I assume you are arriving on an International flight on UA or a Star Alliance airline. If so, yes, you go through Immigration/Customs/Baggage Recheck in the C Concourse. You then go through TSA security and come upstairs across from gate C7.

At that point most folks go to their connecting flight. But you are perfectly free to follow the signs for BAGGAGE CLAIM/GROUND TRANSPORTATION. That will take you to the AeroTrain station - take the train two stops to the TERMINAL - exit security area into Baggage Claim - go down the exit ramp to the second sidewalk where most of the shuttles pickup (hotel, car rental, parking) and look for the overhead 2E sign. That is where the shuttle to Udvar-Hazy stops.

When you return to IAD from the museum, look for the "Dulles Diamond" TSA security checkpoint between baggage carousels 8 & 10. (That checkpoint closes at 6pm.) Then head for your gate.

Feel free to message me with any questions.

Thanks for your advice. But what happens after 6pm? I come in at 3:30pm and take off again at (ca) 11pm. Are there other options to get in or do I simply have to be inside by 6?

The regular security lines are downstairs, just follow the signs whenever you are ready to head through. Two warnings, it takes a little bit of time to get out to the terminal after you pass security and walk to your gate (budget 15 min at a minimum) plus tsa reduces staffing later iin the evening and can cause 15-20 min waits at security. You just never know what you'll encounter. Enjoy your trip.

Most, but not all, airline clubs I know of sell one-day passes for $40-50. Check the Web sites of the various airlines there. Otherwise, you normally have to be a registered/paid club member, or be at a very high level of the airline's frequent flyer program, or be flying international business or first class ("just" flying domestic first class isn't usually enough). The exact amenities vary, depending on the airline and exact location.

Marriott just outside the airport entrance is expensive, but you lose nothing by checking. If you rent a car, you could drive to the museum, or park at the airport, and then go to the museum and return to meet your brother. 40 minutes to some parts of DC could be doable, but double that (at least) at rush hour. If you're using the airport you can use the free airport access road from the Beltway (I-495), vs. the toll road, but be prepared to show something or at least have the info of the fight you're meeting, in case a cop pulls you over (they regularly give tickets to people illegally using the access road to avoid tolls).

Can you go into DC and wait for your brother there? There is a Metro bus from the airport to Herndon, Rosslyn and L'Enfant Plaza (DC). Check www.wmata.com. It is (or at least was) a regular bus, with no extra space for luggage. But cheap. Next chapest alternative is probably the Washington Flyer bus to the West Falls Church Metro station, then Metro into DC. If you are staying in DC, parking is expensive, even at hotels. My wife and I stayed at a hotel in Rosslyn (Arlington), just across the Potomac from DC, and paid $10 for parking. Used our car to go shopping in outlying areas, etc., but used Metro to go into DC itself. Paid about the same when we stayed in Silver Spring, Maryland, north of DC.

To go on to NYC, train, or "Chinatown" bus (Google it) or renting a car at a closer-in airport (e.g., DCA, or even BWI, which is accessible by MARC commuter train or Amtrak from DC) would be options.