UAS Reliability and Maintenance

Maintaining Reliability and Maintenance of UAS

Summary

This paper examines maintaining the reliability and maintenance of UAS since this system is increasingly adopted in the National Airspace System (NAS). The issue is examined on the backdrop of increased adoption of UAS in commercial and civilian domains though they were initially designed for military application. The discussion seeks to promote an in-depth understanding of UAS operations, understand UAS capabilities and limitations, and develop suitable procedures for maintenance and enhanced reliability of UAS.

UAS refers to a system whose components do not carry a human operator and are piloted remotely or fly independently. It was initially designed and adopted for military applications but has since grown to be used in civilian and commercial domains. However, the increased use of UAS in these settings has generated concerns regarding safety and reliability. Maintaining the reliability of UAS requires conducting reliability assessments using either deductive or inductive approaches. Deductive reliability assessment like FTA focuses on detecting high-level system failure events and all lower-level incidents that could directly contribute to a failure incident. Inductive assessments like FMEA focuses on identifying the failure modes of system components at the lowest level possible.

UAS maintenance is defined as any activity carried out on the ground prior to or after the flight to promote and ensure the successful operation of the system as well as its safety. Scheduled and unscheduled maintenance are the two broad categories of UAS maintenance activities. UAS maintenance can be achieved through various recommendations including the establishment of tailored maintenance training, redesigning the roles of the UAS operator and UAS maintenance personnel, and adopting preventive maintenance approaches. On the other hand, deductive reliability analysis like FTA is the recommended approach for reliability.

Maintaining Reliability and Maintenance of UAS

The idea of conventional aircraft design has changed dramatically over the past few decades. In essence, the introduction of unmanned aircraft systems (UAS) is a reflection of the evolution of the overall architecture of aircraft design. However, many modern aircraft have some components and systems that are similar to the conventional models. Since their introduction, unmanned aircraft systems have attracted growing consumer attraction in terms of ownership and operation. Wicker et al. (2019) note that the emergence of UAS has the potential of generating significant economic and social benefits to the United States. Despite the potential social and economic benefits, UAS has attracted concerns regarding safety and reliability. These concerns are partly attributable to the fact that their manufacture does not conform to a type design (Ley, 2016). This paper examines the issue of maintenance and reliability of UAS in relation to safety concerns.

Overview of Unmanned Aircraft System (UAS)

Lu et al. (2019) state that UAS is an application that represents independent technologies in the aviation industry. This system is developed by the unmanned aerial vehicle (UAV), data links, ground control station, and recovery and launch system. UAS, which is also known as unmanned aerial system, refers to a system whose components do not carry a human operator (Gupta, Ghonge & Jawandhiya (2013). UAS are either remotely piloted or fly independently. This implies that UAS have an associated ground control station as well as a data link between the board and the ground. Therefore, UAS has a system that is characterized by command, communications, and control. While UAS air vehicles and their associated equipment do not carry a human operator, it’s operations require necessary personnel for remote piloting or to fly autonomously.

UAS comprises three major features or components i.e. unmanned aircraft, the data link or command and control link, and ground control station (Hobbs & Herwitz, 2006). Unmanned aircraft is essentially a powered vehicle without a human operator or an aircraft with no pilot on board. As previously indicated, the aircraft can be operated remotely i.e. by a pilot/operator at a ground control station or fly independently based on pre-programmed plans (Gupta, Ghonge & Jawandhiva, 2013). UAS can be recoverable or expendable as well as carry a deadly or non-lethal payload. However, cruise missiles, unattended sensors, ballistic or semi-ballistic vehicles, torpedoes, satellites, artillery projectiles, and mines are not classified as UAS or UAVs.

