The phrase "fly across" means to travel across a long distance by flying. There are a number of synonyms for "fly across" that can be used to add variety to your writing. Some of these include soar over, traverse by air, wing across, traverse by plane, cross the sky, and glide across. These synonyms all imply the act of traveling by flight, and can be used in a range of contexts. Whether you're describing the migration of birds or the journey of a plane across the ocean, using synonyms can help to make your writing more engaging and interesting.