01.043.019 – HYDRAULIC SYSTEMS – THE MARVIN PROTOCOL

01.043.019.001 — State Pascal’s law and its practical consequence in an aircraft hydraulic system.

A. Pascal’s law states that pressure applied to a confined fluid is transmitted undiminished throughout the fluid and acts equally in all directions. In practice, this means that actuator force is equal to flow multiplied by piston area, so force can be multiplied by using a larger piston.

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B. Hydraulic lock is the condition in which an actuator is held stationary because fluid is trapped on both sides of the piston and no flow can enter or leave the actuator chambers.

Correct Incorrect Correct answer

C. Hydraulic systems provide high power-to-weight capability, efficient remote transmission of force through small lines, compact installation, precise control of large aerodynamic surfaces, and good compatibility with redundant system architecture.

Correct Incorrect Correct answer

D. Pascal’s law states that pressure applied to a confined fluid is transmitted undiminished throughout the fluid and acts equally in all directions. In practice, this means that actuator force is equal to pressure multiplied by piston area, so force can be multiplied by using a larger piston.

Correct Incorrect Correct answer

01.043.019.002 — Explain why increasing hydraulic system pressure allows smaller actuators for the same output force.

A. For a given required force, increasing system pressure increases the piston area needed to produce that force.

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B. A constant-pressure variable-displacement pump uses a pressure-compensated swashplate. As outlet pressure rises to the selected value, the swashplate angle reduces, decreasing piston stroke and pump displacement until only the required flow is produced.

Correct Incorrect Correct answer

C. For a given required force, increasing system pressure reduces the piston area needed to produce that force.

Correct Incorrect Correct answer

D. Hydraulic fluid types must not be mixed because seals, hoses, elastomers, coatings, and other system materials are selected for compatibility with a specific fluid family.

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01.043.019.003 — State the main operational advantages of hydraulic systems in aircraft.

A. Hydraulic systems provide low power-to-weight capability, efficient remote transmission of force through small lines, compact installation, precise control of large aerodynamic surfaces, and good compatibility with redundant system architecture.

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B. Hydraulic lock is the condition in which an actuator is held stationary because fluid is trapped on both sides of the piston and no flow can enter or leave the actuator chambers.

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C. Aircraft hydraulic fluids must have low effective compressibility, controlled viscosity across a wide temperature range, good lubricity, fire resistance, corrosion inhibition, and compatibility with system materials and seals.

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D. Hydraulic systems provide high power-to-weight capability, efficient remote transmission of force through small lines, compact installation, precise control of large aerodynamic surfaces, and good compatibility with redundant system architecture.

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01.043.019.004 — State the main disadvantages of hydraulic systems.

A. Pneumatic systems are relatively expensive, vulnerable to leakage, sensitive to fluid contamination, and require tight tolerances, filtration, and careful maintenance.

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B. Hydraulic systems are relatively expensive, vulnerable to leakage, sensitive to fluid contamination, and require tight tolerances, filtration, and careful maintenance.

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C. A Power Transfer Unit transfers hydraulic power from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

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D. Aircraft hydraulic fluids must have low effective compressibility, controlled viscosity across a wide temperature range, good lubricity, fire resistance, corrosion inhibition, and compatibility with system materials and seals.

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01.043.019.005 — State the essential properties required of aircraft hydraulic fluids.

A. Aircraft hydraulic fluids must have low effective compressibility, controlled viscosity across a wide temperature range, good lubricity, fire resistance, corrosion inhibition, and compatibility with system materials and seals.

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B. Aircraft hydraulic fluids must have high effective compressibility, controlled viscosity across a wide temperature range, good lubricity, fire resistance, corrosion inhibition, and compatibility with system materials and seals.

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C. Hydraulic systems provide high power-to-weight capability, efficient remote transmission of force through small lines, compact installation, precise control of large aerodynamic surfaces, and good compatibility with redundant system architecture.

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D. Hydraulic systems are relatively expensive, vulnerable to leakage, sensitive to fluid contamination, and require tight tolerances, filtration, and careful maintenance.

