The Federal Aviation Administration has granted Archer Aviation a key certification that gets the electric air taxi maker closer to eventually flying travelers, the company said Wednesday.

Archer is making electric vertical takeoff and landing aircraft, or eVTOLs, and won orders and backing in 2021 from United Airlines, which says the new technology could reduce carbon emissions.

Carriers have been investing in or ordering eVTOL aircraft, which take off and land vertically like helicopters and whose developers say they can cut down on emissions in congested areas. United, for example, says passengers could take them to and from the airport in big cities, such as between Manhattan and United’s hub in Newark, New Jersey.

“Today we have received the Part 135 certification, which allows us to effectively become an airline so we can carry passengers,” Archer CEO Adam Goldstein told CNBC.

The process has taken Archer about two years: It submitted more than 2,000 pages of documents and 14 manuals outlining operational procedures, training and maintenance.

Now Archer has to get its four-passenger aircraft, called “Midnight,” certified by the FAA, which the company is currently working on, Goldstein said. That could put the air taxis into service as early as next year, the company estimates. Goldstein said he couldn’t give an exact time frame but when asked about certification delays on variants of older aircraft, he noted that Archer’s aircraft are much simpler with far fewer components than commercial jets.

Archer’s demonstrator aircraft, Maker, can fly up to 60 miles at top speeds of 150 mph. The company’s Midnight aircraft has a range of 100 miles, though Archer aims to use it for shorter distances.

United is working with Archer on what it would look like to enter the electric aircraft into service.

“This is not something that is a push of a button,” said Andrew Chang, managing director of United Airlines’ venture arm. “It’s matching how quickly [Archer] can progress the operational side and how to fit that within our airport hubs.”

Archer has partnered with automaker Stellantis to produce hundreds of the electric air taxis.

Archer’s rivals have also made strides. Joby Aviation received its Part 135 certificate two years ago, has a partnership with the U.S. Air Force, and has won orders and backing from Delta Air Lines. On Tuesday, Joby said it plans to acquire the autonomy division of autonomous aviation company Xwing.