The Gremlin starts only one engine on the plane even though there are 2 engines.
In the book "Victory Thru Hare Power", Bugs Bunny reads this sentence as he shows it to the audience: "A constant menace to pilots are the gremlins who wreck planes with diabolical sabotage." Grammatically speaking, the subject of the sentence, "A constant menace", is singular. So the sentence should read "A constant menace to pilots IS the gremlins ...". The fact that "gremlins" is plural is irrelevant.