Every 2 1/2 to 3 years, specifically every 2,000 flight hours, regulations require that both wings of a DC-3 be removed for a major inspection. The purpose of the inspection is to look at all of the attach angles, stringers, ribs, and the complete structure inside and outside of the wing. If any problems are found, such as corrosion or damage, it is repaired and brought back up to airworthy standards.
One of MFI's turbine DC-3s, N500MF, is currently down for a Wing Pull inspection. Below is a picture showing the left wing detached from the airplane with mechanics doing a preliminary inspection. The airplane will be out of service for 2 to 4 weeks while the inspection and repairs are being completed.