FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
A section of hydraulic tubing between two shutoff valves is isolated with both valves closed while the aircraft sits in direct sunlight. A technician later finds that fluid pressure in that isolated segment has risen significantly above normal system pressure. Which component is specifically designed to prevent damage in this situation?
- AThermal relief valve
- BSystem relief valve
- COrifice check valve
- DSequence valve
Show answer & explanationAnswer & explanation
Correct answer: A. Thermal relief valve
A thermal relief valve is installed to protect line segments that can be isolated by shutoff valves on both ends. As trapped fluid heats and expands, the thermal relief valve opens at a set pressure to relieve the excess back into the system, preventing line or fitting damage.
Why the other options are wrong
- B. The system relief valve protects the main system, not isolated trapped segments between two closed valves.
- C. An orifice check valve controls flow rate in one direction; it does not relieve pressure from thermal expansion.
- D. A sequence valve controls the order of operation of actuators, unrelated to thermal expansion.
Thermal Relief Valve
A small relief valve installed to protect a length of tubing that can be isolated by shutoff valves on both ends, relieving pressure caused by fluid thermal expansion.
- Protects trapped, isolated fluid segments
- Opens at a lower pressure than the main system relief valve
- Common near landing gear and other valve-isolated sections
Memory trick: Thermal relief = the 'pressure release valve on a soda bottle left in the sun.'