4
The Yak-42 was grounded for roughly 2 years due to stabiliser malfunction.
The de Havilland Comet was grounded for OVER 4 Years (Aug 1954 - Sept 1958)
because of blown out windows, causing decompression and in-flight disintegration.
3
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), ...
2
The Space Shuttle was grounded for 36 months following the Challenger disaster and again for 29 months after the loss of Columbia.
Whether that qualifies as an aircraft or as a falling brick is debatable, though.
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