Background Information of UAS

In the initial years of development, UAS was adopted by military planners to conduct surveys and/or attack missions. The history of UAS can be traced back to 1916 when the first unmanned air vehicle (UAV) was developed by the Americans Lawrence and Sperry (Gupta, Ghonge & Jawandhiva, 2013). The development of the first UAV marked the beginning of attitude control, which played a critical role in the automatic steering of an aircraft. Lawrence and Sperry named the first UAV aviation torpedo and flew it a distance exceeding 30 miles. However, the end of the 1950s marked a significant period in the history of UAS as full-scale research and development of UAVs was carried out until the 1970s. The research and development of UAVs during this period was influenced by the Vietnam War. After this War, the United States and Israel commenced the development of smaller and cheaper UAVs. These UAVs were small aircraft with small engines like those used in snowmobiles or motorcycles. They can be regarded as the prototype of the current UAS as they had video cameras transmitted images to the ground operator.

According to Lum & Tsukada (2016), the use of UAS technology started to grow beyond the military domain in recent decades. The growth was fueled by increased interest in commercial and civilian domains. The commercial UAS industry has been characterized by the exponential growth of small unmanned aircraft. Small unmanned aircraft has received increased interest in this industry because of its potential to carry out tasks that would have formerly required larger aircraft (Hobbs & Herwitz, 2006). The improved capabilities of small unmanned aircraft are attributable to technological advancements like miniaturization of sensor equipment and autopilots as well as advances in battery technology. Additionally, small unmanned aircraft are based on cheaper hobby store model aircraft that sometimes involve an autopilot.

The commercial UAS industry has witnessed rapid growth because small unmanned aircraft have several potential uses and can be utilized in different sectors and applications. Some of the potential uses of these aircraft include traffic monitoring, search and rescue, homeland security applications, power-line inspection, border surveillance, agriculture, policing and firefighting, aerial photography, wildlife monitoring, and minerals exploration. Additionally, UAS are also used in sports events film coverage, research by university laboratories, pipeline survey, and communications relay (Gupta, Chonge & Jawandhiya, 2013). UAS utilization has expanded to more civil domains because of their convenience and low-cost attributes (Lu et al., 2019). As a result, UAS in civil domains has outnumbered UAVs substantially with estimated several millions annually. The growth of non-military UAS applications is projected to increase in the near future once airspace regulations are established (International Civil Aviation Organization, 2011). UAS market for civilian and commercial applications is projected to grow by up to $7.5 billion in the near future.

Problem Statement

UAS offers convenience and low-cost attributes, which has contributed to its increased adoption in commercial and civilian domains. However, the increased use of these applications in commercial and civilian domains has generated numerous safety concerns and considerations. UAS in these domains is threatened by a series of safety considerations, especially maintenance issues involving human activity and operations. Despite improvements in engines, current UAS reliability approaches are still regarded as fatalistic. Currently, sophisticated UAS systems are deemed to have an overall rate of failure of 25% (Petrioli, Leccese & Ciani, 2018). According to Mrusek, Kiernan & Clark (2018), the safety impact of UAS remains a major issue despite the growing consumer focus on owning and operating them. The introduction of UAS into the National Airspace System (NAS) has generated several challenges in the maintenance of airworthiness. Current UAS maintenance efforts are characterized by challenges relating to the lack of civil regulatory requirements and the lack of dedicated and qualified maintenance personnel. These challenges have contributed to difficulties in maintaining the reliability and maintenance of UAS.

Significance of the Problem

UAS operators and other stakeholders face challenges in maintaining the reliability and maintenance of this system. These challenges increases safety concerns and issues with the airworthiness of UAS. Therefore, the identification of effective reliability and maintenance approaches is essential to enhance the safety and worthiness of UAS. Through addressing this problem, the relevant stakeholders in the aviation industry will identify suitable actions or approaches for maintaining the reliability and maintenance of UAS. UAS missions need to achieve an acceptable safety and reliability level to promote their increased use in the National Airspace System (NAS). Addressing this issue would also help to promote the development of reliable and realistic risk evaluation methods for improved reliability of UAS. Additionally, this issue is significant for UAS missions as it plays a critical role in promoting and ensuring the airworthiness of these systems. The improved airworthiness of UAS would play a critical role in their safe operations and increased integration in NAS.

