Definition
A hydraulic control valve that ensures one action in a hydraulic system is completed before a second action begins. It senses pressure in the primary circuit and opens to allow fluid to flow to a secondary actuator only after the first actuator has reached the end of its travel and pressure has built up to a preset level.
Plain English
A valve that makes hydraulic actions happen in the right order. It holds back fluid from the next step until the first step is finished.
Context Anchor
Seen in aircraft hydraulic systems, especially landing gear, door, brake, and flap systems where parts must move in a set order.
Derivation
Sequence comes from the Latin sequi, meaning to follow. The valve enforces a following order — one action follows another, never at the same time.
Why Pilots Care
Prevents structural damage or jamming by guaranteeing components move in the correct sequence.
Analogy
Think of a sequence valve like a doorman who only lets the next person through after the first person has cleared the doorway. The valve is not deciding the whole plan; it is simply making sure the next step does not start too soon.
Intuition Check
Do not read “sequence” as just a list of steps on paper. In this term, a sequence valve is a physical valve that controls when hydraulic fluid is allowed to move to the next part of the system.
Example Sentence 1
The sequence valve held pressure off the gear actuator until the gear doors had fully opened.
Example Sentence 2
On flap retraction the sequence valve keeps the flaps from moving until the slats have reached the stowed position.