Electric flight operations for interisland mobility

Several studies have shown that flying electric between the so-called ABC-islands in the Caribbean (i.e., Aruba, Bonaire and Curaçao) is feasible with the upcoming first generation of battery-electric aircraft. This paper presents a real-world case study that deals with the technical and operational characteristics of electric flight in that region. With that purpose, the Aruba Airport Authority (AAA) commissioned this investigation, which involved numerous local stakeholders, such as airlines, energy providers and navigation services. This study involves two commuter electric aircraft under development, aiming to investigate how they fit in the current operational scheme of three local airlines and three conventional aircraft types in terms of technology, capacity, schedule, performance, CO2 emissions and fuel costs. Conclusions indicate that a transition to batter-electric aircraft is feasible with regards to the aforementioned criteria and with the current technology and energy density of batteries.


Introduction
Motorized flying has been possible since 1903, and in the following century aerospace innovations have occurred at fast pace.Aviation has become an indispensable part of transportation of humans and goods, but one thing has remained the same: the use of fossil fuels for propulsion.Even though engines nowadays are highly efficient, aircraft emissions have grown due to an overall increase of the volume of flights [1].Using fossil fuels comes with a penalty, namely contribution to climate change, depletion of raw materials and the deterioration of air quality [2].A large part of next generation aircraft is currently being developed with the use of hydrogen, electricity, or Sustainable Aviation Fuel (SAF) as primary energy sources, or in hybrid configurations, combining electrification and some forms of fuel [3].Several airports in the former Dutch Caribbean have the ambition to make interisland air transportation more sustainable and affordable with the introduction of electric aircraft operations.Similar studies have highlighted the benefits and challenges of electric aviation [4,5].The benefits include the reduction of CO2 and non-CO2 emissions as grids increasingly rely on renewable energy sources, while major challenges include the specific energy of batteries, thermal management of propulsion systems and the overall architecture of electrical components.Various initiatives and studies have already shown that in the short-term flying electric between the ABC-islands is feasible [6].In fact, this area of the Caribbean would be particularly suitable due to the stable weather and relatively short distance between the islands.Aruba International Airport is preparing for electric flight operations, however knowledge, expertise, and infrastructure currently make such an endeavour challenging.Therefore, a case study on the characteristics and complexities of performing sustainable interisland flight operations is presented in this paper.First, the technical characteristics and performance of electric aircraft were identified.This forms the basis for the study and explains what the general potentials and limitations of electric flight are.Several technical values were then determined for the proposed aircraft

Characteristics of electric aircraft
The main difference between conventional and electric aircraft is the mode of propulsion.This section looks at the various components in the electric propulsion system, their properties, and the general performance of electric aircraft.

Comparison of powertrain architecture.
As already mentioned, the major difference lies within the powertrain.Energy for propulsion is not obtained from fuel but from an electrical charge, which is used to drive a motor.A major advantage of the electric powertrain is that it entails a clean process, therefore there are no CO2 emissions at a local level.Figure 1 illustrates the process in both cases, of a small, conventional aircraft and of a battery-electric aircraft.To be noted that one needs to take into consideration the electricity production for correct efficiency bookkeeping.

Electric motor considerations.
Electric motors use electrical energy, derived from a potential difference and current flow, and convert this into mechanical movement.Construction is simple and consists out of various components.Overall, the only moveable part in an electric motor is the rotor, compared to the often, hundreds or thousands, in a combustion engine.This makes the electric motor more reliable and less maintenance sensitive.Cooling is provided by means of conduction, convection and/or cooling liquid [8].Furthermore, the electric motor has a high power-to-weight-ratio and where almost half of the potential energy is lost in the thermodynamic process of a combustion engine, the electric motor achieves an efficiency of around 98% [7].

Controller system.
The controller manages all systems for the desired performance.Some functions that the controller can perform are starting/stopping the motor, selecting forward or reverse drive, regulating speed and torque.It ensures that the right amount of energy is delivered from the battery to the engine when prompted.Various environmental factors are present in aviation, so controllers are protected with several features, such as liquid cooling systems, useability in non-pressurized operations, redundant electronics, and protective casings [9].

