8-71. ICE AND RAIN (TYPICAL).
While in icing conditions, if there is an
unexplained 30% increase of torque needed
to maintain airspeed in level flight, a
cumulative total of two or more inches of ice
accumulation on the wing, an unexplained
decrease of 15 knots IAS, or an unexplained
deviation between pilots and copilots
airspeed indicators, the icing environment
should be exited as soon as practicable. Ice
accumulation on the pitot tube assemblies
could cause a complete loss of airspeed
The following conditions indicate a possible
accumulation of ice on the pitot tube assemblies and
unprotected airplane surfaces. If any of these conditions
are observed, the icing environment should be exited as
soon as practicable.
(1) Total ice accumulation of two inches or more on
the wing surfaces. Determination of ice thickness can be
accomplished by summing the estimated ice thickness on
the wing prior to each pneumatic boot deice cycle (e.g. four
cycles of minimum recommended !&inch accumulation.
(2) A 30 percent increase in torque per engine
required to maintain an desired airspeed in level flight (not
to exceed 85 percent torque) when operating at
recommended holding/loiter speed.
(3) A decrease in indicated airspeed of 15 knots
after entering the icing condition (not slower than 1.4
power off stall speed) if maintaining original power setting
in level flight. This can be determined by comparing pre-
icing condition entry speed to the indicated speed after a
surface and antenna deice cycle is completed
(4) Any variations from normal indicated airspeed
between the pilots and copilots airspeed indicators.
a. Typical Icing. Typical icing occurs because of
supercooled water vapor such as fog, clouds or rain. The
most severe icing occurs on aircraft surfaces in visible
moisture or precipitation with a true outside air temperature
between -5°C and +1°C; however, under some
circumstances, dangerous icing conditions may be
encountered with temperatures below -10°C. The surface
of the aircraft must be at a temperature of freezing or below
before ice will stick to the aircraft. If severe icing
conditions are encountered, ascend or descend to altitudes
where these conditions do not prevail. If flight into icing
conditions is unavoidable, proper use of aircraft anti-icing
and deicing systems may minimize the problems
encountered. Approximately 15 minutes prior to flight into
temperature conditions which could produce frost or icing
conditions, the pilot and co-pilot windshield anti-ice
switches should be set at normal or high temperature
position (after preheating) as necessary to eliminate
windshield ice. Stalling airspeeds should be expected to
increase when ice has accumulated on the aircraft causing
distortion of the wing airfoil. For the same reason, stall
warning devices are not accurate and should not be relied
upon. Keep a comfortable margin of airspeed above the
normal stall airspeed with ice on the aircraft. Maintain a
minimum of 140 knots during sustained icing conditions to
prevent ice accumulation on unprotected surfaces of the
wing. In the event of windshield icing, reduce airspeed to
226 knots or below.
b. Rain. Rain presents no particular problems other
than restricted visibility and occasional incorrect airspeed
c. Taxing. Extreme care must be exercised when
taxiing on ice or slippery runways. Excessive use of either
brakes or power may result in an uncontrollable skid.
d Takeoff. Extreme care must he exercised during
takeoff from ice or slippery runways. Excessive use of
either brakes or power may result in an uncontrollable skid.
e. Climb. Keep aircraft attitude as flat as possible and
climb with higher airspeed than usual, so that the lower
surfaces of the aircraft will not he iced by flight at a high
angle of attack.
f. Cruise Flight.
(1) Prevention of Ice Formation. Prevention of ice
formation is far more effective and satisfactory than
attempts to dislodge the ice after it has formed. If icing
conditions are inadvertently encountered, turn on the anti-
icing systems prior to the first sign of ice formation.
(2) Deicer Boots. Do not operate deicer boots
continuously. Allow at least one-half inch of ice on the
boots before activating the deicer boots to remove the ice.
Continued flight in severe icing conditions should not be
attempted. If ice forms on the wing area aft of the deicer
boots, climb or descend to au altitude where conditions are
g. Landing. Extreme care must be exercised when
landing on ice or slippery runways. Excessive use of either
brakes or power may result in an uncontrollable skid. Ice
accumulation on the aircraft will result in higher stalling
airspeeds due to the change in aerodynamic characteristics