Related Works

As UAS is increasingly becoming common in the NAS, its maintenance and reliability have become increasingly essential. Numerous studies have been carried out in recent years on the reliability and maintenance of this system.

UAS Reliability

Petritoli, Leccese & Ciani (2018) contend that reliability is a dynamic concept that is applicable in many disciplines including technical and non-technical fields. Reliability is defined as the probability of a subsystem, system, or part can carry out its specific functions in a pre-determined time and based on pre-established conditions (Petrioli, Leccese & Ciani, 2018). Over the past few years, UAS applications and operations have been characterized by the growth in significance of reliability. As part of enhancing the reliability of UAVs, engines have become more robust while avionics have been improved. However, current reliability approaches are still regarded as fatalistic as the overall rate of UAS failure is 25% (Petrioli, Leccese & Ciani, 2018). Freeman (2014) notes that current low-cost UAS are unreliable, which demonstrates the need to improve reliability.

According to Petritoli, Leccese & Ciani (2018), reliability, availability, maintainability, and safety (RAMS) assessment is essential in the development of unmanned aircraft. Such an assessment is critical to lessen repair and maintenance costs as well as avoid all kinds of failures such as severe, catastrophic, moderate, and soft failures. Reduction of the rate of failure is a major consideration in UAS reliability. Lum & Tsukada (2016) note that improving the reliability of UAS applications enabled regulators and operators to understand how to lower the rates of failure over time. Therefore, UAS personnel face the need to develop effective approaches to maintain reliability.

Existing literature demonstrates that there is no one-size-fits-all approach for reliability assessment and improvement of UAS (Freeman, 2014). However, maintaining the reliability of UAS requires the use of the existing measures for reliability assessment. Reliability assessment techniques used for UAS are classified into two broad categories i.e. inductive and deductive approaches. Inductive or bottom-up reliability analysis focuses on identifying the failure modes of system components at the lowest level possible. During this process, the effects of these failures on higher-level subsystems are determined forward to higher subsystems. This process is carried out repeatedly for various initiating causes until all probable failure modes are identified. On the contrary, deductive or top-down reliability analysis entails detecting high-level system failure events and all lower-level incidents that could directly contribute to a failure incident. This process is carried out repeatedly until flowing causes of failure incidents down to components at the lowest level are determined (Freeman, 2014). Deductive reliability analysis is advantageous as it focuses on one or more unwanted events and their probable causes. Existing reliability assessment techniques used to help maintain the reliability of UAS fall under either of these two categories or reliability analysis.

UAS Maintenance

UAS maintenance is defined as any activity carried out on the ground prior to or after the flight to promote and ensure the successful operation of the system as well as its safety (Hobbs & Herwitz, 2006). This implies that UAS maintenance covers a broad range of issues and ground support activities such as assembly, software updates, fueling, and pre-flight testing (Hobbs & Herwitz, 2008). The maintenance of UAS is critical to promote the safety and enhance airworthiness of unmanned aircraft in the military, civilian, and commercial domains.

Martinetti, Schakel & van Dongen (2018) note that UAS maintenance strategy should be customized to the technical characteristics. In this case, the maintenance strategy should consider each individual component of the system and its operational situations and conditions. Hobbs & Herwitz (2006) further contend that UAS maintenance personnel should ensure that the reliability of the whole system covers the unmanned aircraft vehicle, the communication equipment, and the ground control station.

Mrusek, Kiernan & Clark (2018) notes that the introduction of UAS into NAS has generated several challenges in maintaining airworthiness. These challenges relate to the safety impact of UAS, which remains a major issue despite the growing consumer focus on owning and operating the system. Ley (2016) states that one of the major challenges in the development of an effective UAS maintenance strategy is the lack of civil regulatory requirements. The unmanned aircraft sector has no civil legislative requirements that coerce manufacturers of these systems to design, create, and support their aircraft to a particular standard of testing and approval. As a result, most of the existing UAS do not conform to a type design or meet some requirements relating to the continued airworthiness of the system vis-à-vis civil regulation. The lack of such regulatory requirements and a standardized type design has contributed to numerous challenges in UAS maintenance practices and efforts. Currently, the availability and quality of maintenance procedures, standards, and technical documentation to support UAS is unknown (Ley, 2016).