Battery and health management.
Batteries store energy that provides power to the engines.Noticeable is that during a flight, the weight of a battery will not decrease, in contrast with fuel.The most common type of battery found in electric vehicles (EV) is lithium-ion (Li-ion), due to its high energy density.Li-ion batteries are sensitive to cold and warm environments.In a warm environment, the capacity of a Li-ion battery slightly increases, with the adverse effect of accelerating the lifecycle degradation.A cold environment increases the internal resistance of the battery, resulting in a lower efficiency [10].Battery temperatures should therefore stay between ideal operating temperatures.To achieve this, battery cooling is provided during operation to achieve thermal stability.The biggest risk of current batteries is the chemical 'thermal runaway' reaction.To reduce this risk, thermal management is of significant importance, as well as precautions measures taken with the battery design.Batteries used for aerospace application feature a battery management system (BMS).This system keeps track of all the important battery parameters.State of Health (SoH) and State of Charge (SoC) are two important battery parameters; the first indicates how much of the initial capacity is available, the latter tells what percentage of the available capacity is charged.In general, batteries could make use of various data analytics concepts encountered in conventional aircraft [11,12,13].

Battery density and developments.
As illustrated in figure 2, Li-ion batteries have one of the highest energy densities within the family of batteries, namely 100-265 Wh/kg to date [14].Performance of Liion batteries is determined by the design of cells and materials used.It is expected that in the coming years, development of Li-ion batteries, based on the current materials and designs will reach its peak.In comparison, the energy density of kerosine lies around 12.000 Wh/kg [15].In terms of energy density, a major limitation becomes visible for the application of batteries in electric aircraft.For the same amount of kerosene instead of battery weight, 45,3 times more potential energy can be carried.Even if the low efficiency of the fossil powertrain is considered, it is still much more with fossil aircraft.However, two new battery types are expected: advanced Li-ion and solid-state batteries, of which the latter is the most promising technically [16,17].

