UAS Reliability and Maintenance: Approaches and Best Practices

Overview of Unmanned Aircraft Systems

The idea of conventional aircraft design has changed dramatically over the past few decades. The introduction of unmanned aircraft systems (UAS) reflects the evolution of the overall architecture of aircraft design. Many modern aircraft, however, retain components and systems similar to those found in conventional models. Since their introduction, unmanned aircraft systems have attracted growing consumer interest in terms of ownership and operation. Wicker et al. (2019) note that the emergence of UAS has the potential to generate significant economic and social benefits to the United States. Despite these potential benefits, UAS has attracted concerns regarding safety and reliability — concerns partly attributable to the fact that their manufacture does not conform to a type design (Ley, 2016).

Lu et al. (2019) state that UAS is an application representing independent technologies in the aviation industry. This system is composed of the unmanned aerial vehicle (UAV), data links, a ground control station, and a recovery and launch system. Also known as an unmanned aerial system, UAS refers to a system whose components do not carry a human operator (Gupta, Ghonge & Jawandhiya, 2013). UAS are either remotely piloted or fly independently, meaning they have an associated ground control station and a data link between the airborne vehicle and the ground. As a result, UAS is characterized by command, communications, and control. While UAS air vehicles and their associated equipment carry no human operator, their operations require personnel for remote piloting or autonomous flight.

UAS comprises three major components: the unmanned aircraft, the data link or command-and-control link, and the ground control station (Hobbs & Herwitz, 2006). The unmanned aircraft is essentially a powered vehicle without a human operator — one that can be operated remotely by a pilot at a ground control station or fly independently based on pre-programmed flight plans (Gupta, Ghonge & Jawandhiya, 2013). UAS can be recoverable or expendable and may carry lethal or non-lethal payloads. However, cruise missiles, unattended sensors, ballistic or semi-ballistic vehicles, torpedoes, satellites, artillery projectiles, and mines are not classified as UAS or UAVs.

Background and Growth of UAS Applications

In the early years of development, UAS was adopted by military planners to conduct surveillance and attack missions. The history of UAS can be traced to 1916, when the first unmanned air vehicle (UAV) was developed by the Americans Lawrence and Sperry (Gupta, Ghonge & Jawandhiya, 2013). This development marked the beginning of attitude control, which played a critical role in the automatic steering of aircraft. Lawrence and Sperry named the first UAV the "aviation torpedo" and flew it a distance exceeding 30 miles. The end of the 1950s marked a significant period in UAS history, as full-scale research and development of UAVs was carried out through the 1970s — a period heavily influenced by the Vietnam War. Afterward, the United States and Israel began developing smaller, cheaper UAVs resembling small aircraft with engines similar to those used in snowmobiles or motorcycles. Equipped with video cameras that transmitted images to a ground operator, these vehicles can be regarded as the prototypes of the current UAS.

According to Lum & Tsukada (2016), UAS technology began to grow beyond the military domain in recent decades, fueled by increased interest in commercial and civilian applications. The commercial UAS industry has been characterized by the exponential growth of small unmanned aircraft, which have attracted interest because of their potential to perform tasks that would formerly have required larger aircraft (Hobbs & Herwitz, 2006). The improved capabilities of small unmanned aircraft are attributable to technological advancements such as the miniaturization of sensor equipment and autopilots, as well as advances in battery technology. Additionally, small unmanned aircraft are often based on cheaper hobby-store model aircraft that sometimes incorporate an autopilot.

The commercial UAS industry has witnessed rapid growth because small unmanned aircraft have numerous potential uses across different sectors. These uses include traffic monitoring, search and rescue, homeland security, power-line inspection, border surveillance, agriculture, policing and firefighting, aerial photography, wildlife monitoring, and mineral exploration. UAS are also used in sports event film coverage, university research, pipeline surveys, and communications relay (Gupta, Ghonge & Jawandhiya, 2013). UAS utilization has expanded into more civil domains because of its convenience and low cost (Lu et al., 2019), resulting in civil UAS outnumbering military UAVs substantially — with estimates in the several millions annually. The growth of non-military UAS applications is projected to accelerate once airspace regulations are established (International Civil Aviation Organization, 2011), with the civilian and commercial UAS market projected to grow to up to $7.5 billion in the near future.

