an automatically deployable step apparatus particularly usable with vehicle such as a fire truck having a passenger compartment at substantial distance from the surrounding environmental surface which provides powered movement between a deployed position and a retracted position and can be mounted in a relatively small restricted area in order to avoid interference with a truck cab which tilted to provide engine access , said apparatus utilizing a unique over - center locking mechanism to provide secure locking of the step in the deployed position while maintaining an apparatus having limited overall dimensions . a unique linkage system is provided for facilitating accurately powered automated movement .