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01.043.019.006 — Compare synthetic phosphate-ester hydraulic fluids with mineral-based hydraulic fluids.

A. Synthetic phosphate-ester fluids are used mainly on transport aircraft because they offer improved fire resistance. Mineral-based fluids provide good lubricity and corrosion protection but are less flammable and are therefore used only where the aircraft design and certification basis permit them.

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B. In an open-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

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C. Synthetic phosphate-ester fluids are used mainly on transport aircraft because they offer improved fire resistance. Mineral-based fluids provide good lubricity and corrosion protection but are more flammable and are therefore used only where the aircraft design and certification basis permit them.

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D. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at high altitude, high flow demand, or low temperature.

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01.043.019.007 — Explain why hydraulic fluid types must not be mixed.

A. Pneumatic fluid types must not be mixed because seals, hoses, elastomers, coatings, and other system materials are selected for compatibility with a specific fluid family.

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B. A Power Transfer Unit transfers hydraulic power from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

Correct Incorrect Correct answer

C. Hydraulic fluid types must not be mixed because seals, hoses, elastomers, coatings, and other system materials are selected for compatibility with a specific fluid family.

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D. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at high altitude, high flow demand, or low temperature.

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01.043.019.008 — Compare open-centre and closed-centre hydraulic systems.

A. In an closed-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

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B. In an open-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

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C. Synthetic phosphate-ester fluids are used mainly on transport aircraft because they offer improved fire resistance. Mineral-based fluids provide good lubricity and corrosion protection but are more flammable and are therefore used only where the aircraft design and certification basis permit them.

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D. A single-acting hand pump delivers hydraulic fluid on only one stroke of the handle, whereas a double-acting hand pump delivers fluid on both strokes.

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01.043.019.009 — Compare single-acting, double-acting, and balanced double-acting actuators.

A. A single-acting actuator is hydraulically powered in one direction only and returns by spring or external force. A double-acting actuator is hydraulically powered in both directions, but its forces and volumes differ between extension and retraction because of the rod area. A balanced double-acting actuator uses rods on both sides of the piston so that the effective area is equal in both directions.

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B. The items perform essentially the same function; the distinction is mainly their installation location rather than their operating principle.

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C. For a given required force, increasing system pressure reduces the piston area needed to produce that force.

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D. In an open-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

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01.043.019.010 — State the purpose of a double-acting tandem actuator.

A. A double-acting tandem actuator provides actuator-level redundancy by incorporating two independent pneumatic sections powered by separate hydraulic systems within one assembly.

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B. Hydraulic filtration removes contamination from both the pressure side and the return side of the system in order to protect pumps, valves, actuators, and reservoir fluid.

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C. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at high altitude, high flow demand, or low temperature.

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D. A double-acting tandem actuator provides actuator-level redundancy by incorporating two independent hydraulic sections powered by separate hydraulic systems within one assembly.

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01.043.019.011 — Define hydraulic lock and state one operational consequence.

A. Pneumatic lock is the condition in which an actuator is held stationary because fluid is trapped on both sides of the piston and no flow can enter or leave the actuator chambers.

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B. Hydraulic systems provide high power-to-weight capability, efficient remote transmission of force through small lines, compact installation, precise control of large aerodynamic surfaces, and good compatibility with redundant system architecture.

Correct Incorrect Correct answer

C. Hydraulic lock is the condition in which an actuator is held stationary because fluid is trapped on both sides of the piston and no flow can enter or leave the actuator chambers.

Correct Incorrect Correct answer

D. Pascal’s law states that pressure applied to a confined fluid is transmitted undiminished throughout the fluid and acts equally in all directions. In practice, this means that actuator force is equal to pressure multiplied by piston area, so force can be multiplied by using a larger piston.

Correct Incorrect Correct answer

01.043.019.012 — Compare single-acting and double-acting hand pumps.

