US startup Ampaire has begun flying a testbed for its electric propulsion conversion for regional aircraft. Unveiled publicly June 6 in Camarillo, California, the modified Cessna 337 Skymaster made its first flight on May 23.

The twin-boom Skymaster has a “push-pull” configuration with Continental IO-360 piston engines mounted fore and aft in the fuselage. In the Ampaire 337, the rear engine is replaced with an electric motor powered by lithium-ion batteries.

In the test aircraft, the batteries are mounted in the cabin, but Ampaire is aiming for supplemental type certification (STC) of a conversion for the Skymaster in which the batteries are housed in an underfuselage pannier now used for cargo.

The Ampaire 337 will use the forward combustion engine and aft electric motor in a parallel-hybrid propulsion system in which the piston engine is optimized for cruise operation and the electric motor provides a power boost for takeoff and climb.

“This will allow the combustion engine to sip fuel and eliminate the corner cases where high power is required [from the engine],” co-founder and CEO Kevin Noertker said. The modified aircraft has a 200-mi useful range and the company is anticipating fuel savings of more than 50% from hybrid propulsion.

Ampaire plans to begin test flights by year-end on a route flown by Hawaiian regional carrier Mokulele Airlines. This will use a second modified Skymaster closer in configuration to the final aircraft, for which the company anticipates receiving the STC by the end of 2021.

The second, pre-production aircraft will be flown for six months on Mokulele’s route between Kahului and Hana on the island of Maui. Operations on the 28-mi. flight between Kahului and isolated Hana are limited by poor economics with current aircraft, Noertker said.

Such short routes “are the biggest early opportunities for electric flights,” he said. Ampaire is working with Puerto Rican regional Vieques Air Link to establish a pilot project and says it has letters of interest from 14 other airlines around the world.

Ampaire plans to follow the six-seat Skymaster conversion with other STCs to retrofit popular aircraft types operated by smaller regional airlines such as Mokulele, which flies Cessna Caravans and Pilatus PC-12s. “We plan to scale up to a larger aircraft next year,” Noertker said.

The startup’s business plan is to begin by upgrading existing aircraft, then forward-fitting electric propulsion systems into new “green” aircraft provided by their manufacturers. Long term, Ampaire expects to develop clean-sheet electric aircraft, “but that is in the future,” he says.

As the Cessna 337 is not in widespread regional-airline use, Ampaire plans to source and convert Skymasters for resale. But some airlines operating larger types want to convert their existing aircraft so the company has relationships with organizations that modify the targeted types, he says.

With the beginning of test flights, the startup has launched a Series A financing round to fund the Ampaire 337 through to certification. “We estimate this will take about $20 million,” Noertker said, noting this is “an order of magnitude less” than the funding required to certify an all-new electric regional aircraft.

Graham Warwick