If anything, the first image is overexposed, so the original metering was wrong. I suppose the best way to remedy the problem is to have it on single shooting, and focus individually for each shot, which would also provide an accurate exposure reading for every shot taken.
I took these from the same position (pretty much), in similar lighting, and the single shot method seemed to work for me fine.
View attachment 59297View attachment 59298View attachment 59299
Another way to solve the problem, even if it's not the most ideal solution, is if you're using a DSLR, shoot in RAW mode, and alter the exposure afterwards. Those shots, especially the underexposed one, don't seem to be out of the realms of simply altering the exposure level to bring it up to standard.
Edit: upon looking twice, I see your shots contain different airframes, therefore not from the same sequence of shots. Would it be possible to provide EXIF data, including the shooting mode (AV, TV, program, etc)? Would perhaps be able to see exactly what's wrong, then.
I adjusted the contrast on the first one a little too much I think, my camera has a tendency to wash out images a little and they always require a little extra contrast and some sharpening in PSP afterwards.
All of these are crops and resized for SoH, I'll post the exact images on my site with some links shortly.
I'll be honest, my camera is pretty crummy when it comes to shots like this, it really is pushing it to the edge of its limits, most spotters laugh when I get it out ( camera that is ! ) but in the right light and with a little luck the old girl can turn a few blinders in now and again, not bad for a pocket point and shoot
http://www.dpreview.com/reviews/canona620/ I also have the newer A640 10Mp but it seems to have lost something over this little rugged predecessor.
Most were shot with +1 exposure rating as I like to see some detail underneath and all of these were shot with aperture priority of F4 to hopefully give a better depth of field and negate some focus hunting.
The ambient light seems to have a dramatic effect, even though there are clouds, if your under one then your images are generally poor, if your in sunlight then there generally good.
When it gets it right it gets it right

. I need more arrivals and chance to try shutter priority next time and then full auto, I've found that setting down from the largest pixel format really does increase sharpness, all my previous at Mildenhall were at 3072x2074, most of these were 2592x1944.
I choose the larger size so I can crop as the preview screen goes blank when the picture is taken for a brief moment !! and thus pan tracking is impossible so you have to use the optical sight which has a parallax error so you mentally need to offset all the time....I really do miss all of my old 35mm SLR stuff, would have made mince meat of these shots, keep looking at a new Eos with a 200mm lens, which should cover all shots I want, though not easy to stick in your pocket and smuggle about work up cranes LOL.
I checked through the image data and whilst aperture remains constant it is adjusting the shutter speed on the fly so it is trying to compensate for each image, I do just think its the background sky and patchy cloud that makes or breaks them, sadly I do like 40/60 cloud cover as a back ground, gives it a bit more feel to me.
Heres the links to unedited ones
http://homepage.ntlworld.com/mickoo/Images_Planes/IMG_1731.jpg
http://homepage.ntlworld.com/mickoo/Images_Planes/IMG_1739.jpg
http://homepage.ntlworld.com/mickoo/Images_Planes/IMG_1815.jpg
http://homepage.ntlworld.com/mickoo/Images_Planes/IMG_1826.jpg
http://homepage.ntlworld.com/mickoo/Images_Planes/IMG_1835.jpg
As you can see from 1815, they suddenly went dark, 1826 was only a few minutes behind ?, virtually the same lighting conditions yet an acceptable image to begin some work on. Other than the focal length ( zoom ) and pixel size the aperture and shutter are identical, yet one underexposed, the other is ok...ish

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Beauty of digital is you can just delete the ones you don't want

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Best
Michael