The Changing Electromagnetic Environment Onboard All-Electric Aircraft, an EMC Perspective

Leonardo Malburg University of Twente

Enschede, the Netherlands l.correamalburg@utwente.nl

Niek Moonen University of Twente

Enschede, the Netherlands

Frank Leferink University of Twente

Enschede, the Netherlands THALES Nederland B.V., Hengelo, the Netherlands

Abstract— All-electric aircraft (AEA) is an emerging subject, due to its environmental contributions and economical appeal, thus, such technology is progressing at a fast pace towards commercial applications. The changing electromagnetic environment (EME) which such aircraft will endure, encompass not only current technologies, but will experience new EMI effects, originating from future mobile communication, power conversion, and increase in air-traffic. As a consequence of an operation relying solely on electric and electrical systems (avionics), together with the implementation of a high-power electric powertrain, AEA will experience increased levels of EMI. Therefore, to regulate the EMI changes onboard AEA, current aerospace standards must be assessed in order to identify possible limitations and bottlenecks. This paper presents an insight into the future EME, its EMC issues, and the intricacies towards the implementation of AEA for regional commercial flights.

Keywords—All-electric aircraft, Electromagnetic interference (EMI), Electromagnetic compatibility (EMC), Avionics, Electric powertrain

I. INTRODUCTION

The recent technological developments of the aerospace industry have depicted major interest towards electric transportation. Whether influenced by economical or environmental aspects, the sector has massively invested its efforts in the pursuit of aircrafts relying solely on electric propulsion. The architecture of all-electric aircraft (AEA). (Fig. 1) differs from typical internal combustion models mainly due to its high-power electric powertrain, which encompass several different components, e.g. electric motor, power converters, batteries, and high-power cabling.

Fig. 1. Envisioned Electrical System Architecture for All-Electric Aircraft. A) Avionics, B) Batteries, C) Converters, D) Electric Motors, E) EWIS.

Complementarily, AEA implements electrical andelectronic systems (avionics) in control and Level A functions over traditional approaches, namely, hydraulic, mechanical, and pneumatic systems [1], [2], [3]. Such developments have made AEA much more power demanding, with its

configuration and operation presenting multiple challenges faced by several disciplines. Ensuring Electromagnetic Compatibility (EMC) in AEA is of utmost importance due to the high risks involved. The high-power components operating within such aircraft are a major source of electromagnetic interference (EMI) [4], and the avionics its victims. The electric powertrain, due to the increased power levels, faster switching speeds, and smaller components, will increase power density, consequently resulting in high electromagnetic emissions [5]. Mitigation is often done using power line filters that contribute significantly to the power electronics bulkiness, thus filter optimization is required [6].

As a consequence of such power level changes, regulatory agencies have been organizing specialized committees in order to address probable EMI issues, as is the case with SAE International [7]. The most relevant, and currently applied EMC standards in the aerospace industry are the RTCA DO-160G (civil) [8], EUROCAE/ED 14G (civil) [9], MIL-STD-461G (military) [10], and the NATO STANAG 4370 – AECTP [11]. Additionally, for instance Airbus and Boeing uphold their in-home developed standards. Besides communal issues, such guidelines encompass specific occurrences from internal combustion propulsion only. Broadband emissions produced by jet engine igniters are widely covered, however, specific AEA emissions are yet to be included. Whilst new systems are implemented in such aircraft, its internal electromagnetic environment (EME) changes to other than the one envisioned in current standards. Thus, modifications in this scope, brought by the interaction of avionics and high-power electric powertrain, must be addressed. Furthermore, the implications of changes in the external EME must be foreseen.

In this paper, an overview is presented about the new EME that requires investigations to ensure EMC. Thus, allowing the proper mitigation solutions to be implemented in early design phases, ensuring the compatibility of AEA systems and components. In addition, such solutions must comply with the general aerospace design particularities, where aspects influencing the weight and volume are critical. Thus, depicting the importance of applying optimized, high power density filtering solutions. Therefore, with the presented technological forecast, this paper presents an insight into the future EME, identifying EMC issues, which will be reflected in the context of the AEA implementation for regional commercial flights.