A. A single-acting hand pump delivers hydraulic fluid on only one stroke of the handle, whereas a double-acting hand pump delivers fluid on both strokes.

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B. A single-acting hand pump delivers pneumatic fluid on only one stroke of the handle, whereas a double-acting hand pump delivers fluid on both strokes.

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C. In an open-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

Correct Incorrect Correct answer

D. Synthetic phosphate-ester fluids are used mainly on transport aircraft because they offer improved fire resistance. Mineral-based fluids provide good lubricity and corrosion protection but are more flammable and are therefore used only where the aircraft design and certification basis permit them.

Correct Incorrect Correct answer

01.043.019.013 — Compare gear pumps and piston pumps used as engine-driven pumps.

A. A gear pump is a simple positive-displacement pump that provides robust, continuous flow, but its efficiency is more sensitive to wear and internal leakage. A piston pump is also positive-displacement, but it generally provides lower capacity and better performance at higher pressures, which makes it more suitable for main aircraft hydraulic systems.

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B. A gear pump is a simple positive-displacement pump that provides robust, continuous flow, but its efficiency is more sensitive to wear and internal leakage. A piston pump is also positive-displacement, but it generally provides higher capacity and better performance at higher pressures, which makes it more suitable for main aircraft hydraulic systems.

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C. In an open-centre system, pump flow returns to reservoir through an open path when the valves are in neutral, and functions tend to operate one at a time. In a closed-centre system, there is no open return path in neutral, pressure is maintained, and several services can operate simultaneously.

Correct Incorrect Correct answer

D. Synthetic phosphate-ester fluids are used mainly on transport aircraft because they offer improved fire resistance. Mineral-based fluids provide good lubricity and corrosion protection but are more flammable and are therefore used only where the aircraft design and certification basis permit them.

Correct Incorrect Correct answer

01.043.019.014 — Explain how a constant-pressure variable-displacement pump regulates output and why it does not require an ACOV.

A. A constant-pressure variable-displacement pump uses a pressure-compensated swashplate. As outlet pressure rises to the selected value, the swashplate angle increases, decreasing piston stroke and pump displacement until only the required flow is produced.

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B. A constant-pressure variable-displacement pump uses a pressure-compensated swashplate. As outlet pressure rises to the selected value, the swashplate angle reduces, decreasing piston stroke and pump displacement until only the required flow is produced.

Correct Incorrect Correct answer

C. For a given required force, increasing system pressure reduces the piston area needed to produce that force.

Correct Incorrect Correct answer

D. A Power Transfer Unit transfers hydraulic power from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

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01.043.019.015 — State the function of a PTU and explain why it does not transfer fluid between systems.

A. A Power Transfer Unit transfers hydraulic fluid from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

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B. A Power Transfer Unit transfers hydraulic power from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

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C. A hydraulic accumulator stores energy by compressing nitrogen and releases hydraulic fluid when demand rises or system pressure drops. Correct pre-charge is essential because too little pre-charge reduces usable stored energy and damping, while too much reduces the available fluid volume.

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D. Hydraulic systems are relatively expensive, vulnerable to leakage, sensitive to fluid contamination, and require tight tolerances, filtration, and careful maintenance.

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01.043.019.016 — Explain the purpose of a pressurised hydraulic reservoir.

A. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at low altitude, high flow demand, or low temperature.

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B. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at high altitude, high flow demand, or low temperature.

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C. Hydraulic filtration removes contamination from both the pressure side and the return side of the system in order to protect pumps, valves, actuators, and reservoir fluid.

Correct Incorrect Correct answer

D. Hydraulic fluid types must not be mixed because seals, hoses, elastomers, coatings, and other system materials are selected for compatibility with a specific fluid family.

Correct Incorrect Correct answer

01.043.019.017 — State the function of a hydraulic accumulator and explain why correct nitrogen pre-charge matters.

A. A hydraulic accumulator stores energy by compressing nitrogen and releases hydraulic fluid when demand rises or system pressure drops. Correct pre-charge is essential because too little pre-charge increases usable stored energy and damping, while too much reduces the available fluid volume.