In an earlier study, Hobbs & Herwitz (2006) contend that UAS maintenance is currently unregulated and carried out by professionals who lack formal maintenance qualifications. This implies that there is no proper oversight and defined standards for UAS maintenance, which in turn increases the risk of operational and structural failures. The increased risk of operational and structural failures, in turn, places air and personnel safety at huge risks. Moreover, manned aircraft maintenance practices cannot be achieved or are not entirely applicable to the maintenance of UAS.

Hobbs & Herwitz (2006) further contend that one of the major impediments to UAS maintenance is the lack of dedicated and qualified maintenance personnel. Generally, most UAS operations do not have qualified and designated maintenance personnel. Most of the operational tasks are carried out by a small team of multi-skilled professionals who carry out the full range of tasks relating to preparing the unmanned aircraft for flight. While some UAS operators have people who carry out maintenance tasks, these individuals are not necessarily maintenance technicians and do not possess the relevant knowledge and expertise. Additionally, some small UAS manufacturers provide maintenance training courses, by most of the people who carry out these activities have no formal preparation for their duties. These people possess a diverse range of skills including engineering, radio control aircraft operations, and electronics. Therefore, their specialty and expertise is not maintenance. Many of the required knowledge and skills in UAS maintenance lies in the avionics field. UAS maintenance personnel also need to possess skills and knowledge relating to preventative servicing duties. The diversity of skills and expertise required for UAS maintenance contributes to regulatory challenges.

Alternative Actions

As evident in the review of existing literature, there is no one-size-fits-all approach to enhancing the maintenance and reliability of UAS (Freeman, 2014). Existing literature demonstrates that the reliability of UAS can be achieved through the establishment of a proper and effective reliability risk evaluation method. However, the establishment of such a method requires the identification of some of the risk factors facing these applications. An accurate understanding of the risk factors would help enhance the reliability of UAS by allowing regulators and operators to address each of them individually. Risk factors for UAS operations and applications are classified into three major categories i.e. human error, system failure, and environmental factors.

Human error includes inadequate operator response, improper UAS maintenance, and mission planning mistakes (Lum & Tsukada, 2016). Over the past few years, human error has become the dominant risk factor in UAS operations followed by system failure. While UAS does not carry an onboard human operator, operational experience shows that human error poses threats or hazards to their operation. Currently, the rate of accidents for unmanned aircraft is significantly higher than that of manned aircraft (Hobbs & Herwitz, 2006). Human error has become the dominant risk factor for UAS because operators have less real-time information and fewer alternatives for fault recovery. Human error or human factor issues in UAS reliability are basically personnel issues that affect the operations of unmanned aircraft. Some of these personnel issues include complacency, lack of direct pilot reports, and model aircraft culture.

System failure is a broad term referring to various factors ranging from hardware or mechanical failure to system issues. Some examples of hardware or mechanical failure include loss of link, engine failure, and damage to the control surface. According to Hobbs & Herwitz (2006), hardware or mechanical failures are linked to various factors in UAS operations including whole-of-system approach, wide use of computer hardware, system assembly, battery maintenance requirements, composite materials, modular design, fuel mixing and storage, and composite materials. On the contrary, some examples of system issues include software update challenges and software failure (Lum & Tsukada, 2016). System failure could result in unexpected or abnormal system behavior. Additional system failure issues include lack of maintenance documentation, lack of reporting systems, and poor standard of maintenance documentation. Potential hazards that could be brought up by system failure include ground control station failure and flight computer failure.

