* "A new airplane" is the subject of the sentence.
* "was" is the verb.
* "your conclusion about strange lights sky" is the predicate nominative, which means it renames the subject.
There is no pronoun in this sentence. The phrase "your conclusion about strange lights sky" is a noun phrase acting as the predicate nominative.
It seems like the sentence is missing a word or two to make sense. It might be trying to say something like:
* "A new airplane was your conclusion about the strange lights in the sky?"
* "A new airplane was your explanation for the strange lights in the sky."
In these revised sentences, "your" is a possessive pronoun.