I came across the following weird restrictive relative clause in which ‘of which’ is used in place of ‘whose’. Actually, l am not quite sure whether it is correct or not.
The plane (the right engine of which broke down) has been repaired in Cairo Airport.
I don’t know of any reason why that sentence should be considered syntactically incorrect in formal written English. “Of which” is not a very natural start to a relative clause, but it is an acceptable one in this kind of English: it uses the relative pronoun which, which can stand for an inanimate noun phrase like “the plane”, and the preposition “of”. The preposition comes before the relative pronoun because of what linguists call “pied-piping“. There are some restrictions on pied-piping, but I don’t think this sentence violates any of them. Maybe some speakers/writer have stricter restrictions on pied-piping in this context than I do.
If you convert it to separate sentences, it makes sense and is clearly grammatical:
(The right engine of the plane broke down.) The plane has been repaired in Cairo Airport.