Performance of electric aircraft.
This study focused on 9-and 19-seaters, as according to research performed by NACO & NLR, the Eviation Alice (Figure 2a) and the defunct Heart Aerospace ES-19, which was recently reconfigured to a new type called ES-30 (Figure 2b).Based on various sources [18,19], technical values of both aircraft could be derived.Unknown values were calculated or assumed.
It is important to clarify that despite the fact that Heart switched from ES-19 to ES-30, the scope and conclusions of this investigation remain relevant, as they highlight the operational aspects of batteryelectric aircraft, independently of their type.Below is a concise overview presented of how the values in table 1 have been approached, for the full calculations please refer to the original case study [20].
Battery Capacity.The initial maximum battery capacity is derived from the NACO & NLR research and is 820 kWh for the Alice and 900 kWh for the ES-19 [7].In order to determine the available capacity at a SoH of 80% (end-of-life), the initial capacity was multiplied by the factor 0,8.By doing so, the capacity for the Alice is reduced to 656 kWh and for the ES-19 to 720 kWh.According to the same sources, the reserve energy requirement is 30% of the original available capacity. Specific reserve energy requirements have not yet been established for electric aircraft; however, EASA recently published a proposal regarding this topic [21].
Technical Range.Since the mass of the battery does not change during the flight, the range equation of electric aircraft is simplified in comparison with conventional aircraft.Eviation claims a range of 440 nautical miles, which is equal to approximately 820 km, however this is based on an empty aircraft, while commercial operations will always involve payload.Therefore, in these range calculations the Maximum Take Off Weight (MTOW) has been assumed for both the Eviation Alice and Heart Aerospace ES-19.Under these circumstances, the Alice can fly 470 km and the ES-19 448 km.Energy Consumption.It is important to understand the amount of consumed energy for a specific distance.To calculate this, the consumption per km travelled was determined for cruise, climb and descent.This value only looks at the consumption for the propulsion of the aircraft itself for ideal conditions.There are also on-board systems that require additional energy, but since no relevant information is available, these have not been considered in this preliminary study.In addition, real operational conditions will produce differentiations in the consumption figures, but these will only be explored during flight testing.
-Cruise.Cruise speed represents the optimal balance between energy consumed and distance travelled, in this scenario the maximum range is achieved.As the initial battery capacity was known, and the technical range has been calculated, the average consumption during cruise could be derived.This is done by dividing the initial capacity by the technical range.For cruise (with full payload), the Alice consumes 1,75 kWh/km and the ES-19 2,01 kWh/km.
-Climb.Currently there are no flight performance charts available for either the Alice or ES-19.
During climb drag increases as a result of the increased angle of the aircraft, it is a very demanding and energy consuming flight phase.In the scientific paper of the German Aerospace Centre a comparison is made on performing flight segments with a simulated electric regional aircraft [8].
Although it is not a one-to-one comparison, it does give a good idea of the difference in requested power from the motor.By analyzing segment data, it could be derived that the energy consumption per km ground distance travelled is approximately 3,29 times higher during the climb phase.To assume values for climb as close as possible to realistic scenarios, this factor has been applied to the cruise consumption.For climb, the Alice then consumes 5,74 kWh/km and the ES-19 6,62 kWh/km.The climb phase is seen as a separate segment and will therefore not be deducted from the total travel distance for flight calculations.Eviation mentions a climb rate of 2.000 ft/min for Alice, no data from Heart Aerospace is given on the ES-19, but the same value of the Alice is acted realistic and therefore assumed.
-Descent.Since the engines of electric aircraft can also act as a generator, it is possible that propellers can recover some of the energy during the descent phase.This recovery function will entail more drag as the propeller is rotated by the airflow.Due to the increase in resistance, the descent rate of the aircraft will also increase.Therefore, it is realistic to expect that the descent phase starts later and will be steeper than the one of conventional aircraft.However, such operations can only be accommodated if Air Traffic Control (ATC) makes this possible with special approach procedures.Despite the ability to recover energy, overall drag of the windmill propellors is high and efficiency low; only around five percent can be recovered per flight, depending on the propulsion system, flight distance and altitude [7].Because exact numbers are not yet known, neither if the proposed aircraft support this principle, an energy consumption of 0 kWh per km ground distance will be used for calculations.The descent phase is also seen as a separate segment and will therefore not be deducted from the total travel distance for flight calculations.As descent speeds are also unknown, the same duration from the climb phases are used for these calculations.Payload.In order to determine the available payload, as stated in table 1 above, the average weight of flight crew needs to be subtracted from the nominal values.According to EASA, the standard flight crew weight is 85 kg per pilot.This means that when assuming two pilots behind the deck, the Alice can accommodate 930 kg and the ES-19 1635 kg.

Sustainable interisland flights
This section focused itself on the question how interisland flight operations are currently performed between the ABC-islands, what amount of electric energy is needed to fly the routes with electric aircraft and how electric flight operation compare to conventional operations.A combination of interviews, onsite visits, and literature [20,21,22,23,24] has been used.

Air transport between the ABC-islands.
On the ABC-islands there are three main international airports: Aruba Airport (AUA), Bonaire Airport (BON) and Curaçao Airport (CUR).In terms of flight distances, CUR and BON airport are closest to each other with a flight distance of 76 km, followed by the connection of AUA and CUR at 120 km.Currently there is no scheduled service between AUA and BON, which are 194 km apart.Flights are operated with small aircraft that can accommodate up to a few dozen passengers.

