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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).

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.

B3.3.4 General checks
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 you can touch a wide sector of the tank area by hand. If clear ice is detected, the wing upper surface should be de-iced and then re-checked to ensure that all ice deposits have been removed.
It must always be remembered that below a snow / slush / anti-icing fluid layer there can be clear ice.
During checks on ground, electrical or mechanical ice detectors should only be used as a back-up advisory. They are not a primary system and are not intended to replace physical checks.
Ice can build up on aircraft surfaces when descending through dense clouds or precipitation during an approach. 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. Under freezing fog conditions, it is necessary for the rear side of the fan blades to be checked for ice build-up prior to start-up. Any discovered deposits should be removed by directing air from a low flow hot air source, such as a cabin heater, onto the affected areas.
When slush is present on runways, inspect the aircraft when it arrives at the ramp for slush/ice accumulations. If the aircraft arrives at the gate with flaps in a position other than fully retracted, those flaps which are extended must be inspected and, if necessary, de-iced before retraction.
As mentioned above, the Flight Crew Operating Manual allows takeoff with a certain amount of frost on certain parts of the aircraft (a frost layer less than 3mm (1/8 inch) on the underside of the wings, in the area of fuel tanks and a thin layer of rime or a light coating of powdery (loose) snow on the upper surface of the fuselage.) This allowance exists to cope mainly with cold fuel, and humid conditions not necessarily linked to winter operations. However, when the aircraft need to be de-iced, these areas must be also de-iced.
　
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