Environmental factors refer to threats in the operational environment that could affect the safety and reliability of UAS (Hobbs & Herwitz, 2006). Unlike conventional aircraft, unmanned aircraft is not usually stored outdoors where they would be exposed to environmental or external threats. However, UAS operations may involve exposure to harsh environments including high altitude and extremely low temperatures. Such harsh environments generate new maintenance demands to safeguard the airworthiness of UAS. According to Lum & Tsukada (2016), one of the most common environmental factors affecting UAS is bird strikes. Bird strikes are regarded as one of the greatest risks for aircraft during taxi, take-off, and landing. Bird strikes represent a major risk factor for unmanned aircraft since most UAS operations take place below 400 AGL (Lum & Tsukada, 2016). Therefore, bird strikes represent a potential non-trivial risk factor for UAS operations.

Approaches to Maintaining Reliability of UAS

Reliability assessment is an important step in enhancing or maintaining the reliability of UAS. This is primarily because the assessment plays an important role in facilitating the development of effective approaches to maintain reliability. As evident in existing literature and current best practices, UAS reliability is achieved through conducting reliability assessments. As previously indicated, reliability assessments are classified into two major categories namely: deductive reliability analysis and inductive reliability analysis. These categories entail different approaches for conducting the assessment and have different techniques. The techniques in each of these categories are regarded as alternative actions in maintaining the reliability of UAS. In light of this, there are some approaches that can be used to improve UAS reliability as follows:

Fault Tree Analysis (FTA)

The most commonly used deductive reliability analysis technique is fault tree analysis (FTA), which is common in aerospace and other safety-critical industries. FTA is commonly employed in these settings because of the straightforward nature of extending it with probabilistic risk analysis when the rates of failure events are well known. FTA is a renowned risk assessment technique applicable to UAS operations because it establishes system dependability (Abdallah, 2019). When employed to help maintain the reliability of UAS, FTA entails analysis of system failure beginning with a top event and progressing towards the leaves of the tree. This process is carried out to help identify basic events that are not the root causes of the failure event identified at the top. This reliability analysis technique is a graphical representation of the logical connections/links between failure events and their causes. FTA shows how a mixture of various components failure and environmental situations can result in overall system failure. Fault trees basically comprise different types of nodes i.e. events, gates, and transfer symbols (Abdallah, 2019).

Reliability analysis using FTA can be resumed in two categories i.e. qualitative or quantitative level. Qualitative level of analysis is achieved by constringing fault trees and changing them into minimal cut sets (MCSs). MCSs act as the total sum of products based on the smallest mixture of the basic issues contributing to the top event. On the other hand, quantitative level of analysis entails calculating the likelihood of occurrence of the top event through referring to the rate of failure of each system component. Therefore, quantitative level of analysis gives indicators regarding the most influential components on system reliability. Insights obtained from such analysis enable analysts to give more priority to critical components in order to lessen the probability of failure. Through this analysis, analysts identify redundant components that affect system reliability.

Failure Modes and Effects Analysis (FMEA)

One of the industry-standard reliability assessment techniques is the failure modes and effects analysis (FMEA). FMEA is an inductive reliability analysis technique that is used to examine and identify all probable failure events of a system, their impact on the system, and measures to correct and/or lessen the failures and their impact on the system (Freeman, 2014). The ranking of the likelihood and severity of the failure event influences correction and mitigation priority (Abdallah, 2019). It is an efficient tool for the identification of probable failure modes and their impact in order to enhance the safety and reliability of complex systems (Damanab et al., 2015). Unlike FTA, this deductive reliability analysis approach is limited in scope as it focuses on system hardware failures and their effects. As a result, software failures and human factors may be excluded from this reliability assessment process though their consideration is vital to ensure overall system reliability. However, the purpose of FMEA is to identify failure modes and their impact, to determine suitable corrective actions, and to develop an efficient maintenance system to lessen the likelihood of probable scenarios relating to the failures (Damanab et al., 2015).

System dependability is an important part of maintaining the reliability of UAS. FMEA is essential in the process of determining and ensuring system dependability as it is commonly employed in the early stages of system development. This approach is employed in these stages to help enhance understanding of the probable failure modes and develop strategies to lessen them. The use of FMEA in the early stages of system development is influenced by the belief that reliability issues can be usually more easily addressed during these stages. However, for mature UAS operations, this deductive reliability assessment technique can be carried out as part of system improvement measures. In such scenarios, the analysis helps to identify necessary design changes or hardware upgrades for future designs. On the contrary, FMEA is implemented throughout all stages of system development for safety-critical systems.

