Ampaire, Inc., a Los Angeles-based startup that has partnered with Mokulele Airlines to launch a hybrid electric plane, marked a milestone with its first test flight of a retrofitted Cessna 337 Skymaster in southern California this morning.

The Ampaire 337, described by the company as “the highest-capacity hybrid electric aircraft ever flown,” took to the skies above Camarillo Airport in California before a crowd of engineers, investors, and journalists. Ampaire said it was a significant step for aviation because “never before has a hybrid-electric aircraft this large flown.”

Ampaire’s long-term goal is to make the hybrid planes commercially available for short-haul flights in markets like Hawaii in order to reduce greenhouse gas emissions by 2021. Ampaire is planning to test its hybrid plane on the route between Kahului and Hana, Maui, later this year in partnership with Mokulele Airlines.

“Imagine that in just a few years you will be able to buy a ticket for a flight that is clean, quiet and inexpensive,” said Kevin Noertker, CEO of Ampaire, in a news release. “Ampaire is proud to lead the aviation industry in transportation electrification, and we recognize the importance of electric aviation for climate change and community connectivity.”

The Cessna 337 Skymaster was retrofitted with Ampaire’s proprietary electric propulsion system and is powered by a lightweight battery system. One of the aircraft’s two combustion engines was replaced with the battery-powered electric motor.

The result is a “parallel hybrid,” meaning the internal combustion engine and electric motor work in concert to optimize power output as the plane flies, said Ampaire. A test pilot and flight engineer flew the experimental plane.

Thursday’s test flight follows the Federal Aviation Administration (FAA) airworthiness approval in May to begin a flight test program. The test flights will take place several times per week from June through August, and will provide important data on electric propulsion performance characteristics.

A newly retrofitted Cessna 337, further refined with data from the test flight program, will be tested on Maui.

Ampaire was one of the 20 cohorts selected last year by Elemental Excelerator, a Honolulu startup that funds companies that fit its mission of improving lives through innovative solutions. Each cohort is awarded up to $1 million.

In addition to the upcoming project on Maui, Ampaire is working with Vieques Air Link (VAL), a regional airline in Puerto Rico, to establish a pilot project in the region.