An aeroplane flying along the horizontal at a velocity `v_(0)` starts to ascend, describing a circle in the vertical plane. The velocity of the plane changes with height h above the initial level of motion according to the law `v^(2) = v_(0)^(2) - 2a_(0)h`. The velcity of the plane at the upper point of the trajectory is `v_(1) = v_(0)//2`. Determine the acceleration a of the plane at the moment when its velocity is directed vertically upwards.