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Vibrating-reed ice detectors from the 1980s are designed for controlling inflatable rubber boots, but today’s UAVs require a more modern approach.

The limited surface area for ice sensor probes on a small UAV can complicate the location and installation of any kind of metal ice sensor vis-a-vis the aircraft’s sensitive radio antennas. Solving this problem, the external sensing probe of modern aviation ice sensors is fabricated of non-conductive plastic that is transparent to radio signals, and poses no radio interference problem for the host UAV.

Modern optical ice sensors generally consist of a unitized plastic probe with an air gap, circuit board, housing, and cable. The probe is a delrin plastic cantilever that holds two optical windows and a reflecting wall below the wing, out into the airstream. In operation, optical ice sensors detect the H2O phase-change between liquid water and solid ice. Producing the maximum possible sensitivity, in-flight ice molecules form directly on the probe’s optical surfaces.

Figure 4. Saturation ice on a modern optical ice sensor probe.

An optical sensor-excitation signal is created and received by the interface board. The earliest formation of ice molecules on the optical surfaces perturbs the sensor’s excitation signal, on a molecular level. The board interprets and outputs transducer signal variations on three discrete logic wires as no ice (000), ice alert (001), more ice (011), or saturation ice (111). The probe’s inboard end mates with a small interface board buried in solid epoxy inside the housing, completely submersible in water. A lightweight blue cable connects the unit to its host system.

Optical ice sensors are small, lightweight, have no MHz clock, and no moving parts. They install from inside the wing, extending down, air gap facing forward into the air stream. The entire unit is fixed in place with a 5/16"-24 thread and nut, just as an ordinary outside air temperature gauge installs in a general aviation aircraft.

Optical ice sensors owe their high degree of sensitivity to the fact that they are pencil thin, and so create minimum ram-air heating effect on airborne H2O molecules. For this reason, they attract solid ice molecules before fatter, warmer airframe members, such as fuselage, wings and struts. During NO-ICE conditions, ambient wind removes liquid water from the sensor optics, but ice sticks to it and accumulates for detection.

NASA Glenn’s Icing Research Tunnel, the world’s largest such tunnel in Cleveland Ohio, has tested and documented optical ice sensors according to a matrix of temperature, humidity, altitude, air speed, liquid water content, drizzle drop diameter, and air pressure. Test tunnel matrix and report available upon request.

Shape-Changer

Ice formations on an exposed optical surface in an icing domain can be either clear ice or rime ice, depending upon atmospheric variables. But optically clear ice or opaque rime ice makes no difference to the sensor. Optical ice sensors can change their shape according to the type of ice formation and report it. When shipped from the factory, optical ice sensors have one shape, but when installed in a UAV, flown aloft and faced with ice, they can change to a different shape. Because they are completely solid and lightweight vs vibrating sensors, optical ice sensors are extremely robust vs shock and vibration. They create less aerodynamic drag than vibrating sensors, and they add lightness to any aircraft. One of the reasons for their lightness is they eliminate the weight of many-turns-of-fine-wire magnetic coils required for an electro-magnetically driven sensor probe.

Not only is the weight of the copper eliminated, but also the weight of the metal frame to contain and mount the vibrating assembly. Optical ice sensors employ very simple direct-sensing technology, have no moving parts, and are simple to manufacture and test. Manufacturing cost is much lower than vibrating sensors. What’s more, optical ice sensors substantially reduce the power budget of any aircraft. Simple to design into any UAV, they operate on just one single DC voltage anywhere between six and 30 volts. At standard 24 VDC input, they draw less than 100 mA; you can power it with a 5 Watt solar panel. Output logic levels for the three dedicated wires is zero volts to 3.3 volts DC.

Absence of installation-template restrictions affords optical sensors a great deal of flexibility. Probes can be separated from their electronic interface boards, and conveniently integrated directly with UAV running lights and Pitot tubes. Because they are potted solid with two-part epoxy, they are explosion-proof. Because system integration is so simple, sensors are shipped with their connecting wires simply stripped-and-tinned at the end. No requirement for MIL-SPEC connectors.

Modern ice sensors are offered as commercial off-the-shelf (COTS) products. Because of an unfortunate absence of any published FAA TSO specification for in-flight ice-sensors, the aviation community relies upon defacto standard SAE aerospace specification AS-5498, core paragraph ¶5.2.1.1.1. Optical sensors are also listed in SAE aerospace information report AIR-4367 paragraph ¶ 4.11.

As unmanned aerial vehicles become more and more popular throughout the aviation community, modern ice sensors are becoming equally important to help operators of the aircraft avoid the hazards of ice-induced tailplane stall and other ice-related hazards. The recent advent of modern optical ice sensors to supplant primitive vibrating sensors promises to aid the state of the art.

This article was written by Richard Hackmeister, Vice President, New Avionics Corp., (Fort Lauderdale, FL). For more information, Click Here.

For more information on ice-induced tailplane stall, interested readers may wish to view a 23-minute NASA video entitled “TAILPLANE ICING” that excellently describes, illustrates and dramatizes the hazards of ice-induced tailplane stall, and its effects on aircraft safety while landing. For information, contact NASA Glenn at email This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://icebox.grc.nasa.gov/education/products.html. Also, a limited number of digital video disks are available directly from the author, email This email address is being protected from spambots. You need JavaScript enabled to view it.