You will have to shoot at small aperture as well, when shooting at such slow shutter speed.
I try to shoot 'prop' planes at 1/125 for decent prop blur; 1/250 will keep the props from looking like the plane's standing still but is usually not slow enough to give you 'good' prop blur.
It does take some practicing to shoot at such a low shutter speed and get sharp pics, but that's what makes air shows fun to shoot 
One last thing...you might want to shoot in manual mode, so you can set the shutter speed where you want it then adjust your aperture accordingly.
And, if it's a really bright/sunny day without clouds, I often use a polarizer filter/lens which cuts down on the bright light and allows for slower shutter speed and not having to shoot at higher than f/13 or so.
FWIW...
Jim K.