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B. A Power Transfer Unit transfers hydraulic power from one hydraulic system to another in order to restore or maintain pressure in a low-pressure system. It does not transfer fluid because the two sides are connected only mechanically through a common shaft.

Correct Incorrect Correct answer

C. Hydraulic filtration removes contamination from both the pressure side and the return side of the system in order to protect pumps, valves, actuators, and reservoir fluid.

Correct Incorrect Correct answer

D. A hydraulic accumulator stores energy by compressing nitrogen and releases hydraulic fluid when demand rises or system pressure drops. Correct pre-charge is essential because too little pre-charge reduces usable stored energy and damping, while too much reduces the available fluid volume.

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01.043.019.018 — State the purpose of hydraulic filtration and explain the significance of filter bypass operation.

A. Hydraulic filtration removes contamination from both the pressure side and the return side of the system in order to protect pumps, valves, actuators, and reservoir fluid.

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B. Pneumatic filtration removes contamination from both the pressure side and the return side of the system in order to protect pumps, valves, actuators, and reservoir fluid.

Correct Incorrect Correct answer

C. A double-acting tandem actuator provides actuator-level redundancy by incorporating two independent hydraulic sections powered by separate hydraulic systems within one assembly.

Correct Incorrect Correct answer

D. A pressurised hydraulic reservoir maintains positive pressure at the pump inlet in order to prevent cavitation and ensure stable pump operation, especially at high altitude, high flow demand, or low temperature.

Correct Incorrect Correct answer

01.043.019.019 — Distinguish a non-return valve from a pressure relief valve.

A. A non-return valve allows flow in one direction only and promotes reverse flow. A pressure relief valve opens when pressure exceeds a set limit and protects the system from overpressure.

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B. For a given required force, increasing system pressure reduces the piston area needed to produce that force.

Correct Incorrect Correct answer

C. A constant-pressure variable-displacement pump uses a pressure-compensated swashplate. As outlet pressure rises to the selected value, the swashplate angle reduces, decreasing piston stroke and pump displacement until only the required flow is produced.

Correct Incorrect Correct answer

D. A non-return valve allows flow in one direction only and prevents reverse flow. A pressure relief valve opens when pressure exceeds a set limit and protects the system from overpressure.

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01.043.019.020 — Explain why entrained air is more harmful to hydraulic response than the small compressibility of the fluid itself.

A. Hydraulic fluid is only slightly compressible, but entrained air is highly compressible.

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B. Pneumatic fluid is only slightly compressible, but entrained air is highly compressible.

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C. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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D. If hydraulic fluid is trapped between closed valves and temperature rises, the fluid expands and pressure can increase rapidly.

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01.043.019.021 — Explain why insufficient accumulator pre-charge increases ACOV cycling in a closed-centre constant-delivery system.

A. With insufficient pre-charge, the accumulator stores more usable energy and cannot smooth small pressure losses effectively.

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B. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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C. With insufficient pre-charge, the accumulator stores less usable energy and cannot smooth small pressure losses effectively.

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D. Case drain flow carries internal leakage from the pump housing.

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01.043.019.022 — Explain why filter bypass operation is considered an abnormal condition even though hydraulic flow is maintained.

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. Case drain flow carries internal leakage from the pump housing.

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C. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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D. Filter bypass preserves system flow, but it allows unfiltered fluid to circulate through valves, actuators, and pumps.

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01.043.019.023 — Explain why trapped hydraulic lines may require a thermal relief function.

A. If hydraulic fluid is trapped between closed valves and temperature rises, the fluid expands and pressure can decrease rapidly.

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B. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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C. Hydraulic fluid is only slightly compressible, but entrained air is highly compressible.

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D. If hydraulic fluid is trapped between closed valves and temperature rises, the fluid expands and pressure can increase rapidly.

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01.043.019.024 — Explain why case drain flow is a useful indicator of pump condition.

