American Airlines Group Inc. is planning to retire 45 of its oldest Boeing 737 jets over the period of next two years as part of remodeling that will reduce the age of its workhorse narrow-body fleet.

The carrier company plans to withdraw 12 of the aircraft in next year and 33 aircraft in 2020. American’s oldest 737-800 carriers are almost 19 years old, although the group’s average age is 8.1 years which has been reduced by recent deliveries according to the airline’s annual report. American had 304 Boeing models at the end of last year, which makes one-third of its main jet fleet.

American spokesman Josh Freed said Thursday “It’s strictly an age-based retirement at this point “We have some that we took back in 1999.” Older planes tend to require more maintenance than newer models.

American is planning to add 40 Boeing 737 Max aircraft and 50 Airbus SE A321neo jets between the end of this year and the end of 2020 according to a filing of regulatory.

The changes are part of American’s broader effort to balance deliveries of new planes with retirements of old ones to hold its mainline fleet at a number of 950, although the figure is predicted to drop to 935 in 2020. Since it merged with US Airways at the end of 2013, the carrier has received 469 new planes. Lowering its average fleet age to 10 from 13, it has retired the same number.

ALSO READ: