Getting Charged Up – About Electric Propulsion for Training Aircraft

The race to achieve safe and economical sustainable aircraft is quickening. OEMs and their industry teams are making important progress to place electric, hydrogen and hybrid aircraft into commercial service as early as this decade.

First, a spoiler alert. The airline industry is desperately in need of decarbonization solutions. One solution set – electric propulsion – while quickly advancing, has limited use cases. While the initial tranche of electric-powered planes has limited range compared to fossil fuel-powered planes, early results are encouraging OEMs and their battery industry suppliers to move to second-generation propulsion solutions for their next aircraft models. Indeed, of importance is the increasing interest within commercial aviation training to ‘go green’ by investing in electric aircraft for their training fleets. Beyond this trend are emerging, major implications for training maintenance personnel who will be charged with providing air carriers with safe and ready-to-operate sustainable aircraft.

Next, an important definition or two (with the author’s promise to minimize the physics, chemistry, metallurgy and other sciences being brought to bear).

We’re increasingly hearing volumetric energy density (density) discussed in the eVTOL, eCTOL and adjacent markets – an attribute with huge implications. Density simply refers to the amount of energy that can be contained within a given volume. Increasing the volumetric energy density of batteries allows electric aircraft to travel further without increasing the size of the battery pack. Conversely, it can allow the aircraft to travel the same distance with a smaller battery pack, thus saving space, weight, and manufacturing costs, according to the US Department of Energy’s Office of Energy Efficiency and Renewable Energy. The real story in the evolving batteries-for-aviation propulsion sector is the arc of progress – think Moore’s Law in the adjacent microchip sector. As the DoE office reported “Li-Ion [Lithium-Ion] battery capacity increased approximately eight (8) fold from 2008 to 2020 and it would be expected these technological advances will continue.”

Two recent industry battery developments provide some context to the DoE office’s sector observations.

Cuberg, a part of Sweden-based Northvolt, reported the first round of internal testing on their first module was completed in December. “Against rigorous internal requirements, Cuberg demonstrated that its 20 Ah lithium metal cells, which have a specific energy of 405 Wh/kg, work as intended when assembled into an aviation battery module.”

Cuberg’s baseline module technology, which can be customized for each customer, boasts “industry-leading” performance metrics on specific energy and energy density. Selective details of the company’s cell of the item’s footprint and other attributes:

Mass: 16.4 kg [36.2lbs]
Size: 95mm [3.7in] x 280mm [11in] x 540 mm [21in]
Rated energy: 4.6 kWh
Energy density: 320 Wh/L
Specific energy: 280 Wh/kg

The battery and system supplier noted, “Aircraft manufacturers will take note of the specific energy of the Cuberg module, which, at 280 Wh/kg, is up to 40% higher than comparable modules based on lithium-ion technology.” Cuberg is also focused on advancing one ‘sweet spot’ in commercial training: “This significant improvement in specific energy translates to increased flight range which, in turn, enables new use cases for electric and hybrid aviation. Some operators could see their practical range more than double, depending on their aircraft and powertrain design.”

One valuable end-user perspective on electric aircraft flight range was provided by Nick Wilson, PhD, Associate Professor, Aviation at the University of North Dakota. The community expert said, while he doesn’t consider himself a technical expert in battery power systems and specifically battery density, “I can provide an operational context and state that an electric aircraft which has a battery duration of 2.0 hours plus required FAA reserves would cover over 85% of our single-engine (SE) training flights without considering any modifications to curriculum.” Practically, a two-hour battery duration plus reserve would cover all local flight lessons at UND.

“If battery capacity increases to 2.5 hours plus reserve, that would cover over 90% of our SE training flights, and only leave cross-country flight lessons at the tail end of that use-case. I would imagine similar metrics exist at other training providers; however, there are differences in airport design, student training volumes, curricula and other variables which could impact other flight school metrics.”

This April, the reported largest manufacturer of lithium-ion batteries for electric vehicles, China’s CATL, announced the launch of a high-energy-density “condensed battery” technology. CATL is targeting the electric passenger aircraft market deliberately and incrementally – after mass production for the electric vehicle market starts this year, the company says.

CATL, which declined CAT’s request for additional information on this new product, noted in a press release the showcased battery has “an energy density of up to 500 Wh/kg at the cell level and a high level of safety. This compares with less than 300 Wh/kg for current lithium-ion batteries, which use a flammable liquid electrolyte.”

