A&P Hydraulic and Pneumatic Systems Questions and Answers — Questions and Answers
Question 1: A technician observes that a hydraulic system's pressure gauge fluctuates rapidly and the system operates erratically, especially upon initial actuator movement. Which component is most likely failing to perform its function of dampening pressure surges?
- Sequence valve
- Pressure relief valve
- Accumulator (Correct answer)
- Selector valve
Correct answer: Accumulator
A primary function of a hydraulic accumulator is to absorb pressure shocks and dampen fluctuations in the hydraulic system. An accumulator that has lost its nitrogen pre-charge or has a ruptured diaphragm/bladder cannot perform this function, leading to erratic pressure and jerky operation.
Question 2: Which of the following is the primary purpose of a hydraulic sequence valve in a landing gear system?
- To ensure the gear doors are open before the gear begins to extend. (Correct answer)
- To regulate system pressure to a predetermined maximum.
- To allow fluid to flow in one direction only.
- To store hydraulic fluid for emergency gear extension.
Correct answer: To ensure the gear doors are open before the gear begins to extend.
Sequence valves are used to control the order of operations in a hydraulic circuit. In a landing gear system, a sequence valve ensures that the gear doors are fully open before hydraulic pressure is directed to the landing gear actuator, preventing mechanical damage.
Question 3: A technician is servicing a large transport category aircraft hydraulic system that uses a purple-colored, phosphate-ester based fluid (Skydrol). What type of seal material MUST be used for components within this system?
- Neoprene
- Butyl rubber or Ethylene Propylene (EPDM) (Correct answer)
- Buna-N
- Natural rubber
Correct answer: Butyl rubber or Ethylene Propylene (EPDM)
Phosphate-ester based hydraulic fluids, such as Skydrol, are chemically incompatible with petroleum-based seals like Neoprene and Buna-N, which will swell and deteriorate rapidly. Systems using Skydrol require compatible seals made of materials like Butyl rubber or Ethylene Propylene Diene Monomer (EPDM).
Question 4: In a high-pressure pneumatic system, what is the primary function of a moisture separator?
- To add oil to the air for lubricating pneumatic components.
- To remove entrained water vapor to prevent freezing and corrosion. (Correct answer)
- To regulate the downstream air pressure to a constant value.
- To store a reserve of compressed air for emergency operations.
Correct answer: To remove entrained water vapor to prevent freezing and corrosion.
When air is compressed, its temperature rises and its ability to hold moisture decreases, causing water vapor to condense. A moisture separator is designed to remove this liquid water from the pneumatic system to prevent it from freezing at high altitudes, which could block lines, and to prevent corrosion of system components.
Question 5: An aircraft hydraulic system is powered by a constant-displacement pump. To prevent pump and system overload when no services are being operated, what component diverts the full output of the pump back to the reservoir at low pressure?
- System pressure relief valve
- Shuttle valve
- Unloading valve (Correct answer)
- Check valve
Correct answer: Unloading valve
A constant-displacement pump moves a fixed volume of fluid with each revolution. An unloading valve is used to automatically divert the pump's output to the reservoir at low pressure when system pressure reaches a set point. This 'unloads' the pump, reducing power consumption and heat generation, while a check valve maintains pressure in the system. A pressure relief valve is a safety device for over-pressurization, not for normal unloading.
Question 6: What is the mandatory, first procedural step before checking the nitrogen pre-charge pressure of a hydraulic system accumulator?
- Operate the system until normal operating temperature is reached.
- Ensure all hydraulic system pressure is completely discharged. (Correct answer)
- Pressurize the hydraulic system to its maximum rated value.
- Cycle a nearby hydraulic actuator several times.
Correct answer: Ensure all hydraulic system pressure is completely discharged.
To get an accurate reading of the nitrogen pre-charge, all hydraulic pressure on the fluid side of the accumulator must be released. If hydraulic pressure remains, it will compress the nitrogen, leading to a falsely high pressure reading on the charging gauge. Discharging system pressure is a critical safety and accuracy step.
A&P Hydraulic and Pneumatic Systems Questions and Answers
A technician observes that a hydraulic system's pressure gauge fluctuates rapidly and the system operates erratically, especially upon initial actuator movement.
Which component is most likely failing to perform its function of dampening pressure surges?