The majority of 50 Boeing 737-900ERs and 737 MAX aircraft Alaska Airlines just ordered will be for fleet replacement, the carrier’s CFO tells Aviation Week.

“We plan to use two-thirds of these aircraft for fleet replacement, and one-third to position us for very modest growth over the next decade,” Brandon Pedersen says of the Oct. 10 firm order for 20 737-8s, 17 737-9s and 13 737-900ERs, in a deal worth $5 billion at list prices.

In addition to the 50 aircraft on firm order, Alaska says it has 25 737-900ERs on backlog from a previous order, indicating that this latest deal also includes the conversion of three -800s to -900ERs. Delivery of the first -900ERs from that order is expected to begin this month.

“The -900ERs will be have 181 seats in a two-class configuration,” says Pedersen. “This offers us a range beyond our existing aircraft with more seats,” he says, signaling the carrier’s growth plans.

Alaska will begin retiring its fleet of -400s and -700s in the next three to five years, says Pedersen.

The oldest aircraft of the carrier’s fleet of -400s are 20 years old and account for 30 of the carrier’s 120 737s. According to the Aviation Week Intelligence Network fleet database, Alaska also operates 17 -700s, 61 -800s and 12 -900s, most of which are relatively new.

The re-engined 737s provide significant cost opportunities, Pederson says. “The MAX is a great airplane for us. It will allow us to save 13% on our fuel bill.” Seat configurations for the -8 and -9 aircraft have not been determined, he adds.

“We wanted to make sure we were early in the delivery stream to maintain our industry-leading fuel efficiency,” Pedersen says. Alaska will begin taking delivery of its -8 and -9 aircraft in 2018.