Is my usage of the imperfect and the compound past correct here? I'm having trouble distinguishing which one to use in situations like this. Thank you for your help!

 

The sentence seems correct this way, but it all depends on what you were trying to say! What would it be in English?

 

Your sentence means that at the moment referred to in the past, the plane had not yet left. You might also have written devait partir.

On the other hand, if you meant that the plane departed, then you would use the compound past instead of the imperfect: est parti.

Cheers!