and an Airbus A320-sized single-aisle would need 2 kWh/kg. Electric power is only suitable for small aircraft while for large passenger aircraft, an improvement of the energy density by a factor 20 compared to li-ion batteries would be required. Such batteries can reduce the overall operating costs for some short-range flights. For example, the electricity used in the Harbour Air Beavers costs them around $ Canadian per kWh compared to $2.00 per liter for gas, providing of energy with a 44 MJ/kg fuel and a 0.75 density Avgas, $ per chemical kWh or $ per shaft kWh with an efficiency of one third. Jet fuel is cheaper and large gas turbine are more efficient, though. In 2021, beyond-lithium-ion technologies such as Solid-state battery (lithium-sulfur, LSB) and lithium-air batteries (LAB) have become increasingly promising areas of research for more competitive battery-electric aircraft performance. The SAE International AE-7D committee was formed by Electro.Aero in 2018 to standardise electric aircraft charging and energy storage. One of the first documents developed was the AS6968 standard for sub-megawatt electric aircraft charging of electric aircraft. The AE-7D committee is also developing Aerospace Information Report AIR7357 for megawatt power level charging. Some airports have charging stations for electric cars which can also charge aircraft. Ultracapacitors An ultracapacitor is a hybrid electrochemical energy storage system bridging batteries and capacitors, and has some advantages over batteries in being able to charge and discharge much faster with higher peak currents, while not being as limited in the number of charge-discharge cycles, as the reaction is not just chemical but also electrical. Their energy density, typically around 5 Wh/kg, is however well below that of batteries, and they are considerably more expensive, even when their longer lifespan is factored in. Fuel cells A fuel cell (FC) uses the reaction between two chemicals such as hydrogen and oxygen to create electricity, much like a liquid-propellant rocket motor, but generating electricity in a controlled chemical reaction, instead of thrust. While the aircraft must carry the hydrogen (or a similar fuel), with its own complications and risks, the oxygen can be obtained from the atmosphere. Propulsion Electric motors Almost all electric aircraft to date have been powered by electric motors driving thrust-generating propellers or lift-generating rotors. While the batteries weigh more than the equivalent in fuel, electric motors weigh less than their piston-engine counterparts and in smaller aircraft used for shorter flights, can partly