Incendiary

Incendiary (IN) ammunition creates a large area of fire on the battlescape equal to its blast diameter. Incendiary rounds will burn for two turns on non-flammable terrain. Flammable terrain will burn for longer, and the fire is likely to spread to any adjacent flammable tiles. Aliens and unarmored X-COM agents can catch fire as well.

Incendiary rounds do very little direct damage -- between 0 and 10 points per hit -- but continue to do damage every turn that a unit is in fire or on fire. Due to the "funky fire" bug, units in or on fire will also take damage every time an incendiary round goes of anywhere on the Battlescape.

Source

Incendiary rounds are available for the following weapons:

Some tiles may already be on fire at the start of the mission if the UFO crashed.

Blast Diameter

Auto-Cannon incendiary rounds have a blast diameter of 5 tiles; note that the southern tip of the "diamond" will always be missing.
Heavy Cannon incendiary rounds have a blast diameter of 7 tiles.
Incendiary rockets have a blast diameter of 9 tiles.

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AC-IN        HC-IN          RL-IN

Smoke

A burning fire can also be used as an impromptu smoke screen. No smoke appears immediately, but smoke will appear outside the edges of the fire, as the fire slowly spreads in subsequent turns. The smoke begins on the first turn after the turn the incendiary round is fired. The smoke will spread vertically (upwards) as well as laterally. Smoke can block line of sight.

Light

A single square of fire will illuminate the area within the grey line.

Fire not only damages obstacles and aliens hiding in dark corners of the map, but also illuminates the area for several turns. This is especially useful during Night Missions.

Any tile on fire will provide illumination within a 9 tile radius. A large patch of fire will illuminate everything within 9 tiles of the outer edge of the fire. Curiously, aliens on fire do not provide illumination. Maybe they're made of ethanol or something.

Damage

Incendiary damage mechanics are not fully understood and are being re-investigated by Zombie. The information below is part of the picture but not the completely accurate picture. Treat with caution! More discussion is on the Talk page.

Brush and wood on fire will eventually be consumed, leaving a flat scorched-earth tile.

If a Zombie is killed by the explosive impact of any form of Incendiary munitions, the Chryssalid larva will be burnt up instead of hatching. However, Zombies have high hit points and will have to be near death to be killed by an IN round, per se.

UFO interiors and dropped items are fireproof, even if their owners aren't. Silverblade recommends combining this fact with the diagonal wall bug and an incendiary rocket to roast them in the shell.

Damage caused by fire does not alter Morale level.

All forms of manufactured X-COM armor will completely block the effects of fire -- including direct hits from incendiary rounds. Soldiers not wearing armor take Health damage.

Soldiers wearing armour, if wounded, can die from spreading fire. Apparently if the damage the fire would inflict exceeds the soldier's remaining health, the soldier will die. If the damage is not enough to kill the soldier that turn, no damage will be inflicted. This bug only applies to spreading fire, and will not affect a wounded soldier already standing in fire.

Certain creatures take increased or decreased damage from incendiary rounds:
- More Vulnerable:
  - 170% - Reaper
- Less Vulnerable:
  - 80% - Chryssalid
  - 70% - Ethereal, Snakeman
  - 40% - Tanks/Hovertanks
  - 0% - Silacoid (rocks don't burn!)

Initial "impact" damage from incendiary ammunition is either for no points (unit does not catch fire), or between 5-10 points (unit catches fire). Since the distribution is randomly chosen from the set [ 0, 5, 6, 7, 8, 9, 10 ] (seven values), the probability of remaining unharmed is 14% (1/7), while there is an 86% chance (6/7) a unit will take some damage. The average damage is 6.4.

