Abstract: A method and apparatus for determining a change in velocity of a body following a power disruptive event. An accelerometer (10, 50) includes quartz crystals (16 and 18, 60 and 62), which produce output signals indicative of the acceleration to which a body connected to the accelerometer is subjected. The acceleration measured is directed along the sensitive axis of the accelerometer. The quartz crystals are selected to have different scale factors, K.sub.1 and K.sub.2, which define the change in frequency of the quartz crystal from its no-load resonant frequency as a force is applied to it. The quartz crystals are connected between a supporting case (14) and a proof mass (12) so that a given acceleration applied to the proof mass along its sensitive axis causes one of the crystals to experience a tension force and the other to experience a compression force.