Air Net is a 148/152 long-range radio channel that is used exclusively by aircraft and any relevant personnel on the ground, e.g. the FAC. This channel allows the aircraft to coordinate their actions with one another, while also allowing them to maintain orderly communications with ground forces through the FAC or whoever else is listening.
Pilots and Co-Pilots must listen to Air Net at all times. It is usually set to the right ear. Most also listen to Command Net on the left ear, to keep up to date with the actions of various units on the ground. Pilots use Air Net to let each other know what they're doing (preventing conflicts in the air), point out targets and enemy anti-aircraft assets, and announce RTB or Mayday when necessary.
Additionally, pilots use Air Net to stay in contact with ground forces. A FAC will usually be listening and transmitting on Air Net in all communications with aircraft, informing them of ground unit positions, enemy positions, and any tasking he has for them. Should a FAC be killed, Command may take over the responsibility of communicating with aircraft.
Aircraft are not restricted to Air Net, and will occasionally speak on Command Net if they wish to immediately contact a ground unit - or if contact with the FAC has been lost for any reason.
Who uses Air Net
Air Net is used by all Pilots, Co-Pilots and FAC. Additionally, Command will usually listen in on Air Net, but will leave communications on that net to the FAC.
Pilots
Pilots must listen to Air Net at all times. The information exchanged on Air Net is crucial to the effectiveness and survival of their aircraft. If an aircraft has a co-pilot, the pilot himself will rarely transmit on Air Net unless attempting to clarify something (or when addressed personally), leaving that duty to the co-pilot instead.
A pilot is addressed by their aircraft's callsign. In cases where several aircraft belong to the same group and share the same callsign (e.g. "Phantom 1", "Phantom 2" etc.), pilots will generally use only the number suffix to identify themselves and refer to other aircraft.
Co-Pilots
When a Co-pilot is present, one of their duties is to handle all Air Net communications. This reduces the workload for the pilot, allowing them to concentrate on flying and operating the plane itself. The co-pilot relays any orders and questions to the pilot, and then relays any responses over Air Net to the other aircraft or the FAC.
Even when a co-pilot is present, the pilot will still listen to Air Net as a redundancy.
Co-pilots use the same callsigns that pilots would otherwise use.
FAC
The FAC uses two radio channels almost simultaneously: Command Net and Air Net. The former is used to receive orders from Command and keep track of the situation on the ground; The latter is used to relay that information to the aircraft and receive updates from them. Aircraft typically wait for information to be repeated on Air Net before acting on it, even if they've already heard it on Command Net.
On Air Net, the FAC uses the callsign "FAC" in all communications.
Command
Command may also be listening in on Air Net, though they typically leave communications on this channel to the FAC.
Should the FAC be incapacitated, Command may have to assume that role and converse with aircraft, splitting their attention between Air Net and Command Net. In such cases, the workload on Command can be overbearing, and pilots are generally expected to become more independent and share that workload. If necessary, aircraft may stop contacting ground forces on Air Net altogether, instead conversing with Command on Command Net.
On the Air Net, Command may identify himself as "Command" or "Ground", and will answer both callsigns. If Command has assumed the FAC role, they may also answer to the callsign "FAC" if called on the radio.
Information Exchange
The purpose of a separate Air Net is to allow aircraft to coordinate with each other and with the Ground forces. This involves a substantial exchange of all kinds of information, as well as orders.
Aircraft to Aircraft
When multiple aircraft are involved in the mission, they must use Air Net to coordinate with one another. Aircraft will exchange multiple types of information to ensure that no collisions occur, and that each Aircraft attacks only the correct target at any given time. When aircraft fly in a formation or pair, coordination between them on Air Net becomes even more important.
First and foremost, Aircraft will notify one another of anti-aircraft assets they encounter. Whenever an aircraft is fired upon by an enemy, they will report the position and type of the threat immediately (and usually also mark it on the map). This informs attack aircraft of a high-priority target that should be destroyed if possible, while also warning all aircraft to be careful when approaching the marked position (or to avoid it altogether).
When an aircraft is attacking a target, the pilot informs all other aircraft of that attack. This prevents additional aircraft from attacking that same target until the first attack is concluded, preventing a waste of ammo as well as potential collisions between attacking aircraft. Once the attack run is concluded, the pilot announces this on Air Net so that other attack aircraft (if present) can make their own attempt if necessary.
