Definition

A shutoff valve (SOV) is a valve that stops the flow of bleed air or fluids such as fuel, oil, and hydraulic fluid.

Description

SOVs are installed in various aircraft systems to shut off bleed air, fuel, oil, hydraulic fluid, etc. SOVs are normally electrically operated, but in some cases are manually or mechanically operated. In some designs, an SOV can be operated both electrically and mechanically. For example, in some models of the C-130 Hercules transport aircraft, there are bleed air SOVs that are electrically closed and mechanically opened. Electrically operated valves may be actuated either by an electric motor or a solenoid.

Electrically operated SOVs can be designed either fail-safe open or fail-safe closed, depending on the overall system design. A fail-safe open valve is spring-loaded to the open position, and it must have electrical power to remain closed. A fail-safe closed valve is spring-loaded to the closed position, and it must have power to remain open. 

The use of a fail-safe open or a fail-safe closed valve will depend on its location and purpose. For example, a hydraulic valve might be fail-safe open, to prevent hydraulic failure during an electrical power loss. On the other hand, a a firewall fuel valve might be fail-safe closed, to prevent fuel leaks or fires.