I sent email to FRA
Airport customer service and to Bundesgrenzschutzamt Flughafen Frankfurt/Main Sachgebiet Grenzpolizei (border police?
I've asked them if I will have enough time to go thru customs and back. Both of them told me that 5 hours is more than enough to visit the deck. Both responded within minutes. I have to say that they have really good customer service at FRA
"If you are an US-Citizen and holding an US-passport, it will be no problem to leave the transit area in order to visit the Visitors terrace in Terminal 1, Hall A"
"Five hours overlay time are more than enough time to enjoy a stay at our visitor's terrace.
Our visitor's terrace, located in the public part of terminal 1, is open March to October 08:00 am to 08:30 pm, last admission 30 minutes before closing."
My other question is: If the sun on the deck is good only afternoon, is there any place inside the terminal where I can shoot from? I will be there from 10:30 am until 4pm.