Air Operators for interisland travel.
As stated in table 2, the airlines that operate flights between one or more ABC-islands are Divi Divi Air (DVR) and EZ Air (EZR).Flights between Aruba and Bonaire are operated through CUR, the hub for all interisland traffic.The flight schedule is fixed throughout the year, as there are no seasonality effects.The fleet of Divi Divi Air consists of three DHC-6 Twin Otters (19 Passengers -PAX) and two BN-2 Islanders (8 PAX).The current fleet of EZ Air consists out of three Saab 340B (34 PAX).The aircraft used on the routes have a standard seating configuration, however in practice less seats are sold than there are available.This has to do with the fact that travellers are heavily packed and as a result max payload is easily achieved.From interviews with the two airlines average fuel consumption, desired flight times, and assumptions on payload values could be derived.Also, with the site visits it became clear what their operation entailed in practice, such as the turnaround process.In table 3, an overview of the most important operational parameters of the different aircraft are given.

Comparison of energy sources.
Electric flying can be sustainable only if the used energy is also renewable.On the ABC-islands, electricity is generated by a mix of energy from fossil fuels and renewable sources.For this section, calculations were made on the fuel usage, price, and the emissions the current flights.The emissions per kg of AVGAS or Jet A-1 were calculated with basic chemical reaction equations.Combustion of 1 kg AVGAS results in 3,088 kg CO2 and combusting 1 kg Jet A-1 produces 3,106 kg CO2.As almost all refuelling takes place in Curaçao, prices of the airport supplier CUROIL were consulted.The costs of both fuel types were equal at that moment, namely, $1,56 per litre (price level: 1 May 2022).Average flight times for routes per aircraft and airline were derived from flightradar24 data.As all uncertain values were now known, fuel usage, total price and emissions could be calculated for the current operations.In table 4, an overview is given of the calculated average values.Each island has a different mix to get their required amount of electrical energy.An interview with Aqualectra (Curaçao), revealed that fossil generators for production had an average consumption of 0,22 litres of heavy fuel oil per kWh of electric energy.This oil has an emission factor of 3,310 kg CO2 per litre [23].Fuel consumption for Aruba and Bonaire was assumed to be equal.Because the energy mix and fossil production emissions were known, the average emissions per kWh of energy could be calculated.Prices for electrical energy were derived from the current published tariffs on their site (price level: 1 May 2022). -

Discussion
This paper examined the feasibility of electric aircraft operations in the Dutch Carribean.Batteries are one of the most critical components, due to their sensitivity to environmental conditions and low energy density, which dictates the regional nature of such aircraft types.Two conceptual aircraft designs were considered, and their energy requirements were examined at their maximum payload for different flight phases.The associated emissions and costs from local electricity production were calculated and a comparison was made with the current conventional aircraft types operating in the region.We see notable improvements in CO2 emissions per seat for the examined missions, which we expect to further improve with the decarbonization of the local grids.

Conclusions
This study highlights the feasibility of electric aircraft operations between the ABC islands.A direct comparison with the conventional aircraft types in operation in the region revealed that the considered electric designs have the range, payload capacity, and operational characteristics to replace the conventional aircraft when they enter service at the end of this decade.As the electric designs increase in maturity, more accurate calculations will be possible regarding their energy requirements for various mission profiles.However the obtained preliminary results were encouraging about the environmental benefits of electric aircraft operations in the region.

Figure 3 .
Figure 3. Interisland mission profiles for the four routes.

Table 1 .
Performance values of the proposed aircraft intended for interisland flights.

Table 2 .
Interisland operators on flight routes, their frequency and aircraft used.

Table 4 .
Average fuel, emission and cost calculations for current flight operations.
[20]ion profiles for interisland routes.One of the main research objectives was to identify the amount of energy needed to fly the interisland routes with the proposed electric aircraft.This is important to determine whether the routes are feasible in different conditions as well as to examine if charging is possible in the desired turnaround time.Comparison with conventional operations can be made on energy prices and emissions for performing the flights.To calculate the amount of electrical energy, flight time and emissions per flight segment for the Alice and ES-19, a model was developed[20], considering multiple preconditions.For example, the most severe wind influence that could be realistically expected was applied to the cruise segment.Furthermore, two pilots behind the flight deck, maximum payload configuration and current flight altitudes for the routes.Calculated energy and times are stated in the mission profiles of Figure3.

Table 5 .
Average fuel, emission and cost calculations for electric flight operations.