Fly down is a phrasal verb that means to descend from a higher point, usually referring to aircraft or birds. There are many synonyms for this phrase that can be used in different contexts. For example, "touch down" implies a smoother landing, "drop" suggests a sudden descent, and "descend" is a neutral term that can be used for any situation. Other synonyms include "land," "alight," "settle," "come in for a landing," and "get down." Depending on the context and tone of the sentence, any of these synonyms can be used in place of "fly down" to create a specific mood or emphasis.