When diabolical dark creatures assault an airplane, one spunky yet meticulous teenager misjudges his technical abilities and unknowingly makes the devastation worse, consequently he must travel back in time using an enchanted relic to undo the damage and defeat the dark creatures.

This logline is getting better, now to cut the word count and get specific.
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1) Demonic and dark is redundant, use one or the other (or just use the word demon)

2) Spunky yet Meticulous: A spunky person flies by the seat of their pants, they go-for-it, a meticulous person doesn’t; I would keep spunky and drop meticulous

3) Misjudges his technical abilities: This isn’t needed, drop it

4) —and unknowingly: Drop this, if he doesn’t know it, how does he know to go back in time and change it.

5) Makes the devastation worse: Be specific, this is the crux of the logline. How does he make the devastation worse? ?Does he open a portal to hell? Does he accidentally set the auto-pilot to crash the plane into a mountain? Tell us how he makes the devastation worse. This is the inciting incident and very important to the logline.

6) Consequently: Drop this

7) Using an enchanted relic: Be specific.?What relic? You should watch Buffy the Vampire slayer, every relic had a name, every relic had a history— Orb of Thesulah, gem of amara— you should tell us what relic sends him back in time, give it a name.

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“When his flight is attacked by demons, a spunky Greek curator accidentally opens a doorway to hell and must use the gem of Chronos to travel back in time to undo the damage and stop the dark-ones from taking over the world.”
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Hope that helped, good luck with this!