I don't have any experience of flying in a sail plane but I do often take pictures from a paraglider, I fly as a passenger with my husband. We can be sometimes 3,000ft above the ground where the air is smooth, but there is less detail to take pictures of. I usually find I spot the best shots when coming into land when the camera is tucked safely in its bag! Always the way!
I used to use my 28-300 but it's one of the older ones and the IQ wasn't brilliant. They are great from a 'not having to change lens' point, but in reality I found that I rarely used and got good results from the long end. Unless the air is really clear the haze usually gets in the way, plus it's more cumbersome. I now tend to use my 18-125. I sometimes take my 10-24 as I feel it really conveys the wide open space up there.
A polarizing filter has mixed blessings. If the weather is dull and overcast then it can reduce the light levels too much to get fast shutter speeds, but it helps cuts through the haze sometimes and, as I'm sure you know, it makes the best of a blue sky/big cloud day. I guess it might help with reflections from the glass canopy. That all said, I struggle to turn a polarizing filter and trying to line up a shot whilst when moving about. Changing a lens could certainly be tricky.
Use a big card - you won't want to change card, far too fiddly and time consuming, the trip will go so fast - and put the camera on continuous shooting. I've not yet had much chance to fly with A77 but when I have I find it will hunt when trying to photograph other flying things against the sky, so I often have it on manual focus.
If you want to see some airborne images, I have a couple of 'flying' sets on Flickr (http://www.flickr.com/photos/16629154@N03/collections/72157626270626568/
) in which there are a few images. There's many more from abroad because I enjoy that more photographically and flying wise. A friend of mine had a flight (http://www.flickr.com/photos/spwalker/sets/72157631602437999/
) over London last September in a plane. The exif data might give you some ideas for lens length, although I don't know what height he was at.
If possible I would speak with the flying club and find out where you might be flying over, and again if possible to see photos others have taken on similar trips. My husband had a flight in a glider once and the club walls were covered in pictures – great for inspiration.
I hope this helps