October 1, 2003
By Thomas A. Horne
It's 5 a.m. Mountain time, and National Center for Atmospheric Research (NCAR) meteorologist Frank McDonough is on the telephone at his Boulder, Colorado, office. At the other end of the line a NASA flight crew in Cleveland huddles around a speakerphone, hanging on McDonough's every word.
"There are some cold tops over Sandusky, and a north-south line of clouds over central Ohio," McDonough says. "Yesterday we had minus 40-degree cloud tops and some convection there, but right now a high cloud layer is blocking the view of conditions below. We'll have a clearer picture when the sun comes up, but with a vorticity max east of Columbus I'd say head southwest toward Mansfield. Looks like the freezing level is at 7,000 to 8,000 feet. At 10,000 it's minus 3 degrees [Celsius]. Tops are minus 12 to minus 15 degrees, so 10,000 looks like a good icing altitude."
The crew takes little time to decide: To Mansfield it is, at an altitude of 10,000 feet. It may sound suicidal to deliberately seek out the worst icing conditions, but for the pilots and crews at the NASA Glenn Research Center (GRC), it's the stuff of their daily lives in the colder months of the year. Today is no different, except that I'm tagging along to get a taste of what icing research is all about.
Our testbed is NASA Glenn's 1966 de Havilland Twin Otter, a hard-working truck of an airplane fitted out with probes, sensors, gauges, cameras, and other specialized gear — all of it designed to quantify icing encounters, determine safe flying procedures, and assess the quality of icing forecasts. Acquired from NASA's Langley Research Center in 1982, the Twin Otter has logged more than 4,000 hours in some of the most nightmarish icing ever recorded, performing some of the most hair-raising maneuvers imaginable.
No, I haven't moved to Australia and, no, the laws of meteorology haven't been repealed. In August's " Wx Watch: Air Blocks," I referred to the "counterclockwise flow" of air around Bermuda highs. Obviously, this was an unfortunate slip of the pen, for which I apologize. Of course, air flows clockwise around high pressure — unless you really are in the Southern Hemisphere. If there's a bright side to this gaffe, it was having the chance to correspond with so many interesting readers. Among them was Lester Zinser, a pilot who spent years flying around thunderstorms for the National Center for Atmospheric Research. Lester, I especially liked your story about being sucked into a thunderstorm whilst flying a Queen Air on a research mission. Hope you got good data. — TAH
Pilot Kurt Blankenship heads up today's mission. As a test pilot, Blankenship is unusual in that he comes from a civilian flying background. He projects a calm psychic center, something necessary in a pilot whose job is milling around in ice clouds. He and flight test engineer Tom Ratvasky take me on a walkaround to show me the Twin Otter's many modifications.
The probes are the first to get your attention. On the left wing is a forward scattering spectrometer probe for measuring water droplet sizes. This uses a laser beam to detect smaller cloud droplets. The nose has a liquid water content (LWC) probe. Like so many others, it's a heated probe. But its heating is governed by ice accretions: The more heat required to deice this probe, the higher the LWC. The nose also has an air temperature probe, some still camera viewports, and a Rosemount ice-detection probe. The Rosemount probe vibrates; when ice adheres to it the vibrations slow down, signaling the crew of the onset of an icing encounter. Another spectrometer — for measuring large cloud droplets — is on the right wing.
There are video cameras, too. One is mounted atop the fuselage in a rotating mount. It's used to view and record ice buildups on the wings. A tail-mounted camera is for examining tailplane ice accretions.
Of course, the airplane has a complete ice-protection package. Pneumatic boots are on all aerodynamic leading edges, and NASA added extra boots to the airplane's wing and landing gear struts to further eliminate the drag of ice accretion. The wing and tail carry chordwise and radial markings used to delineate the extent of ice buildups. On the horizontal stabilizer, for example, chordwise station marks are at 5-percent intervals. The tailplane is also fitted out with perforated tubing designed to detect shifts in pressure distribution.
Blankenship climbs into the Twin Otter's cockpit and fires up the 500 shaft-horsepower Pratt & Whitney PT6A-27s. The cockpit's most striking visual aspect is the massive control column — a huge pillar that rises from the floor, then sprouts into two control wheels that look like they'd be more at home on a tractor. The panel: Pure disco era, with the exception of Blankenship's control yoke, which is festooned with switches for recording flight data.
The Twin Otter makes a good icing testbed. "It's well booted, handles the low-speed regime very well, is benign in its control response, and performs well with all our equipment," Blankenship says.
Back in the Twin Otter's cabin, it's a cramped world, a jumble of test equipment, video monitors, and racks of electrical components. The gear that Ratvasky and research engineer Sam Lee pore over draws about 200 ampere/hours of electricity. The screens are capable of displaying just about every variable in the flight and atmospheric environment. This includes video imagery of ice accretions on the airplane's wings and tail, plus a long list of data crucial to describing an icing event.
