The invention relates to a method for determining the key crash phases for triggering a passive safety device in a vehicle. According to the invention method, an acceleration signal (ax) representing an acceleration (ax(t)) in the longitudinal direction of the vehicle is determined, and two window integrals are simultaneously set up at the beginning of the crash-related deceleration of the vehicle, by which means the deceleration signal (ax) is added or integrated over a pre-determined time window. The first time window is smaller/equal to the minimum triggering time for triggering the safety device, and the second time window is significantly larger than the first window. The period during which the window integrals have the same value and during which said value is higher than a threshold value is defined as the first crash plane.