Ice formation on wings during descent.

[ISO100]

On a recent flight I noticed what appeared to be a thin layer of ice forming on the wings of our A320. This was as we descended through a cloud layer on approach to Manchester at night. It is hard to guess our altitude but flaps had been partially extended so I would make a guess of about 5000 ft. The ice, (if that is what it was) soon cleared after we descended below the cloud.

Possibly this was an optical illusion but I was wondering what systems the aircraft would have to detect ice build up and deal with it.

[SNS3Guppy]

ISO100,

Did you see frost forming on the wing, or ice on the leading edge?

Conditions need to be fairly extreme to get ice forming on the wing aft of the leading edge. Frost or a frosting appearance can form on top of the wing, and often forms under the wing after landing. When passing through a cloud, a temperature drop occurs across the top of the wing, along with a pressure drop, and you may see a misting, or sometimes the appearance of a light frost-type formation which typically quickly dissipates. It's usually found, if at all, along the aft part of the wing.

Generally any ice that builds inflight will occur along the leading edges of the wing. These are heated in most airplanes (some use rubber boots which inflate to break off the ice), causing the ice to slip away or melt. A trace ice buildup on the leading edge isn't common, but will usually disappear as soon as one is out of the visible moisture.

[ISO100]

Thanks SNS3.

Yes it was more like a frosting just in front of the flaps.

[SNS3Guppy]

That makes a lot more sense. Any ice which does form over the wing is typically less than trace and sublimates right away. The area around the flap is a low pressure area, especially above and behind the flaps. Most flap systems incorporate a slotted system, where air passes through the flap assembly or between flap sections to reduce drag somewhat, enhance lift, and prevent airflow separation. These things tend to reduce the pressure differential back there, and consequently reduces the amount of ice that might form, to some degree. All the same, that area of the wing does experience a slight pressure drop (you can get a sense of it sometimes when arriving in very humid conditions, when you see the air "stretched thin" and a vapor trail forming between flap sections or around the edge of wingtips or flaps). This same effect can also produce trace ice under the right conditions...but it invariably sublimates, or turns to gas, right away.