I am considering using the apple supplied automobile cigarette lighter adaptor in my airplane to charge my iPad, but am concerned that I may do damage to the iPad as my plane uses a 24 volt system. I could buy an adapter to convert the cigarette lighter to 12 volts if necessary.

Any advice or experience with this problem?

There are a lot of converters available, I actually have used a few that I got from people locally. One thing you have to be careful of is companies advertise online 12/24 volt adapter cigarette lighter plugs, but its misleading as it will say in fine print "12/24 volts, output depends on input volts"

Removable 24 volt to 12 volt DC/DC converters (regulated) from 100 to 150 Watts non-isolated for 24V and 28V airplanes and trucks.
A Man's Toyshop
12 VOLT ACCESSORIES

just for some ideas, on the last link scroll down to the bottom, its on the left side of the page

I have no idea what's inside an airplane as far as electronics. I wired up a German Liebherr crane when I had my stereo installation business and just about everything was 24 volt. But it had a computer system running on 12 volt and actually had a power supply, once I located it I wired the head units (one in the cab, one in the operators cab) and also installed a generic 12 volt "cigarette lighter" socket to charge the operators phone.

If you have a 12 volt supply somewhere you could just wire up a socket, just two wires and a couple of bucks. But if you're anything like me, nothing works out the easy way lol