Both should have and must have can be used and they are correct...
Regarding your misery, we use -miþ when we assume things, or hear from another person, but not witnessed ourselves...
Ali eve gelmiþ bile.. means you were not expecting that he would come this early, you open the door, you see him, and turn around to tell your friend that he has already come back to home....
Orada bir kaza olmuþ - You haven´t seen the accident, but somebody has seen it and told you, and now you are narrating it to someone else.
Likewise in your examples, you haven´t seen the plane landing and you haven´t heard from anyone, but you know the plane was to land at a particular time, so you are assuming that the plane must have landed, because it was supposed to land at - 6 o´clock and now it is 6;30.
Thank you Merih. Let me ask one follow up question to that then.
Is there a whole lot of difference between saying "Uçak inmiþ" and "Uçak inmiþ olmalý" - what I´m thinking is that in the second example you´re making the assumption about the plane yourself and in the first somebody told you about it? Is that right?
Merih only wishes he´d look the same 
ay she´s that toxic