Pipistrel's Velis Elector has found favor at flying schools around the globe, with “the active fleet approaching 100 aircraft in 14 countries.” Source: Pipistrel

One Aircraft OEM’s Insight

Pipstrel’s Velis Elector is finding increasing favor with training enterprises around the globe. Tine Tomažič, Director of Engineering and Programs at Pipistrel, told CAT the Velis Electro is used by flying schools throughout the world, with “the active fleet approaching 100 aircraft in 14 countries.”

The engineering executive pointed out the Velis Electro comes with a Pipistrel type-certified electric engine E-811-268MVLC (TC No. EASA.E.234), with the 57.6kW liquid-cooled electric engine providing power to the aircraft. “The batteries are in the 400-volt class and can power the Velis Electro for up to 50 minutes of flight time, plus a VFR [visual flight rules] reserve. Each battery is approximately 12 kWh, comprised of 1,152 battery cells in an enclosure that is fault tolerant and approved to DO-311A. Battery design, including the battery management system, is Pipistrel’s in-house proprietary technology,” he added.

Tomažič pointed out the Velis Electro is the world’s “first and currently only EASA type-certified electric-powered aircraft” and is mainly intended for pilot training. He continued: “The Velis Electro is an ideal aircraft to train student pilots towards their LAPL [light aircraft pilot license] and PPL [private pilot licenses]. It is optimized for local flights around the aerodrome, for example for takeoff and landing sorties. The EASA exemption for pilot licensing and training allows for the Velis Electro to be utilized in flight training environments and assuring transitions of pilots to and from Velis Electro even before they have obtained a pilot license.”

Comparable to the operational advantages CAT has presented about early tranches of eVTOLs preparing to enter commercial service, Pipistrel notes the Velis Electro is a “game-changing” aircraft in terms of technological innovations and cost-efficiency. The executive explained the aircraft “is exceptionally quiet, making it possible to bring flight training much closer to urban areas without adversely affecting communities’ quality of life. With noise levels of only 60 dBa, it is considerably quieter than other airplanes and produces no combustion gases at all.” Further, savings on every flight hour “are significant so that even with the battery overhauls included the operation is not only more environmentally friendly and quieter, but also more cost effective.”

In terms of a training organization maintaining a fleet of this aircraft, and additional implications for maintenance training, Tomažič said the Velis Elector was designed “to be simple to operate and maintain.” He continued, “The Velis Electro achieves the highest levels of safety, even surpassing those required for conventionally powered aircraft. Employing Pipistrel’s type-certified electric engine, the Velis Electro delivers power instantly and without hesitation – using a simplified user interface in a cockpit that maintains the same look-and-feel of its conventionally powered siblings. The simplified user interface and handling make the aircraft more user-friendly for pilots in training.”

As noted, the battery-for-aircraft propulsion sector is in its embryonic stage. To point, Tomažič candidly offered another challenge for maintenance in this early, current generation of batteries for aircraft propulsion. When it comes to maintaining chemistry-based Lithium-Ion propulsion batteries, it is best to avoid leaving an aircraft with a nearly full battery in a hot climate above 45°C (113°F). “In this condition, the battery would self-discharge over time. It doesn’t cause damage immediately, but when repeated over a longer period of time, it will cause degradation of battery life.”

Yet, in a Pipistrel plane, the information and data on the electrical system will be showcased through indications on every power-up. Each line-replaceable unit (LRU) on the aircraft has a built-in test (BIT) function. When the aircraft is engaged, it will report its function and status and will flag any issues. Tomažič added it is also possible to work through Pipistrel’s cloud interface by reviewing data logs and error messages, and connect to Pipistrel’s technicians. “Pipistrel has dedicated personnel on standby who can help test and troubleshoot the system. The maintenance of the battery system is purely on condition, linked to performance indexes that the battery system conveys to the user. No prescriptive fixed-period maintenance schedule applies.”

The Pipistrel executive left no doubt his company is on a path to migrate to larger capacity (denser) batteries for its electric aircraft. Tomažič emphasized Pipistrel is continually improving its battery system technology, both in-house, as well as through collaborative projects such as HighSpin [a project led by the Austrian Institute of Technology], Helena and Matisse – all focusing on battery performance increases. “Pipistrel is no stranger to building much higher-performance batteries than the ones used in the Velis Electro today. However, not all battery chemistries are certifiable or economically sustainable. The Velis Electro will certainly continue to receive battery system upgrades in the future, and we are excited that almost all of the current owners are already enjoying the second generation of the battery, which started shipping in 2022.”

Other OEMs, including Ampaire, are advancing hybrid-electric and electric-propulsion designs for aircraft that, in some cases, may enter commercial service later this decade.

CAT will be following the significance of these developments for the commercial aviation training and maintenance communities.