Translation:I am flying to the United States tomorrow.
NO, NO, NO. Polish is not English! That's not because it is a perfective in Polish that a continous present is needed in the translation. Teaching people that "Lecę" can never be translated by "I fly" is completely wrong!
While I understand what you mean and agree with you, it seems to me for the purpose of this course, it is useful to reinforce the distinction for those of us learning Polish
Actually, I just got this question again, and it really doesn't seem to matter in this case. When using present as immediate future, "I fly tomorrow" and "I'm flying tomorrow" mean the exact same thing. But whether i will ever remember whether it's lecę or latam jutro, who knows?