Anyone know a good mechanic? The Pentagon grounded the U.S. military’s entire fleet of F-35 Joint Strike Fighters after discovering a crack in the engine of one earlier this week. Though it’s considered the Pentagon’s main future fighter, this is the fourth full or partial stand-down of F-35s in just two and a half years. There are around 100 F-35s in three models. Because the three models share an engine design, a crack in one could signal a problem with all the others.