FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
An aircraft's main wheel brakes are normally powered by the primary hydraulic system, but an emergency pneumatic system using a compressed nitrogen bottle is installed for use if hydraulic pressure is lost. Which valve automatically connects whichever source (hydraulic or pneumatic) is providing pressure to the brakes, while blocking flow back into the unpressurized source?
- ASequence valve
- BPressure reducing valve
- CShuttle valve
- DThermal relief valve
Show answer & explanationAnswer & explanation
Correct answer: C. Shuttle valve
A shuttle valve has a free-floating shuttle that moves to whichever inlet has higher pressure, sealing off the low-pressure (or unpressurized) port and routing the pressurized source straight through to the actuator. This allows automatic switching between normal hydraulic brake pressure and emergency pneumatic brake pressure without pilot action to select a source.
Why the other options are wrong
- A. A sequence valve controls the order of operation of two actuators, such as gear doors before gear extension.
- B. A pressure reducing valve lowers pressure for a subsystem needing less pressure than the main system.
- D. A thermal relief valve only relieves pressure buildup from fluid expansion in an isolated, closed line section.
Shuttle Valve
A valve with a free-floating shuttle that automatically routes whichever of two pressure sources is higher to a common outlet, isolating the lower-pressure source.
- Used for emergency brake or gear systems with dual pressure sources
- Shuttle physically blocks the unpressurized/low-pressure port
- Requires no pilot selection—operates automatically
Memory trick: Shuttle 'runs' to the winning (higher) pressure side