While using the ST1500SF the trimmer head flew off. I had not noticed it being loose at any point. I had just loaded the string in the head and I would have thought I would have noticed if it was loose. I cannot find the flange, it must be lost in the grass somewhere. I called about this and I am being told I need to BUY the part. Shouldn't this be under the warranty? Is this a design flaw? I am concerned that this maybe on ongoing issue since I am seeing others are having the same issue. I don't want to get hit by it if it fly's off again. Maybe there should be a nylon type lock nut or something? I just don't understand why the unit fails and I am paying for parts. Just what does the warranty cover?