Ice feathers

From Glossary of Meteorology

ice feathers

(Also called frost feathers.) A type of hoarfrost that is formed on the windward side of terrestrial objects and on aircraft flying from cold to warm air layers.

Ice feathers are made up of single, columnar ice crystals, some of which grow out from others at large angles and thus build up a delicate spatial array of tiny crystals.
See air hoar.