Harnessing Alpha Particles to Spot Aircraft Icing

Photo: Nicholas Eveleigh/Getty ImagesA 10-cm-long streamlined electrode fixed to wing of meter-long model aircraft for wind-tunnel tests.

Given North America’s recent record-breaking cold with its thousands of cancelled airline flights, ice and aviation are very much in the news—especially the ice that cakes up on wings and tails, requiring trips to de-icing stations on the ground and constant vigilance aloft. Now a research team from the University of West Florida (UWF) and West Virginia University (WVU) has extended the principles of the household smoke detector to build and test a new way of warning fliers of impending danger.

Aloft, ice forms as super cooled water flows around an airfoil’s leading edge (it also forms deadly embolisms in engines, but that’s a different story). The crystallization starts at the most tightly curved part of the surface and works its way outward into plaque that can kill the wing’s lift or jam control surfaces. Most icing is easily visible from the cockpit: according to a handy NOAA primer on aircraft icing, grainy, white rime ice accounts for 72 percent of in-flight ice buildup.  About 21 percent of the time, however, water freezes into clear ice, which is nearly invisible. (The remaining cases are mostly mixtures of rime and clear ice).

To be sure, there are already instruments that help solve the problem: over the past decade, the avionics industry has developed several acoustic and optic sensors that detect wing icing. Most notable are pencil-stub-size spectrometers that shoot an infrared beam across a small, open-air gap and measure the signal returning from a reflector. Opaque rime ice blocks the beam, cutting off the return signal. Clear ice acts like a spliced-in chunk of optical fiber, reducing the beam’s dissipation and strengthens the return. The sensor reports either deviation.

Ezzat G. Bakhoum, leader of the UWF team, calls these solutions quick and reliable, but they have a common draw-back: These sensors are not embedded in the lifting surfaces, but are mounted separately. They detect icing in the sensor itself, not icing on the airframe. With graduate student Kevin Van Landingham and WVU’s Marvin H. M. Cheng, Bakhoum devised a system that responds to the amount of ice actually building up on the wing.

As they report in IEEE Transactions on Instrumentation and Measurement, the group turned to alpha particles, the bundles of two protons and two neutrons emitted by some radioactive isotopes as they decay. Alpha emitters include Americium 241, the element often used in household smoke detectors. Americium shoots out alpha particles, which have relatively high energies ( 5500 kiloelectronvolts, compared to the 100 keV or so of a medical x-ray photon) but high mass, low speeds, and little penetrating power. Indeed, a sheet of paper, the topmost layer of your skin, or a 50-micrometer-thick sheet of water or ice will block the beam.

With two protons and no electrons, alpha particles carry a positive charge, which is the key to their usefulness in sensors. To build their ice detector, the UWF researchers affixed a thin wafer of Americium (a chunk they describe as just slightly larger than used in smoke detectors) to the forward surface of a test plane’s wing, and mounted an aluminum electrode a few centimeters above it. If there is no ice or water on the wing, the stream of alpha particles pours into the electrode, transferring their charges to it. If even 100 micrometers of water coats the wing over the Americium wafer, it cuts off the alpha stream. By connecting the electrode to the gate of an n-channel MOSFET transistor, they can continuously monitor the electrode’s charge, and so instantly know how much of the alpha-particle stream is getting through. If the electrode charge drops to zero, the device flips a switch and sends a “wet” signal, indicating water or ice is blocking the beam.

That’s not enough to sound a klaxon in the cockpit, of course. Additional data and analysis have to be built into the instrument. The device’s output voltage fluctuates—and fluctuates more as airspeed increases.  In rain, the output fluctuates widely around “Gee, I’m wet,”  for example, while an ice-covered wing produces a steadier signal. With some built in signal-analysis logic and an attached thermometer, the ice sensor can reliably distinguish among a variety of environmental conditions.

Bakhoum and colleagues wind-tunnel tested the detector under seven different sets of conditions—clean dry air, very humid air, clouds, rain, ice crystals, dust, and lightning—at airspeeds of 0 to 900 kilometers per hour. Tests with dry, humid, cloudy air, and even dusty air, produced outputs that oscillated just above the 0-voltage “dry” reading.  Simulated rain produced output that jumped and spiked just below the 6-volt “wet” value. Only ice produced the steady “wet” value, seconded by the thermometer, and prompted  an icing alarm. (As for lightning: the artificial bolts confirmed that protective components did indeed keep the circuit from frying.)

Bakhoum calls the device “very robust and reliable” compared to optical detectors, and he expects that the device will be commercialized in the near future.


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