In This Issue
Summer Bridge Issue on Aeronautics
June 26, 2020 Volume 50 Issue 2
The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come.

Hybrid Electric Aircraft to Improve Environmental Impacts of General Aviation

Thursday, June 25, 2020

Author: Jean J. Botti

According to two recent reports (ATAG 2018; IHLG 2019), the aviation sector represents a major economic factor for the global economy: its economic value is estimated at $2.7 trillion and it generates 65.5 million jobs. In 2018 airlines carried more than 4.3 billion passengers on scheduled flights, an increase of 6.4 percent from 2017; the number of departures grew from 36.7 million to approximately 38 million, an increase of 3.5 percent; and traffic in revenue passenger per kilometer expanded by 4.1 percent. International air traffic is expected to continue increasing, with average growth forecasts for the next 20 years of 4.1 percent per year.

With this growth and traffic, aviation is now responsible for 2 percent of global carbon dioxide emissions. Increasing concerns about global warming are pushing airlines and manufacturers to design new solutions to reduce pollution. Over the past 50 years, aircraft fuel consumption (and therefore CO2 emissions) has been reduced by almost 80 percent per tonne-kilometer and noise at source by 20 dB.

To reduce aviation’s environmental impact, the European Commission (2011) established the following objectives for aviation to reach by 2050:

  • 75 percent reduction in CO2 emissions per passenger per kilometer,
  • 90 percent reduction in NOx emissions,
  • elimination of emissions during aircraft taxiing, and
  • use of alternative fuels with improved environmental performance.

These commitments require efforts on several fronts, including research on technologies and operations and the development and adoption of new energy sources (e.g., biofuel, electric).

Customer requirements must also be met:

  • Safety through complete redundancy on the ­kinematic chain (electrical, mechanical, or hybrid);
  • Reduction of noise pollution and emissions, ­especially around airports; and
  • Savings in energy and operating costs.

A New Era of Propulsion Systems:
Hybrid Electrification

Noise Reduction

The reduction of noise pollution associated with aviation is essential, particularly for pilot training, which is a significant potential source of noise pollution given frequent rotations (takeoffs and landings). Commercial traffic is a major noise source, but general aviation noise must also be addressed. In France, for example, certain general aviation airports around Paris, like ­Toussus-le-Noble (about 25 km southwest of the city), close on Sundays to limit noise disturbances; only planes equipped with silencers are permitted to fly.

Aircraft noise comes from multiple sources (e.g., the fan, the airframe), and the use of electric power reduces some noise more than others. Hybrid electric aircraft will allow quieter operation while maintaining the capacity to cover desired flight distances:

  • On the ground and during takeoff, the use of electric traction in the wheels (etaxiing) reduces fuel consumption and eliminates noise.
  • On initial climb, in flight, and during landing, electric motors are quieter than an internal combustion engine.

Lower Costs

The cost of operating an aircraft depends on many parameters, including

  • consumption and type of energy used (e.g., aviation gas [avgas], unleaded automotive gasoline for traditional piston engines, Jet-A1 [kerosene] for turbines and diesel engines, electricity);
  • maintenance (parts, labor) and service;
  • hangar, landing, and parking fees;
  • insurance (depending on the pilot’s experience and coverage guarantees); and
  • other items such as personnel costs and air traffic control fees.

For controlling recurrent costs, electricity brings undeniable advantages. In France, for example, the cost of electricity is €0.13/kWh, which is less than the cost of avgas (€0.21/kWh) (these rates are roughly comparable throughout Europe).

Here is an example to illustrate the difference in cost: A Cessna 172 consumes 10 gallons/hour of avgas at 110 knots, or 19 liters/100 km. At a cost of €1.83/liter, the total cost is €35/100 km (the energy in the 19 liters of avgas is 204 kWh).

In France 20 kWh of electricity at the grid is €2.6 (€0.13/kWh), so for an equivalent 204 kWh the total cost would be €26.52/100 km. There is thus a 24 percent saving with electricity over avgas. Once the infrastructure is in place, most general aviation aircraft will recharge at the grid in the hangar.

Intelligent management of an aircraft’s electrical network can optimize energy consumption, with a more precise allocation of resources and parasitic loss reduction. This makes it possible to substantially reduce the energy needs of the main and auxiliary networks.

Furthermore, the relative simplicity of an electric motor makes it possible to envision reduced maintenance and a significantly longer service life than current configurations that use the thermal engine as the main propulsion element.

Overall, hybrid electric aircraft will

  • prompt the development of new products and services for general aviation,
  • stimulate and guide the establishment of a competitive industry in energy storage technologies for aircraft propulsion (whether electric or hybrid) and system management,
  • spur technologies to advance for adaptation to ever larger aircraft,
  • support intensive aviation training—by reducing noise at small airports where novice pilots train—to meet the growing need for pilots generated by con­tinued increases in air traffic, and
  • enhance the ability to meet environmental ­regulations.

