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Keeping ice off airplane wings.

Sophisticated devices such as magnetostrictive, electromagnetic, and ultrasonic sensors are helping to increase the safety of air travel by preventing ice buildup on aircraft.

Despite all the advances that have made air travel safer than ever, the accumulation of ice on airplane wings continues to be one of aviation's most insidious problems. In the past 25 years, 35 air-carrier incidents worldwide have been attributed to inadequate ground deicing. Icing was blamed for the 1992 crash of USAir Flight 405, which killed 27 people after a failed takeoff in a snowstorm. Still more accidents take place when ice accumulates while a plane is in flight.

Some airplanes cannot use engine heat to melt ice, so any ice that accumulates in-flight is removed by pneumatically inflating rubber bladders installed along the wings. For planes on the ground, maintenance crews use various chemicals to melt ice and prevent it from reforming.

Traditionally, crews visually determine whether either deicing method is needed. Visual inspection is sometimes unreliable, however, because ice can be difficult to see. As a result, airlines are increasingly relying on various devices - including magnetostrictive, electromagnetic, and ultrasonic sensors - to alert them to the presence of ice and the need to implement deicing measures.

Ice affects a plane's performance by hampering the wings' lift capability. Even a relatively small amount of ice can have a serious impact on performance; planes can face the threat of chunks of ice breaking off, flying into the engines, and causing extensive damage. Even in cases where accumulation isn't severe enough to jeopardize safety, it still forces the aircraft to burn more fuel to compensate.

In the air, ice accumulates on surfaces as the plane flies through icing clouds, which contain water droplets suspended in a supercooled, unstable liquid state. Water droplets in an icing cloud remain suspended in the liquid state as long as there is nothing for the ice crystals to form on. When this state is disturbed - such as when an aircraft passes through - the droplets freeze.

The Federal Aviation Administration (FAA) quantifies icing severity by measuring the distance an object can travel before accumulating 1/2 inch of ice. A 1/2-inch layer over 40 miles is considered light icing; medium icing is a 1/2-inch layer that forms within 10 miles. Anti-icing systems generally need to be activated when there is medium icing, but they are ineffective during more severe conditions, so the plane must either land or not take off at all.

In-flight icing isn't a significant problem for large, fast commercial aircraft, which use turbine-engine bleed air to melt any ice that might start to form on the wings. Furthermore, the cruising altitudes of these aircraft are generally well above any precipitation, so ice would only be a potential concern during takeoffs and landings. In-flight ice buildup is a much more serious problem for slower, propeller-driven aircraft, which cruise at altitudes where ice is likely to form and are not able to melt ice with engine bleed air.

Ice formation when a plane is on the ground is much more common, affecting aircraft of all sizes and speeds. Airlines normally remove ice from wings by coating them with various deicing and anti-icing solutions. A glycol-based chemical known as a type 1 solution is used to take off any ice already on the plane. Such solutions are often heated, and they can be diluted up to 50 percent with water if conditions aren't severe enough to warrant a full-strength solution.

Deicing can be required again within 15 to 20 minutes of precipitation. Airlines therefore often use an icing suppressant - a type 2 solution - to coat the wings with a thin film that keeps ice from re-forming. Precipitation dilutes the film and ice can form again if the solution becomes too diluted, but it still takes much longer for ice to accumulate than it would without the coating. The coating itself hampers aircraft performance, but it shears off when the plane reaches 70 knots and is gone before the plane is in the air.

During a severe storm, an airline could use thousands of gallons of these chemicals. Anti-icing and deicing solutions are expensive and can seep into the groundwater, so airlines use them only when necessary. Maintenance crews, however, don't always know when the solutions are needed, because ice can be difficult to see. Even in the best lighting conditions, the ice might still be invisible if it forms in a thin, laminar sheet.

GIVING ICE THE BOOT

The main way aircraft handle ice in flight is by avoiding cloud formations in which ice is likely to form. When that's not possible, planes use rubber bladders - a pneumatic boot - on the wings to crack any ice that has formed.

The boots can be automatically activated at periodic intervals, but frequent use causes unnecessary mechanical wear. Also, if the boots are used too often, ice could form a hollow cavity around the bladders, rendering them ineffective. Furthermore, operating the anti-ice system consumes a lot of energy.

Although boots of various designs have been used successfully for decades, they are not without their flaws. The pilot needs to know when to activate the boot, and ice is not always visible. In addition, the boot does not work equally well for ice of all thicknesses. In general, the boot should be activated when the ice is 1/4 to 1/2 inch thick.

The SmartBoot, a device developed by BF Goodrich Aerospace in Uniontown, Ohio, has a magnetostrictive system that determines when the pneumatic boot should be activated. With magnetostriction, certain ferromagnetic materials change dimensions under the influence of a fluctuating magnetic field. The SmartBoot is the primary ice-detection system in all Boeing aircraft.

The sensor's principal component is a 1/4-inch-diameter probe that protrudes about 2 inches through the strut on the ice detector. Inside the strut are two coils, one of which vibrates ultrasonically at 40,000 hertz. As ice builds up on the probe, the probe's vibrational frequency decreases. The second coil senses the change in frequency. At a specified frequency shift, which is related to the ice mass on the probe, an output signal is generated to instruct the pilots to activate the deicing system. "Being able to read icing conditions across the 3-foot length of the deicer provides a whole new level of detection," said Dick McMurry, ice-protection-systems general manager. "It's a great alternative to single-point ice sensors, where localized detection may not be sufficient."

