These periodic inspection rules vary based on the aircraft type, but all craft, from small private and business jets to military and commercial, must abide by them. Informally called “checks,” the inspections follow schedules based on flight hours and cycles—that is, specific numbers of takeoffs and landings—or prescribed calendar periods if those arrive first. A-checks and B-checks are the lighter inspections, typically executed overnight and involving visual examinations of engines, landing gear, control surfaces, and other key systems as well as routine maintenance, such as fluid checks and examination of safety systems, like evacuation slides. C-checks are more involved. These require taking the plane out of service for a few weeks roughly every two years. D-checks are major events, occurring approximately every decade of active use, that involve taking apart most components, from the engines to the electronics to the landing gear and even the interiors, for a more thorough, deep-tissue examination. These checks can last several months and cost millions of dollars, including parts replacement and assorted heavy-maintenance steps.