In the film Donnie Darko, a jet engine lands on Donnie's room in the beginning of the movie. I thought that it was the engine of the plane in which his mother and sister were travelling, which gets crashed and was sent from the future by Donnie through the portal as an act of self sacrifice to save others. If this was the case, he had successfully altered the future and the crash should not have happened in the future. If the crash itself had not taken place, how did he send the engine? Or did he transfer that object from tangent universe to the primary universe (I think this is highly unlikely).
CHAPTER : THE ARTIFACT AND THE LIVING
When a Tangent Universe occurs, those living nearest to the Vortex will find themselves at the epicenter of a dangerous new world. Artifacts provide the first sign that a Tangent Universe has occurred. If an Artifact occurs, the Living will retrieve it with great interest and curiosity. Artifacts are formed from metal, such as an Arrowhead from an ancient Mayan civilization, or a Metal Sword from Medieval Europe. Artifacts returned to the Primary Universe are often linked in religious iconography, as their appearance on Earth seems to defy logical explanation. Divine Intervention is deemed to be the only logical conclusion for the appearance of the artifact.
The whole Tangent Universe thing is happening because of a problem that has occurred with the 4th dimension. The details of it we are never told. It is the purpose of the Living Receiver to send the artifact to the Primary Universe to restore balance. Donnie does not send the artifact which landed in his home back to the Primary Universe. That stays in the Tangent Universe. He sends the duplicate copy from the Tangent Universe back to the Primary Universe. Yes, the Living Receiver has the power to do this in the Tangent Universe.
So where does the artifact come from to the Tangent Universe? Well that is the text mentioned in Roberta Sparrow's text - Divine Intervention. There is no logical explanation. A copy of the artifact will exist seemingly from nowhere. Roberta doesn't understand how either. It is caused by the tear in the fabric of 4th dimension.
As far as the Primary Universe is concerned, they will always continue to wonder where that Jet Engine came from. They will get no answer. But for the Primary Universe, there is no Receiver or Artifact. Spacetime has corrected itself and life goes on.
This is probably one of the better explanations I have found, from user Bon Gart:
The Jet Engine comes out of the divergent time line involving the events of the movie (and from his mother's plane), and is supposedly sent back in time through the same wormhole that Donnie uses to time travel with. Unfortunately, Donnie was miles away when he actually traveled back in time. The vortex might have been forming over his house, but that's not where he was. It was where he ended up... but the vortex and subsequent wormhole should have followed him... or he should have returned to it at that moment he decided to travel through time. The Jet Engine most certainly traveled through that wormhole... it came from the future to kill him in the past. IE... more proof of the actual Time Travel theory, as opposed to the Future Visions theory. This again points to the paradox and says "See what you made me do?" If Donnie never got out of bed and was killed by the Jet Engine, how could he be killed by a Jet Engine in the first place when it came off a plane hitting the atmospheric turbulence created by the vortex that was there from Donnie corrupting the time line and NOT getting killed by the Jet Engine? Answer? Paradox!