Why do multiple aircraft models have the same type rating?
If additional models are certified based on the original FAA type certificate, then they all have the same type rating. To qualify for the same type rating, the aircraft must have a majority of systems common to all models. If there are too many differences, then the FAA determines that a new type rating must be obtained. This also means that a type-rated pilot may serve as pilot-in-command of any model covered by that type rating.