The implementation of FMEA in the early stages of development is carried out for various purposes. Petritoli, Leccese & Ciani (2017) state that some of the purposes include comparing alternative designs, planning logistic support strategies, identifying probable design weaknesses, examining life-cycle costs, optimizing conditions, and establishing objectives for additional reliability tests. In addition, implementing FMEA in these stages helps to predict the occurrence of failures, potential effects, and measures to mitigate their likelihood and impact. Therefore, this approach provides baseline data or insights regarding system reliability.

FMEA reliability assessment process incorporates several major steps beginning with documentation of the functions of all components of the system. During this step, an understanding of the various system operational environments is developed. This step of the process helps to identify all probable failure modes for each system component. The next step in this process is the determination of the impact of each failure mode. In this stage, more emphasis or focus is given to any common-mode system failures. The analyst usually assigns a qualitative severity or criticality ranking to potential failures and their impact based on their likelihood of occurrence and/or damage potential. The process involves the use of several different severity criteria based on industry standards and application. The final major step in FMEA is the revision or modification of system design to better mitigate identified risks. This process is carried out based on information obtained from the previous two steps (Freeman, 2014).

UAS Maintenance Approaches

As evident in the review of related works, UAS maintenance is considered critical to enhancing the safe and successful operations of the system. UAS maintenance also plays an important role in determining UAS airworthiness. Hobbs & Herwitz (2008) state that UAS ground support maintenance activities are carried out in three major locations i.e. in the field, workshop, or at specialist/manufacturer facility. There are two major approaches used for the maintenance of UAS as follows:

Scheduled Maintenance

Scheduled maintenance is one of the major categories of ground support activities for the maintenance of UAS. Scheduled maintenance includes routine inspections, timely replacement of system components, and adjustments. These tasks also include planned ground support activities like pre-flight system assembly, pre-flight functional tests, fuel mixing, and battery charging (Hobbs & Herwitz, 2008). Scheduled maintenance activities are common as they are always carried out. While these activities are common and routine, they have a high probability of absent-minded mistakes due to memory lapses.

The development of scheduled maintenance programs is viewed as an essential part of the maintenance of UAS applications/missions. Such programs may not necessarily require regulatory changes since some of the maintenance activities for manned aircraft can be applied to UAS (Mrusek, Kiernan & Clark, 2018). Despite the applicability of some maintenance processes for manned aircraft, UAS maintenance activities are slightly different because of the slight variations in airborne components. Generally, the components of unmanned aircraft include the airframe, control surfaces, propulsion and fuel system, actuators, radio communication equipment, and electrical system. In addition, unmanned aircraft also comprises a feature known as flight termination system, which should be considered when carrying out scheduled maintenance activities.

Given the differences in UAS airborne components, ground support activities relating to maintenance incorporate standard pre-flight visual assessments and engine runs. Some pre-flight ground support activities that are vital in scheduled maintenance of UAS include verifications that connections are properly made during assembly and checking the movement of control actuators (Hobbs & Herwitz, 2008). The other activities include examining the deflection of control services, charging batteries, testing flight termination system activation, examining the aircraft’s weight and balance, and fueling the aircraft. The accurate measurement of fuel during UAS maintenance is essential because many small unmanned aircraft do not have fuel gauges.

While fueling the aircraft is increasingly critical, significant advancements have been made in the recent past in relation to battery performance. These advancements have contributed to the heavy reliance on batteries for UAS communication and navigation equipment, payloads, and propulsion systems. Therefore, battery charging has emerged as a critical component of UAS maintenance. Consequently, batteries are seemingly involved in most UAS mishaps for airborne and ground-based components. However, UAS maintenance activities should go beyond battery charging to incorporate careful attention to battery charging or discharging cycles. This is primarily because some types of batteries for unmanned aircraft contain lithium, which can be harmful if proper procedures are not adhered to. The failure to pay attention to charging or discharging cycles could result in a fire that may begin after a period of time rather than immediately.

