I am flying from New Orleans to Dulles (IAD) on United Express, and from IAD to Frankfurt on United. I have a short connection (45 minutes) in IAD.
I'm trying to figure out how can I make it:
Will I need to go throughout security and border check since I'm leaving the US? How big is IAD airport? and are there different terminals for domestic and international flights? If so, how long would it take to walk between them?
There are no exit formalities for leaving the U.S., and there is no distinction between international and domestic departures at Dulles, as is the case at most U.S. airports. Dulles is spread out but not unusually so, and all of the midfield concourses at Dulles (A/B and C/D) are connected airside, without the need to re-clear TSA security.
The main trouble you have is variability. United-operated international flights almost always depart from gates on Concourse C or D, but if Lufthansa or another partner is operating the flight, it could depart from B. United Express regional jets mostly use Concourse A, but they can also arrive at Concourses C or D. That makes for a lot of combinations, which wouldn't be so bad except that which modes of transportation— foot, AeroTrain, or "mobile lounge"— you can use to transfer from one to another, and which ones are recommended, will vary depending on the pair.
United's terminal map is not to scale, which doesn't help with planning.
If I had to place bets, I'd say you will probably arrive on Concourse A and depart from Concourse C, which makes the transfer straightforward: find the AeroTrain station at Concourse A, take it outbound to Concourse C, then proceed immediately to your departure gate.
Here are a couple of tricks if that is not the case:
The midfield terminals are some distance from the main terminal, so it is never worth your while to take a train or mobile lounge back to the main terminal to transfer back out again. Staying midfield also negates any risk of accidentally leaving the secure area and having to re-clear TSA.
There is no way to get between A/B and C/D on foot.
The AeroTrain station does not serve Concourse D. It serves Concourse C but because the (35-year old) Concourse C is a "temporary" terminal, the train overshoots it by a couple hundred hundred yards, stopping near the future "permanent" terminal. For lower-numbered D gates, therefore, it can be faster to take the mobile lounge from A to D and backtrack than to take the AeroTrain from A to C and walk down and over. On the flip side, the trains run every three or four minutes, whereas the "mobile lounges" (which may be referred to as "shuttles," or by aficionados as "moon buggies") are much less frequent.
In the event you do not receive a boarding pass for your IAD-FRA flight when you first check in, do not waste time trying to print it out at Dulles until you get to the gate for your connection; if there are no kiosks, the gate agent will assist you.
Assuming you booked this all on a single ticket, your chances of making the connection are reasonably good; United does not make money by instigating misconnections for their own sake. If your inbound flight is canceled or delayed, speak to a customer service agent on the phone, or preferably in person at the airport. They will work to accommodate you on a later flight on United or one of its transatlantic partners at no extra cost to you, though it will not necessarily be the next available flight out. If you are forced to stay overnight because of a problem caused by United (e.g. staff unavailability or aircraft maintenance), they should also provide you with meal and hotel vouchers.
If you bought both sides of the connection as part of the same ticket, from the same airline (and yes, United & United Express count as the same for this purpose), that means the airline thinks you can make it.
It also means that if the first flight is delayed long enough that you can't make it, the airline will work with you to help you reach your destination (though the extent of accommodation will depend on the reason for the delay).
There is usually no special exit/review/emigration process for leaving the US, other than that when you check in for the first flight you will need to show your passport to the airline representative, probably at the ticket counter outside security in New Orleans. They will probably mark your boarding pass for the international flight "DOCS OK" and you'll need that for boarding the next flight.
When you land at IAD, go directly to the connecting gate, checking whatever gate-listing flight status screens are nearest the gate you arrive at to find out exactly which one that is (note: it might have changed during your flight in). Once you're there you can see how much time you have for trips to a restroom, food vendor, etc. if any. Some people study terminal maps ahead of short connections; you can do that if it helps you feel more prepared.
Also make sure you do not exit the secure area, so you won't have to go through security again. There are usually clear signs ("Once you pass this point, you must continue to exit" with a large red triangle containing an exclamation point) so this mistake is relatively easy to avoid.
Oh my God, I hate that connection.
It is an incredibly quick way to get back to Europe from certain parts of the US, but the connection is hair-raising.
The overwhelming problem is
Like many US airports, IAD is comically out of date, a complete shambles.
You move between the bizarre terminal arrangement on these whacky overland-busses which kind of thread between the aircraft.
They come and go every 10 or 15 minutes or so, so it all depends on timing luck.
As choster explains you will almost certainly be going A->C. You DO have to use the aero-bussy device. (DON'T try the other possibilities.)
To avoid safety and cost regulations, actual airlines in the US use "partner" airlines for short flights. Your first flight on united "express" will be comic.
Get off the plane and run like crazy
On your first comic flight on united "express". If you have a typical rollon "full size" carry on bag, it does not, repeat not, fit inside the aircraft.
Right at the gate, they will put it in the luggage tray of your "airplane".
When you arrive at IAD, you have to wait quite a few minutes until the (polite) ground handlers pull the "valet'd" bags out and hand them out.
This will cost you a good ten minutes.
I travel with nothing, but for this very purpose I specifically have a small carry on bag (with wheels!) which does fit inside these commuter planes, if I have to carry a few laptops or whatever.
When you get out of the first flight,
run like hell to the stupid Aero-Bussy. ask clearly where it is to get to C and don't consider any other options to get to C
pray you connect quickly with the Aero-Bussy. If you see one pulling away as you run towards it, you're probably buggered
Frankfurt inevitably seems at the far left end of C - you likely have to literally run
As with any tight connection, everything could go perfectly smoothly and you'll have no problem at all.
But missed connections are so common now in the US you might as well plan for the worst.
Say your first flight is on ComicUnitedExpress100. Use the awesome data sites to examine the recent flight history of CUE100. (Simply google "recent flight history CUE100") You will learn if it tends to be late.
and finally the ultimate tip:
carefully check regarding your first flight. Is there another one earlier?
Normally you can call United and the conversation goes like this. "As you can see I'm on 100 then 747 to frankfurt with that (insert knowing laugh) connection at IAD. Would you mind changing me to the earlier flight 69 - I just had a knee operation and I'm not able to run."
They'll normally waive the fee and change it for free (well, that's happened to me every time in similar situation with AA or United).
Of course, there may be no earlier flight or you may not be able to take it.
Good luck!
Wear track shoes.
Don't forget the "Will my luggage get 'valeted'?" issue, which is critical.
Finally to cheer you up remember this - you could be connecting in Chicago.