Aircraft flying in formation must inform one another of their position and actions. Typically it is the lead aircraft in the formation who will announce his actions so that other aircraft can react appropriately or simply keep the formation.
While communication is very important during dogfights, it is of paramount importance for transport helicopters as they make their Approach to land. The lead aircraft will announce when they are balancing, flaring, and which position exactly they intend to land on. The lead aircraft may also suggest landing positions for each of the following aircraft. Each aircraft on the ground will also announce when it takes off and in which direction, letting others know where to go when they take off (or to wait for a few moments before taking off, to prevent collisions).
Finally, Aircraft will also inform one another when they rejoin a formation, leave a formation, or cross the path of another aircraft (whether below, above, or at the same altitude).
Aircraft to Ground
When Aircraft contact ground forces -- typically the FAC -- it is less important to announce the aircraft's position as it has little bearing on the ground forces (unless the aircraft is leaving or approaching the battlefield), but far more important to announce an aircraft's actions and the location of enemy targets.
All aircraft announce their arrival over the battlefield by calling "On Station", and leaving the battlefield by calling "Off Station" or "RTB". This lets ground forces know that an aircraft is available for Tasking or will not be available for tasking temporarily.
Furthermore, aircraft announce "Bingo" when they have effectively run out of fuel, or "Winchester" when they have run out of one or more types of ammunition. They will usually request permission to RTB at this point, which may or may not be authorized by the FAC depending on circumstances.
Observation
Whenever an aircraft spots a target on the ground, they typically announce it on Air Net before taking any action, especially if the target is very close to ground forces. If the target is far away from ground forces, it may or may not be announced at all. Aircraft will rarely report insignificant targets such as a lone Infantryman, but will almost always report a tank or another kind of dangerous vehicle. If the target is far from friendlies, the aircraft may interdict it without reporting anything on Air Net.
Attack
Aircraft announce any attack they make on targets close to ground forces. This gives ground forces one last chance to call off that attack (in case friendlies are too close to the target).
After an attack run, an Aircraft will report whether they believe the target has been destroyed, damaged, or escaped damage. Aircraft may also request confirmation of the damage by ground forces, if it is within their line of sight.
Landing
An approach to land is very complicated and delicate, and often requires close coordination with ground forces to ensure that no harm comes to the aircraft, and that the loading/unloading of troops can occur quickly and efficiently. A lot of information is exchanged during the landing process.
Once an aircraft has been called in to land, it will announce an estimate of the time it will take to touch down. This informs ground units of how long they have to get ready to board or unload the aircraft. If the initial announcement was made a long time before landing (e.g. "one minute"), the pilot may announce the ETA again closer to the actual touchdown. After taking back off, the aircraft may make another call on Air Net (or sometimes on Command Net) to make sure they have picked up everyone they needed to.
Ground to Aircraft
Ground forces must also relay some information back to the aircraft. This is usually the role of the FAC, or in his absence whoever took over those duties. In case there is no one designated to contact aircraft, players on the ground may switch over to Air Net or relay the information on Command Net.
The most common information sent to aircraft is the position of any enemy units detected by ground forces that could serve as a good target for aircraft. They will not bother reporting small groups of enemy infantry, but will report larger units as well as any enemy vehicles. Since much of this information is typically relayed on Command Net as well, repeating it on Air Net can technically be redundant but nevertheless important.
Orders from Command to the aircraft is also relayed on Air Net, to avoid cluttering Command Net.
Although aircraft can track the general position of ground units on the map, it is important to relay detailed information on exact positions whenever the aircraft is ordered to attack targets close to ground units. This is also important when aircraft are coming in for a pick-up in a Hot LZ, as knowing the players' exact position can help the pilot adjust their landing spot to make the pick-up smooth and quick, reducing time on the ground and thereby risk to the aircraft and passengers.
If an aircraft is landing at a tight location without much space to maneuver or margin for error, the FAC can help the aircraft avoid damage by indicating which direction the aircraft should shift to from his vantage point on the ground. This is also important when transport helicopters attempt to drop under-slung cargo, or when attempting to lift such cargo with the sling.
Finally, ground forces must inform any approaching aircraft of potential threats, such as enemy vehicles threatening (or about to threaten) an LZ, or when anti-aircraft units have been spotted nearby. Even enemy infantry, if it is close enough, could threaten an incoming aircraft with anti-tank launchers or other weapons. In case the threat becomes immediate and extreme, ground forces could instruct the aircraft to "Wave Off" (abort the landing) and try again later.