Obviously inspired by the current 737 Max situation, what is the longest period of time that any individual type of airliner has been grounded for safety concerns (excluding those that never returned to service)?
After the crash of Air France 4590 (July 2000), the Concorde was grounded until November 2001 (same link) - 16 months.
Space Shuttle was grounded after Columbia crash from 2003 February till July 4, 2006 with only single test mission over this time in 2005.
Columbia crashed while flying at Mach 19.5 (23,278 Km/hr; 14,464 Mph), at 209,800 feet altitude (63.9 km; 39.73 mi). It is probably possible to argue that at the time of crash it was flying as an aircraft (glider), interacting heavily with the air, and it disintegrated because of aerodynamic forces.