CLEAR ICE PHENOMENON
11.1 Detection of Clear Ice
Under certain conditions, a clear ice layer or frost can form on the wing upper surfaces when the aircraft is on the ground. In most cases, this is accompanied by frost on the under wing surface. Severe conditions occur with precipitation, when sub-zero fuel is in contact with the wing upper surface skin panels. The clear ice accumulations are very difficult to detect from ahead of the wing or behind during walk-around, especially in poor lighting and when the wing is wet. The leading edge may not feel particularly cold. The clear ice may not be detected from the cabin either because wing surface details show through.
11.2 Severity of Clear Ice
The following factors contribute to the formation intensity and the final thickness of the clear ice layer:
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Cold fuel that was added to the aircraft during the previous ground stop and/or the long airborne time of the previous flight, resulting in a situation that the remaining fuel in the wing tanks is below 0° C.

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Abnormally large amount of remaining cold fuel in wing tanks causing the fuel level to be in contact with the wing upper surface panels as well as the lower surface, especially in the wing tank area.

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Temperature of fuel added to the aircraft during the current ground stop, adding (relatively) warm fuel can melt dry, falling snow with the possibility of re-freezing. Drizzle/rain and ambient temperatures around 0°C on the ground is very critical. Heavy freezing has been reported during drizzle/rain even at temperatures of 8 to 14°C (46 to 57°F).

11.3 Vulnerable Areas
The areas most vulnerable to freezing are:
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The wing root area between the front and rear spars,

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Any part of the wing that contains unused fuel after flight,

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The areas where different wing structures are concentrated (a lot of cold metal), such as areas above the spars and the main landing gear doubler plate.

11.4 General checks
11.4.1 A recommended procedure to check the wing upper surface is to place high enough steps as close as possible to the leading edge and near the fuselage, and climb the steps so that one can touch a wide section of the tank area by hand. If clear ice is detected, the wing upper surface should
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be de-iced and then re-checked to ensure that all ice deposits have been removed.
11.4.2 It must always be remembered that below a snow/slush / anti-icing fluid layer there can be clear ice.
11.4.3 During checks on ground, electrical or mechanical ice detectors should only be used as a back-up advisory and they are not intended to replace physical checks.
11.4.4 Ice can build up on aircraft surfaces when descending through dense clouds or precipitation during an approach.
11.4.5 When ground temperatures at the destination are low, it is possible that, when flaps are retracted, accumulations of ice may remain undetected between stationary and moveable surfaces. It is, therefore, important that these areas are checked prior to departure and any frozen deposits removed.
　
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