Personally speaking 250mm will often times be too short for a lot of shows. I say this as I find 300mm is mostly to short too. you will be OK for formations or larger aircraft, but most of the fast jets and classic fighter types as well as anything aerobatic will mostly just end up as small dots. I would really suggest that if you can rent either the Sigma 50 or 150-500mm OS or the 100-400 L IS lenses. These are not lnown as the best airshow lenses for nothing. I have been struggling with the Sigma 28-300 and an old T mount 500mm mirror lens, but took the hire option for the last show I went to, and took a 100-400. The difference was amazing, there really is no substitute for focal length, and at times even the 400mm was a bit short, although at least when the aircraft were directly opposite me I was able to get a Spitfire to pretty much fill 2/3 of the frame.
For props really 1/160 or less is where you need to be for decent prop blur, or 1/80 for helicopters. otherwise as fast as you can get within reason, but at least 1/FL if you would otherwise be having to up the ISO excessively. Back button focus is also a good option.
Most airshow's have fairly steady light, and the aircraft are usually in the same light as you are. I would suggest shooting in manual exposure mode, by taking a partial or spot reading from some grass. Nice green grass is usually a decent mid tone for exposure, also shoot RAW and ETTR if you can, although the sky may look pretty blown out in the preview it can be pulled back quite well especially in the new LR4 or ACR version that is now available. Also remember that if you are looking mostly at the bottom of the aircraft then it will need a bit more exposure to bring out the details (meter about 2/3 to 1 1/3 over on the grass for this), but you need to do this without blowing the detail on the top of the aircraft, where you can see both halves. In that situation better to let the sky go a bit, as there are ways of fixing it in post that will look better than the noise you will get from boosting something that was way under exposed. It is a dynamic range problem which there is no getting around.
If you really are not happy with shooting in manual mode then use Av, pick what you think will be a suitable ISO, and then set the aperture so that you have the shutter speed in roughly the right ball park. With long telephotos you are going to be working with minimum of f5.6 or even 6.3 on some of the cheaper lenses. If you shoot Tv mode there is often the possibility that you will set a shutter speed that would need a wider aperture than is available on your lens, and you get an under exposed shot. With Av you are unlikely to get to 30s exposure. You might even get way with a relatively slow shutter speed and if it is a prop plane a really nice full disk.
As well as you r 18-55 take your flash as well in case you need some fill light, you never know who or what you might want to photograph.
Alan

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