During the development of modern hydraulics water was the fluid used most often. Freely available and cheap to acquire, water was the obvious choice. However, as technology moved on mineral oil became the fluid of choice; from the 1920s mineral oils were chosen due to their lubricating nature and higher viscosity than water. Mineral oil also has a much higher boiling point than water, so could be used in applications where heat was an issue for hydraulic machinery using water. Mineral, natural and synthetic oils are still used today as the base for all hydraulic fluids on the market, and ingredients are added to create a range of hydraulic fluids suitable for many different purposes. It is very important in mechanisms like brakes that the fluid is not hygroscopic, meaning that it will not absorb any water. Braking creates heat, caused by the friction between the callipers and discs. This heat can boil and vaporise any water in the brake fluid, leading to the presence of a vapour in the system that will cause brake failure as that vapour is compressed by the action of the mechanism. It is for this reason that brake fluids are mineral oil or silicone based, and no other type of hydraulic fluid should be used in brakes or other mechanisms where such failure could be catastrophic. Safety is also a concern in industries where there is a high risk of fire. Hydraulic fluids can be very flammable, but water-glycol and polyol-ester based fluids have naturally fire-resistant properties, meaning they are excellent for use in hydraulic firefighting equipment and other places where high heat, oxygen and combustible materials are routinely used. There is a similar concern for safety in environmental applications, such as agriculture, fish farming and other marine based industries. The failure of a seal or even a small leak in a pipe will cause hydraulic fluid to escape from the system and enter the environment, so it is imperative that biodegradable and non-toxic fluids are used in case of a spillage or leak, as these will not harm the surrounding habitats. Hydraulic fluids are chosen for the properties that are needed in a specific application. In the examples above fire resistance, biodegradability and hygroscopic properties are very important, but there are several other factors to consider when choosing between fluids. Viscosity and lubricating properties are important in applications where it is not easy to change the fluid regularly (for example sub-aquatic machinery) as the hydraulic fluid also needs to help maintain the mechanisms. Lubricating fluids also have a low tolerance for water ingress so are good for systems where stopping action is just as important as the movement itself. Aircraft controls use hydraulics to make the transfer of power from the pilots hands to the machinery easier for the pilot; many systems require a large amount of force to operate and this is unsustainable, especially on long flights or in the military. Hydraulic actuators initiate the systems, which are in turn controlled by valves operated by the pilot, flight crew or autopilot. The hydraulic power is generated by the engines and by a backup system that can be deployed in the event of engine failure or during ground maintenance, meaning that even in the event of main engine failure the control systems can still be operated. Aircraft hydraulic fluid must be handled very carefully to avoid contamination, as even a small amount of water or dirt in it could cause catastrophic failure of the systems while the aircraft is in flight. The fluids are checked during every routine maintenance check to ensure they are free from contamination. The types of fluid that can be used on any given aircraft are specified by the manufacturer, and should not be substituted for any other type.