near Mt. Fuji in Japan. Also, in 1968, a Fairchild F-27B lost parts of its wings and empennage, and a Douglas DC-8 lost and engine and wingtip in 1992.
Rotors, specifically, have been cited as contributors to accidents in commercial, military, and general aviation (GA), Grubisic says. Experienced pilots know about them and avoid them. But rotors and waves remain particularly dangerous to pilots who are unaware of them.
As the Federal Aviation Administration (FAA) has aptly put it, "Your first experience flying over mountainous terrain (particularly if most of your flight time has been over the flatlands of the Midwest) could be a never-to-be forgotten nightmare [italics in the original] if proper planning is not done and if you are not aware of the potential hazards."
Besides the aviation hazards that rotors pose, Grubisic says, "we're doing this because it's one of the unsolved problems in atmospheric research." The outstanding questions include not only why rotors form, or how they do, but also how they often get so strong. It seems, she adds, that rotors pick up their "intense rotation" from the "boundary layer" of air next to the earth's surface. But explaining exactly how this happens has been "one of the more puzzling questions," and has become one of the principle research aims.
Moreover, the numeric modeling of certain atmospheric conditions from T-REX could lead to better forecasting of rotors and waves. Indeed, another question occupying researchers' minds is just how predictable rotors will come to be.
Current attempts at numerical simulations are not the best because they use "idealized assumptions" of atmospheric conditions, Heise explains. In addition, the lack of "sufficient empirical data" makes it difficult to develop certain parameters, a problem that T-REX's more precise measurements should ameliorate. This should lead to better forecasting of rotors and waves, and enhanced flight safety.
Steve Nelson, NSF's program director for physical and dynamic meteorology, agrees that there's a lot of unknowns with mountain rotors, adding that T-REX eventually could have significant implications for aviation safety. He draws an analogy between the scientific inquiry into the dynamics of rotors and similar inquiries into two other troublesome atmospheric phenomena, downbursts and microbursts. Some years ago, the scientific knowledge base for these second two was similar to what exists today for rotors, he tells Regional Aviation News. But a long stream of related research projects led to better radar and wind-detection systems at airports, greatly reducing the hazards. If a similar research stream gets going in the wake of T-REX, rotors someday may subject to far more accurate forecasting and become much easier for pilots to avoid.
The Sierra Nevada mountains are especially ideal for studying rotor and wave formation because they are "the tallest, steepest, quasi two-dimensional topographic barrier in the contiguous United States," according to the T-REX Web site. Thus, rotors and waves grow particularly large and strong there. Additionally, prior research shows that they are especially frequent in the Sierra Nevada in March and April.
There also are two aspects to T-REX's current phase of data collection, Grubisic explains. One involves the data being read by several ground-based stations. The second involves the readings coming from three aircraft. One is a Beechcraft King Air turboprop, owned and operated by the University of Wyoming. It can take readings from 500 ft. to 28,000 ft. above ground, and it is flying for T-REX while based at Bishop, Calif. The craft is doing about 25-30 flights for the study. On the mountains' lee side, Bishop also is the T-REX operations center.
Above the range of the Beechcraft at altitudes reaching 35,000 ft. is a British Aerospace BAe 146. It's based in Fresno and is making about 10 flights for the study.
The third craft, which can take readings up to 45,000 ft., is the new Gulfstream V HIAPER, which stands for "high-performance instrumented airborne platform for environmental research". NSF developed and modified the craft specifically to enhance its environmental research needs in the coming years (and indeed, T-REX also is expected to yield data to help fight pollution). It's being operated and maintained for NSF by the National Center for Atmospheric Research (NCAR) in Boulder, Colo. The craft will make its dozen-or-so data-gathering flights from a base just south of Boulder in Jefferson County (which is part of metro Denver). T-REX also represents the craft's maiden use for scientific research.
The HAIPER is especially suited for its role in T-REX because it's the only aircraft that can reach such heights while deploying GPS Dropsonde technology and other instruments to measure certain meteorological parameters, says Jim Huning, program officer for NSF's Lower Atmospheric Observing Facilities. Dropsonde, which was developed at NCAR, drops a sensor that is equipped with a little parachute below the craft to get measurements of such factors as atmospheric pressure, horizontal wind, and moisture. Coupled with GPS, those readings can now be tied to very specific points in space and time.
"It gives a very accurate idea of what's going on," Huning tells Regional Aviation News, and should help especially with precise measures of rotor dynamics. The National Oceanic and Atmospheric Administration (NOAA) has found the technology very useful recently in hurricane research.
The mid-altitude BAe is also deploying the GPS Dropsonde sensors, while the low-altitude Beechcraft King Air is equipped with a special Dopplar radar sensor for studying clouds. Not only will its readings reveal where the clouds are, but the wind velocities within the clouds.