Ice detector
An ice detector is an optical transducer probe available for aviation and other applications where it is important to sense the formation of naturally occurring ICE in hazardous environments, and taking swift corrective action in response to its formation in a timely manner.
Ice formations on aircraft in flight are extremely hazardous, and pilots’ response to it can mean life or death. Many ice-induced airplane crashes have been documented[1] as causing the loss of hundreds and even thousands of human lives both inside airframes and on the ground.
Other types of ice sensor probes include plastic,[2] steel,[3] refrigerator defrost,[4] and wind turbines.[5]
An ice detector has no moving parts, is completely solid and its principle of operation is entirely optical. Intrusive to the airstream and hermetically sealed, it uses un-collimated light to monitor the opacity and optical refractive index of the substance on the probe. It is de-sensitized to ignore a film of water.
Ice detectors work as a combined optical spectrometer and optical switch. A change in opacity registers as rime ice. A change in refractive index registers as clear ice. Optical components are made of acrylic, which is the material used for aircraft windshields. The wavelength of the transducer's excitation light is not visible to the human eye so as not to be mistaken for any kind of navigational running light.
Installation
Optical ice detectors can be installed on any type of air vehicle with enough air speed to keep water from accumulating on the optics, such as rotorcraft, general aviation aircraft and unmanned aerial vehicles. They can be embedded into host aerospace systems such as an altimeter, antenna, anti-icing system, flight data recorder, jet engine inlet, pitot tube, stall warning indicator or weather system.
Optical ice detectors operate on 3.3 V DC at 100 mA. Installation requires the probe to be mounted in the airstream beyond the boundary layer, and in a location easily accessible to the pilot for occasional cleaning with a cotton swab and isopropyl alcohol.
Operation
The device works as a rudimentary go/no-go ice indicator or as an icing rate indicator for pilots who may have inadvertently entered icing domains.
Electronic signal-level comparators sense the transducer's output and activate LEDs to indicate relative icing rates to the pilot. The pilot can then judge the rate of ice accumulation and make a piloting decision. Once the aircraft has left the icing region, the ice either ablates or melts and blows away. The icing rate display sequence then reverses itself.
Transducer probe de-icing can be hastened by incorporating turns of resistance wire and dissipating a few watts into the probe, which weighs ¼ ounce. This resets the transducer in anticipation of the next icing event.
Optical ice detectors offer substantial adjustment range of drive level and returned signal amplification. Thus, they can be applied to operate in a wide variety of applications and sensitivities, down to 0.001" of ice.
Testing
Testing at NASA Glenn Icing Research Tunnel in Cleveland, Ohio, has verified the performance of optical ice detectors.[6]
Other technologies
Other technologies for ice detection on aircraft include:
1. Daytime visual Pilot visually observes ice accretion on the unprotected portion of the windshield, windshield wiper, or some protruding element in the pilot's field of view.
2. Nighttime visual For night ice detection, airplane-mounted illumination of airplane surfaces that are critical relative to ice accumulation is usually provided.
3. Obstruction A scraper rotating on a surface. As ice accretes on the surface, the torque required to rotate the scraper increases. At a preset torque, a signal is generated setting ice alert.
4. Differential Pressure A probe senses air pressure through small orifices on its forward face. As ice blocks the orifices, the pressure differential signal is created setting ice alert.
5. Latent Heat A periodic current pulse is sent through a resistance element to heat a probe. If ice has accreted on the probe, the temperature increase will be temporarily halted at 0°C and set ice alert.
6. Vibration Ice on a vibrating reed slows down its resonant frequency, which is detected and used to set ice alert.
7. Microwave A microwave transducer consisting of a resonant surface waveguide is embedded into a surface on which ice accretes. It acts as part of the waveguide, changes its phase, and sets ice alert.
8. Electromagnetic Beam Interruption An EM source is placed on one side of a flattened tube, and directed at a sensor on the opposite side of the tube. As ice accretes on the tube, the signal is blocked and sets ice alert.
9. Ultrasonic Sound waves are reflected from an ice/air interface. If ice is present, the reflected waves will be received by a transducer and set ice alert.
10. Capacitance A total impedance ice detector uses the capacitance of a surface-mounted electrical circuit to determine the presence and thickness of ice and set ice alert.
11. Optically Occluding An optical source directs radiation at an optical receiver. An accreting surface in close proximity to the beam and accreting ice is senses when ice blocks the path of the beam, and ice alert is set.
12. Optically Refractive An optical source sets ice alert by using un-collimated light to monitor the opacity and optical index-of-refraction of whatever substance is on the probe. Desensitized to ignore a film of water, it has no moving parts, and is completely solid. The wavelength of the transducer's excitation light is not visible to the human eye so as not to be mistaken for any kind of navigational running light.
References
- Huffstutter, P. j; Pae, Peter; Times, Los Angeles (February 12, 2009). "Wing icing preceded 'pitch and roll' in Buffalo plane crash". Houston Chronicle.
- "9732 PLASTIC | ice detector sensor".
- "9732-STEEL | ice detector sensor".
- "9734 REFR | ice detector sensor".
- "9734-SYSTEM | ice detector sensor".
- NASA Lewis (Glenn) video document D040 dated 23 September, 1997, range time = 18:43:37 hours