In FRA, passengers from international flights exit the plane into a part of the terminal where everyone has already officially "left" Europe by passing through exit immigration, or has not yet "entered" Europe by passing through entry immigration. So you should not need a visa - HOWEVER, I don't know what the airline requires. Also, even if a visa is not required, you might want to get a transit visa anyway because if there is some delay and you are forced to spend the night (or longer) in Frankfurt, you would not be able to exit to Germany to find a hotel. A visa would allow you to do that.