Air combat occurs when an air unit enters into combat with land unit (or vice versa), or with another air unit. Air combat doesn't occur until quite late in the game, during the Modern Era, when the first Flight technologies are researched. Its rules differ considerably from both land and naval combat.
The arrival of airplanes changes warfare in modern times. Based in cities or aircraft carriers, they are capable of attacking from several tiles away, thus being immune to the counter-attacks of the army on the field. But be careful - their attacks are considered melee, allowing their targets to damage them in return. If used properly, however, the damage they receive is far less than that they deliver. Basically you can consider aircraft the advanced units of the late game, having mixed ranged-melee qualities. They're capable of attacking at great distances, concentrating on a single unit (or city), and wiping it out before it can react.
The most common air units are the airplanes. Because of the technological challenges to moving through the air (and the amount of fuel this takes), airplanes have very special movement rules.
Due to their fuel limitations, airplanes can't stay outside of their bases (on the open land) for extended periods of time, like all other units can. That means that airplanes always have to end their turns in a base, or if they want to move to another base, they have to expend their turn to do so. In other words, airplanes may only "move" between bases, while what would be normal movement for other units, for them is effectively attack range, within which they can attack a unit each turn, then return to base. Every airplane model can "re-base" (move) to a maximum number of tiles - that means it is unable to instantly move to the other side of the world in one turn. More advanced airplanes can re-base to bases further away, up to the Stealth Bomber that can literally move across continents and oceans.
There are two valid bases for airplanes:
- Cities - This includes all of your empire's cities, including Puppet cities, but not Allied city-states. Any city may base up to 6 Airplanes, or 10 if it has an Airport.
- Carriers - These big ships' main purpose is to provide bases and support for airplanes and move them across the ocean. Normally Carriers can base 2 airplanes, but promotions can extend their capacity to 5 planes.
From a base, airplanes may attack any tile within their operative radius. This radius is larger the more advanced the unit, and may also be increased via a special promotion. Once the airplane finishes its attack, it returns to its base (if it survives, that is).
Note that airplanes cannot be attacked while inside their bases, and since they always finish their turns inside bases, then it follows that they can't be attacked at all. Airplanes can only be damaged via Interception by their targets in retaliation to their attacks, or by nuclear weapons.
Unlike other military units, you may stack many airplanes in a single tile/base. Up to six airplanes may base within any city; an Airport's hangars add to the storage capacity, increasing that number to ten.
A Carrier can also base a limited number of airplanes, starting at two, and increasing via promotions.
Helicopter Gunships don't follow these rules, although they're also considered air units. Helicopters move just like land units, with the difference being that they can hover over terrain, including mountains, ignoring all movement penalties (and bonuses) and moving 1 tile per MP. Helicopters are also able to remain outside of bases indefinitely, like other normal units. They attack according to the rules of melee combat, but for bonus purposes are considered air units (so, for example, units that have bonuses fighting air units are much more dangerous to helicopters).
The Atomic Bomb is also considered an airplane unit, because it's delivered via a bomber. It is subject to the same rules as airplanes, including Interception.
Air combat occurs when an airplane conducts an attack from its base of operation. A series of events may then be triggered, according to certain rules.
Any airplane may use its Air Strike ability within its operational range to attack a land unit or city. It will leave base, perform the attack, then return to base. Note that the attacking airplane is prone to a retaliation attack from its target - all unit types can retaliate, including cities, just as if they were attacked in melee. However, most units' efficiency at retaliation to Air Strikes is very low - only the specially designated anti-air units such as the Anti-Aircraft Gun have dangerous retaliation strength.
Airplanes may also attack naval vessels within their range, with the same results.
The best air-to-land attack units are the bomber-type airplanes. Besides having a hefty Combat Strength, they can acquire a series of promotions that increase damage against all sorts of land or naval targets.
The only real defense against air strikes is Interception. This is an automatic ability which certain units possess, and consists of a bonus attack which is triggered when a hostile airplane enters the effective interception radius of the unit. That unit then automatically attacks the airplane. Note that only one interception attack may normally be done in a turn, though an extra attack may be possible after acquiring certain promotions. Also note that Interceptions don't count towards the normal move/action of the unit in a turn; they're an extra attack.
Interception special abilities are available in all three classes of military units:
- Land - Anti-Aircraft Guns and Mobile SAMs
- Naval - Destroyers and Missile Cruisers
- Air - Triplanes, Fighters, and Jet Fighters
When there is more than one interceptor in range of the attacking airplane, only one will perform an Interception! This way the other interceptors may react to other airplane attacks.
The best interceptors by far are fighter-type airplanes. Not only can they acquire promotions increasing their damage when intercepting, but they can also acquire an additional Interception per turn - something none of the other units can do.
The mechanics of evasion vs. interception are as follows:
- First, a random number is generated between 0 and 99 inclusive; if it is less than the defending unit's evasion chance, the defender evades (so a unit with Evasion 100 can never be intercepted).
- If the defender does not evade, a random number is generated between 0 and 99 inclusive; if it is less than the attacking unit's interception chance, the attacker intercepts.
- Attacking an intercepting land unit (e.g. a Mobile SAM) is an exception; it will go into "air strike" combat, which has a different mathematical model.
Fighter airplanes have the special ability Air Sweep, which is specifically designed to counter interception so that other aircraft can attack safely. When using the ability, the fighter attacks a tile in such a way as to trigger a nearby interceptor's bonus attack, but evade most of its normal damage. Fighters are especially good when targeting land or naval interceptors, and suffer practically no damage during the sweep. That's not the case when they encounter another fighter unit. What occurs then is known as "Dogfighting" - the two attack airplanes engage in spectacular air-to-air combat. Regardless of the outcome, an Air Sweep consumes one interception for the defending army.
Promotions can improve a fighter's ability to Air Sweep.
As mentioned above, these are only possible against Helicopter Gunships. Any land, or naval unit may attack a helicopter in its range, with different success (specialist anti-air units are very dangerous when attacking helicopters). Otherwise, airplanes may only be attacked via Interception.
All fighter units and the Stealth Bomber have a special ability - Air Recon. They make automatic reconnaissance sweeps in the beginning of each turn (which don't count towards their move), which gives you permanent vision over a great range of tiles around their base. This ability is very important ability for the late game, as it allows you to better plan your offensives.
Guided Missiles are a late-game special weapon, which are essentially another type of air unit. Just like airplanes, missiles may only be stationed in bases, and attack from there within their operational range. Unlike normal airplanes, ballistic missiles can't acquire promotions, and are usable only once - they're one-shot, expendable weapons, and are destroyed after their first attack.
The bases possible for missiles are a little different: they cannot be transported by Carriers, but they may be stationed in cities, in Missile Cruisers (3 capacity), or in Nuclear Submarines (2 capacity).
Their operational range is quite large, comparable to that of a Fighter. The greatest quality of ballistic missiles is, however, the fact that they can't be intercepted, and are essentially a sure-hit weapon.