Pascal's Principle and Hydraulics Hydraulic systems use a incompressible fluid, such as oil or water, to transmit forces from one location to another within the fluid. Most aircraft use hydraulics in the braking systems and landing gear.
Pneumatic systems use compressible fluid, such as air, in their operation.
Some aircraft utilize pneumatic systems for their brakes, landing gear and movement of flaps. Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is any equal increase at every other point in the container. A container, as shown below, contains a fluid. There is an increase in pressure as the length of the column of liquid increases, due to the increased mass of the fluid above. For example, in the figure below, P3 would be the highest value of the three pressure readings, because it has the highest level of fluid above it.