The other posts are all quite right, only one problem is adequate viewing of the aircraft. I have tried this several times... the trouble with dulles is that the airport owns so much property it is hard to get a look at the planes coming and going. Once the a/c park one can view them to some extent. AF and ANA are easily viewed, as is VS at the B terminal. Unfortunately once airlines like BA and LH arrive at their gates, they are all but impossible to see! they are hidden in the old midfield terminal. The following 747s grace IAD with their massiveness each day: LH 742 (744 in summer), BA 742, ANA 744, VS 742 and 744.
Saudi and Korean fly the 744 several times per week.
Other favourite large ones of mine are the Swissair and Sabena 330's, the AF 777 and A340.
If you find a place to spot near runways let me know!!
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