8-57. ICE AND RAIN (TYPICAL).
W A R N I N G
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 recommendation ½-inch accumulation.
(2) A 30 percent increase in torque per engine
required to maintain a 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 Ice. 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
he 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
b. Rain. Rain presents no particular problems other
than restricted visibility and occasional incorrect airspeed
c. Taxiing. Extreme are 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 be 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 be iced by flight at a high
angle of attack.
f. Cruising Flight. 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.
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 an
altitude where conditions are less severe.
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
and increased weight of the aircraft due to ice buildup.
Approach and landing airspeeds must be increased