It's simpler to think of it that лететь is used when you are talking about something flying in a specific direction (usually at a specific time) and летать is used in every other sense of the word. Самолет летит над океаном means "right now there is a plane flying above the ocean". But in another context you could say самолеты летают - for example: я живу рядом с аеропортом - самолеты часто летают над моим домом. You might also use самолеты часто летят in this sentence, but it would have an unusual meaning, like planes always fly in the same direction above your house.