A. Case drain flow carries internal leakage from the pump housing.

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B. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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C. Filter bypass preserves system flow, but it allows unfiltered fluid to circulate through valves, actuators, and pumps.

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D. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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01.043.019.025 — Explain why reservoir quantity indication may decrease when the hydraulic system is pressurised.

A. When the system is pressurised, fluid is distributed into actuators, lines, and other pressurised components, and the reservoir reaches a new internal equilibrium.

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B. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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C. If hydraulic fluid is trapped between closed valves and temperature rises, the fluid expands and pressure can increase rapidly.

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D. Hydraulic fluid is only slightly compressible, but entrained air is highly compressible.

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01.043.019.026 — Compare the function of a pressure reducer with that of a pressure relief valve.

A. A pressure reducer maintains a higher controlled downstream pressure during normal operation. A pressure relief valve remains closed in normal operation and opens only when pressure exceeds a safe limit.

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B. A pressure reducer maintains a lower controlled downstream pressure during normal operation. A pressure relief valve remains closed in normal operation and opens only when pressure exceeds a safe limit.

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C. A pressure-maintaining valve protects essential services by restricting or cutting off supply to non-essential consumers when system pressure falls below a threshold.

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D. Pressure rise usually indicates that the first actuator has reached the end of its travel or that the required upstream condition has been achieved.

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01.043.019.027 — State the purpose of a pressure-maintaining valve in a hydraulic system.

A. Its primary purpose is to increase the associated load or pressure and allow unrestricted operation rather than to regulate, isolate, or protect the system.

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B. A pressure reducer maintains a lower controlled downstream pressure during normal operation. A pressure relief valve remains closed in normal operation and opens only when pressure exceeds a safe limit.

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C. Pressure rise usually indicates that the first actuator has reached the end of its travel or that the required upstream condition has been achieved.

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D. A pressure-maintaining valve protects essential services by restricting or cutting off supply to non-essential consumers when system pressure falls below a threshold.

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01.043.019.028 — Explain why a sequence valve is often triggered by pressure rise rather than by time alone.

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. As altitude increases, oxygen partial pressure falls further and the body reaches incapacitating hypoxia much more quickly.

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C. A pressure reducer maintains a lower controlled downstream pressure during normal operation. A pressure relief valve remains closed in normal operation and opens only when pressure exceeds a safe limit.

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D. Pressure rise usually indicates that the first actuator has reached the end of its travel or that the required upstream condition has been achieved.

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01.043.019.029 — State the purpose of a hydraulic fuse and its main system benefit.

A. A hydraulic fuse automatically closes or restricts flow when abnormally low downstream flow indicates a line rupture or major leak.

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B. Blow-out panels allow rapid pressure equalisation between compartments during decompression.

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C. Fusible plugs protect against tyre burst by melting when wheel temperature becomes excessive, allowing controlled tyre deflation.

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D. A hydraulic fuse automatically closes or restricts flow when abnormally high downstream flow indicates a line rupture or major leak.

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01.043.019.030 — Explain why a closed-centre hydraulic system with a constant-delivery pump requires an ACOV.

A. With all selector valves in neutral, the open-centre system blocks return flow, but the constant-delivery pump still tries to deliver fluid.

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B. It can de-stroke automatically as system pressure reaches its selected value.

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C. With all selector valves in neutral, the closed-centre system blocks return flow, but the constant-delivery pump still tries to deliver fluid.

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D. The ejector relies on motive flow from the low-pressure pump to create the Venturi suction effect.

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01.043.019.031 — Explain why a variable-displacement constant-pressure pump does not need the same unloading philosophy.

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. It can de-stroke automatically as system pressure reaches its selected value.

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C. With all selector valves in neutral, the closed-centre system blocks return flow, but the constant-delivery pump still tries to deliver fluid.

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D. The ejector relies on motive flow from the low-pressure pump to create the Venturi suction effect.

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01.043.019.032 — Explain why contamination is especially harmful to servo and selector valves.

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. These components rely on very small clearances and precise internal movement.

