It is no secret that 50-seat regional jets have been falling out of favor as higher fuel costs make them uneconomical on a growing number of routes, which is why many U.S. regional carriers have been trying to shift their fleets to larger aircraft. But a closer look at some numbers shows why regional airlines remain vulnerable on their 50-seat fleets, especially if fuel costs continue to rise.

For all of their size-shifting efforts, 50-seaters still accounted for about half the U.S. regional airline fleet as of last July, according to Regional Airline Association statistics (RAA).

A significant percentage are used on routes of 600 mi. or more, an Aviation Week analysis shows. That matters because of the economics—Delta Air Lines already has pulled back its usage of 50-seaters to routes under 750 mi. and has talked about pulling them back to routes under 600 mi. That means those longer routes would be the most vulnerable if other major carriers follow Delta’s lead.

Delta still has about 60 routes of 600 mi. or longer, all of which are operated by Pinnacle Airlines, according to Aviation Week’s analysis.

The moments of truth will come with the expiration of the contracts a regional carrier has for flying the aircraft for a major airline partner (or sooner, if the major airline finds a way out of the contract). Over the rest of this decade that will happen for more than 70% of the 50-seater fleet. If the aircraft are not coming off lease, the regional carriers will have to find contract terms the major carriers are willing to accept or find another use for them.

Pinnacle, however, talks about these contract expirations as an “opportunity,” because its fleet also includes larger and more cost-efficient Bombardier CRJ900 regional jets and Bombardier Q400 turboprops. “Some of these [50-seater] contracts will be renewed or offered to other regional airlines and some will be replaced with larger regional jets,” Pinnacle explains, while noting that its first mass expiration of contracts will not occur until 2017. “We intend to actively compete to obtain profitable regional jet and Q400 flying during this period of transition within the industry, and we believe our history of strong operating performance with a competitive cost structure will position us to succeed.”

Watch for more regional news on AviationWeek.com this week as the RAA annual conference kicks off today in Nashville.