It may be sold for a car but it will work fine in an airplane. Most receiver sand servos are compatible with 5-cell/6V batteries. My main concern would be the battery condition. How old is it? How many times has it been charged? What is the actual capacity? Cycling is the best way to find out the battery condition. Still, if you are in doubt, new batteries don't cost all that much when you consider the alternative of losing an airplane.