It will be the first time (that) I have flown to America.
The above example sentence is borrowed from this earlier post.
Depending on context, there can be other variations as follows:
It is the first time (that) I have flown to America.
It was the first time (that) I had flown to America.
The parentheses are there to indicate that that is optional.
What exactly is this that-clause? Is it an ordinary relative clause? If it is, why is it not allowed to use "when" instead of "that"?
It will be the first time when I have flown to America. (?)
Edit: I'm leaning toward the relative clause.
The only thing that bothers me is that a relative clause generally allows "when" instead of "that" insofar as the antecedent is some kind of "time". Are there some other relative clauses that don't quite allow "when" for a "time" antecedent?