On departure from Belfast, when the aircraft was ready to move off from the stand, the ground engineer disconnected the GPU power cable and collected the wheel chocks which he stowed on the GPU. He then got into the tractor used to tow the GPU and drove clear. Once positioned clear of the aircraft he signaled this to the crew and then drove a short distance further on to the terminal building. However, at arrival at the terminal he noticed that the GPU was not attached to the tractor. Meanwhile the aircraft had received taxi clearance and had started to move forward. Very shortly after this the right hand propeller struck the GPU which was still in position on the right side of the aircraft. It was subsequently determined that the tractor, which had originally positioned the GPU, had then been disconnected from it for use elsewhere before being put back again prior to the aircraft departure. The GPU towbar had not been reconnected to the tractor and this had not been noticed by the driver.On departure from Belfast, when the aircraft was ready to move off from the stand, the ground engineer disconnected the GPU power cable and collected the wheel chocks which he stowed on the GPU. He then got into the tractor used to tow the GPU and drove clear. Once positioned clear of the aircraft he signaled this to the crew and then drove a short distance further on to the terminal building. However, at arrival at the terminal he noticed that the GPU was not attached to the tractor. Meanwhile the aircraft had received taxi clearance and had started to move forward. Very shortly after this the right hand propeller struck the GPU which was still in position on the right side of the aircraft. It was subsequently determined that the tractor, which had originally positioned the GPU, had then been disconnected from it for use elsewhere before being put back again prior to the aircraft departure. The GPU towbar had not been reconnected to the tractor and this had not been noticed by the driver.