Definition 1 of 2
Definition
An automatic selector valve in a hydraulic or pneumatic system that connects a single output line to whichever of two input lines has the higher pressure. When the normal pressure source is operating, the valve directs that flow to the actuator. If the normal source fails and an alternate or emergency source is applied, the internal element shifts to block the failed side and admit the emergency pressure to the same actuator.
Plain English
A small valve with two inputs and one output. It automatically passes through whichever input has higher pressure, so the same brake or landing gear can be operated by either the normal system or an emergency backup without the pilot needing to switch anything manually.
Context Anchor
Seen in aircraft systems that have a normal pressure source and a backup pressure source, such as brakes, landing gear, or other hydraulic equipment.
Derivation
The name comes from the way the internal piston or ball 'shuttles' back and forth between two seats depending on which side has higher pressure. 'Shuttle' originally described something that moves quickly to and fro, like a weaver's shuttle on a loom. The valve element behaves the same way.
Why Pilots Care
It enables reliable operation of critical systems such as landing gear when the primary hydraulic source is lost.
Analogy
It is like two hoses feeding one sprinkler through a small sliding gate. Water from either hose can reach the sprinkler, but the gate blocks water from being pushed back into the other hose.
Intuition Check
Do not read “shuttle” as a transport vehicle or as a pilot-controlled switch. In this term, it means a small valve part that shifts automatically when pressure comes from one side or the other.
Example Sentence 1
When the pilot applied the emergency air brakes, the shuttle valve shifted and isolated the normal hydraulic system from the brake line.
Example Sentence 2
The mechanic confirmed the shuttle valve moved freely during the hydraulic system inspection.