"Terminate lease" option is below the aircraft picture. If you don't see it you have to wait few ( ? ) game months. I didn't know about this feature until I checked. My newest aircrafts don't have the terminate option, but aircraft that is 0.8 years old has it.
You'll have to keep a leased aircraft for a certain percentage of the duration of the lease depending on how long the lease lasts. If you leased it for one year, the mandatory period is 1.2 months. The longer you lease it (3+ years) the longer that mandatory period is and the longer you'll need to keep the airplane.
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