Rule: In if clauses of real conditions, be to + infinitive adds the idea that the contingency has been planned or pre-arranged.
Example of error:
(1) If extra-terrestrial beings are to appear on our planet, we cannot know if they will be friendly or not.
Be to in its 'future meaning has a strong connotation of pre-arrangement by reason of some authority, schedule, or plan, e.g.:
There's to be a meeting tomorrow.
This not only predicts the future event, but also implies that it has been planned or scheduled. This idea of pre-arranged future is closely related to the notion of purpose, and it is in these terms that Leech (1971:98) describes the meaning of be to in the if clause of real conditions in sentences like
If we are to win the competition, we must start training now.
According to Leech:
The meaning of the if clause here is close to 'in order to win the competition or 'If we are going to (i.e. intend to) win the competition.
In (1), however, be to would imply that extra-terrestrial beings are expected and desired, which was not the author's intended meaning. Since it is a rather unlikely condition to be fulfilled, he may have been confused by the possibility of using was/were to as an alternative to the simple past in if clauses of unreal conditions. In this case the correct form of (1) would be:
If extra-terrestrial beinzs were to appear on ourplanet, we cannot know if they would be friendly or not.
Thus, if we understand the sentence as an unreal condition, be to + infinitive is merely an alternative to the simple tenses, but adds the idea of pre-arrangement, which would not be appropriate in (1).