The lack of transition training has been cited as a causal factor in many GA accidents. Accidents frequently result from pilots being unprepared for challenges presented by the new, or different, aircraft they are flying. Even when pilots are legally certificated to operate aircraft within a specific category and class, significant differences can exist among different types of aircraft within that category and class — thus necessitating the need for effective transition training.

Fact Sheet: https://www.faa.gov/news/safety_briefing/2018/media/SE_Topic_18-06.pdf