Delta Air Lines’ tentative collective bargaining agreement with its pilots union is worded to ensure that the carrier will have to cut its regional airline services if it reduces domestic mainline services, under a formula that could spread to union contracts at other carriers.

The tentative deal also could set other precedents. For example, the contract includes language to automatically ensure that Delta pilots retain an “appropriate percentage” of the flying in any new joint-venture agreement—an issue that the Delta Master Executive Council for the Air Line Pilots Association (ALPA) is referring to as “big scope” because it deals with international widebody flying, rather than “small scope” regional aircraft operations.

On “small scope” issues, some union contracts used to include limits on how much mainline operations could be cut, and sometimes those limits were not linked to the size of the regional operations. These airlines, however, usually negotiated elimination or loosening of these restrictions during Chapter 11 restructuring.

The proposed deal with Delta establishes a ratio between domestic mainline and regional flying block hours that Delta must maintain. The national ALPA office says some other agreements also include ratios—based on block hours or hulls—but adds that the Delta deal is “unique” because the ratio would change if Delta revises the composition of its regional fleet to include larger aircraft. The Delta ALPA unit says it initiated the ratio concept.

The agreement lets Delta add 70 more 76-seat aircraft to its regional fleet, if it adds 88 mainline narrowbody aircraft and cuts its 50-seat fleet by more than 200 aircraft. But as Delta adds more 76-seat aircraft, the minimum ratio rises.

For example, when Delta adds its first 76-seat aircraft, the ratio starts out as a requirement for Delta to provide 1.1 block hours of flying for every hour of block flying outsourced to Delta Connection carriers. But the ratio increases for every 10 additional 76-seaters that Delta adds to Delta Connection service, ending at a mandate for 1.56 hours of mainline block hours for every hour at Delta Connection if Delta adds at least 61 new 76-seaters to regional carrier operations. Once a new ratio is established, Delta cannot reduce it under most circumstances.

That guarantees mainline pilots at least 60.9% of the domestic block hours if Delta adds 61-70 of the 76-seaters, the Delta ALPA leadership says.

That means—unlike the big shift that U.S. airlines implemented during and after the recession in the early 2000s—Delta would not be able to move domestic services from mainline to regional. That has meaning, Delta union spokesman Buzz Hazzard says, because it would ensure “fairness through proportionality” if Delta were to downsize, and because “how we get paid and how we get employed is block hours.”

The contract includes a provision that allows Delta to go lower than the ratio for a “circumstance over which the company does not have control.” But the contract explicitly states that those allowable circumstances do not include the price of fuel or aircraft, the state of the economy, the company’s financial state or “the relative profitability or unprofitability of the company’s then-current operations.”