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C. Air extracted from a higher compressor stage is generally hotter.

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D. Air removed from the compressor is no longer fully available to the main propulsion cycle.

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01.043.019.033 — Explain why a PTU is valuable in a multi-system aircraft even though it cannot replenish hydraulic quantity.

A. It can restore pressure and useful hydraulic power to a degraded system by using energy from another system.

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B. It can restore pressure and useful pneumatic power to a degraded system by using energy from another system.

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C. With all selector valves in neutral, the closed-centre system blocks return flow, but the constant-delivery pump still tries to deliver fluid.

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D. Air extracted from a higher compressor stage is generally hotter.

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01.043.019.034 — Distinguish the typical indications of an internal hydraulic leak from those of an external hydraulic leak.

A. An internal pneumatic leak is typically indicated by temperature rise, reduced efficiency, slow actuator response, pressure decay, or frequent regulation cycling. An external hydraulic leak is typically indicated by loss of reservoir quantity and visible fluid leakage.

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B. An internal hydraulic leak is typically indicated by temperature rise, reduced efficiency, slow actuator response, pressure decay, or frequent regulation cycling. An external hydraulic leak is typically indicated by loss of reservoir quantity and visible fluid leakage.

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C. Frequent ACOV cycling usually indicates that system pressure is decaying too quickly in standby.

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D. Abnormally high case-drain flow is a strong indication of internal pump wear or impending pump failure.

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01.043.019.035 — Interpret frequent ACOV cut-in and cut-out cycling in a closed-centre hydraulic system.

A. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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B. Frequent ACOV cycling usually indicates that system pressure is decaying too quickly in standby.

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C. Excessive shoulder wear is a typical indication of under-inflation.

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D. The gear is down and locked, but the red indication suggests the abnormal selector or door configuration associated with emergency extension.

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01.043.019.036 — Assess the significance of abnormally high case-drain flow from an engine-driven hydraulic pump.

A. Abnormally low case-drain flow is a strong indication of internal pump wear or impending pump failure.

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B. The landing gear becomes more susceptible to shimmy during ground operations.

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C. Abnormally high case-drain flow is a strong indication of internal pump wear or impending pump failure.

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D. Frequent ACOV cycling usually indicates that system pressure is decaying too quickly in standby.

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01.043.019.037 — Identify the likely operational effect of a clogged hydraulic filter after bypass-valve opening.

A. Pneumatic services may continue to operate, but unfiltered fluid will circulate through the system.

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B. The landing gear becomes more susceptible to shimmy during ground operations.

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C. Hydraulic services may continue to operate, but unfiltered fluid will circulate through the system.

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D. An internal hydraulic leak is typically indicated by temperature rise, reduced efficiency, slow actuator response, pressure decay, or frequent regulation cycling. An external hydraulic leak is typically indicated by loss of reservoir quantity and visible fluid leakage.

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01.043.019.038 — Assess the system-level benefit of a hydraulic fuse after a downstream line rupture.

A. A hydraulic fuse isolates the low-flow leak and helps preserve fluid and pressure for the rest of the system.

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B. A non-return valve prevents reverse flow and preserves the intended source priority between branches or systems.

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C. A hydraulic fuse isolates the high-flow leak and helps preserve fluid and pressure for the rest of the system.

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D. Blow-out panels allow rapid pressure equalisation between compartments during decompression.

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01.043.019.039 — Discuss the safety value of a non-return valve in multi-source hydraulic logic.

A. A non-return valve promotes reverse flow and preserves the intended source priority between branches or systems.

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B. The negative pressure relief valve protects the aircraft structure when ambient pressure becomes significantly higher than cabin pressure.

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C. The positive pressure relief valve protects the fuselage by opening automatically if cabin differential pressure exceeds the structural limit.

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D. A non-return valve prevents reverse flow and preserves the intended source priority between branches or systems.

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01.043.019.040 — Analyse the consequence of insufficient accumulator pre-charge on emergency braking capability.

