The Southwest Airlines Maintenance and Engineering (M&E) operation runs on a 24-hour schedule. Every few days, the overnight M&E Employees undertake the exciting task of changing an engine. Southwest's engine changes take place at the Maintenance Hangars at DAL, HOU, PHX, MDW, and ATL. However, AMTs must occasionally travel to other Southwest locations to perform onsite engine changes as well. During an engine change, the engine is removed entirely from the aircraft and replaced with another engine. The process can take as few as four and as many as seven hours to complete. Teamwork is crucial; everyone has a job. Like most tasks that AMTs perform, an engine change is an extremely controlled procedure that requires not only skill, but a strict compliance with complex federal regulation. In the past year alone, Southwest AMTs performed 154 engine changes—excluding aircraft retirements.