The State of the Art

The design and manufacture of commercial aircraft with hybrid or electric propulsion require the development of technologies and skills to test concepts and select the best technical choices.

In recent years, multiple projects for the development of all-electric light aircraft have been launched, but many were private initiatives with relatively limited industrial prospects. The first demonstrations were developed by the Airbus Group (the personnel involved now work at VoltAero):

  • The electric Cri-Cri (presented at the 2011 Paris Air Show) is a 180 kg single-seat piloted airplane equipped with a standard electric motor and bat­teries for a flight duration of 25 minutes. The aircraft was utilized as a flying testbed and performance laboratory, allowing R&D teams to acquire experience in battery integration, energy management, energy recovery, and the variable pitch of the propellers.
  • The experimental prototype E-Fan was presented at the 2013 Paris Air Show and completed a historic English Channel crossing on electric power in 2016. The two-seater plane, designed to provide tangible proof of the concept’s effectiveness, was made entirely of carbon composite material, 6.67 meters in length, with a wingspan of 9.5 m and a weight of 650 kg. It had a range of 48 minutes and the propulsion was provided by two electric motors with a maximum combined power of 60 kW, driving two ducted propellers (eight blades) of fixed pitch.

Today, there are more than 200 electric aircraft projects worldwide and the aviation electrification market is expected to be valued at $4–$5 billion over the next decade. Table 1 shows just a few projects for electric or hybrid electric aircraft under development.

Table 1 

The domain of electric/hybrid aircraft remains in an emerging phase. The major challenge is that the range and certification standards have yet to be defined, as no all-electric aircraft have been certified, because of a lack of sufficient technological capacity. However, hybridization can significantly reduce operational risks with the redundancy brought by an internal combustion engine.

Figure 1 

In this context, VoltAero is working to develop ­Cassio, a family of 4- to 10-seat modular electric hybrid aircraft intended for initial pilot training as well as the general aviation sector (figure 1; UBS Investment Bank 2019). Using an existing airframe for its flight testbed, VoltAero’s baseline design retains the original aircraft’s “push-pull” propulsion system, and because the airframe is already completely certified, the company can focus on development of the hybrid propulsion system.

The Cassio hybrid propulsion system is composed of two electric motors located on the wings, powered by battery packs (in each wing) that deliver an output of 60 kWh each. The combustion engine is in the aft fuselage, delivering 300 kW, combined with an electric motor of 180 kW in a hybrid power module, for a total output of 600 kWh, as the propulsion is distributed between the wing-mounted motors and the aft-fuselage hybrid power module. Through the energy it produces, the power module’s combustion engine also recharges the batteries during flight (which would not be possible with a fully electric system), offering 3.5 hours of flight autonomy (with the possibility of extending to 5 hours).

With this propulsion configuration, the aircraft can taxi and take off on full electric energy while the combustion engine remains in idle mode, ready to take over in case of failure or to contribute when the cruise phase starts. Landing also will be fully electric. The battery discharge at takeoff is estimated at 20 percent; the discharge limit should be 50 percent (e.g., during the cruise phase). The discharge/recharge cycles occur at 50–85 percent so that the battery is preserved during the cruise phase.

The electric/thermal hybrid aircraft will achieve substantial energy savings while in cruise and demonstrate excellent performance during the flight cycle. Maintenance costs may be reduced because electric motors are more reliable than internal combustion engines and require little maintenance.[1]

For commercial use (based on the range required) the Cassio aircraft could be configured in three modular concepts:

  • pure electric (range of less than 200 km)
  • mild hybrid (range of 200–600 km)
  • heavy hybrid (range greater than 600 km).

Fuel economy and CO2 reduction will vary based on these configurations.

Technological Challenge: Batteries

Energy Density

Batteries are a key technology for hybrid and electric propulsion, but they will have to reach a level of energy density of around 350 Wh/kg at the level of the entire battery pack within the next 5 years. New batteries are being studied to improve energy density in order to reduce weight.

Today, automotive batteries reach energy densities on the order of 100 Wh/kg (at pack level), and approximately 30–50 Wh/kg for commercial aviation (as a supplementary power source). These differences result from certification and qualification constraints. As explained in the next section, aviation must be very conservative in its qualification of batteries, so the safety margins for battery charging and discharge are very stringent. Reaching 350 Wh/kg is thus a real technological challenge.

Battery Charging

The standard charging system for batteries in current prototypes requires approximately 3–4 hours for a full charge.

In the case of hybrid aircraft, the industry needs to create a charging system that is adapted to the specific characteristics of the batteries that will be developed and that can be used for recharging during flight.

Currently, there is no adequate electrical infrastructure at airports for charging batteries. The definition and implementation of a dedicated system is of strategic importance. Connectivity to the network, aircraft access to the charging terminal, the safety and ease of the procedure, the billing system…all are subjects that require study and development.

Two challenges to be solved concern the heating and cooling of batteries and preservation of their durability and safety over time. This is why recharging methods (strategies, management, and controls) will need to be adapted to both the uses (e.g., frequency and charging time) and the technical specifications of the batteries (e.g., power, optimal recharging time, aging).

