I have used air filters on airplanes... usually I don't bother. The reason for the filter is to keep dirt out. If you are flying in an area that has a lot of dust to deal with, the filter can extend your engine life. If you are flying off a paved runway and don't have noticeable blowing dust... its probably unnecessary. If you are flying off a dirt runway that has loose duret which the prop is always kicking up... you need a filter.