Alas, Paradox has not included aircraft in the military form into the game. There has been talk of trying to incorporate it as a brigade (like artillery or guards) or even creating another unit, but it's unlikely to happen.
I believe the reason for the lack of aircraft is that, during Victoria's time period, they really hadn't reached a truly useful pinnacle of power. I mean, in World War One, airplanes were used mainly for observation of enemy troop movements. Just because there were fighters doesn't mean that hundreds of thousands of Germans were killed by French aircraft strafing the trenches, and just because there were bombers doesn't mean that London and Paris were laid waste by a primordial Luftwaffe. Airplanes in this era simply hadn't reached the strength and utility necessaryt for incorporating them into the game.
If there is one thing aircraft could do, though, it is show you more accurately into which province an enemy army is moving, or perhaps they could let you see one province farther into the fog of war. But aside from these rather simple changes, no, I don't believe aircraft should be in the game, nor are they.