Units standing in a blaze (as opposed to being on fire themselves) will receive between 1-12 damage points, most likely determined by the flammability rating of their tile (MCD offset 45). The chance of catching fire is determined by an if-then statement: IF Random(100)<40*Damage Modifier/100 THEN Catch Fire. So for example, a normal unarmored soldier (with a damage modifier to IN=100) has a probability of catching fire of: Random(100)<40*100/100 = Random(100)<40 which means that rookie will catch fire 40% of the time. From a previous 6000+ trial conducted by Zombie soldiers have a 33% chance of catching fire. It's unknown why these numbers don't match, but it's possible the sample size was still too small.

Once lit, units burn for a slightly random amount of time, taking 5-10 damage points each turn. (If units fall unconscious, they do not continue to burn.) Thus the average total damage is 21.5 points (3 x 7.5), but could be as high as 50. Compare that alien health values range from 30 to 120... it can kill or seriously damage Sectoids, Floaters (if on ground!), and maybe Snakemen, but barely warms up Mutons. Terrorists are hardly touched (but now you might have their attention). Unsurprisingly, the equation used to determine how long a unit will remain on fire depends on the damage modifier of the unit in question: MaxTime=5*Damage Modifier/100, Duration=Random(MaxTime). So for example, if a tank (damage modifier to IN=40) catches fire it will remain lit for MaxTime=5*40/100=2; Duration=Random(2) or 1-2 turns.

Smoke does not snuff out units on fire.

Ultimately, in comparison with other ammo types, incendiary ammo is both weak and slow. Most of the time you will want to use it for other purposes (light, blocking LOS, clearing areas) and consider any damage a plus. However, it can be particularly effective against Sectoids - so have plenty of IN for your early-game missions. Barbecue spits are optional.

Spreading

Fires can spread one tile in every direction, in ideal conditions (such as wheat fields and haylofts). However, since no map is a pure wheat field and many tile types are unlikely to burn, they eventually go out.

Unarmored soldiers can walk through fire unscathed, but don't be left standing on or next to a burning tile at the end of the turn. That's when the fire-spreading code runs. Incendiaries also restrict alien movement; they, too, try to avoid fire.

Explosives (any weapon with damage type HE) can start fires, particularly in mountains (due to a bug). Copious use of explosives on thick cover can flush out the enemy - but you have an 80-item limit.

Explosives will not put out fires, per se. You can directly destroy tile flammability via explosives so that areas can't burn (or stop burning), but it's tricky. (How accurate is your throwing?) In theory, it's possible for your team to corral or even direct flames, but how many folks have tried this? Please put any good war stories along these lines on this page's Discussion.

Quirks, Tips, And Tricks

XCOM makes a brave attempt to model fire. However, there are some quirks:

If a unit (alien or friendly) is on fire or standing in flames, an incendiary explosion anywhere else on the map will damage it, even if you just shoot an IN round at a random patch of ground.

Friendly units standing in smoke will take STUN damage every time an incendiary round explodes, even if armored. Alien units are not affected (needs further testing).

Aliens not facing you will not respond with reaction fire when hit (directly or indirectly) by an incendiary round (see Reaction fire triggers).

Smoke and flames do not mix. Only one type may exist on a particular tile on a particular turn. To quickly smother a blaze simply drop a primed Smoke Grenade in the center. This is a fairly useful trick for unarmored soldiers who want to safely cross a blaze without waiting for it to burn out by itself. But remember, you can cross a blaze safely - just don't stop in or next to one.

There is an upper limit to how many squares can be on fire or in smoke (400 squares total for both). Once you have a good sized blaze going (forest fire, anyone?) further incendiary rounds will seem to fizzle, because no more fire can be created. Impact damage will still be dealt and units can still be set on fire. Still, existing smoke-covered tiles can be turned into fire instead of smoke, and vice versa.

The damage values listed in the UFOpaedia do not determine how powerful an incendiary round deals damage; it only determines how wide an area will be blanketed with flames. Fire is unlike other damage; it works at initial blast and then over time, as described above.

Shots made with IN rounds never count as "hitting" for the purposes of firing accuracy experience, but reaction shots are counted and can go towards better reaction stats.

Incendiary rounds (and other forms of fire) do not damage valuable equipment/loot; even corpses will not burn.