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# Attrition and accidents

Attrition is the loss of equipment — not manpower — during the course of a war when fighting on the offensive. While only land units suffer from attrition, air units may suffer accidents, especially when moving in a harsh weather, resulting in loss of equipment.

## Causes

• Training: 6% attrition.[1]
• Terrain: Up to 50% attrition.
• Weather: Up to 70% attrition and +60% rate of air accidents.
• Resistance: Up to 25% attrition.
• Out of supply: Up to 30% attrition.[1]

## Equipment loss

Higher reliability reduces equipment loss from attrition.

### Land units

For land units, the average rate of equipment loss per day as a proportion of total equipment is

${\displaystyle {\frac {\text{equipment lost per day}}{\text{number of equipment}}}=10\%\cdot {\text{attrition}}\cdot \left(100\%-{\text{reliability}}\right)}$

So for 100% attrition and 0% reliability, about 10% of equipment will be lost per day. At 80% reliability, 100 days of training (enough to go from Green to Trained, or halfway from Trained to Regular) will cost about 15% of equipment.

### Air units

While aircraft do not suffer from attrition, they have a chance of accidents when taking off/landing. This is

${\displaystyle {\text{accident chance}}=0.1\%\cdot \left(100\%-{\text{reliability}}\right)}$

or equal to 1% attrition for land units. Carrier-based aircraft suffer twice the accident rate. It can quickly turn into a major issue, chewing down through the pool of accessible planes regardless of how well the air combat goes.

### Naval units

Ships do not suffer from attrition, but the reliability corresponds with probability of getting critically hit by enemy attack.

## Methods to reduce equipment loss

There are several ways to reduce the effects of equipment loss. One way is to improve reliability by attaching a Maintenance Company to a division or creating new variants of equipment, such as tanks, ships, or planes, with increased reliability. Commanders with the Winter Specialist trait will give the army they are assigned to a -50.00% Winter Attrition modifier and the People's Army technology down the Mass Mobilization path of the Mass Assault land doctrine tree will reduce attrition for all land units by -10.0%.

### Custom difficulty settings

Adjusting the strength slider of a major country prior to a game will reduce the amount of attrition that that country's units will receive. However, doing so disables achievements.

## Footnotes

1. See in /Hearts of Iron IV/common/defines/defines.lua