The MU-2 has a unique history.
As a matter of general regulations, FAA requires a “type rating” to fly any airplane over 12,500 lbs. That is aircraft-specific training and a checkride and an endorsement on your license. When this reg came into effect eons ago (1950s?), the idea was that this weight cut-off was a decent dividing line between airplanes simple & slow enough that any pilot could jump into it and fly it safely, versus ones where there was so much complexity and performance that you needed specific schooling on it to be safe.
Of course since then several things have changed. Manufacturers got real good at building 12,400lb airplanes to avoid their model being saddled with extra expense for the operators.
Also, the overall complexity of high end GA airplanes and especially avionics has skyrocketed as has the performance. Mostly due to the advent of the larger ICE engines and especially the advent of turboprops.
And last of all, starting in maybe the 1970s insurance companies became very interested in pilot proficiency. The FAA may allow any yokel who has flown 1 kind of light twin just few hours to jump cold turkey into a different manufacturer’s medium twin & go for a flight. But the medium twin’s insurance company will nullify the insurance unless the would-be pilot attends some airplane specific school. Not to the level of a formal type rating, which cannot exist without FAA involvement, but most airplane owners will balk at letting some yokel take their now-uninsured airplane for a spin. Manufacturers of fancy airplanes also got into the act, insisting that buyers of their new planes take specific training before being allowed to take possession.
The MU-2 fell into a crack in that evolution. It weighed less than 12,500 lbs, so no type rating. But it was hot and it was unusual and it came out before the insurance companies and manufacturers had twigged to it being in their own interests to ensure only well-trained people ran risks with their stuff.
So after enough MU-2 accidents, the FAA came up with a special FAR amendment just for MU-2s. Which mandated a type rating to fly them, despite the fact they weighed lass than the magic 12,500. The MU-2 has the distinction of being the only such airplane.
Certainly many other complicated and performant high-end GA airplanes below 12,500 lbs have been invented since. But they all came out after the insurance companies & manufacturers got wise. So they enforce the quality of the pilots & training. So the FAA doesn’t have to. Which suits FAA just fine.
