Which characteristic is most important for hydraulic fluid in terms of safety?

The most important characteristic for hydraulic fluid in terms of safety is having a high flash point. The flash point refers to the lowest temperature at which the vapor of a liquid can ignite when exposed to an open flame or spark. A hydraulic fluid with a high flash point is less likely to ignite in case of a leak or spill, reducing the risk of fire hazards in operational environments such as aircraft.

In aviation and other industries that use hydraulic systems, ensuring safety from fire is paramount. Hydraulic systems can operate under high pressure and temperatures, and the presence of fluids with low flash points would increase the likelihood of ignition in such conditions, leading to severe safety risks.

Other characteristics such as low viscosity, high corrosiveness, and the ability to freeze at low temperatures do not directly address the critical aspect of fire safety as effectively as a high flash point does. Low viscosity is important for the efficient operation of the hydraulic system, but it doesn't relate to fire risk. High corrosiveness would be detrimental to the integrity of the system and its components. Lastly, while freezing points are important for fluid behavior in extreme conditions, they are less significant in the context of overall safety compared to the flash point.