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DENVER (KDVR) — United Airlines is celebrating its first commercial passenger flight using non-petroleum engine fuel.

The Dec. 1 flight made history thanks to two Denver-based pilots who work at the United Airlines Flight Training Center in Denver’s Central Park neighborhood. 

Every year, more than 10,000 United pilots receive training at the 550,000-square-foot complex near Quebec Street and Martin Luther King Jr. Boulevard.

“There’s a very large operation here,” said Ryan Smith, director of strategy and flight test at United Airlines.

Smith joined Joe Heins, United’s managing director of test, technology and engineering, for the historic flight.

Heins and Smith are test pilots who were approved to fly the experimental flight. They work to help United reduce its greenhouse gas emissions in an industry that produces about 2% of the world’s carbon.

“We’re taking first steps, as United Airlines, to reduce that [carbon] footprint and set an example for the industry,” Heins said.

Heins and Smith took off from Chicago for the nation’s capital with more than 100 travelers, including their boss, CEO Scott Kirby. The plane they flew — a Boeing 737 Max 8 — used 100% non-petroleum-based sustainable aviation fuel in one of its engines.

“We ran jet fuel in the left engine, and we ran this 100% sustainable fuel in the right engine so we could show that there’s no difference in the performance of the airplane as it operates in the flight,” Smith said.

Heins said there was no difference between the two fuels as it relates to pilot, crew and passenger experience. United regularly mixes the biomass solution with conventional fuel for its flights.

“[The sustainable fuel] comes from feedstock,” Smith said.

United said scaling up sustainable fuel production and regulatory approval are needed as it flies into an eco-friendly future.

United said the work and research happening at the flight training facility in Denver are key to helping the company reach its goal to eliminate its greenhouse gas emissions by the year 2050.