The U.S. Air Force grounded over 500 T-38C jet trainers for five days, after two of them crashed in an eight day period. The air force hustled to find out if there was a connection between the two crashes, and if there was any kind of fundamental flaw in the aircraft. There wasn't.
The T-38 has been in use as a trainer for 47 years, and has a good safety records. Existing T-38s are undergoing structural and electronics upgrades, to keep them going for another twelve years. Production of some 1,200 T-38s ended in 1972, so all existing models are over 36 years old. The five ton aircraft is actually a variant of the F-5 fighter aircraft (a few of these are still in service). Several other nations use the T-38, but many newer trainer designs are beginning bought as replacements.