"They're kind of the market-maker," said Robert Agnew, president and chief executive of aviation consultant Morten Beyer & Agnew.
"If Delta weren't there, the airplane might be struggling."
Values have risen more than 17 per cent this year, to range from US$7.5 million ($11.9 million) to US$10.5 million, while lease rates soared 22 per cent after bidders jumped in when Sweden's SAS put nine planes on the market, according to Ascend. Delta got three, Qantas snagged two and four went to budget carrier Volotea.
Once known as the MD-95, the 717 was the last jetliner developed by McDonnell Douglas before Boeing acquired the company 18 years ago. By the time the plane debuted in 1999, airlines were tilting away from 100-seaters for short-haul flying in favour of bigger jets packing in more travellers.
"It's a very good aircraft," said Jonathan Bogaard, a founding member of law firm Vedder Price's global transportation finance team. "It just never caught on."
Boeing still provides 717 engineering and spare-parts support, which is common. More unusual - it owns 103 of the aircraft and holds operating leases worth about $1.5 billion.
The Chicago-based company has helped find new homes for 717s as operators such as SAS's Blue1 dropped the model.
"We're part of the market and that helps us stay close to the operators of our airplanes," said Beth Notari, managing director for portfolio marketing with Boeing Capital.
Carriers like the 717's performance on shorter flights, while the five-abreast cabin layout is favoured by travellers because there's only one middle seat per row.
The plane's revival signals that the 100-seat segment of the jet market "is more robust than many believe," said Addison Schonland, an analyst with AirInsight.
The 717 didn't become a hot commodity until Delta - known for spotting value in little-loved used planes - decided the model could replace cramped, 50-seat regional jets. It's flying them between Los Angeles and San Francisco, among other routes.
- Bloomberg