The testtng and valadatzon of formal speczficatzons of hzgh-assurance real-tzme systems repiizres a n ertenszble szniulatzon enrwonment wzth support for users to generate events, customzze dzsplays and monztor cwnt traces Thzs paper descrzbes MTSzm an customzzable szmulation platform for the Modechart Toolset ( M T ) MTSzm prozlzdes support for "pluggzng zn" user-defined vzeu'ers as well as user partactpataon i n the generatzon of szmiilatzoiis by allowtng users t o znject events znto fhe erecutton trace Moreover, MTSzm provzdes rnonitorzng and assertion checkzng of eaecutzon traces and the zntiocatzon of user-speczfied handlers upon assertzon vzolatzon Thzs paper also zntroduces a MTSzm component called WebSzm. a suzte of simulation iools for MT. nnd n n applzcntzon-speczfic component of MTSzm, whzch dzsplays fhe cockpzt of an F-I8 azrcraft and models zts bomb release functzon