AMT Hydraulic and Pneumatic Systems Questions and Answers — Questions and Answers
Question 1: An aircraft hydraulic system's reservoir is pressurized. What is the primary reason for this pressurization, especially for aircraft operating at high altitudes?
- To increase the thermal capacity of the hydraulic fluid.
- To provide a positive fluid flow and prevent pump cavitation. (Correct answer)
- To automatically purge any trapped air from the system.
- To serve as a secondary source of pressure for the brakes.
Correct answer: To provide a positive fluid flow and prevent pump cavitation.
At high altitudes, the ambient atmospheric pressure is significantly lower. Pressurizing the hydraulic reservoir ensures a constant, positive supply of fluid (head pressure) to the inlet of the hydraulic pumps, preventing pump cavitation, which is the formation of vapor cavities in the fluid that can damage the pump and reduce system efficiency.
Question 2: A technician is servicing an aircraft that uses Skydrol, a phosphate-ester based hydraulic fluid. Which of the following is a critical safety and maintenance consideration when handling this type of fluid?
- Skydrol is compatible with natural rubber seals and hoses.
- It is non-toxic and requires no special personal protective equipment.
- It is highly flammable and must be kept away from ignition sources.
- Skydrol is an excellent solvent and can damage many paints, plastics, and adhesives. (Correct answer)
Correct answer: Skydrol is an excellent solvent and can damage many paints, plastics, and adhesives.
Skydrol is known for its excellent fire-resistant properties, but it is also a very aggressive solvent. It is incompatible with many common materials, including certain paints, plastics, and elastomers like natural rubber. Technicians must use Skydrol-resistant materials for seals (like Butyl or Ethylene Propylene) and exercise care to prevent it from contacting incompatible aircraft surfaces or wiring insulation. It is also irritating to human tissue, requiring proper PPE.
Question 3: What is the primary function of a hydraulic accumulator in an aircraft system?
- To filter contaminants from the hydraulic fluid before it enters the pump.
- To cool the hydraulic fluid by radiating heat into the surrounding structure.
- To store hydraulic fluid under pressure to supplement the pump or be used in emergencies. (Correct answer)
- To mix air with the hydraulic fluid to enhance its compressibility.
Correct answer: To store hydraulic fluid under pressure to supplement the pump or be used in emergencies.
A hydraulic accumulator is a pressure storage reservoir that holds hydraulic fluid under pressure from an external source, typically compressed nitrogen gas. Its main functions are to supplement the engine-driven pump during peak demand, dampen pressure surges, and provide a limited source of hydraulic power for essential components (like brakes or landing gear) in the event of a pump failure.
Question 4: In a high-pressure pneumatic system, what is the purpose of a relief valve?
- To regulate the downstream pressure to a constant, usable level for specific components.
- To add moisture to the compressed air to prevent seals from drying out.
- To protect the system from damage by venting excess pressure above a preset limit. (Correct answer)
- To direct the flow of air to different actuators within the system.
Correct answer: To protect the system from damage by venting excess pressure above a preset limit.
A relief valve is a safety device designed to protect a pneumatic system from over-pressurization. If the system pressure exceeds a predetermined set point, the valve opens automatically to vent the excess compressed air, preventing damage to components or lines.
Question 5: A hydraulic actuator with a piston area of 5 square inches is required to produce a force of 15,000 pounds. Based on Pascal's Law, what is the minimum system pressure required to achieve this force, neglecting any friction or back pressure?
- 75,000 PSI
- 5,000 PSI
- 3,000 PSI (Correct answer)
- 1,500 PSI
Correct answer: 3,000 PSI
Pascal's Law states that pressure in a confined fluid is transmitted equally throughout. The relationship between pressure, force, and area is given by the formula: Pressure = Force / Area. In this scenario, Pressure = 15,000 pounds / 5 square inches = 3,000 PSI.
Question 6: Which of the following is a primary method for removing moisture that has condensed in an aircraft pneumatic system?
- Injecting a desiccant fluid into the pneumatic lines during operation.
- Using drain valves located at low points in the system. (Correct answer)
- Heating the entire system with bleed air to evaporate the water.
- Increasing system pressure to force the water back into a gaseous state.
Correct answer: Using drain valves located at low points in the system.
As compressed air cools in a pneumatic system, moisture condenses into liquid water, which can cause corrosion and other damage. A primary and common method for removing this liquid water is to install drain valves at the lowest points of the system, such as in receiver tanks and along air lines, where the water naturally collects. These can be operated manually or automatically.
AMT Hydraulic and Pneumatic Systems Questions and Answers
An aircraft hydraulic system's reservoir is pressurized.
What is the primary reason for this pressurization, especially for aircraft operating at high altitudes?