I believe it may have been touched on during the tutorial, but a lot of players overlook bits of the tutorial. Planes are a bit more complicated than can be fit into little tooltip, so the best place to learn how to use them effectively is here on the forum.
The behavior of planes also is changed/updated from time to time as the game is improved and balanced, so information can easily become outdated.
Here is a quick summary of plane mechanics:
For best results due to the x factor and SBDE limitations it is best to have a group containing 5 interceptors and 5 tactical bombers patrol together, sometimes adding strategic or naval bombers as needed.
While on patrol planes have their blue "circle" and a 15 minute timer.
At the end of the timer the plane will "tick" off a patrol attack on all units within the blue patrol circle. This includes ground units and other planes including planes whose patrol circle overlaps by just a tiny bit, or are passing through the patrol circle. These attacks are done at 25% of the potential direct attack value listed in the plane's stats.
When planes are on patrol they have a green patrol zone that they can maneuver in. If you select the patrol button again with your plane already on patrol this green zone will show. The plane can be placed on patrol anywhere within the green zone and it will move their directly and resume patrol. Moving to a location outside the patrol zone will cause the plane to have to return to base first. Note that planes on patrol do not have to refuel and can patrol indefinitely.
Planes that are on patrol defend themselves quite well, so never attack them directly with your own planes. Instead patrol in the same area.
After an attack or when relocating there is a brief refueling period when they are vulnerable to being hit by your own bombers without returning fire. They are especially vulnerable to rockets during the refueling time as well, so beware of refueling once an enemy has level 2 and up rockets in range.
To defend yourself from enemy planes patrol your own planes over whatever they are targeting. This will allow your planes to hit them when they attack their target, and your own planes won't be damaged.
Planes that are parked on the ground (not refueling) are still planes, and will defend themselves. Sending your own planes to attack a large group of enemy planes is not wise, and will result in losing a lot of planes yourself.
Enemy planes that are parked on the ground without any ground units to protect them are very vulnerable to a ground attack. If a high speed unit such as an Armored Car can avoid other ground units and capture the province where airplanes are parked on the ground those planes will be turned into convoys and be quite vulnerable, much like refueling planes. It will take the armored car some time to kill the convoys, but a hit from a few bombers or a rocket works nicely.
For best results due to the x factor and SBDE limitations it is best to have a group containing 5 interceptors and 5 tactical bombers patrol together, sometimes adding strategic or naval bombers as needed. Some of the combat mechanics at work here are:
In every battle there is an X-factor that means an attack might do from zero to 100 percent of the listed damage. This is in addition to the effects of terrain, forts, and home defense. This matters partly because of rounding. When you have only a few planes on patrol their combined strength/damage done is low to start with, and the X-factor can make it lower.
>> There is a limit to how many units of each type are most effective per group, called the State Based Damage Efficiency. This means that having more than the most efficient number of units makes their combat strength lower.
You might also look at the air combat guide pinned to the top of this general discussion section
