FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
A hydraulic system uses a constant-delivery (fixed displacement) engine-driven pump along with a pump unloading (pressure control) valve. When system pressure rises to the cutout setting of 3,000 psi, what does the unloading valve do?
- AShuts the engine-driven pump down completely until pressure drops
- BOpens a relief port to dump pressurized fluid overboard
- CIncreases the pump's internal displacement to lower output pressure
- DDiverts pump output back to the reservoir at low resistance, allowing the pump to run unloaded
Show answer & explanationAnswer & explanation
Correct answer: D. Diverts pump output back to the reservoir at low resistance, allowing the pump to run unloaded
Because a fixed-displacement pump cannot reduce its own output, an unloading valve senses system pressure and, at the cutout setting, redirects the pump's constant flow back to the reservoir at near-zero resistance. This lets the pump continue spinning without building further pressure, until pressure drops to the cut-in setting and normal delivery resumes.
Why the other options are wrong
- A. The engine-driven pump physically cannot be shut off while the engine runs; it is only unloaded, not stopped.
- B. Fluid is never dumped overboard; it is recirculated internally to the reservoir.
- C. Fixed-displacement pumps cannot vary their own displacement; that describes a variable-displacement pump.
Pump Unloading Valve
A pressure control valve used with fixed-displacement pumps that diverts pump output to the reservoir at low resistance once system pressure reaches the cutout setting.
- Used with constant-delivery pumps
- Cutout: diverts flow to reservoir at set high pressure
- Cut-in: restores normal delivery at a lower set pressure
Memory trick: Unloading valve = pump treadmill, spins free once it's 'full.'