Ground tests will come first, examining how the turbogenerator, batteries, and motors work together, followed by demonstration flights on a pre-production ES-36 that Heart plans to fly in 2028. Heart said the data will support ES-36 certification and inform FAA certification of future hybrid-electric aircraft. The company plans to certify the ES-36 under FAA Part 25 and put it into service in 2031.

With the ES-36, Heart has returned to the series-hybrid architecture it first proposed for the 30-seat ES-30 in 2022 before switching to an “independent hybrid” layout in 2024. That switch eliminated the turbogenerators the original ES-30 carried in its rear fuselage, replacing them with two turboprops that drove the outer propellers directly while electric motors turned the inner pair. The ES-36 brings back the turbogenerators, with one in each of its two nacelles. Heart calls the turbogenerator its own design, though it said in September that each unit runs on a commercially available gas turbine. It has not revealed the supplier.

The CLEEN award follows a $4.1 million grant the FAA gave Heart in August 2024 under its $291 million Fueling Aviation’s Sustainable Transition (FAST) program. That grant funded development of Heart’s Total Hybrid Electric Management and Integration System (THEMIS), a power management system originally intended for the ES-30. Heart now says THEMIS will coordinate the ES-36’s motors, turbogenerators, and batteries.

Heart is one of five U.S. companies that received two-year agreements when the FAA launched CLEEN Phase IV in September and the only one developing a hybrid-electric airliner. The others are Boeing, GE Aerospace, Honeywell Aerospace, and blended-wing airliner developer JetZero. Since 2010, the program has mostly funded established manufacturers and suppliers working on conventional engines, airframes, and sustainable aviation fuel.