Diamond Aircraft is facing challenges to reaching agreement with regulators on requirements for certification of its eDA40 electric trainer. Battery safety is a key concern for certification, which is planned for 2023.
The two-seat eDA40 is a derivative of the already certified four-seat, piston-powered DA40NG. The affect of the changes on the original certification basis is a subject of debate with regulators, Kevin Bruce, Diamond’s director of quality and airworthiness, told a conference in late July.
Diamond launched development of the all-electric eDA40 in October 2021. The Austrian company previously experimented with hybrid-electric propulsion, flying three test aircraft from 2011 to 2018, but ended up waiting until battery performance, reliablity and safety were improved, he said.
“About two years ago, we launched the program to start electrifying our fleet and the choice was the DA40 for a lot of different reasons,” Bruce told the Electric Aviation Symposium organized by the Vertical Flight Society.
“We envisioned breaking into the training market because the reality is that with the current technology of the battery systems, you are not going to get much more than 90 min. of flight regardless of the size of airplane,” he said. Diamond projects operating cost savings up to 40% for the eDA40.
Diamond has a two-seat trainer in its lineup, Bruce noted, but the DA40 made a good platform because it has room to accommodate the batteries. They are housed in a pod under the fuselage and in the nose along with the electric-motor and battery-management systems. Safran is providing its EngineUS 100 motor and Electric Power Systems (EPS) its EPiC modular battery system.
The company needed a 90-min. flight time to cover a 60-min. training mission with a 30-min. energy reserve, and had to restrict the aircraft to two seats to carry enough batteries. “In the end we’re going to get a 90-min. airplane but the rest of how it performs—handling, speed, stall characteristics—will remain the same because we haven’t changed anything else,” he said.
Diamond’s approach to certifying the eDA40 is to modify the existing DA40-type design. The electric engine and new fixed-pitch propeller each require type certification, but the battery system is planned to be approved under FAA Technical Standard Order TSO-C179B.
The intent is to certify the new electric propulsion system under the Subpart E powerplant regulations in the latest European Union Aviation Safety Agency (EASA) CS-23 Amendment 5 (FAA Part 23 Amendment 64) rules for small aircraft, leaving the rest of the original DA40 certification untouched.
“This is in question with EASA and the FAA,” Bruce said. “They’re saying it’s too big a change, you have to go all-new Amendment 64. That’s a big problem for us because in Amendment 64 there are new stall/spin characteristics which our airplanes weren’t designed for.”
Diamond also has issues with how Subpart E was written. “There’s redundancy after redundancy. . . . It’s causing some issues,” he said, adding that other regulations and standards for electric propulsion including EASA’s Special Condition E-19 and ASTM International’s F3239 have gaps and overlaps.
But a key concern for the regulators is battery safety. The eDA40’s 85-kWh system comprises 22 modules in two redundant strings. Each module has 24 lithium-ion pouch cells and an energy density of 205 Wh/kg including casing, connectors and thermal runaway protection.
Safran’s 130-kW motor is air-cooled and has a redundant design with two electric machines in one housing, each side powered by a different battery string. The design allows for emergency operation with the loss of one side, Bruce said, the motor providing 75-80% of maximum power.
“Thermal runaway is the Number One issue with the regulators,” he said. “The approach we chose to buy into and EPS is moving toward is modularizing the battery so the system can go into complete thermal runaway, and the only problem is how to manage what comes out.” An ejection port on top of each module connects to a duct that pipes hot gases off the aircraft.
Bruce said EASA is looking at two levels of protection against thermal runaway—no propagation to adjacent cells and containment at the battery system or installation level. According to EASA, the aircraft design should incorporate a battery explosive fire zone.
“Our approach is the module itself is the explosive zone,” he said. “EASA doesn’t agree. We’ll see where that goes because they’re looking for two levels. They’re really pushing on no propagation cell-to-cell, which is not easy to do and increases weight if you have to go that way.”
Clarifying the proposed regulations for propulsion batteries, Jannes Neumann, EASA senior project certification manager for general aviation, told the conference: “I see three different scenarios for the batteries: either full containment or no propagation or a mixture, which means if no-propagation cannot be shown, there has to be containment for a time sufficient to perform a descent and lending.”
Bruce said EPS has designed other features into its battery system to protect against thermal runaway, including manufacturing and how it screens the cells. “Probably the Number One issue in battery technology is manufacturing and having foreign material within the cell. That’s probably the most common way you are going to see a thermal runaway, other than charging,” he said.
EPS is developing the fast-charge system, a ground unit completely separate from the aircraft that plugs in to provide both charging and cooling of the batteries. Change time is expected to be 20-30 min., allowing flight schools to turn the aircraft around quickly between training missions. Battery useful life on the aircraft, to 80% capacity, is expected to be 2,000-3,000 charge/discharge cycles.
It is uncertain whether authorities will accept Diamond’s and EPS’ approach of containing the thermal runaway within a unit within the aircraft, Bruce said. Another issue is testing, and whether regulators will require a thermal-runaway test on the aircraft itself. Pilot displays and crashworthiness with an 800-volt electrical system and 400 lb. of batteries under the aircraft are other concerns for regulators.
Another issue cited by Bruce is unusable energy. Under Part 23, unusable fuel is determined through flight testing. “With battery technology, we’ve got to figure that out. When do you say you have no energy left, but you actually do have energy? How do you determine that,” he asked.
A battery’s capacity drops over time as it is used and Diamond plans to replace the battery at 80% capacity remaining. “From a usable energy standpoint, do you define it at the 80% level or do you let the maintenance team and the pilot know how that energy level is degrading over time, so that every day I get in the airplane I’m going to have a different endurance than the day before?” Bruce said.
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