Note: Air units need an airfield to take-off. They use air damage values when airborne or when stationed in a province with an airfield. Ground damage values (Fighter: 10%, Bomber: 7%) are used when on ground without air base. Balloons mobility is limited to the routes to which all ground troops are bound.
I feel that if your army is able to catch a group of planes sleeping on the ground (with airfield), then the planes should not get their air values but rather their ground values. Planes can move large distances quickly. A smart player would move them away from the front whilst offline. If their planes get caught sleeping by a slow marching army, they deserve to fight on ground values.
I used 100 men to attack a base with two armies of about 20 men each plus two groups of planes, the larger of which had three bombers and two fighters. Within the first attack my army of 100 men reduced to 60. I was surprised and checked the manual to find the above was not as I expected it to be.