JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
obvious that parasite drag will significantly increase. The buildup of ice on various structural
parts of the aircraft can result in vibration, causing added stress to those parts. This is especially
true in the case of propellers and rotors, which are delicately balanced. Even a small amount of
ice, if not distributed evenly, can cause great stress on the propeller and engine mounts.
Icing is not restricted to airfoils and other external structure. Engines, fuel, and instruments may
also be affected by ice formation.
Figure 4-9 — Pitot Tube Icing
Ice associated with freezing rain or drizzle can accumulate beyond the limits of an ice protection
system. If you encounter any type of freezing rain or drizzle, the best course of action is to leave
the area.
Structural icing can block the pitot tube (Figure 4-9) and static ports. This can cause a pilot to
either lose or receive erroneous indications from various instruments such as the airspeed
indicator, VSI, and altimeter. For example, if the pitot tube becomes blocked with ice, the “total
pressure” input to the system remains constant. Therefore, during a descent, as the “static
pressure” input to the system increases, the airspeed indicator gives an erroneous indication of
decreasing airspeed. The opposite would be true during a climb.
During flight, it can be difficult to detect ice on areas such as the empennage that may be
impossible to see. Some cues which signal the potential for icing include the following: (1) ice
on windshield wiper arms or projections such as engine drain tubes, pitot tubes, engine inlet lips,
or propeller spinners, (2) decreasing airspeed with constant power and altitude, and (3) ice
detector annunciation.
Icing on rotary wing aircraft is related to those involving wings and propellers. Ice formation on
the helicopter main rotor system or anti-torque rotor system may produce serious vibration, loss
of efficiency or control, and can significantly deteriorate the available RPM to a level where safe
landing cannot be assured. In fact, a 3/16-inch (4.8-mm) coating of ice is sufficient to prevent
some helicopters from maintaining flight in a hover.
OTHER TYPES OF AIRCRAFT ICING
Induction icing – In flights through clouds that contain super-cooled water droplets, air intake
duct icing is similar to wing icing. However, the ducts may ice when skies are clear and
temperatures are above freezing. The reduced pressure that exists at the intake lowers the
temperature to the point that condensation and or deposition take place, resulting in the
formation of ice. The degree of temperature decrease varies considerably with different types of
engines. However, if the free air temperature is 10° C or less (especially near the freezing point),
Version 3.2/Dec 08 4-13