This little mod introduces a 20% penalty to firing and 10% to throwing accuracy of some units when they are flying. The rationale is that they don't have a proper foothold, and these flying suits are bound to have some bobbing. Also, the recoil is gonna affect flying units to a greater extent than standing on something firm.
That was the "realism" reason for the mod. The gameplay reason was that I wanted to nerf the flying suit somewhat. It already makes your life a lot easier, putting you out of reach of Chryssalids.
This mod has two versions: "Flying accuracy penalty" per se and "Flying accuracy penalty Plus".
The first one affects only X-COM soldiers, but not any other units: massive hovertanks and cyberdisks are not gonna be affected by this, and neither are "natively" flying aliens, since they would have some adaptations in their motor coordination because flying is their natural means of locomotion. Also, this version should work with all flying armors, vanilla or added by mods (without the need for these mods' authors to provide compatibility), since it checks whether a soldier is flying at the moment, and if yes, it applies the penalty.
The second one can be made to interact with other mods to affect also enemies. The drawback is that by default it affects only vanilla flying suit. If you have a flying armor from some mod, it is not gonna be affected by this penalty, unless its author knew of this mod and included the needed tag in his armor rule (or unless you do it yourself). The upside is that mod authors can apply the penalty to enemy units as well as X-COM soldiers, since the check is made on a per armor basis. (See readme.txt).
Do not install both versions alongside each other!
One warning: with both versions the chance of success of psi attacks is handled by the engine as accuracy. So this mod also decreases the chance of success of psi attacks. At first I thought of inserting a condition that would exclude psi attacks from this decrease. But then I thought that mental attacks are gonna take a lot of concentration, and it's gonna be hard to concentrate on such things when you are dangling several meters above the ground supported by something like a jetpack. So the penalty for psi attacks stays
Many thanks to Yankes and Meridian for explanations of the script engine and correction of my mistakes!
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Upd. Attached a new archive. It contains the "Plus" version of the mod in addition to the original.