Heading that list is LWC (expressed in terms of grams of water per cubic meter), water droplet size (expressed in microns), outside air temperature, airspeed, altitude, angle of attack of wing and tail surfaces, and duration of exposure to an icing event. The higher the LWC and droplet sizes, the closer the outside air temperature to the minus 5-to-zero-degree range, the higher the angle of attack, and the longer the flight in these conditions, the more dangerous the potential consequences. Soon I'd have a first-person confirmation.
I'd seen video and still imagery of some of GRC's icing testing, but it didn't prepare me for what was to come. We take off in light rain, ascend into the gloom, and head for Mansfield, climbing to 10,000 feet. Ratvasky gets his equipment running, and starts collecting data. At 5,000 feet it's 6.2 degrees C, still raining, and all the water is running back on the wings. Passing through 7,000 feet, he notices the temperature has dropped to 2 degrees C. McDonough back in Boulder provides an update.
Ratvasky picks up his satphone to get the report from McDonough, who says, "There's a rain band at Mansfield now, 25 dBZ reflectivity [this indicates moderate rainfall]. Cloud tops are minus 10 degrees. There are pireps of moderate ice at 9,000. The moist area is 7,000 to 14,000. You should hit freezing drizzle at 7,500 feet."
Blankenship and Ratvasky perk up. McDonough is describing an environment that GRC crews know well. It's the large-droplet environment, one they've flown in many times as part of the research efforts after the 1994 crash of an ATR-72 twin turboprop commuter. That crash — and GRC's tests — proved that there are icing scenarios far worse than previously believed. The results of flight tests like ours prompted the FAA to extend the ATR-72's deice boots farther aft on the wing.
I peek out the side window. We're still in solid instrument meteorological conditions (IMC), but now it's snowing. At 8,300 feet we encounter freezing drizzle, and it's right at zero degrees C. "You're in the band!" says McDonough. A slushy ice buildup encrusts the Twin Otter's windshield wiper blades. A glance at the propeller spinners shows a deepening ring of ice. The wing boots are picking up a good amount of ice too. Blankenship and I exchange glances. "It handles ice well!" he says. "Wanna fly?"
So strange a feeling. Here I am, where decades of lore and learning have urged me never to be. And yet, the crew's professionalism has a calming effect. We already know where we'll go if the ice builds too much. "The escape route is down," Blankenship, Ratvasky, and McDonough say — almost in unison — during the preflight briefing. That's where the warmer temperatures are. The Twin Otter motors on like a train.
After reaching 9,000 feet we encounter mixed icing. It's minus 3 degrees C and we're still in cloud. Now it's time to fly a few racetrack patterns to collect more data. "It's mostly ice crystals now," says Ratvasky. The temperature drops to minus 4 degrees and Ratvasky watches the LWC. "It's .5 to .25 — no, wait, I just saw 1.6 to 1.8. Incredible! Some of the highest readings I've seen in a while!" He ought to know. Ratvasky was one of the lead scientists in the NTSB's investigation of the ATR-72 accident, and did seminal research work on tailplane icing. For his efforts in GRC's Tailplane Icing Program and other initiatives the American Institute of Aeronautics and Astronautics awarded Ratvasky its 2002 Losey Atmospheric Science Award.
The ice on the wipers is now about one inch thick. Our normal cruise speed would be 127 knots, Blankenship figures, but now it's down to 118 knots. I can't believe what Blankenship says next.
"OK, it's time for some stalls." Normally, the Twin Otter stalls at 68 knots in the clean configuration. Blankenship, who has been hand-flying the whole time (an autopilot can mask abnormal control feel caused by ice accretions), pulls aft on the yoke and notes when the airplane starts shaking: 77 knots. A subsequent test, with more ice on the airplane, shows the stall speed has risen to 80 knots.
The day's work over, we head back to GRC's home base at Cleveland-Hopkins International Airport. By the time we descend out of 7,000 feet, all the ice sheds from the airplane.
Icing flight tests may be the most glamorous part of GRC's mission, but the others are no less important. While a recounting of its work over the years would fill a book (in fact, one has been written: We Freeze to Please by William M. Leary and published by the NASA History Office, Washington, D.C., under the code NASA SP; 2002-4226), here are some of NASA Glenn's highlights:
Although little documented by historians, and little recognized or appreciated by the general aviation community, the GRC has been at the forefront of the world's body of icing wisdom. In the upcoming months we'll talk more about ice-avoidance strategies, and procedures for flying an iced-up airplane. Thank the GRC for the scientific grunt work that makes this kind of advice possible.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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