Hydraulic and Pneumatic Systems Flashcards

An aircraft's hydraulic system reservoir is found to be pressurized with bleed air from the engine's compressor section. What is the primary reason for pressurizing the reservoir?

Answer: To prevent hydraulic fluid from foaming at high altitudes.

At high altitudes, the ambient atmospheric pressure is significantly lower. This reduced pressure can cause dissolved air in the hydraulic fluid to come out of solution, creating foam. Foaming reduces the hydraulic system's efficiency and can cause pump cavitation (the formation of vapor cavities in the liquid). Pressurizing the reservoir ensures a positive pressure on the fluid, keeping the dissolved air in solution and preventing foaming.

A technician is servicing a large transport category aircraft and needs to identify the type of hydraulic fluid used in the main system. Which of the following is the most reliable method to determine the correct fluid type?

Answer: Consulting the aircraft manufacturer's maintenance manual or the placard on the reservoir.

The most accurate and authoritative source for identifying the correct hydraulic fluid is the aircraft manufacturer's maintenance manual or the instruction plate (placard) affixed to the hydraulic reservoir or the unit being serviced. While fluid color can be an indicator (e.g., mineral-based is often red, phosphate ester is purple), it is not a foolproof method due to potential dye variations or contamination. Assumptions should never be made in aviation maintenance.

What is the fundamental operating principle that allows a hydraulic system to transmit a small input force into a large output force?

Answer: Pascal's Law

Pascal's Law is the foundational principle of hydraulics. It states that pressure applied to a confined, incompressible fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel. This principle allows a small force applied to a small area (input piston) to generate a much larger force on a larger area (output piston).

During a pre-flight inspection, a technician notices that the brake system pressure is holding steady even though the main hydraulic pumps are offline. Which component is responsible for maintaining this pressure for a short duration?

Answer: An accumulator

An accumulator is a device that stores hydraulic fluid under pressure. One of its key functions is to store energy to provide pressure for short-term or emergency use, such as for the parking brake, when the main hydraulic pumps are not operating.

A technician is troubleshooting a hydraulic system that is operating sluggishly. The fluid in the reservoir appears milky or foamy. Which of the following is the most likely cause?

Answer: Air is entering the system, likely through a leak in a suction line.

Milky or foamy hydraulic fluid is a classic symptom of air or water contamination. Air entering the system, a process known as aeration, can be caused by low fluid levels or, more commonly, a leak in the pump's suction line. This contamination reduces the fluid's incompressibility and leads to sluggish and erratic system operation.

Which of the following is a primary advantage of a pneumatic power system compared to a hydraulic system on an aircraft?

Answer: Air is universally available and there is no need for a return line, saving weight.

A significant advantage of pneumatic systems is that the operating fluid, air, is readily available and can be exhausted to the atmosphere after use. This eliminates the need for a return line, which saves both weight and complexity compared to a closed-loop hydraulic system that must return fluid to the reservoir.