Remote Sensing of In-Flight Icing Conditions
Operational, Meteorological, and Technological Considerations
CHARLES C. RYERSON
1.0 EXECUTIVE SUMMARY
In-flight icing is a significant aviation risk despite
sities. The absolute range of cloud liquid-water content
improving icing forecasts and onboard ice protection
in clouds of various genera is generally well known,
systems. A remote-sensing system designed to detect
but its vertical and horizontal distribution within clouds
icing conditions in the flight path could allow aircraft
and cloud masses is not well understood. The cloud gla-
to avoid and exit hazardous conditions. Ground-based
ciation process is also imperfectly understood, as is the
near airports or airborne, such systems would be most
effect of mixed-phase conditions on aircraft icing and
useful to low, slow-flying aircraft that frequently
on cloud remote sensing. The shape of the drop-size
encounter icing, such as turboprops and helicopters.
spectrum is not well characterized in icing conditions,
Development of an icing remote-sensing system
especially in supercooled large drops, and the median
requires consideration of the operational environment
volume diameter does not often provide an adequate
within which it is used, the meteorological environment
description of the drop-size spectrum, especially if the
it senses, and the technology available for sensing icing
distribution is bimodal. Though it is generally known
conditions.
that static temperature changes more rapidly vertically
Operationally, pilots need information in the cock-
than horizontally, there has been very little characteri-
pit for making risk-management decisions. Displays must
zation of temperature distribution in icing conditions.
evoke proper pilot decisions and provide clear, unam-
The ability to detect temperature change ahead of an
biguous warnings of severe conditions for avoidance.
aircraft is critical, because the temperature within liquid
Human factors and cockpit and aircraft integration
water determines whether icing will occur. Character-
issues must be developed in addition to avoid-and-exit
ization of icing conditions is expensive, typically requir-
protocol and training. Dispatchers and meteorologists
ing research aircraft. More reliable and less costly
need integration of icing remote-sensing systems into
instrumentation is needed to replace first-generation opti-
the weather system infrastructure. Cost, maintenance,
cal probes, and to reduce cost, coordination is needed
power, weight, and space are a concern of manu-
with other federal programs that make cloud microphys-
facturers, operators, and regulators, as is the evalua-
ical measurements. Such cooperation has started
tion of aircraft flight envelopes in icing conditions.
between the Canadian Atmospheric Environment Ser-
An icing remote-sensing system detects conditions
vice, Transport Canada, NASA, and the FAA.
conducive to icing, including cloud and precipitation
The core of an icing avoidance system is the tech-
liquid-water content, the drop-size spectrum, and tem-
nology used to sense icing microphysical conditions.
perature. An icing metric algorithm would convert these
Radar and microwave radiometers are the most viable
measurements into an estimate of icing potential for
technologies. Ranging capability makes radar an attrac-
cockpit display. To develop specifications, the absolute
tive technology for detecting liquid water, drop size,
magnitudes of cloud microphysical conditions and the
and possibly temperature. Multiple-band radars, such
spatial and temporal variability of icing weather condi-
as X and Ka bands to retrieve liquid-water content using
tions, must be understood at multiple scales. Icing cloud
microphysics have been measured since the 1940s by
bands to retrieve liquid water and drop size using neu-
NACA, NASA, the FAA, NCAR, and several univer-
ral nets, currently appear most viable. Information-
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