This is a big, big deal that will have a lot of impact on how we fly.

What happened: Delta Air Lines has an agreement in principle to sublease the 88 Boeing 717s that AirTran currently flies. Southwest, of course, recently acquired AirTran. The deal is subject to approval by Delta’s pilots as part of a tentative agreement that also allows the carrier to increase the number of large (76-seat) regional jets its commuter associated can fly.

Why this matters for Delta: This deal, if approved, will allow Delta to replace a lot of inefficient, older 50-seat regional jets with larger aircraft (either 76-seat Bombardier CRJ900s and Embrair E175s, or 110-seat Boeing 717) that are more efficient to operate and that offer a first class section. It also puts a lot of pressure on United (UA), American (AA), and US Airways (US) to also cut deals with their pilots unions to do the same sort of thing — if they can. Whether UA, AA, and US can work out such deals is very questionable. If they can’t, this could be a huge win for Delta.

At the margin, this also makes a American/US Airways merger more difficult. Why? US Airways plan is to cut a deal with American’s unions by offering less pain than what American’s management has offered. The pain level may have just got upped and what US Airways and American’s unions have agreed to may no longer make sense from US Airways’ perspective.

How this matter for Southwest: It gets rid of an oddball fleet type, allowing Southwest to remain a pure Boeing 737 airline. It also ups the pressure on Southwest’s own unions — its flight attendants just rejected a proposal to allow the airline to fly internationally (!) and to Hawaii. So Southwest can no say: Unless you allow international flying and Hawaii, there are less jobs for you because those will be planes we don’t need. Note also that Southwest just deferred deliveries on 30 new 737s…

CLT impact: Another sign to expect less not more Southwest here. The 137-seat minimum aircraft size at Southwest also limits the routes that it can make work from here.

Bonus thought: Fewer overall aircraft does not bode well for Delta’s smaller hubs in Cincinnati and Memphis. Delta has three flights a day to each from Charlotte, mostly on 50-seat regional jets.