FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
During landing gear retraction on a light twin, the gear doors must open fully before the landing gear begins to retract into the wheel well, to avoid structural interference. Which hydraulic component is specifically designed to enforce this order of operation?
- AA hydraulic fuse
- BA priority valve
- CAn orifice check valve
- DA sequence valve
Show answer & explanationAnswer & explanation
Correct answer: D. A sequence valve
A sequence valve controls the order in which two or more actuators operate within the same hydraulic circuit. In a landing gear system, the sequence valve prevents pressure from reaching the gear retraction actuator until the gear doors have fully opened, ensuring the correct mechanical sequence and preventing structural damage.
Why the other options are wrong
- A. A hydraulic fuse isolates flow after a line rupture; it has no role in normal sequencing of actuators.
- B. A priority valve allocates flow between essential and non-essential systems, not the order of operations within one system.
- C. An orifice check valve controls flow rate in one direction to slow an actuator, not to enforce an operational sequence between two actuators.
Sequence Valve
A valve that ensures one hydraulic actuator completes its motion before pressure is allowed to another actuator, enforcing a required order of operations such as gear doors opening before gear retraction.
- Common in landing gear door/gear circuits
- Opens only after upstream pressure reaches a set value, indicating prior actuator has completed its stroke
- Prevents mechanical interference between components
Memory trick: Sequence valve = 'traffic light' making doors go green before gear moves.