Layover in Frankfurt for 1h40m - is it enough? What happens otherwise?

Well, if it's on the same ticket, operated by Lufthansa, they certainly expect you to make it, and they'll sort you out if you don't (due to delays or whatever it might be).

For your own sanity, for a domestic-to-international transfer like that at Frankfurt, the estimate is around 45 minutes. So you'll have a bit of leeway around that anyway.

I'm not sure if this is still true, but I remember that I once realized that there was no airside connection from terminal A/B to C, which means that you will also have to pass through security, further cutting into your time.

Also, the smaller aircraft more often get outfield parking positions, then you will be bussed to the terminal, which also takes more time than getting off via a jetway (on my last connecting flight from FDH, it was 30 minutes between touchdown and actually reaching the terminal building. Consider that the maximum, the plane was parked at the very end of the airport grounds).

As the others have mentioned, Lufthansa will rebook you if you miss the connection, but given that there is only one direct flight to Chennai every day, I would rather try to get an earlier flight into Frankfurt.

Depending on where you are staying the night before your travel, and how you will be getting to the airport, it might be worth taking the train instead of a connecting flight: it gives you much more flexibility, plus the train ride from Stuttgart main station to Frankfurt airport is roughly one hour, while the ride to Stuttgart airport takes about 30 minutes; so heading straight to Frankfurt airport is actually faster.

Normally, the airline is very interested that you make your connection, as many others explained, and for most places in the world that would do. But especially Lufthansa in Germany has unfortunately a history of not caring much. Those connector flights are often delayed significantly, and arrive only after the international connection left already.

If this happens, Lufthansa will book you on the next day's flight, and pay you a hotel for the extra day in Frankfurt; that may be of interest to you, but if you live in Stuttgart or Nuremberg or so, this is not of much value, and if you need to be somewhere the next day, it's not fun either.

I know dozens of people that have missed their connection in Frankfurt that way (myself included), and have started to book an earlier connector because of that. If you care to catch this flight, I recommend you try to rebook your connection to an earlier one (there should be at least one per hour from Stuttgart to Frankfurt), so you have 3 or more hours layover in Frankfurt.

I did this yesterday morning. Took me 32 minutes from the plane stairs (apron parking, so we were bussed to the terminal A) to exiting immigration and walking to a gate in terminal B.

I can confirm that there is no security (and this would have been my main concern time-wise). As far as I can tell, you only go through security if you come from a non-Schengen/USA/Canada flight.

All the answers date back to 2016. I traveled in a similar leg recently: Munich - Frankfurt - Chennai.

Scenario 1:

I arrived in zone A of Frankfurt airport at 8 AM. My flight to Chennai was supposed to be at 10:15 AM from C15. During my transit from Zone A to C15, I had the following (in no particular order):

• 1) Passport Control
• 2) Regular Security Check
• 3) Took the train from Zone A to C
• 4) Walked for atleast 20 minutes
• 5) Had the final passport/boarding pass check by
  Lufthansa before letting into C15.

All these together took me 45 minutes exactly. So, yes, it was all fine for me.

Scenario 2: I had my flight canceled the previous day and they were giving me a flight from Munich that would reach Frankfurt at 9 AM. I was skeptical about making it to my Chennai flight on time, hence, I asked them for a flight that reaches Frankfurt at 8 AM from Munich.

They also told me, if I miss the connecting flight to Chennai because of any delay caused in between, they would put me on the next flight.