ft altitude to collect video imagery; that altitude put the Hunter above most anti-
aircraft artillery during Kosovo operations. Hunter operators learned to recognize
the occurrence of icing on the aircraft by these events: the camera freezes over;
rapid altitude loss; air speed loss; and/or porpoising (fluctuations in altitude).
(Freezing [partial or complete] of the pitot tube results in erroneous airspeed
feedback to the on-board computer. As the UAV tries to maintain airspeed, it
porpoises up and down, and may even go into an intentional dive to regain
airspeed. A Hunter UAV suffered extensive structural damage as a result of
altitude fluctuations [Nascimento 2000].) Conference participants noted that even
if the aircraft could fly in icing conditions, the camera could not obtain useful
imagery because the camera faceplate would ice over.
Ironically, icing-related limitations on UAV operations are not known to the
Army's general aviation community. Apache instructor pilots visiting CRREL
advised that the enemy knows by the weather conditions when Apaches will and
will not be flying. The pilots stressed that UAVs need to be flying in conditions
that ground Apaches in order to provide intelligence and to suppress the enemy.
The CRREL presentation included results of a DARPA-funded study
indicating that 58% of wintertime UAV flights in Kosovo would be affected by
icing. The audience response was that 58% is too small a number given the actual
experience of Hunters in Kosovo; one remark was that the icing problem was so
severe that it is questionable whether Hunters should have been used. Even
during the warmer months of April through October when the Hunters were
flown in Kosovo, 25% of flights were adversely affected by icing or rainfall.
The United Kingdom UAV program's planned enhancements for its Phoenix
UAV include an ice warning capability and protection of vital systems (carbu-
retor, pitot tube) by heating them. The objective is to enable the Phoenix to
escape icing, with the anti-icing system expected to "buy enough time" for
Phoenix to fly elsewhere without first being overcome by the effects of ice
forming on the aircraft.
The Air Force Predator, a medium-altitude (maximum 25,000 feet), 48-foot
(14.65-m) wingspan, medium-endurance UAV, has a fielded deicing system
consisting of a "weeping wing" system that continuously pumps a film of deicing
fluid onto the wing. The Air Force's high-altitude, high-endurance UAV is the
Global Hawk, which has a wingspan of 146 feet and flies as high as 65,000 feet.
This aircraft has no airfoil ice protection system.
Although UAVs may be operated above icing conditions, they may have to
ascend and descend through icing conditions. Since UAVs have a lower climb
rate (about 150 feet per minute) than Army rotary- and fixed-wing aircraft, they
are more susceptible to the formation of an ice layer sufficiently thick to destabi-