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Ultrasonic ice detection from Goodrich

The UWID transducer is centered between two rivets to minimize guided wave interference phenomena. Click to enlarge.

An in-flight wing ice accretion detector based on newly ultrasonic guided wave sensor technology has been developed and tested by Goodrich Corp. and The Pennsylvania State University. The system was designed to detect the accumulation of ice on the aircraft wing's leading edge and determine when such ice is shed and/or sublimated.

During a normal flight, ice can be encountered when the aircraft is climbing and detected by an in-flight ice accretion detector. The types of detectors available include flush-mounted surface and probe-style ones mounted on the fuselage, wing, or engine inlet. Bleed air ice protection is activated and the stall protection boundaries are set to conservative angle of attack limits. When icing conditions are exited, as indicated by the in-flight ice detector, bleed air ice protection is turned off and the stall protection boundaries are automatically reset to normal levels.

The need for an icing detector is evident in situations where the bleed air ice protection fails, but stall protection is set to conservative values. Unable to confirm the absence of ice, the crew would be obliged to continue the entire flight with the conservative settings. The risk of such an event can eliminate some destinations and airports from the aircraft's service range. The resulting reduction in achievable city pairs could reduce the airplane's value to some operators. The consequence of an actual ice protection failure could be an unscheduled refueling/repair stop or a turn-back.

The wing ice detection sensor operating principle is based on the use of ultrasonic wave propagation, which has been used traditionally to detect defects in materials. Using a specially designed transducer, ultrasonic guided waves are applied to the aircraft skin. The waves interact with the reflective geometry of the skin and a portion of the waves scatter back toward the transducer, where they are received. The waves are composed of both vertical shear and longitudinal components, out-of-plane or in-plane displacement, respectively. Surface contaminants attenuate one or both of these components. By monitoring the amplitude, frequency change, and phase velocity of the received wave, bonded contaminants such as ice can be distinguished from those of water or deicing fluids. By controlling active ultrasonic guided wave mode type and frequency, the vibration on the outer surface of the leading edge can be made either sensitive or insensitive to certain contaminant types.

System hardware includes two ultrasonic wing ice detection (UWID) sensors and a processing unit. Cables run inside each wing and out to the movable slat to connect the processor to the transducers. Each UWID sensor includes an ultrasonic transducer and a reflector that are bonded to the native surface of each slat. The distance of travel from the transducer to the reflector and back is approximately 330 mm. One sensor-reflector combination is installed on the inboard and unheated slat 3 and one is installed on the outboard and heated slat 7. The heating mechanism for slat 7 is the wing anti-ice/bleed air system (WAI). This system takes high-temperature air from the engine and transports it to the outboard slats via piccolo ducts that run inside the movable slat.

The UWID processing unit that is used to generate the transducer excitation signal, condition the received signal, digitize and process the data, and output the results, is located inside the aircraft fuselage. The processor excites the transducer over a range of frequencies that are sensitive to bonded ice. The amplitude of the signal at each frequency depends on the amount of ice bonded to the aircraft skin. The amplitude values of all the frequencies are processed, and the result is a digital signal that can be experimentally correlated to an ice thickness. Data collected during wind tunnel testing suggest that the UWID signal will rise above zero when 0.5 mm of ice is present on the slat surface. The maximum thickness before complete signal saturation is estimated to be 2 mm or less. The actual thickness depends on the type of ice formed on the slat surface.

Researchers demonstrated the system during two flight trials aboard an Airbus A340. According to Goodrich and The Pennsylvania State University, the devices performed well, indicating readiness for field use.

Information was provided by Derrick Hongerholt and Gary Willms, Goodrich Corp., and Joseph Rose, The Pennsylvania State University.

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