Delta Air Lines Inc. is buying nine used MD-90 jets from Japan Airlines Corp. to replace older DC-9s that are being retired.

Deliveries will start next January after the planes are refurbished to match the rest of Delta's fleet, and Delta will "continue to look for opportunities to acquire used MD-90s," Nat Pieper, vice president of fleet strategy, wrote earlier this week on the Atlanta-based airline's internal employee website.

The 160-seat, twin-engine MD-90s will be flown on domestic routes instead of the DC-9s, which are an average 34 years old, said Pieper, who didn't disclose financial terms. Newer planes offer better fuel efficiency, and Boeing Co.'s last MD-90 was delivered in 2000.

"MD-90s have appraised values of $5 million to $9 million, depending on vintage, and it is highly unlikely that Delta would be paying more than that range for the Japan Airlines aircraft," said Douglas Runte, managing director at Piper Jaffray & Co. in New York, who follows aircraft transactions.

A Delta spokesman, Trebor Banstetter, declined comment beyond confirming the website posting's authenticity.

Delta plans to purchase about 50 of the used MD-90 jets over the next two years because it is a "very capital-efficient aircraft," President Ed Bastian said at a December investor presentation.