As previously indicated, scheduled maintenance activities are carried out in three different locations i.e. in the field, workshop, or at specialist/manufacturer facility. Scheduled maintenance activities in the field include fuel mixing, assembly, adjustment, and calibration. The scheduled maintenance activities at the operator’s workshop include updating autopilot software, preventative maintenance, and replacement of spark plugs while scheduled engine overhaul is carried out at the manufacturer/specialist facility.

Unscheduled Maintenance

The second important category of UAS maintenance programs is unscheduled maintenance, which includes identifying damage and replacing/repairing components (Mrusek, Kiernan & Clark, 2018). As the name suggests, unscheduled maintenance activities are usually not planned and are carried as and when the need arises. These tasks usually depend on the nature and extent of the damage itself. Therefore, unscheduled maintenance is less predictable and carried out less frequently. Unlike scheduled maintenance, unscheduled maintenance is usually complex as it entails fault/failure identification and diagnosis. This process tends to be time-consuming, especially when the failure or damage involves avionics or computer systems (Hobbs & Herwitz, 2008). Given the complexities involved in unscheduled maintenance, its activities can generate greater mental demands on the technician. Therefore, UAS maintenance personnel should possess problem-solving skills and be ready to deal with unfamiliar situations.

While scheduled maintenance is planned maintenance, unscheduled maintenance is reactive since it is carried out once faults or damages are identified. The nature of UAS operations complicates efforts toward fault detection and diagnosis. UAS maintenance personnel also face challenges in this process of unscheduled maintenance since they do not have a clear distinction between the two types of maintenance. In most cases, UAS maintenance activities are unscheduled because of a lack of standardized maintenance procedures.

Unscheduled maintenance accounts for a high proportion of UAS maintenance activities because unmanned aircraft are exposed to different risk factors. Some of these risk factors include handling damage, exposure to weather events or environmental factors, mishaps, and impacts (Hobbs & Herwitz, 2006). The most common factor for unscheduled maintenance of UAS is hard landings or handling damage. Even though most unscheduled maintenance tasks are carried out in the operator’s workshop, some of them are handled by the manufacturer. Unscheduled maintenance activities like testing of autopilot or specialized engine repairs require the manufacturer to ship the UAS component to the manufacturer. The most common unscheduled maintenance tasks include troubleshooting and correcting failures, repairing electrical connectors, responding to the changes in payload, correcting fuel system issues, and responding to overheating avionics.

Similar to scheduled maintenance, unscheduled maintenance activities are carried out in the field, at the operator’s workshop, and at the manufacturer’s facility. Unscheduled maintenance tasks in the field include troubleshooting operational mistakes and minor repairs. On the contrary, unscheduled maintenance activities at the operator’s workshop include minor repairs and modifications. At the manufacturer/specialist facility, such activities include the repair of laptop hardware.

Recommendations for Maintenance and Reliability of UAS

One of the recommendations for maintaining reliability and maintenance of UAS is tailored maintenance training. Currently, scheduled and unscheduled maintenance activities for UAS are affected by various factors including the lack of qualified maintenance personnel. Tailored maintenance training should be carried out as part of the integration of scheduled maintenance programs with unscheduled maintenance activities. As evident in the previous discussion, UAS maintenance is classified into two major categories i.e. scheduled and unscheduled maintenance. The lack of proper training has contributed to difficulties by maintenance personnel to easily distinguish between scheduled and unscheduled maintenance. Through such training, UAS maintenance personnel will not only distinguish these two categories of maintenance but also possess relevant knowledge and skills to carry out these activities in a proper and effective manner. Tailored maintenance training for UAS should incorporate a wide range of skills relating to maintenance activities and their operations such as avionics. According to Wardrop (2021), tailored training would help overcome some of the challenges relating to scheduled and unscheduled UAS maintenance.