Automatic activation device (AAD) in skydiving terminology refers to an electronic-pyrotechnic or mechanical device that automatically opens the main or reserve parachute container at a preset altitude or after a preset time. AADs are typically used to open the reserve parachute container at a preset altitude if the descent rate exceeds a preset activation speed. This indicates that the user has not opened his or her parachute, or that the parachute is malfunctioning and is not slowing the descent rate sufficiently. The older style mechanical AADs are falling out of fashion in favour of the newer style electronic-pyrotechnic models. The built-in computers in the newer models can make much better estimates of the altitude and the vertical speed, therefore making the units more reliable than the older types.