**1. Introduction**

Aircraft maintenance is an important part of the global aviation industry. It sometimes entails complicated tasks to be carried out by Aircraft Maintenance Engineers (AME) often with considerable time constraints [1]. In recent times, the aviation industry has experienced a rapid advancement in technology such as highly automated and integrated systems, which increases the burden on the AMEs to maintain both old and new fleets. AMEs also need to constantly improve their knowledge compared to the AMEs in previous times [1]. These advancements in technology have the tendency to introduce new types of maintenance errors, this is because hindsight cannot always be used to assess the system safety of new and complex designs. However, it should also be noted that other advancements such as fail-safe systems, enhanced hardware and in recent decades the use of health monitoring technologies on engines, systems and even structures have contributed to the reduction in maintenance and inspection workload [2].

### *1.1. Study Background, Accidents and Maintenance-Related Events*

The Nigerian Civil Aviation Regulations (Nig. CARs) [3] define commercial air transport and general aviation as: "an aircraft operation involving the public transportation of passengers, cargo or mail for remuneration or hire" and "an aircraft operation other than a commercial air transport operation or an aerial work operation", respectively.

Aviation activities in Nigeria increased post-independence in 1963 after the Federal governmen<sup>t</sup> of Nigeria obtained all Nigerian Airways shares [4] and the increase was especially observed in commercial air transport. In 2006, by the Civil Aviation Act, the Nigerian Government established Nigerian Civil Aviation Authority (NCAA) as an autonomous regulatory body and the Accident Investigation Bureau which has the sole responsibility of independently investigating accidents in Nigeria in accordance with International Civil Aviation Organisation (ICAO) Annex 13.

Nigeria has experienced various accidents and serious incidents over the years. The most recent fatal accident occurred when a Sikorsky S−76C + operated by Bristow Helicopters (Nigeria) Limited crashed at Oworonsoki area of Lagos in 2015 and unfortunately claimed all 12 souls on board [5]. Daramola [6] analysed all accidents that occurred in Nigeria from 1985 to 2010 using the Human Factors Analysis and Classification System (HFACS) which is a taxonomy framework widely used in aviation. Findings from Daramola's research showed that skill-based error, inadequate supervision and environment were the three most contributory factors.

While this study focuses on the analysis of accidents, serious incidents and occurrences that must be reported to the regulatory authority, e.g., "Mandatory Occurrence Reporting" in Nigeria, the previous studies focusing on European and global datasets enabled us to use the previously developed taxonomies based on the analysis of such data. Therefore, understanding the trends outside of Nigeria is also important. The European Aviation Safety Agency [7] Annual Safety Review (ASR) identified aircraft maintenance as a safety issue a ffecting Key Risk Areas (KRA). The KRAs a ffected are aircraft upset, runway excursion, and aircraft environment in commercial and non-commercial operations. Aircraft maintenance also had higher risk occurrence in comparison to other safety issues identified [7]. Analysis of 120 accidents and incidents which occurred from 2014 to 2018 revealed that aircraft maintenance contributed to 17 of them [7]. Unfortunately, maintenance errors were not considered in European Aviation Safety Agency (EASA)'s ASR prior to 2014.

The UK Air Accidents Investigation Branch (AAIB) includes aircraft maintenance and inspection as one of the safety recommendation topics in their ASR. The maintenance- and inspection-related recommendations are shown in Table 1.

**Table 1.** UK Air Accidents Investigation Branch (AAIB) maintenance- and inspection-related safety recommendations, adapted from AAIB [5,8,9].


Top findings from the International Air Transport Association [10] safety report for 2018 show that maintenance events were the sixth most significant threat by 13% contribution to the total number of accidents between 2014 and 2018 but they contributed to only 7% of the fatal accidents during the same period.

The same report also showed an interesting discrepancy about the contribution of maintenance events to total number of accidents between IOSA-registered operators (17%) and non-IOSA-registered operators (10%) revealed an interesting fact. This is rather contradictory to the di fference in overall safety performance between the IOSA-registered and non-IOSA-registered operators as the latter's performance is significantly poorer and this is used by International Air Transport Association (IATA) to promote the IATA Operational Safety Audit (IOSA) programme.

A total of 586 lives out of 4.3 billion passengers were lost in 2018, due to events in commercial air transport. This is in contrast to the 67 fatalities recorded out of 4.1 billion passengers in the year 2017—tagged the "safest year ever" [7]. In the general aviation category, EU-registered aircraft with Maximum Certificated Take-O ff Mass (MCTOM) above 2250 kg experienced events that saw the loss of 12, while EU-registered aircraft with MCTOM under 2250 kg experienced the loss of 159 lives [11]. Maintenance contribution to flight safety has been identified as one of the current and emerging safety risks by IATA in the 2018 annual review [12]. Details can be found in Appendix A.

In Africa, the average fleet age for di fferent operators is in the range of 6 to 28 years and Nigerian based airlines were found to operate the oldest aircraft in comparison to air operators based in Ethiopia, South Africa and Rwanda [13]. One of the key challenges the airlines operating aging fleet face is the additional maintenance tasks such as corrosion prevention and control tasks which aim to ensure the airworthiness of the aircraft [14].

African carriers are consistently banned by the European Commission partly due to inadequate authority oversight [15]. In spite of this, a review of various academic journals via Google Scholar, Scopus, Science Direct, Elsevier and Emerald found very limited literature presenting the analysis of aircraft accidents and incidents in Nigeria or Africa. Additionally, Table 2 shows the departures and accidents rate by the International Civil Aviation Organisation (ICAO) Regional Aviation Safety Group (RASG) region of occurrence.


**Table 2.** Departures and accidents rates by Regional Aviation Safety Group (RASG) region of occurrence. Adapted from International Civil Aviation Organisation (ICAO) (2018).

The data for the African continent are in bold to highlight higher than average accident rate, in spite of the lower estimated departures compared to other regions.
