Research on safety issues and safety analysis of new energy aircraft

As the global aviation industry faces increasing pressure on the environment, the International Civil Aviation Organization (ICAO) has implemented increasingly strict requirements for aircraft carbon emissions. The adoption of innovative energy technologies, such as electric power, hydrogen fuel, and sustainable biofuels in the aviation sector, will enable us to achieve zero carbon emission targets and mitigate environmental pollution associated with air transportation. Drawing on statistical data of aviation accidents, this paper analyses the primary causes of flight mishaps and summarizes several safety issues faced by new energy aircraft. Based on these, the new energy aircraft is divided into several subsystems, including power system, energy system, flight control system and so on. Then, safety analysis tools and methods are then used to conduct an analysis of the aircraft’s safety, with the aim of improving overall safety.


Introduction
With the aviation industry continuously expanding, environmental and energy issues are becoming increasingly salient.The future's strict environmental standards present a challenge for traditional fuel aircraft.The use of clean energy to gradually replace fuel power has become an important trend in the development of the aviation industry.In recent years, there has been a global surge in the development of new energy aircraft, like the revolution seen in new energy cars.As the aviation industry continues to expand, environmental and energy issues are becoming increasingly prominent [1].The technology for developing and producing new energy aircraft is advancing rapidly.Airbus, Boeing, Dassault, and other airlines have unveiled new energy concepts.Rolls-Royce has recently successfully tested hydrogen-powered aircraft engines.FAA, EASA, and CAAC have successively established committees that are specifically responsible for setting airworthiness standards for energy aircraft.Additionally, regulations and standards have been approved for these types of aircraft [2,3].New energy aircraft are expected to enter the commercial market within the next 5-10 years.Thanks to the strong promotion of green aviation development, new energy aircraft can develop rapidly.At present, enterprises and research institutions worldwide are primarily focused on technical demonstrations and designs.The battery energy density limits the feasibility of pure electric aircraft for both trunk and regional lines.However, hydrogen fuel technology has been identified as a viable alternative for constructing such lines in other countries, although it is still in its early stages.Hydrogen fuel-powered aircraft is a medium-and long-term development way for new energy aircraft.By around 2050, there will be a complete line of fully autonomous new energy aircraft industry products that cover all markets and product lines [4][5][6].Many of the system technologies lack established stand-    The results indicate that the safety of IOSA-certified airlines has improved to some extent over the past decade, as evidenced by a decreasing number of accidents each year.The number of major damage accidents involving civil jet aircraft is higher than that of turboprop aircraft, and these accidents most frequently occur during the landing and low-altitude flight stages.The safety of turboprop aircraft has not significantly improved over the past five years.

Safety issues for new energy aircraft
The primary features of new energy aircraft are their environmental friendliness and economic viability.The main difference between new and traditional aircraft lies in the utilization of alternative energy sources.The use of new energy sources, such as solar power, electric power, and hydrogen fuel, has resulted in modifications to the power plant and corresponding advancements in aircraft aerodynamics, energy systems, power systems, and flight controls.Modifications made at the design level will have an impact on security concerns.

Energy system safety issues
The energy system of an aircraft can be classified into primary and secondary energy systems [7].The primary energy system of new energy aircraft converts clean sources of energy, such as electricity, hydrogen, and solar power, into the driving force for propulsion.The current focus of new energy aircraft is primarily on small electric aircraft.However, the limited energy density of storage equipment still presents challenges for trunk and regional aircraft, which are still in the conceptual stage.Storing hydrogen fuel in large quantities and safely on airplanes is a challenge.High-pressure gas storage occupies more space in the airframe, while cryogenic liquid storage offers higher energy density but requires demanding conditions.Additionally, high concentrations of hydrogen can easily cause combustion due to its invisible and heatless nature, making it difficult to detect.All these factors have led to serious flight safety accidents.Therefore, it is necessary to implement effective monitoring and protective measures, as well as establish corresponding safety standards for the use and monitoring of hydrogen.For new energy aircraft, the configuration of the energy system differs, requiring a specific focus on safety analysis and tailored problem-specific analyses.

