The empennage of all TriStars as delivered was the same. There was a retrofit larger rudder for the -500 that some operators had to overcome the Vmcg issues on shorter runways. That issue could be worked around in other ways and to some extent was dependant on the state of registry. The UK CAA demanded a more stringent set of certification conditions for Vmcg than the FAA, and this led to a higher Vmcg on UK registered -500s.