*4.4. Level 3—Maintenance Errors*

All 11 accident reports were further coded to identify the top-level system/componen<sup>t</sup> affected and the codes can be seen in Table 8.


**Table 8.** Level 3 Maintenance Errors.

Inadequate and incorrect maintenance were not identified as causal or contributory factors in any of the accidents analysed. However, research has highlighted that these types of omission and commission errors are common in aviation maintenance [19,48]. The absence of these errors may be due to inadequate information provided to the AIB by the personnel involved. Another plausible reason may be due to the difference in phraseology used in taxonomies.

The results show a high presence of Failure to follow procedures which can be classified under "AMM" and "Airworthiness Directive". Failure to follow procedure is a maintenance issue as well as an organisational issue. This is because every team member, including managers who mount pressure on AMEs, are involved in the maintenance chain [49]. This is also in agreemen<sup>t</sup> with organisational factors being one of the maintenance contributory factors. This is a growing concern in the aviation maintenance area.

There was a surprisingly low number of human-factor-related errors in the accident events. During the categorisation, no assumptions were made, hence where there were no human factors mentioned, it was not categorised. According to Sarter and Alexander, human error contributed to 70% of major aviation accidents [50]. The analysis highlights discrepancies in limited human factors considerations during investigations.

Oversight by operators and authorities was identified as the highest causal/contributory factor to maintenance-related events. Research by Dhillon showed that operators and regulator's oversight led to some fatal maintenance-related accidents [51]. Oversight should be a critical aspect of aviation as it aids all personnel involved to obtain second views and opinions.

### *4.5. Analysis of Mandatory Occurrence Report by Using Hieminga's Taxonomy*

All the incidents in the SDIA MOR database were coded in accordance with Hieminga maintenance incidents taxonomy. The results show that a total of 588 incidents were related to maintenance error. This is about 23% of all incidents which occurred from 2006 to 2019. The breakdown of total number of MORs, total number of maintenance-related MORs and the number of most frequent Level 1 category events is shown in Figure 3. There are several findings which are related to the scope of this study.

**Figure 3.** Analysis of maintenance-related MORs (Mandatory Occurrence Reports) between 2006 and 2019.

Firstly, the total number of MORs fluctuates and does not follow a pattern. When such statistics are presented, one important consideration is to normalise the numbers by presenting them as rate of occurrences based on traffic numbers. Nevertheless, it is not always possible to determine the cause of certain data points such as 2014 being the lowest number of mandatory occurrence reports the Nigerian CAA has received.

Secondly, the rate of the total number of maintenance-related MORs has increased in recent years. Particularly, as shown in Table 9, the maintenance-related MORs were more than 40% of the total number of MORs within the last three years the data set covered. While such statistics may be concerned, it is important to understand the further analysis of the data and identify the causal and contributory factors.


**Table 9.** Analysis of Mandatory Occurrence Reports (2006–2019).

Thirdly, one clear finding from this analysis was that "removal/installation issues" have been the most frequent event category. For example, out of 588 maintenance-related MORs in total, 259 of these events were caused by "removal/installation issues". Furthermore, this category appeared as the "most frequent" event within the last 8 years and ten times within the 14 years the dataset covered. This is

certainly in alignment with several other studies including the ones which led to the development of two taxonomies used in this study [34].

The "removal/installation issues" category was followed by working practices which contained most errors related to accumulation of dirt and contamination. This may be caused by the dry season weather conditions in Nigeria. Studies have shown that dust would contribute to aviation safety through corrosion, blockage of Pitot-static tube, etc. [52]. Inspection and testing issues also had a high contribution to maintenance-related incidents, followed by job close up, lubricating and servicing. This correlates with previous studies carried out by Latorella and Prabhu [53].

### *4.6. Results of the SME Survey*

Information was gathered from the responses to the questionnaires. Themes were identified and nodes created to extract the most important elements of the survey. The information identified from three different groups is written below along with brief discussions on them.