Operational Safety and Prediction of Remaining Charge

Beyond the energy density constraint, the development of lithium-ion/lithium-polymer batteries for aviation applications—for propulsion in particular—presents two major obstacles:

1. Achievement of a level of operational safety that allows the battery to be certified

Improvements in safety and reliability are paramount. Overheating of the battery pack would put the aircraft at risk and therefore cannot be tolerated. For aeronautical applications, the most feared event is a fire. ­Standards require that no flames or smoke emerge from the battery pack inside the aircraft. (In comparison, for an automobile, standards under development will require the prevention of flame or smoke for 5 minutes in the passenger compartment only, to give passengers time to get out.)

Thales/Boeing certified the 787 Dreamliner’s lithium-ion batteries, but then experienced several fires during the aircraft’s operation, including a “thermal runaway.” This led to a design review and the decision to provide both better protection around the battery and a means to evacuate gases from the aircraft. These measures increased the airplane’s drag, thus countering the interest in switching to lithium-ion batteries in the first place. SAFT/Airbus was unable to certify the A350 XWB’s lithium-ion battery and had to revert to nickel-cadmium technology. In a lightweight aircraft, Siemens had a battery fire that killed two people, which led to discontinuation of the project.

2. Ability to predict remaining energy in the battery pack with acceptable accuracy

Current lithium-ion battery systems have a state-of-charge indicator. The accuracy of these indicators is generally acceptable at the start of life under standard conditions of use (5–7 percent for smartphone applications, 3–5 percent for automotive applications). However, under extreme conditions of use (for example, in extreme cold or during high power demands), the accuracy of this indicator (as well as the actual power output) can be greatly degraded. Reduced accuracy also can occur at the end of the battery’s life. For an aeronautical propulsion application, estimating the ­battery’s state of charge is a critical function.

Coordination of Other Parameters

Other important parameters for batteries are safety, fast rechargeability by the internal combustion engine, life­span (approximately 2,500 cycles), volume, and price.

Implementation of innovative battery technology will require coordinated efforts involving the aircraft, its electronics, the battery packaging, and integration into the aircraft (taking into account weight/volume constraints and maintenance challenges). The following steps will help guide efforts:

  • Define an optimized electrochemistry based on ideal trade-offs among safety, energy, lifetime, and price.
  • Define the most generic battery cell format possible.
  • Guarantee good “processability” and the potential for industrial manufacturing.
  • Make the best choices in terms of electronic architecture (e.g., redundancy, controls).
  • Integrate all the components of the aircraft’s electrical system (e.g., propulsion systems, control systems, instrument panel) while respecting certification ­standards.
  • Choose the best cooling solution (e.g., heat sink, oil or air cooling, two-phase).
  • Define and optimize the mechanical installation and conditioning of the battery for weight reduction while meeting aeronautical standards (e.g., choice of materials, partitions, connectors; management of mechanical safety, fasteners, and shock and vibration constraints; accessibility for maintenance).
  • Ensure redundancy by creating modules that will cross-feed the engines, which will avoid handling problems for the aircraft if one or more of the engines fails.

A New Way Forward for Pilots

For pilots, the introduction of hybrid electric propulsion will modify the management of energy in flight and the type of information to be processed. It will therefore be necessary to rethink the presentation of information (the amount and nature) and the decision-making mode linked to energy management. These changes in flight management will impact certain aspects of pilot training. This area should be investigated, along with flight safety and program compliance for obtaining pilot licenses. Cockpits will need to be more “intelligent” and provide the right level of information to the pilot for power management.


The environmental impact of aviation is a major concern in terms of both emissions and noise. Some ­people are even seeking to reduce their reliance on air transportation.

Hybrid and electric propulsion for aircraft is an option to reduce both emission and noise levels. However, various challenges must be resolved to enable the introduction of electric aircraft capable of transporting hundreds of people. For example, the energy density of batteries is too low to allow the development of an all-electric aircraft with a decent range. And improving the safety and reliability of electric drive chains is essential for aircraft to be certifiable.

Despite these challenges, several companies around the world are pursuing efforts to develop technologies applicable in the aeronautical field, particularly for general aviation. This process can be considered “revolution by evolution.”


ATAG [Air Transport Action Group]. 2018. Aviation: Benefits Beyond Borders. Geneva.

European Commission. 2011. Flightpath 2050: Europe’s Vision for Aviation. Luxembourg.

IHLG [Industry High Level Group]. 2019. Aviation Benefits Report. Montreal.

UBS Investment Bank. 2019. Green power: Will climate change propel the sector towards hybrid electric aviation by 2028? New York.


[1]  This can be explained as follows: The thermal engine in this context is used as a range extender and as such always functions at its best operating point, which greatly extends its life compared to a conventional propulsion thermal engine. Lithium-ion bat­teries involve very little maintenance: when they are dead they are simply replaced.

About the Author:Jean Botti is founder and chief executive officer of VoltAero.