The paper is structured as follows. In Section II, a brief overview of the current EME endured by typical aircraft. As for Section III, a prospect on the future EME will be given, based on the observed trends. Using these forecasts, in Section IV their impact on EMC will be discussed, with the subsections dedicated to each major component of the AEA. Based on the advancements discussed, a brief evaluation of the suitability of the standards towards AEA will be presented in Section V. Finally, Section VI will eventually present the conclusions regarding the content here discussed.

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme, Grant Agreement No 875504 (EASIER). This

publication reflects only the authors’ view, exempting the European Union from any liability. Project website: http://easier-h2020.eu

II. THE EM ENVIRONMENT OF TODAY

The external EME of typical aircraft (Fig. 2) includes well-known threats, where high-intensity radiated fields (HIRF) (AECTP 258) and lightning strikes (AECTP 254) are considered the major hazards at the aircraft level [11], [12]. Such elements compose the external part of the EME. In this category, interactions between the aircraft and ground communication stations and lightning strikes are approached, assessing such influence towards the aircraft’s radars, altimeters, and other susceptible systems. Furthermore, vital systems must be able to properly operate during and after the occurrence of a strike, and precautions should be taken so resulting currents do not act as fuel igniters [13]. Commonly implemented in current aviation operations, navigation and communication equipment as radio altimeters, automatic direction finder (ADF), very-high frequency omnidirectional range (VOR), and air traffic control systems, can be affected by external sources as radio communication stations and other radar infrastructures [12].

Fig. 2. Aircraft’s Electromagnetic Environment (EME).

The internal EME can be categorized into four types of threats: intersystem, intrasystem, HIRF-induced, and internal lightning transients [12], [13]. Within this category, avionics, wiring and other electrical systems are assessed in order to evaluate their influence on the safe operation of the surrounding equipment. In a typical aircraft, such elements are represented by fly-by-wire flight controls, hotel and gallery loads, radio, and remaining electronic equipment [3]. Lightning strikes, although dangerous, are a phenomenon that occurs on average once per year, or every 3,000 flight hours [14]. Nevertheless, lightning transient effects towards avionics and other components are assessed, as strong currents can flow through those systems causing hard faults, or a momentarily upset. In addition, HIRF induced faults at equipment interface circuits are a great concern, with wiring acting as antennas when f ≤ 400 MHz, and for f > 400 MHz the EM energy typically couples via equipment apertures [12].

III. THE EM ENVIRONMENT OF TOMORROW

All electrical components are subjected to technological advancements, thus changing the EME even when not considering AEA. Current standards, e.g., DO-160G, only encompass elements according to contemporary technology. It will be important to consider the future EME when introducing AEA “only” technologies into the standards, otherwise, an AEA update to the current standard could be already outdated the time it is published.

Following II, a distinction is made between internal and external contributions to the envisioned EME of the nearby future.

A. External

The enhanced communication technologies implementedinto the operation of AEA, the high number of power converters in all kinds of equipment, alongside an increase in aircraft traffic, will undoubtedly affect the external EME. Due to the nature of AEA systems, aircraft can experience different sorts of malfunctioning. There are cases of “nuisance disconnects”, “hardovers”, and “upsets”, which according to [2], could cause component failure, leaving no signs of the issue, while appearing in a non-repeatable pattern.

The external EME is highly influenced by the advancements in mobile communication, i.e., the current 5th generation (5G). The in-flight aircraft interaction with 5G mobile communication, operating in frequency bands up to 28 GHz [15], [16], in an unfavourable scenario could interfere with radar altimeters operation (4.2 - 4.4 GHz) [17]. Such equipment is present in most aircraft and responsible for safety-critical operations [15]. Therefore, an AEA undergoing such EMI endures potential adverse effects, which could have an impact, as in a fly-by-wire aircraft the pilot’s control over its maneuverability could be completely neglected during and after the exposure. This scenario aggravates with the implementation of autonomous or remote controllability, as is the case with the Airbus electric vertical take-off and landing (eVTOL) aircraft, the CityAirbus [18], in which the absence of a direct human response to variations in control and navigation would increase the attention requirements towards a harsh EME. In [19] it was demonstrated that the Pixhawk PX4 autopilot system was disrupted by modulating a 2.001 GHz signal with a 300 Hz PWM waveform.

