Pneumatic systems operate on a principle similar to hydraulics, but with one major difference: air, unlike a fluid, is compressible. For this reason, an air-driven actuator cannot hold a load rigidly in place like a hydraulic actuator does—aircraft control surfaces would be “springy.” Pneumatic systems are also less responsive and support lighter loads than hydraulic systems. However, due to their relative simplicity, reliability, and safety because of the absence of flammable oil in the lines, pneumatic systems are often incorporated into aircraft designs. They may power landing gear, flaps, de-ice boots, windows, doors, etc. Air pressure to drive the system can be provided through engine-driven or electric pumps and is controlled through a series of valves.