A heater in the probe is then energized to remove the ice. When the ice is gone, the heater is de-energized. If ice begins to accumulate after the probe cools down, the probe will again warn the pilot at the appropriate time.

Soon after BF Goodrich began offering the magnetostrictive system, the company developed an ice-detection technology based on water's heat of transformation. The detection system operates by periodically heating a thin-film platinum resistor with a constant power input. Microprocessor-based electronics measure the element's rate of temperature increase by comparing the times for the element to pass through two reference temperatures. Ice forming on the energized sensing element starts to melt at 0 [degrees] C. Since this melting process absorbs considerable energy, the temperature increases at a slower rate, indicating an icing condition. The system was developed to be a less expensive yet equally effective alternative to the magnetostrictive system, but the manufacturer discontinued the product after losing its source of inexpensive platinum resistance elements.

DETECTION ON THE GROUND

Because ice can be difficult to see, airlines don't always know when deicing is necessary. Therefore, several organizations have developed technologies to detect ice while the plane is on the ground.

One technique, developed by inventors Jacques Padawer and Robert M. Goldberg, uses an electro-optical system. A number of discrete light sources and electro-optical sensors are installed at various points on the aircraft surface. Each sensor is an active unit that relies on the presence of a reflective overlying material to couple coded light pulses from the source to its adjacent detector. When a reflecting material, such as ice, covers one or more of these sensors, signals are generated to alert the crew. The system that processes these signals is able to distinguish ice from water, anti-icing fluids, and deicing fluids, and is not confused by sunlight or other extraneous light sources. Optical fibers carry signals between each discrete sensor and a central processor. The developers are working on commercializing the device.

The C/FIMS system from AlliedSignal Aerospace Canada in Toronto operates on the principle that materials can be characterized electromagnetically by two principles: conductivity and permissivity. When an electric field is generated in the material, the behavior of the field is affected by these properties. By knowing how the field is generated and sensing the effect of the field at certain points, the characteristics of the material can be determined.

Each sensor in the system incorporates analog/digital electronics interfacing with a sensor disk, which is mounted flush with the aircraft's outer surface. The sensor consists of a ceramic-substrate disk with a series of electrodes and associated electronic circuits. By energizing the electrodes through various excitation patterns and sensing the resulting complex current, which is sensitive to the electrical properties of the material on the sensor surfaces, the material can be identified. Once a set of measurements has been made, a computer identifies the type and thickness of the material. This use of multiple excitation patterns also allows the sensor to measure and discriminate between layered materials as well as measure the integrity of anti-icing and deicing fluids.

Unlike some surface-mounted ice detectors, this technology does not rely on signal returns from hard boundaries between different contaminants. AlliedSignal has demonstrated that the system can detect light, fluffy snow and measure the properties of a coating of deicing fluid at several distinct heights above the surface. The sensor also ensures that the disk surface remains within 0.1 [degrees] C of the surrounding wing-surface temperature, eliminating any influence on the conditions being monitored.

The system can identify and display all common contaminants found on an aircraft surface, such as ice, snow, and deicing fluids. The cockpit display clearly shows the surface status as either clean or contaminated, and it distinguishes acceptable deicing fluids from those that have failed, which are identified as contaminants.

Rosemount Aerospace in Burnsville, Minn., is in the final stages of developing its own system to detect ground ice formation. The HALO ice-detection system consists of a processing unit, a cockpit-mounted display, and four wing-mounted ultrasonic sensor assemblies. Each sensor assembly consists of an ultrasonic transducer, a sensor plate, and redundant temperature sensors. The two inboard-mounted sensors extend over the wings and are used for detecting ice. The two outboard ultrasonic sensors, located in front of the ailerons, are used in conjunction with the two inboard sensors to detect failures of deicing and anti-icing fluids. Any detected hazard is indicated to the flight crew on the cockpit-mounted display.

The sensors use ultrasonic guided waves. Through a specially designed transducer, short ultrasonic pulses are applied to a sensor plate. These pulses travel along curvatures in the structure like those on the wing's leading edge. The reflected pulses are returned to the processing unit. This pulse-echo technique enables a single transducer to be used rather than separate transmit and receive transducers, reducing the assembly size, complexity, and cost.

The pulses are composed of both vertical and horizontal shear-wave components. The guided waves, sampled over a range of frequencies, yield data on surface contaminants. These signatures are collected from multiple sensors, processed, and combined. A relatively simple algorithm classifies the signature to determine whether ice is present, and the flight crew is notified if necessary.

The system is especially effective in detecting ice in hard-to-see areas. For example, if a relatively large amount of fuel is left on board a McDonnell Douglas MD80 aircraft, the fuel can come in contact with the wing's upper skin. If the fuel is cool, it will create on the wing a cold area prone to ice formation. Ice in this area is especially troublesome: If it falls off during flight, it can enter the engines. Since hard-to-detect clear ice tends to accumulate in this location, FAA has issued a directive that requires crews to perform a tactile inspection.

Several other aircraft have similar problems. Once the system is certified, Rosemount hopes that HALO will eliminate the need for tactile inspection and improve the detection of ice on aircraft before takeoff.
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Title Annotation:deicing instruments
Author:Paula, Greg
Publication:Mechanical Engineering-CIME
Date:May 1, 1997
Words:2082
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