Whether of autonomous control or not, the aircraft dependency on navigation and guidance systems is of major importance. The Airbus Global Market Forecast (2019-2038) [20] estimates that up to 39,000 new passenger aircraft anddedicated freighters will be required by 2038, an averagegrowth of approximately 3.25% p.a.. This means that AEAoperating on commercial routes in two decades would besusceptible to approximately twice nowadays air-traffic(21,000 in 2019). Therefore, in-flight collision possibilities,occurring especially in case of system failures, could increasedue to the considerable electromagnetic susceptibility natureof electrical and electronic onboard systems, and theirexposure to the future EME.

Fig. 3. Forecast of aircraft traffic in number of new aircraft (2020-2038).

Complementarily, but no exclusive of AEA, the structural development introduces novel composites [3], influencing the conductivity properties of aircraft structural shielding [12]. This is significantly troubling when considering lightning strikes, as the discharging current is channeled throughout the aircraft structure. Furthermore, air-traffic control radars at airports will create a much higher field

strength in composite planes compared to conventional full-metal planes.

B. Internal

The analysis of newly introduced systems into AEAallows the prediction of its internal EME. Aside from the conventional elements of typical aircraft, AEA introduces a wide range of new electrical loads, particularly the high-power electric powertrain, and avionics. Their interaction must be addressed within the developed EMC guidelines. Furthermore, every device is/will be fitted with power converters causing also conducted EMI.

When discussing the electrical and electronic systems of AEA, the number of cabling pathways, both power and data interconnections, distributed along its structure is much more extensive. Together with the operation of avionics, it entails a worsening scenario for emissions originating from both inter and intrasystems. While addressing the yet to be developed guidelines for AEA, not only the primary power distribution system and its interconnected components must be taken into account. Substantial advancements have also been made regarding aircraft control, onboard services, as well as structural integrity, which must be equally investigated.

Concerning the control of Level A functions and remaining systems, those rely entirely on electrical devices, performing crucial operations for the proper and safer flight completion. Furthermore, inflight services include enhanced communication technologies and electrical supply for portable electronic devices (PED) [21]. The AEA structure introduces novel composites [3], affecting its conductivity, and eliminating a significant low impedance path, influencing crosstalk occurrences [12], [22]. Complementarily, the internal EME of AEA presents a much higher level of interfering sources, and an increased number of onboard devices while introducing more sensitive flying equipment, hence, justifying the intensified level of attention towards this matter. In addition, in the case of in-flight services, several communication technologies will contribute to the HIRF from inside the chassis. These contributors are the current technologies like 3G/4G/5G, Bluetooth, Wi-Fi 6 (2.4 and 5 GHz), as well as future implementations like 6G, and Wi-Fi 6E (6 GHz). Therefore, the EM field is getting even more complex as new sources are presented.

IV. ALL-ELECTRIC AIRCRAFT AND EMC

Differing from typical aircraft, AEA implement fully electrified systems. This means that traditional equipment are replaced by digital interfaces, and the combustion propulsion system gives way to a complex high-power electric powertrain. Whilst inflight service loads (e.g., entertainment, and catering systems), ignition, and a few other electrical and electronic systems represented the EMC scope of typical aircraft, AEA present a much more complex scenario.

Particularly, in the electric powertrain of AEA, as a result of the high-power propulsion requirements, the rated power distributed is much greater than currently implemented in typical aircraft. Thus, defined as the most dominant source of EMI onboard AEA. Therefore, it will be the focus in the following subsections based on the main components shown in Fig. 1.

