pilots in instrument meteorological conditions (IMC)--
2.4 Relevance
a pilot training problem--many of the accidents may
Current methods of avoiding icing, including meteor-
be preventable with onboard icing-avoidance systems
ological and pilot reports, are extremely ineffective
(Bertorelli 1992). Even a VFR pilot may be able to avoid
(Erickson 1997). Icing information is not provided with
the most serious of icing conditions--freezing drizzle
the detail, accuracy, and timeliness needed for commer-
or freezing rain--with onboard remote-sensing icing
cial and private aircraft to avoid icing conditions effi-
avoidance capability. An instrument flight rule (IFR)
ciently. As a result, either aircraft cannot fly or large
pilot could avoid icing within IMC, or at least avoid
areas of potentially flyable airspace must sometimes be
conditions that tax aircraft ice-removal systems.
avoided because of inadequate spatial and temporal reso-
Helicopters flying low-altitude missions to service
lution of forecasts. Military aviators and civil aviators
offshore oil rigs and in search-and-rescue operations,
in the Far North where bush flying is common also need
for example, are particularly vulnerable to icing. In addi-
to be able to avoid icing autonomously because fore-
tion, limited altitude capability, low speeds, rotating
casts are often unavailable in operational areas (Owen
components, and generally low power reserves make
1997). In addition, increased use of laminar flow air-
them more susceptible to ice than fixed-wing aircraft
foils and more efficient engine designs less tolerant of
(Manningham 1991). However, helicopters' low speed
contaminants make some aircraft more susceptible to icing.
and maneuverability may be an asset if they are
As air traffic increases in volume, new aircraft designs are
equipped with an icing-avoidance system because they
implemented, and new routes are established, more air-
have more flexible course- and altitude-changing capa-
craft that are less tolerant to contamination may be
bility than fixed-wing aircraft. As recently as 1995, only
exposed to icing. To increase aviation safety and
one commercial helicopter was icing-certified, the Aero-
efficiency, and to increase military readiness and air su-
spatiale Super Puma (AHS 1995).
periority, improved methods of avoiding and exiting
In-flight icing is not generally considered a problem
icing are needed.
in Army aviation, despite problems in Bosnia, because
Aircraft flying at 400 knots or greater, which includes
most missions are flown in warm climates, missions
most jets, generally do not have icing problems because
are not flown if ice is predicted, and icing is so infre-
they typically have heated leading edges and fly above
quent that readiness is little affected. However, about 9%
most icing (Taylor 1991). However, jets on approach
of Army medevac flights in Alaska are canceled due to
and departure, 300-kt turboprops, piston aircraft, and
icing, and medevac commanders give icing avoidance
helicopters are all susceptible. Turboprops fly exclu-
a high priority. Mayer et al. (1984) found about 525
sively at lower altitudes and are thus exposed to ice for
icing-related mishaps in the Navy between 1964 and
extended periods. Few light piston-engine aircraft have
1984, with about 70% due to in-flight problems and
deicing capability. Helicopters are probably the most
nearly all due to foreign-object damage from ice. Acci-
threatened of all aircraft because of their unique aero-
dent reports in recent years suggest that the Navy has
dynamics and mission requirements and because they
had fewer icing problems, with more reports of hail-
typically lack deicing capability.
impact damage than airframe ice accretion problems,
Pilots, operators, and manufacturers typically do not
but Lef et al. (1994) state that the Navy is concerned
know when most aircraft reach their performance limits
about the icing threat to carrier-launched aircraft and that
in icing (Erickson 1997). There are generally few clues
helicopter icing accidents are not infrequent. Air Force
provided to the pilot that indicate how close an aircraft
transport aircraft have also experienced icing problems,
is to those limits. This is of particular concern for air-
for example, in tropical cumulus clouds at high alti-
craft operation outside of FAA FAR 25, Appendix C,
tudes. The Coast Guard reports problems with icing in
design guidelines. The result is that many pilots may
search-and-rescue and enforcement missions (Yatto
unknowingly operate their aircraft at or near safety limits
1997).
when in many icing situations, despite the availability
Military and civilian unmanned aerial vehicles
of onboard protection systems.
(UAVs) are special cases in need of icing avoidance sys-
Fortunately, transport-category aircraft are rarely lost
tems. UAVs, especially high-altitude, long-endurance
to icing, although there are reported incidents (Engel-
UAVs, may be required to seek routes through icing
berg and Bryant 1995), but private general aviation does
conditions autonomously (Siquig 1993). Onboard
not fare as well. Aviation magazines carry many reports
weather-sensing systems could be coupled with autono-
of private general-aviation icing incidents and accidents.
mous controls to allow UAVs to avoid or minimize
Between 40 and 60 private general-aviation accidents
icing, which impacts them more severely than it does
annually are attributed to in-flight structural icing, about
conventional aircraft because of their low power and
50% of which are fatal (AVEMCO 1983, Taylor 1991).
high-efficiency airfoils.
Although about 50% of these are visual flight rule (VFR)
3
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