Depends, and depends on what state you're in. Different states have different rules.

First off, if it's in public then it's probably always fair game and making a PA on a plane probably counts as public.

In most places it's probably always illegal to record a conversation if none of the participants know you're doing it.

In some jurisdictions it's legal as long as at least one participant knows.

In some jurisdictions, all participants must be aware.

And IIRC in some jurisdictions all participants must affirmatively consent.