A. Avionics

Whilst traditional aircraft operations rely on hydraulic,mechanical, and pneumatic systems, AEA operability is entirely based on electrical systems and devices, which are

performing individual functions within the aircraft [1]. The operations performed by avionics encompass, amongst others, communication, navigation, and flight-control. When considering the EMC scenario of such avionics within the AEA, vital functionalities could experience adversities as such systems are typically safety-critical (Level A) [2], from being relied upon by pilots to perform emergency flight and landing under risky situations, to in-flight communication, and even weather information gathering [3], [23], [24]. Flight-controls are defined as both primary and secondary types. Primary surface controls are responsible for responsive maneuverability, mainly throughout three types of axial controls, pitch (lateral), roll (longitudinal), and yaw (vertical) [25]. In addition, secondary controls are as typical as primary, allowing the pilot to implement finer flight maneuverability, consisting of wing flaps, leading-edge devices, spoilers, and trim systems. In order to execute such commands, AEA operate on electrically compatible actuators (EHAs or EMAs) [3], which are also implemented in braking, landing gear, and other functions. Nevertheless, both EHAs and EMAs are composed of an electric motor and a converter. EMI characteristics of such components require research in an AEA environment. Brushless DC (BLDC) types are often used in such functions due to their reliability and high-precision controllability [26]. According to [27], BLDC motors, although operating in low-voltage applications, produce significant radiated emissions. Therefore, their implementation must be evaluated, being characterized as sources of EMI within the system. Important is that BLDC’s will not operate under steady-state conditions continuously and variability should be accounted for.

In the sense of EMI susceptibility, secondary flight-controls are of less concern due to the reduced operational risk when compared to primary controls, as the EMI would not be able to introduce a vital safety issue, considering that the aircraft can complete the flight by relying solely on the primary type [24]. However, multirotor eVTOLs, e.g., CityAirbus [18], present unique flight-control characteristics compared to the typical aircraft, as they rely on speed variations of each of its motors in order to provide different torque and thrust configurations [28]. Although its maneuver mechanism is considerably different, variations of primary and secondary flight-controls are still applicable, furthermore, the equal characterization of such aircraft flying system as fly-by-wire type, requires that the same concern presented previously must be applied here.

B. Energy Storage System: Batteries

Defined as a major challenge when enabling AEAtechnologies, batteries currently fall short of minimum gravimetric energy density requirements for commercial flights. When compared to jet fuel (~12 kWh/kg) [29], current commercial technologies offer only around 250 Wh/kg. Nevertheless, it was estimated in [30] that 500 Wh/kg would be the minimum required to successfully introduce AEA in the market as a viable commercially competitive technology, forecasted to occur only by 2030 (Fig. 4).

Fig. 4. Gravimetric energy density (Wh/kg) forecast of battery technologies [30], [31].

When discussing the energy storage system with respect to AEA, its composition comprises not only batteries, but also the module control units (MCU) within the battery management system (BMS), DC/DC converter, an internal charger, and more [32]. The trend in batteries is directed towards high voltage (HV) solutions due to high-power demands, flight range, and extended life cycle [33]. However, higher voltages could possibly mean a higher degree of radiated emissions, both in 3-phase AC, and DC systems [33]. Additionally, in [34] it was identified that integrated circuits (IC) of BMS are likely to be disrupted by EMI injected into the cell input terminals. Therefore, emphasizing the need to perform susceptibility/immunity tests on batteries.

Batteries are part of the noise propagation path for EMI originating in the converters. Therefore, the in-situ common mode (CM) and differential mode (DM) impedance are required for proper modelling and applying effective mitigation techniques. Furthermore, it was demonstrated in [35] that EMI could contribute to battery damages in the formof thermal runway. It occurs through high RF currents forminga resonating circuit, which could affect the AEA avionics. Theproper selection of battery technology and its protection willimpact the resulting emissions due to the current paththroughout the stacked layer elements, i.e., electrodes,electrolyte, and mechanical separator, accounting fordifferences in the devices’ internal impedance [33]. At themoment high power delivering batteries are not yet consideredin aerospace standards.

C. Power Converters

In AEA systems the main newly introduced converters areat the battery and motor interfaces, as these are components of the high-power electric powertrain (Fig. 1). They have to comply with high-power requirements, as the operation of the electric motors demand several hundreds of kilowatts (TABLE I. ). Besides such high-power converters, secondary subsystems, which in this case include all systems that do not contribute to the propulsion, require their own converters. Nevertheless, these are expected to have a significantly lower power rating compared to the propulsion side converters.

