**1. Introduction**

Environmental sustainability problems frequently require the need for practical, "real world" decision-making to compute solutions to situations possessing numerous uncertain factors and unquantified dimensions [1]. This study applies a novel analytical technique to quantitatively examine the carbon emission impacts resulting from a transformation of the aviation industry toward a state of greater airline electrification.

The link between carbon emissions from the aviation industry to climate change was firmly established in the 1992 report of the United Nations Framework Convention on Climate Change (UNFCC) [2]. Today, it is estimated that aviation emissions annually contribute between 2–5% of all global emissions [3–5] with some estimates forecasting that with the current growth trajectory, 25% of all emissions could be attributed to flying by 2050 [5,6]. In fact, while many industry sectors have actively been reducing their carbon footprints, the emissions from the aviation industry have increased by more than 75% from their 1990 levels [5,7]. The biggest culprits in aviation emissions are the long-distance, commercial flights, and this long-haul aviation segment is the hardest to decarbonize, by far [4].

At the recent UNFCC Conference of the Parties meeting (COP26) in Glasgow—in which, ironically, the vast majority of the 26,000 delegates arrived via air—there were strong calls for immediate action to be taken to reduce airline emissions [8]. Consequently,

**Citation:** Kozlova, M.; Nykänen, T.; Yeomans, J.S. Technical Advances in Aviation Electrification: Enhancing Strategic R&D Investment Analysis through Simulation Decomposition. *Sustainability* **2022**, *14*, 414. https:// doi.org/10.3390/su14010414

Academic Editor: Lynnette Dray

Received: 8 December 2021 Accepted: 29 December 2021 Published: 31 December 2021

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decreasing the overall carbon contributions from the aviation industry has become one of the primary initiatives within the current global climate policy formulation and represents a significant component of the overall strategy for achieving climate neutrality by 2050 [2,6,9]. Clearly, technological progress can serve as the most significant, enabling factor for accelerating the pace of air traffic decarbonization. While biofuels have generally been viewed as the primary remedy for emissions from long-haul aviation [10], electrification is now considered the more viable option for regional, short-haul flights [8,11].

In comparison to combustion engines, electric aircraft are less expensive to operate as there is no need to acquire expensive kerosene, and the maintenance of an electric powertrain is far less complicated and cheaper. According to some estimates, the cost per hour from operating an electric aircraft is less than one-third that of an otherwise similarlysized, fueled aircraft. Economically, lower costs translate into lower overall prices, thereby enabling higher traffic volumes. Some experts envision the possibility of completely disruptive innovation in regional traffic flows, where all-electric turboprops provide a substitute for train-, bus-, and car-travel [12]. For example, one interesting development is Airbus' concept of urban air mobility that employs its all-electric, vertical-takeoff, remotely piloted, four-seater CityAirbus. Some airlines that only engage in short distance flights have experimented with switching to an all-electric fleet (e.g., see [13]).

Short-haul flights form a separate business community. So-called business aviation is comprised of charter flights, corporate aviation, and air taxis, though the explicit definition and composition differs depending on the source organization [14]. Short-haul business aviation creates many benefits for businesses, the environment, and economies, in general. These benefits can be expected to intensify in conjunction with an increased electrification of aircraft fleets in combination with consequent price decreases. Improved business aviation can connect many currently isolated communities in rural and remote locations, contributing a significant boost to their economic growth and investments. On-demand scheduling substantially increases efficiencies for business by saving time spent engaging in large airport procedures and avoiding unnecessary waiting time at stopovers [15].

In aviation electrification research, prior R&D investment studies have generally analyzed only selected scenarios that tend to be focused primarily on battery technology. For example, Brdnik et al. [16] focused on the existing specific energy levels of batteries and their impacts on the resulting flying ranges of three different aircraft sizes. Schäfer et al. [17] estimated the economic and environmental consequences of high-level, specific energy batteries. Unfortunately, due to the weight of the batteries, electrification is practicable only for the 20% of commercial flights under a distance of 1500 km [5]. Hence, for commercial aviation implementation purposes, an appropriately realistic balance must be struck between electrification for short-haul flights and combustion engine aircraft for longer distances. Achieving this satisfactory balance between short- and long-haul flight strategies in aviation is analogous to computing a solution to the environmental equilibrium requirements specified in the range limited routing problem of electrical- versus combustion-engine transportation methods in urban logistics planning [18–23].

Consequently, this paper aims to portray a more holistically sustainable aviation electrification picture by concurrently integrating environmental impacts from the ongoing technological developments of electric motors for short-haul flights into the R&D investment analysis. This is achieved by employing a Monte Carlo study in combination with a novel computational ancillary analysis technique to model the flying range of an all-electric aircraft based upon improvements to its batteries together with its electric motor. Monte Carlo simulation has been applied to a wide spectrum of environmental planning problems to incorporate disparate uncertain inputs with their corresponding outputs frequently portrayed visually as probability distributions.

In order to progress beyond a selected scenario approach, the Monte Carlo study undertaken will be extended using the recently introduced, innovative approach called simulation decomposition (SimDec) [24]. The SimDec method has been created to expand the analytical capacity of simulation by significantly broadening its cause-and-effect explanatory powers [24–27]. SimDec provides a very powerful, straightforward approach for visually analyzing the impacts of combinations of variables on output measures [24,26]. It is a generally usable method that is not context-dependent [25]. In SimDec, selected uncertain input variable combinations are used to "decompose" output distributions into a number of state-influenced sub-distributions [24]. These sub-distributions are superimposed onto an output distribution, thereby permitting an explicit visualization of the cause-and-effect impacts of the decomposed multi-variable groups of input combinations and/or their various interactions [24]. The practical contributions from the decomposed visualization facilitates subsequent decision-maker insights with respect to the underlying simulation model. SimDec supplies both sensitivity and scenario contributions that are frequently employed by decision-makers in conjunction with "real world" quantitative analyses [24–27]. At the strategic level, SimDec enables a visual analytic display in continuous numerical space of the simultaneous interaction between multiple different factors that affect the flying range of electrical aircraft, thereby more fully portraying the financial and environmental benefits of aviation electrification to the decision-makers.

The remainder of the paper is structured in the following way: Section 2 provides a general description of the SimDec method; Section 3 summarizes the key aspects associated with aviation electrification; Section 4 describes the computational and Monte Carlo model used for capturing the environmental impacts from electrification; Section 5 provides the results from the Monte Carlo and discusses the environmental impacts discovered in the decomposition of the output; and Section 6 concludes the outcome of the SimDec analysis for evaluating the environmental contributions of aircraft electrification to aviation decarbonization.
