It's correct depending on the context. Your sentence would be appropriate if, for example, you were discussing a recent hijacking and you said, "I was a passenger on a plane that was hijacked," implying a previous incident. If you were discussing a recent incident and you wanted to say you, too, were on that particular flight, you would say what Steve said: "I was a passenger on THE plane that was hijacked". Hope that helps.

I would say "I was a passenger on the plane that was hijacked".