TABLE I. AEA ELECTRIC POWETRAIN - POWER RATING

AE

A

Equipment Rated Power [kW]

Vin [V] Vout [V] AC No. of Phases

DC/DC Converter 400 345-450 DC 540 DC

- 690-900 DC 1080 DC

DC/AC Inverter 400 540 DC 350 AC

3 or 6 1080 DC 690 AC

PM Motor 400 350 AC

- 3 or 6 690 AC

Choices are to be made whether the stored power will be distributed via AC or DC mains. Inherently, this will influence

the applicable cable constellations, size and weight of filter components and thermal dissipation at every stage of the power distribution network. Conventionally, aircraft have been using a 400 Hz AC grid, due to the reduction in size and weight of the transformers. However, with the introduction of high-power semiconductor devices, this is no longer a necessity, thus, allowing flexibility in choosing a different fundamental power frequency. Note, however, deviating from the convention might introduce challenges at secondary power system, as most commercial of the shelf (COTS) devices are designed (and tested) with respect to the conventional 400 Hz system. Therefore, further standards developments should also envision or be applicable when different fundamental frequencies are used.

In Fig. 1, a possible configuration is depicted for an AEA utilizing a DC distribution grid. As is often the case, most avionics currently available expect a 400 Hz AC voltage. However, it is likely converted to DC internally. Therefore, it is expected that the inverters and rectifiers will be fully replaced by DC/DC converters in such applications. Nevertheless, in both cases, the most dominant EMI will be at switching frequencies and their harmonics. Inverters are expected to be localized at the motors, most likely of multiphasic constitutions, i.e., they will generate 3 or more phases allowing precise control of the motor speeds and torque while keeping the generated EMI in check [36].

Looking at the changing EME onboard an AEA, aside from the relatively large number of converters required, the typical EMC challenges at each converter are applicable. There is a trend towards high-power density in electric transportation, which in aircraft is even more apparent due to the limited available space, but perhaps even more significant considering the reduction in weight [3]. Fortunately, considerable research has been done on the mitigation and control of EMI in standalone converters, with basically each type covered. In addition, the main challenge of AEA, concerning power converters, is the combination of all of them.

D. Electric motors

The selection of suitable electric machine technology isfundamental for the proper design and operation of an AEA, where several factors must be considered, from power density to fault-tolerance, thus, evaluating the most delicate trade-offs of the system. Amongst the many possible choices, induction motors (IM) and permanent magnet synchronous motors (PMSM) can be considered the most popular options for electric powertrains [37]. They are also the technology of choice for some current AEA designs, e.g., Velis Electro, and E-Fan projects 1.0 and 1.1 [18], [38]. Nevertheless, whenspecifically addressing aircraft applications, [39] identifiesswitched reluctance motors (SRM) and brushless DC (BLDC)motors as the most suitable choices. However, switchedreluctance motors (SRM) are known to provide higher EMIthan IM and PMSM [37]. Complementarily, motor windingsunder different operating modes regarding the flight phases(Fig. 5), will undergo power variations. This behavior willproduce transient voltages and current fluctuations,originating from sudden changes of magnetic fields,propagating throughout coupling paths, in addition to theeffect of converters [5].

Fig. 5. Power demand per flight phases: A) Climb, B) Cruise, C) Descent. Cases: Epos+ (CS-VLA), Cesna 172 (CS-23), Pilatus PC-24 (CS-23), Fokker 70 (CS-25) [40], [41], [42].

Whether of AC or DC nature, electric motors are likely to interfere with neighboring systems [5]. They can harmfully interact with other devices via mutual inductances and parasitic capacitances, originating from windings interacting with the housing and shaft, introducing CM coupling into the system [43]. Although AC motors are the typical drive choice within the current AEA, DC motors could also be implemented. The brushless DC (BLDC) motors are considered an interesting option, due to reduced bulkiness, high efficiency, and high energy density [37]. This is supported by the current conceptual implementation as in the CityAirbus [18], [44], which could considerably diminish the onboard EMI by reducing AC pathways.

