Tenor
The reason I mentioned shutter priority was so as to override the cameras desire to give compromise settings when on auto. If you are trying to get sharp images with telephoto lenses you need high shutter speeds unless using a tripod. an aperture of f20 (small hole) will give a good depth of field (bigger area in focus) but if you have a shutter speed of 160 sec you will get camera shake. Aim for about 500th sec and f8 that will give a good balance between aperture and shutter speed.
Also a very important thing to consider when shooting aircraft is the light reading. If you are pointing at an aircraft in the sky there will be a lot of the image that will consist of bright sky, the aircraft will be darker against the sky and if the camera works out exposure for mainly sky, the aircraft will be underexposed and darker in the resultant picture. To get around this use the exposure compensation dial +1 should be sufficient. Inversely if the background is dark such as storm clouds and the aircraft brightly lit by sunshine you will have to do the opposite and set the compensation to -1 or the aircraft will be overexposed.