Basic zone won't be sufficient for aviation action shots. You should really be using either M, TV
(depending on experience/situation). Using TV
(shutter priority) isn't as hard as you may think. P will work sometimes but won't produce optimal results under all conditions.
First make sure that in the menu, your quality is set to the top large setting (we'll ignore RAW for now). If your quality isn't set right in the first place then you are up against it even more. That setting is the 6 megapixel fine JPEG one.
Are you shooting with the sun behind you illuminating the aircraft? There are many factors like this that can affect the picture.
Turn the mode to TV
and use the dial on the top of the camera to change the shutter speed. Watch the LCD
display on the back, or the green LED
numbers through the viewfinder. It will show numbers like 10,20,100,200 etc. So if it said 100, that means a shutter speed of 1/100 of a second, 10 will mean 1/10 of a second. You will see the number next to it changing as you change shutter speed. This is the aperture and it's being automatically chosen by the camera according to your shutter speed (because you are in shutter priority mode - the reverse would happen in aperture priority mode (AV))
If the lowest aperture value blinks (depends on your lens as to what it is), it means you need to reduce your shutter speed to correctly expose. If the largest aperture value blinks, you need to increase your shutter speed to correctly expose.
I can't tell you what aperture and shutter speed suit every situation, but for action photography (assuming you want to totally freeze your target), you would need a relatively fast speed. Try something like 1/500 (for arguments sake - it can be lower or higher depending on what you are trying to achieve and the conditions at the time of shooting.....I'm not trying to say that's what you always need to use) and point at the object you're trying to take a picture of and see what the aperture displays as. You should try to avoid very low numbers in full light shots because you'll lose depth of field (not everything will be focused). An aperture around f7-f8 is usually quite a nice spot. The smaller the f number the less will be in focus across the whole shot.
If all that is too much for you, go back to mode P, where the camera will automatically choose both for you, but let you change exposure compensation and white balance and ISO etc. If it's daylight, make sure ISO is set to 100 for the best quality of picture. If the picture is coming out dark, increase the exposure compensation a few stops. Also consider why it's coming out dark.....the 300D does have a tendency to under-exposure slightly, but I wouldn't call it dark. Try to ensure the sun is behind you.
In short, your problem is that you will never get a decent action image of an aircraft in a basic zone - even on a DSLR. Just keep practising and you'll get the hang of it eventually. But please dump the basic zones and try out P at least, and also TV
I'm no expert, but I hope this might help a little