Aircraft Hydraulic System Fail-safes

The operating principle of Hydraulic Systems is that force applied to a single point will be transferred to another point using an incompressible fluid, typically an aircraft oil. A modern system is composed of two pistons and a filled cylinder connecting them. When one of the pistons is activated, the liquid moves through the cylinder and applies pressure on the other piston, causing it to move. The efficacy of oil is high in hydraulic systems, and the majority of the force applied appears at the second piston. The cylinder which connects the pistons can be any size, shape, or length.

Aircraft have separate hydraulic systems that power flight controls, gears, flaps, thrust reverse, brakes, and many more functions. Hydraulics also control the movement of an aircraft in all four directions. There are two ways that a hydraulic system could fail; one is loss of pressure, the other is loss of hydraulic fluid. Hydraulic systems are commonly powered by engine driven pumps. In the event of an engine failure, the aircraft will lose the system powered by that engine. However, aircraft are equipped with standby electrically powered hydraulic pumps to prevent this situation from turning worse.

An engine pump failure means the hydraulic system will no longer be pressurized, yet, once the pilot activates the electrical pumps, the hydraulic systems will function again. On the other hand, fluid loss ensues a different set of challenges. The electrical standby pump provides pressure and not fluid,  meaning if there is a loss of hydraulic fluid from the reservoir, the system will cease to function. Despite this occurrence, there would still be a way to fly a plane in this circumstance. Pilots can increase and decrease the power of the engines to slowly descend the aircraft in order to make an emergency landing.

To prevent any of these issues from occurring, large commercial aircraft will likely have three or four hydraulic systems onboard. Some aircraft are also equipped with power transfer units (PTU). In the event of a malfunction, a PTU can be turned on to power the failed hydraulic. If the aircraft controls are operated by a hydraulic system, there are multiple fail safes in place incase failure occurs. These include mechanical backups, multiple hydraulic pumps, and independent hydraulic systems.