Alaska Airlines Mileage Plan and recent partner Northwest WorldPerks have teamed up to offer serious bonus miles this spring.

Through May 31, you can earn up to 20,000 bonus miles when you fly on Alaska Airlines/Horizon Air and Northwest Airlines. Fly two segments on Alaska/Horizon and two segments on Northwest and earn 2,000 bonus miles; four segments each nets 4,000 additional bonus miles; six segments, 6,000 additional miles; and eight segments, 8,000 additional miles — for a grand total of 20,000 bonus miles.

And of course, through Mileage Plan, that 20,000 miles is enough for a free ticket.

You must register at http://www.alaskaair.com before your first qualifying flight, but there’s still plenty of time.