UAS Reliability: Assessment Techniques and Challenges

Petritoli, Leccese & Ciani (2018) contend that reliability is a dynamic concept applicable in many disciplines, both technical and non-technical. Reliability is defined as the probability that a subsystem, system, or part can carry out its specific functions within a pre-determined time period and under pre-established conditions (Petritoli, Leccese & Ciani, 2018). Over the past few years, the significance of reliability has grown considerably in UAS applications and operations. As part of enhancing UAV reliability, engines have become more robust and avionics have been improved. However, current reliability approaches are still regarded as fatalistic: the overall rate of UAS failure stands at approximately 25% (Petritoli, Leccese & Ciani, 2018). Freeman (2014) notes that current low-cost UAS remain unreliable, underscoring the need for improvement.

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 for reducing repair and maintenance costs and avoiding all categories of failure — severe, catastrophic, moderate, and soft. Lum & Tsukada (2016) note that improving the reliability of UAS applications enables regulators and operators to understand how to lower failure rates over time, creating an ongoing need for UAS personnel to develop effective reliability maintenance approaches.

Existing literature demonstrates that there is no one-size-fits-all approach to reliability assessment and improvement of UAS (Freeman, 2014). Reliability assessment techniques used for UAS are broadly classified into two categories: inductive and deductive approaches. Inductive, or bottom-up, reliability analysis focuses on identifying the failure modes of system components at the lowest level possible. The effects of these failures on higher-level subsystems are then determined and traced forward repeatedly until all probable failure modes are identified. In contrast, 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. This process is repeated until the root causes of failure incidents are traced down to the lowest-level components (Freeman, 2014). Deductive reliability analysis is advantageous because it focuses on one or more unwanted events and their probable causes.

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

Martinetti, Schakel & van Dongen (2018) note that a UAS maintenance strategy should be customized to the system's technical characteristics, with each individual component and its operational conditions taken into account. Hobbs & Herwitz (2006) further contend that UAS maintenance personnel must ensure that reliability coverage extends across the entire system — including the unmanned aircraft vehicle, communication equipment, and ground control station.

Mrusek, Kiernan & Clark (2018) note that the introduction of UAS into the NAS has generated several challenges in maintaining airworthiness. Ley (2016) identifies the lack of civil regulatory requirements as a major challenge: the unmanned aircraft sector has no civil legislative requirements compelling manufacturers to design, build, and support their aircraft to a particular standard of testing and approval. As a result, most existing UAS do not conform to a type design or meet continued-airworthiness requirements under civil regulation. The lack of such requirements and a standardized type design has contributed to numerous challenges in UAS maintenance practices. Currently, the availability and quality of maintenance procedures, standards, and technical documentation to support UAS remains largely unknown (Ley, 2016).

In an earlier study, Hobbs & Herwitz (2006) contend that UAS maintenance is currently unregulated and performed by professionals who lack formal maintenance qualifications. This means there is no proper oversight or defined standards for UAS maintenance, increasing the risk of operational and structural failures and placing both air safety and personnel safety at significant risk. Furthermore, manned aircraft maintenance practices cannot always be applied to UAS maintenance.

One of the major impediments to UAS maintenance is the lack of dedicated and qualified maintenance personnel. Most UAS operations do not have designated maintenance staff; instead, operational tasks are carried out by a small team of multi-skilled professionals who handle the full range of tasks needed to prepare the unmanned aircraft for flight. While some UAS operators have individuals who perform maintenance tasks, these individuals are not necessarily trained maintenance technicians. Some small UAS manufacturers offer maintenance training courses, but most of the people who carry out these activities have no formal preparation for their duties. Their skills span a wide range — engineering, radio-control aircraft operations, and electronics — but maintenance is not their specialty. Much of the knowledge required for UAS maintenance lies in the avionics field, and maintenance personnel also need skills related to preventive servicing. This diversity of required expertise contributes to ongoing regulatory challenges.

Conclusion

UAS refers to a system whose components do not carry a human operator and are piloted remotely or fly independently. The system was initially developed for military applications but has grown to become part of civilian and commercial domains. The increased adoption of UAS has generated numerous concerns regarding airworthiness, reliability, safety, and maintenance — concerns attributable to the fundamental differences between UAS and conventional manned aircraft. The reliability and safety of UAS are threatened by various risk factors, including human error, environmental factors, and system failure. UAS maintenance is also characterized by challenges such as the lack of specific civil regulatory requirements and the absence of qualified maintenance personnel.

As a result, a robust strategy for enhancing UAS reliability and maintenance is required to improve airworthiness and safety. The recommended approaches include tailored maintenance training, redesigning the roles of the UAS operator and maintenance personnel, adopting preventive maintenance approaches, and implementing fault tree analysis as the primary deductive reliability assessment technique. Together, these measures provide a comprehensive framework for addressing the reliability and maintenance challenges facing UAS as they become increasingly integrated into the National Airspace System.

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