Not the first or last person to attempt this, but I thought I would have a go at creating solid rules for use of aircraft units in Diplomacy.
So, without further ado, here are my rules:
1. Aircraft as a unit
In my rules the aircraft unit will have a strenght of zero on the map, where all the other units have one.
The aircraft unit can be considered as sort of a ''ghost unit'' as it can move to an occupied square (occupied by one of his own color)
The aircraft unit is unable to take over a center, even if the center is unoccupied.
Aircraft units are destroyed once a unit of different color moves to the province they are in.
Aircraft units are linked to the number of SCs a player has, but on a different level than the rest of the forces.
For every three centers you get one aircraft unit in addition to your regular fleets and armies.
(right now I decided no army shall have aeroplanes in year 01, but since the rules are likely to change anyway, this is a bit early to say)
2. Aircraft unit orders
Aircraft units can be ordered to:
HOLD
MOVE
STRIKE
PATROL
EMBARK
Hold needs no explanation, so let's skip that. Move is a regular order that allows you to move one square to an adjacent territory. Note that you cannot be blocked out of territory by your own units, but you will be blocked out by another country's unit.
Now, the strike order is what actually makes the aircraft unit useful. This is probably the main idea of the post.
An aircraft unit can strike any territory that is within its reach (being two squares) For example Aircraft London could STRIKE Belgium. And what does ''STRIKE'' do? Basically, it means the aircraft will fly to a province, bomb it and fly back, so the unit itself does not move. The province it has targetted gets occupied with infinite force, so any unit moving there or holding there is blocked or dislodged. Also, no retreats to this region will be possible. So, basically aircrafts are completely unstoppable. The only thing that can stop a plane is another plane, which is given either a STRIKE order or a PATROL order (which basically means you will fly two squares and back without bombing a province)
Now to give a few examples:
German Army Ruhr HOLD -----> dislodged by airstrike
French AirCraft Paris STRIKE Ruhr via Burgundy -------> resolved
but with another aircraft joining the party:
German A Ruhr HOLD -----> resolved
French AC Paris STRIKE Ruhr via Burgundy -------> bounced
German AC Kiel PATROL Burgundy via Munich -------> bounced
It is basically assumed you moved over a square to a square and can be blocked on either one. Making the rules easier, you basically are on two squares at the same time and both have to stay clear of any other aircraft.
The last order: EMBARK allows you to board a fleet. It is a one square move order to a fleet (which needs to be given a hold order for it to work) which gets your Aircraft on the fleet. From now on you can move with the fleet or perform strike orders from the fleet. Note that if a fleet is dislodged while your aircraft was given a STRIKE or PATROL order, it is immediately destroyed.
3. One additional rule that might be cool
It would seem fun to me if you were able to destroy a SC. You would issue a DESTROY order to an Aircraft unit and let him target any SC in range. I think due to the drastic nature of this order, the DESTROYing of a SC can be stopped by a regular army or fleet unit occupying that center. If the order succeeds, however, the game from there on will have one less SC.
I hope my thoughts are structured well enough. Let me know.