Damage is a vital concept featured in all games of the Ace Combat series, which refers to a loss of health suffered as a result of harmful situations.
Whenever an aircraft its hit by enemy projectiles (anti-air artillery and SAMs), it will suffer damage. In most games, damage can be tracked through a plane vector and percentage in the lower right corner of the screen. The vector will change color as damage is suffered: green indicates perfect or high health, yellow indicates medium damage, red means high damage and flashing red symbolizes critical health. Certain sources of damage, such as collisions with the environment or attacks from superweapons, will instantly destroy an aircraft. In Ace Combat Infinity, soft collisions with the ground, as well as flying into a large enemy, such as Stonehenge or the Moby Dick fleet, will increase damage to 98% damage (destroying the aircraft if the damage is already this high), and by 95% from a Nimbus missile.
There are ways to reduce or, in some cases, remove damage completely. In most games, damage can be healed exclusively through return lines. However, this advantage is usually disabled in higher difficulty modes. Ace Combat X: Skies of Deception, Ace Combat: Joint Assault, Ace Combat: Assault Horizon Legacy and Ace Combat Infinity featured armor upgrades that would help limit damage sustained in flight. Joint Assault introduced a special tuning part called the "Ace's Amulet", which regenerates an aircraft's health at certain intervals. Ace Combat 7: Skies Unknown features damage reduction parts and a health regeneration part. The latter slowly repairs the player's aircraft up to 50% health and can only be used in Campaign mode.
Ace Combat: Assault Horizon handles the damage concept differently. The game features a "regenerating health" system, which heals any battle damage in a short period. The healing speed and enemy damage depend on the difficulty. In Ace difficulty, healing is disabled entirely.