*3.1. Research Design*

This study utilized a mixed method approach, specifically an exploratory design, commencing with a qualitative content analysis to provide data for a quantitative ex-post facto study [23]. In this approach, the categorical data from the accidents with maintenance contributions were extracted, and then the narratives were coded further to generate additional categorical variables. Once the data was coded, it was then analyzed in an ex-post facto study, to analyze the distributions in comparison to all aviation accidents to assess if any observed di fferences were statistically significant.

### *3.2. Data Collection, Coding, and Cleaning*

The primary source of data for this work was the set of ICAO o fficial accidents [24]. This list of o fficial accidents includes those used in the ICAO annual safety reports and is made up of all safety occurrences that are accidents, in a scheduled commercial operation, and investigated by the relevant national authority. Entries in the ICAO o fficial accident dataset were cross referenced with the Aviation Safety Network (ASN) to provide narratives for the accidents with maintenance contributions. While the ASN database (provided by the Flight Safety Foundation [25]) includes maintenance as one of the contributing/causal factors (of which there are almost 100 cases), full text narrative searches of all entries in the ASN were undertaken. Advanced searches in both Google and Bing were conducted to the extent of the search engines, including omitted duplicates. The searches used criteria that limited the website searched (using "site:", to the ASN database) and the title of the results (using "intitle:", and "ASN Aircraft accident"). The additional search terms then used were "maintenance", "mechanic", "technician", electrician", "AME" (aircraft maintenance engineer), "LAME" (licensed aircraft maintenance engineer), "incorrect installation", "incorrectly installed", "inadequate inspection", "airworthiness directive", "service bulletin", and "inadequate maintenance". The general term "maintenance" was used last, as it returned the most results, and many of the cases were returned with the other more specific search terms, and hence if already selected were identifiable due to the hyperlink's color change.

Using the details and narratives from the ASN, the 35 ICAO official accidents were coded with:


The maintenance issues used by ASN to code these accidents include:


The ICAO data already includes the number and type of engines. The range of mass categories used in the ICAO official accident dataset was insufficient, so a lookup-table was created for all the aircraft (1277, removing duplicates), including the weight (MTOW) and manufacturer (manufacturer was coded as the current active company responsible for the type, e.g., the DHC-8 is coded as Bombardier). These codes were then added to the records for all the accidents (not just the maintenance accidents). Another lookup-table was created for all the three-letter country codes, to give continent and ICAO region. These were used to code the region of the accident and the region of the operator.

The final data set utilized the downloaded spreadsheet from ICAO [24]. Additional columns were added to this spreadsheet; including: MTOW, manufacturer, the operator's business model and the type of operation, the two regions (operator and occurrence), the phase of flight where the maintenance issue occurred, the date separated (day, month, and year), the year of first flight (and then the difference between that and year of accident giving the aircraft age), the identified maintenance issue (and associated system and sub-system), and the separated occurrence categories (downloaded as a single comma separated text string for each accident) giving a sample size of 60 due to the fact that accidents can be coded with multiple categories.