Electric propulsion system safety issues
The conversion of secondary energy systems from traditional aircraft into electric energy signifies progress towards the electrification of aircraft energy, which is a crucial aspect in the development of sustainable aviation.Electric propulsion technology represents a significant innovation in energy and power systems, marking an advanced stage in the development of aircraft electrification [8].The research on new energy aircraft primarily focuses on electric propulsion units and hybrid power systems.This type of power plant needs to pay attention to electrical hazards and ensure effective measures for high voltage and fire protection [9,10].The American Society for Materials has formulated the ASTM F2840 standard, which sets forth requirements on airworthiness safety issues such as the use of materials and components in electric propulsion devices, establishment of operational limits, environmental conditions, simulation calculations, and airworthiness verification.

Flight control system safety issues
With the advancement of computer technology, the entire aircraft system management platform has been optimized for autonomous control and improved flight performance through real-time monitoring of flight status [11].Flight control system technology is developing towards automation, autonomy, greater efficiency and safety, in order to meet the challenges of integrating new energy aircraft.Therefore, the control system of new energy aircraft must meet the following requirements: a) The flight control computer should have robust computational capabilities and a sophisticated monitoring program to cater to the demands of intricate integrated systems.b) The design of complex and highly integrated systems must have sufficient margins and comply with engineering reliability and airworthiness safety standards, and it is crucial to explore additional methods for conformity verification.c) the Important subsystems and components should be backed up and easily switched to the standby operating system in case of primary system failure.

Energy storage system security issues
The energy storage systems utilized in new energy aircraft primarily include batteries, fuel cells, solar cells, supercapacitors, liquid hydrogen tanks, and biofuel tanks.The lithium-ion batteries, serving as representatives of advanced energy storage systems, possess immense potential and are regarded as the fundamental components of new energy electric vehicles.Based on statistics from 2014 to 2019 regarding electric vehicle explosions, the primary causes include spontaneous ignition, collisions accident, equipment failure, battery charging, cable aging, short circuits, immersion events, human operation and so on, as figure 3.These factors can lead to thermal runaway and ultimately result in an explosion with the battery being identified as the main culprit.
The primary factors contributing to energy storage system accidents can be categorized as follows: a) Battery system deficiencies; b) Inadequate protection systems; and c) Complex operational environments.

Composite material safety issues
The extensive use of advanced composite materials in new energy aircraft is an inevitable trend for future development.China has conducted years of research and development on aviation composite materials, leading to the gradual establishment of a comprehensive technical system that essentially meets the requirements of relevant technical standards for aerospace applications.However, ensuring the structural integrity, damage detection capabilities, and proper material maintenance procedures of composite materials is crucial for upholding aircraft reliability and airworthiness.

Aerodynamic configuration safety issues
The integration of aerodynamic structure and propulsion design is an important trend in the advancement of new energy aircraft, as it endeavours to augment aerodynamic performance [12,13].The primary design and manufacturing directions for new energy trunk and regional aircraft are distributed electric propulsion and wing-body configuration.This innovative arrangement provides structural, material, and power advantages for the aircraft.However, due to a lack of suitable analytical tools and references, ensuring airworthiness safety remains challenging.

Safety analysis method
The safety of new energy aircraft is assessed through the application of a safety analysis methodology, which draws on statistical analyses of flight accidents and their associated safety concerns.For example, for hydrogen-fueled aircraft, it is necessary to pay attention to the safety problems, such as Hydrogen storage tank and Hydrogen storage line, and for pure electric aircraft, electric energy is used for all or part of the propulsion system.Components requiring safety considerations include batteries, motors, cables and related equipment.

Function hazard analysis (FHA)
According to the SAE ARP4961 recommended process for assessing aircraft system safety, the FHA scope extends throughout the development life cycle.For the purposes of the Preliminary System Safety Assessment (PSSA), the FHA considers only functional failure modes (i.e., not their probability of occurrence) [14,15].Considering the difference between new energy aircraft and traditional aircraft, it is imperative to comprehensively identify the functions of the new energy aircraft system and its subsystems, account for potential functional failures as well as combined failures across systems, and assess their impact levels.The safety requirements identified through aircraft-level FHA and system-level FHA can be addressed by utilizing Fault Tree Analysis.The purpose of using FHA is to determine the top event and hazard level associated with a specific risk.The FHA table typically includes the main contents shown in the following table 1.

