Airline maintenance outsourcing is a common practice in the deregulation era of airline industry, and it mainly covers topics across technology, economics, and politics. This dissertation used an explanatory sequential mixed methods research to explore the effect of airline maintenance outsource on aviation safety. The first stage of the research was a quantitative research using a panel data analysis using five models to explore the statistical relationships between the independent variables: amount of outsourced maintenance, airline profitability, and real gross domestic product per capita, and dependent variables: aircraft accident rate and aircraft incident rate for the major U.S. Part 121 passenger air carriers between 1995 and 2019. The second qualitative research was interview with commercial aircraft maintenance professionals, airline manager, civil aviation regulators, and other key stakeholders to seek their interpretation about the first stage research results, opinions and understandings about the current commercial aircraft maintenance practice, and their expectations of the industry. Both stages of research confirmed that airline maintenance outsourcing does not affect aviation safety, and there is a positive relationship between airline financial performance and aviation safety performance. Consequently, airline maintenance outsourcing is not only economically sensible but also conducive to aviation safety if it is done properly. In the second stage research, the researcher found deficiencies in the current oversight system mainly due to lack of funding and personnel which needs to be addressed. The researcher recommended activity-based accounting to solve the funding issues about airline maintenance oversight and the future study can focus on decision-making process for airline maintenance outsourcing/insourcing based on empirical data.