E. Cabling (EWIS)

To meet the power requirements imposed by some of thehigh demanding applications, one of the implemented solutions could be the increase in number of phases to the distribution system. When considering the number of phases for a designed motor, the architecture influence towards commonly known issues must be taken into consideration, as is the case of odd phase numbers, which produce higher torque ripple frequency, and lower amplitude characteristics [45]. In AEA, additional cabling pathways are expected, whether of communication or power distribution, such cables are a main part of the classically envisioned EMC scenario (I), contributing as noise propagation mediums. The cabling increase also contributes to the adverse effects caused by HIRF induced coupling, since it acts as an antenna for frequencies from 1 to 400 MHz [8]. Considering the distribution of cables and expecting increased proximity, crosstalk will likely be intensified [3]. This is potentially devastating in ensuring EMC, due to the combination of highly fluctuating powering levels along with its flight profile phases (climb, cruise, and descent) (Fig. 5) [46]. Whereas these are expected to generate transients in the voltages and currents distributed over the system.

V. STANDARDS SUITABILITY TOWARDS AEA

Certainly, the most referenced civil standard towards environmental aspects of airborne equipment is the RTCA DO-160 [8], currently undergoing the revision “G”. Amongst its subjects, EMC is deeply approached, providing a solid base for the compliance evaluation of typical aircraft and its equipment, which are discussed within sections 15 to 22. However, such guidelines are not directed to aircraft with high-power electric powertrains, which shows the current non-suitability of DO-160G towards AEA. Amongst the sections of interest, the major limitations identified are, power

input (sec. 16), voltage spike (sec. 17), audio frequency conducted susceptibility (sec. 18), and emission of radio frequency energy (sec. 21).

Based on Sec. III and IV of this paper the following changes are expected. In section 16, the power input topic is required to undergo major revisions, as the power demand onboard AEA exceeds the ones approached in the standard. In addition, such aircraft are expected to operate under higher voltage levels, frequencies, and in the case of electric propulsion, an increased number of phases (>3). As for sections 17, voltage spike, and 18, audio frequency conducted susceptibility, the issues stand on higher voltage levels, and different fundamental frequencies.

Revisions in sec. 21 must take place in order to encompass the changes discussed in previous sections, thus, requiring new test procedures. In order to represent the changes in the typical power source, i.e., batteries, test equipment (LISN) specification must also be addressed.

Complementarily, when considering power converters within AEA, they often operate at a frequency range below 150 kHz, a situation not covered by DO-160G. However, this range is considered in the MIL-STD-461G [10], CE102 procedure for conducted emissions, radio frequency potential, and power leads, which is applicable to aircraft [47]. Nevertheless, military standards are much more rigorous guidelines due to the expected high risks, which makes the direct implementation of such standards to the civil industry unfeasible from a cost perspective.

Fig. 6. Overview of frequency ranges of emissions and susceptibility.

VI. CONCLUSION

An overview of the requirements and standardization challenges to be faced towards the effective deployment of AEA for long-range commercial applications was presented. Today’s and future’s EME were briefly discussed concerning any aircraft, while an in depth discussion was provided on a newly introduced set of sources and victims inherent to AEA. These encompass the avionics and high-power electric powertrain, including high-power batteries, power converters, motors and cabling. Furthermore, the technological evolution of essential technologies, in special high-power batteries, was assessed, estimating its maturity for such applications to 2030, thus, a foreseeable future. Therefore, the topics presented in this paper raise a valid discussion regarding the type of power distribution implemented within future AEA. Through the identification of changes in onboard architecture (systems and components) and the EME of AEA, when compared to typical configurations, it was possible to foresee the challenges of its future implementation.

To conclude, in order to properly assess the acceptable noise levels originating from within AEA, and the influence of EME interferences, current standards for civil aviation are unlikely to encompass the intricacies of high-voltage electric

powertrains and avionics in relation to the expected high-level electromagnetic emissions. Therefore, either the proper revision of current civil airborne equipment standards (DO-160G) or the development of new EMC guidelines for AEA is of utmost importance.

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