Failure modes and effects analysis (FMEA)
The FMEA is a structured way of identifying actions that could lead to collapse of a system, subsystem, or component.Its aim is to evaluate the consequences resulting from hypothetical failures of components within the system.The new FMEA standard, which was published in 2019, enhances the implementation process and introduces the concept of Seven-step Method (as table 2).By conducting quantitative analysis of the risk priority number (RPN), it is possible to identify the fault mode posing the highest risk to the system.This risk analysis is achieved by combining it with the action priority (AP) method.Utilizing the research of new energy aircraft system, an FMEA was completed by decomposing the overall system into five subsystems: power system, propulsion system, flight control system, electrical system, and structural system.While each subsystem has technical failure modes only applicable to its respective area, there are noticeable failure modes that repeat amongst each subsystem: manufacturing, human error, material fatigue, and environmental.In accounting for these redundant failures, a separate category for overall system failures was organized where these common subsystem failures were explored separately from the unique technical failure modes.The failure of each subsystem is standardized based on its severity (S), occurrence (O), and detection (D).Consequently, the risk coefficient RNP is calculated by multiplying S, O, and D. This resulted in 4-5 components or functions for cause of failure within a subsystem and 25 overall components to be analysed into potential failure modes and potential causes and effects of these failures.Then, the failure modes requiring action are classified by priority to reduce the magnitude of risk.Here, AP can be categorized into high (H), medium (M), and low (L) priority levels.Table 3 shows the RPN failure modes.

Fishbone diagram
From the information gathered in the FMEA, Fishbone Diagrams based on five subsystem failures were obtained that can be used to study overall system failures.Such graphical diagrams provide a clear depiction of the potential causes of the FMEA and clearly show the major root causes of these subsystems, making it easier to understand how these subsystems work and affect the whole-aircraft system of the new energy aircraft.figure4 shows the overall system Fishbone Diagram.Which looks at the redundant failure modes that affect all subsystems, as well as the technical aspects of each subsystem.IOP Publishing doi:10.1088/1742-6596/2633/1/0120067 making a total of five categories.Hence, individual component failures could be classified into any one of these five categories.It was also able to be seen that subsequent failures could be grouped under their respective subsystems.Not all types of failures apply to all subsystems, such as the Human Safety subsystem not depending on any Manufacturing Defects.After assessing each individual component failure, a separate group for Overall system failures was created.Another fault tree was created to visualize and better understand if a top event was a crash imminent for the new energy aircraft (as figure 5).

Summary and expectation
Given that the new energy aircraft system is still under development, obtaining reliable data has proven to be challenging.To address this issue, we have divided the overall system into several subsystems and compared them with traditional aircraft in study.Additionally, we conducted a comprehensive analysis of failure modes for both the entire system and its subsystems using methods such as FHA, FMEA, FTA etc., completing the safety analysis.Currently, ensuring the reliability and safety of new energy aircraft is of utmost importance, while simultaneously addressing technical challenges to minimize potential faults in accordance with secure development requirements.

Figure 1 .
Figure 1.Failure modes observed in commercial aircraft accidents between 1950 and 2019.

Figure 2 .
Figure 2. The accidents data compiled by IATA for 2010 to 2022.

Figure 2
Figure2reviews the accident data compiled by the International Air Transport Association (IATA) for 2010 to 2022.The results indicate that the safety of IOSA-certified airlines has improved to some extent over the past decade, as evidenced by a decreasing number of accidents each year.The number of major damage accidents involving civil jet aircraft is higher than that of turboprop aircraft, and these accidents most frequently occur during the landing and low-altitude flight stages.The safety of turboprop aircraft has not significantly improved over the past five years.

Figure 3 .
Figure 3. Statistics on electric vehicle explosion accidents from 2014 to 2019.

Figure 4 .
Figure 4. New Energy Aircraft System Fishbone Diagram.