Delta Air Lines says it issued a request for proposals from aircraft manufacturers in late December for the “potential replacement” of 100 to 200 domestic narrowbody aircraft, with options for as many as 200 more.

Delta, which disclosed the RFP in its weekly news line for employees and CEO Richard Anderson’s weekly message Jan. 13, says the airline is looking to take the first delivery in early 2013 and wants large, medium and small narrowbodies. Anderson describes the search as being in its “initial phases.”

Delta has been upgrading its narrowbody interiors and supplementing the domestic fleet with inexpensively acquired MD-90 aircraft, which “has been the right answer” for the short term, Anderson says. But “as we look out over the long term, we know that in a few years, we will need to replace Airbus A320s, DC-9-50s and Boeing 757-200s.”

Delta has 27 A320s with CFM56-5A1 engines that are on average 18 years old and 42 A320s with CFM56-5A3 engines with an average age of 13 years, according to Ascend Online Fleets data last updated on Dec. 15. (Delta also has 57 A319s with an average age of eight years.)

The 34 DC-9-51 aircraft Delta has in service are about 32 years old. The 116 757-200 aircraft without winglets that it has in service have an average age of about 18 years, and the 46 with winglets average 14 years.

In his message to employees, Anderson also notes that the airline has significantly reduced the number of 50-seat aircraft it uses. Delta already is limiting its use of 50-seat regional jets to routes of 750 miles or fewer and has talked about lowering the maximum distance to 600 miles within the next year or two, essentially confining them to providing feeder traffic for larger routes out of its hubs (DAILY, Sept. 27, 2010). That’s not only because fuel prices have made 50-seaters uneconomical for many routes, but also because Delta has decided that making its average aircraft bigger is the only way to keep pace with demand while operating at slot-controlled and congested airports.

Delta has removed more than 70 50-seat regional jets and 25 Saab turboprops from its fleet since 2007. Delta’s lone remaining regional subsidiary, Comair began a big restructuring last year, at a time when it had 69 50-seaters in its fleet, that will reduce that number to 16 by the end of 2012 (DAILY, Sept. 2, 2010).

“We’re upgauging the size of the airline,” Anderson explains in the message to employees. “Where we are headed long term is being certain that we have the gauge we need to replace retiring airplanes and have modest growth when the economy and fuel prices support it.”

The RFP also could be a sign that Delta does not expect Boeing to re-engine its 737s. In an interview last fall, Anderson told Aviation Week the following regarding narrowbody fleet replacement: “I think the MD-90 will serve us well, and at some point down the road we’ll look at a narrowbody replacement, after we see where this re-engining gets us.”