Aviation and air breathing missile fuels are characterized and controlled by specifications and by contract. Specifications for aviation fuels have historically been based primarily on usage requirements as opposed to detailed chemistry. Exceptions would be chemically derived, military fuels such as JP-10, as well as certain other missile fuels. Over the years aviation gasoline (Avgas) has become composed primarily of synthetic components, while turbine fuels are largely straight-run distillates and therefore depend on crude oil type for their primary characteristics. Synthetically derived hydrocarbons in the kerosene boiling range generally have properties that fall within the specification range for aviation turbine fuels. However, these fuels must meet rigorous engine and compatibility tests before they are accepted for use.