A ‘C’ check is a maintenance check usually performed on transport class aircraft.  It is usually performed every 20-24 months or after an amount of flight hours specified by the manufacturer.  It usually requires a large majority of the aircraft systems and components to be inspected for serviceability. The check can require an extensive tooling, test equipment, and special training.  A ‘C’ check will usually ground the aircraft for 10-15 days or more depending on the age and condition of the aircraft.  Other modifications or enhancements such as cabin or interior upgrades may be performed during a C check as well.