TM 1-1510-262-10
due to barometric pressure variations within the storm make the instruments unreliable. Maintaining pre-established
attitude will result in a fairly constant airspeed. Turn cockpit and cabin lights on to minimize the blinding effects of
lightning. Do not use autopilot altitude hold. Maintain constant power settings and pitch attitude regardless of air-
speed or altitude indications. Concentrate on maintaining a level attitude by reference to the light director/attitude
indicator. Maintain original heading. Make no turns unless absolutely necessary.
8-54. ICE AND RAIN (TYPICAL).
While in icing conditions, if there is an unexplained 30% increase of torque needed to maintain air-
speed in level light, a cumulative total of two or more inches of ice accumulation on the wing, an
unexplained decrease of 15 KIAS, or an unexplained deviation between pilot s and copilot s 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 indication.
a. Typical Icing. The following conditions indicate possible accumulation of ice on the pitot tube assemblies and
unprotected aircraft surfaces. If any of the following 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 thicknesses on the wing prior to each pneumatic boot deice cycle
(i.e. four cycles of minimum recommended 0.50-inch accumulation).
(2) A 30 percent increase in torque per engine required to maintain desired airspeed in level light (not to
exceed 85 percent torque) when operating at recommended holding/loiter speed.
(3) A decrease in indicated airspeed of 15 KIAS after entering the icing condition (not slower than 1.4
times power off stall speed) if maintaining original power setting in level light. This can be determined by compar-
ing 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 pilot and copilot s airspeed indicators.
Icing occurs because of super-cooled water vapor such as fog, clouds, or rain collecting on the aircraft surface. The
most severe icing occurs on aircraft surfaces in visible moisture or precipitation with a true outside air temperature
between -5C and +1C; however, under some circumstances, dangerous icing conditions may be encountered
with temperatures below -10C. The surface of aircraft must be at a temperature of freezing or below for ice
to stick. If severe icing conditions are encountered, ascend or descend to altitudes where these conditions do
not prevail. If light into icing conditions is unavoidable, proper use of aircraft anti-icing and deicing systems
may minimize the problems encountered. Approximately 15 minutes prior to light into temperature conditions
which could produce frost or icing conditions, the pilot and copilot 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 Figure 8-2 stall airspeed with ice on the aircraft. Maintain a minimum of
140 KIAS during sustained icing conditions to prevent ice accumulation on unprotected surfaces of the wing. In the
event of windshield icing, reduce airspeed to 226 KIAS or below.
b. Rain. Rain presents no particular problems other than slippery runways, restricted visibility, and occasional
incorrect airspeed indications.
c. Taxiing. 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 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 pitch attitude as lat as possible and climb with higher airspeed than usual, so that the
unprotected lower surfaces of the aircraft will not be subjected to ice accretion due to light at a high angle of attack.
f. Cruise Flight.
8-36