A. Emergency or standby braking capability is reduced because the accumulator stores less usable hydraulic energy.

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B. Emergency or standby braking capability is reduced because the accumulator stores more usable hydraulic energy.

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C. If anti-skid becomes inoperative, the system disconnects and the brakes revert to full manual operation.

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D. Blow-out panels allow rapid pressure equalisation between compartments during decompression.

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01.043.019.041 — Compare a shuttle valve with a non-return valve.

A. A shuttle valve automatically selects the lower of two pressure sources to feed one service, whereas a non-return valve only permits one-way flow and blocks reverse flow.

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B. It protects essential services by reducing or cutting off hydraulic supply to non-essential consumers when system pressure becomes insufficient.

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C. A shuttle valve automatically selects the higher of two pressure sources to feed one service, whereas a non-return valve only permits one-way flow and blocks reverse flow.

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D. A pressure reducer actively maintains a lower downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

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01.043.019.042 — Compare a restrictor valve with a flow-control valve.

A. A restrictor valve gives free flow in one direction and metered flow in the other, whereas a flow-control valve regulates flow to maintain a less constant actuator speed.

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B. A restrictor valve gives free flow in one direction and metered flow in the other, whereas a flow-control valve regulates flow to maintain a more constant actuator speed.

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C. It protects essential services by reducing or cutting off hydraulic supply to non-essential consumers when system pressure becomes insufficient.

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D. A pressure reducer actively maintains a lower downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

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01.043.019.043 — Explain why a pressure-maintaining valve is sometimes described as a priority valve.

A. It protects essential services by reducing or cutting off pneumatic supply to non-essential consumers when system pressure becomes insufficient.

Correct Incorrect Correct answer

B. A restrictor valve gives free flow in one direction and metered flow in the other, whereas a flow-control valve regulates flow to maintain a more constant actuator speed.

Correct Incorrect Correct answer

C. It protects essential services by reducing or cutting off hydraulic supply to non-essential consumers when system pressure becomes insufficient.

Correct Incorrect Correct answer

D. A shuttle valve automatically selects the higher of two pressure sources to feed one service, whereas a non-return valve only permits one-way flow and blocks reverse flow.

Correct Incorrect Correct answer

01.043.019.044 — Explain why a pressure reducer is not simply another name for a pressure relief valve.

A. A pressure reducer actively maintains a lower downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

Correct Incorrect Correct answer

B. A pressure reducer actively maintains a higher downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

Correct Incorrect Correct answer

C. A restrictor valve gives free flow in one direction and metered flow in the other, whereas a flow-control valve regulates flow to maintain a more constant actuator speed.

Correct Incorrect Correct answer

D. A shuttle valve automatically selects the higher of two pressure sources to feed one service, whereas a non-return valve only permits one-way flow and blocks reverse flow.

Correct Incorrect Correct answer

01.043.019.045 — Explain why reservoir pressurisation helps prevent cavitation.

A. It maintains positive pressure at the pump inlet and increases the chance of local pressure falling low enough for vapour cavities to form.

Correct Incorrect Correct answer

B. It maintains positive pressure at the pump inlet and reduces the chance of local pressure falling low enough for vapour cavities to form.

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C. A pressure reducer actively maintains a lower downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

Correct Incorrect Correct answer

D. The non-return valve prevents reverse flow into the APU when another pneumatic source is pressurising the manifold.

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01.043.019.046 — Explain why hydraulic lock is useful in normal operation but problematic after certain failures.

A. In normal operation, pneumatic lock helps hold a surface or actuator firmly in position.

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B. Some high-lift systems require continuous rotary drive through transmissions and gearboxes rather than direct linear displacement.

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C. It ensures that one movement begins only after the required preceding condition has been achieved.

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D. In normal operation, hydraulic lock helps hold a surface or actuator firmly in position.

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01.043.019.047 — Explain why a pressure relay was valuable on older direct-reading hydraulic gauges.

A. It allowed system pressure to be indicated in the cockpit without a continuous free pneumatic line running directly into the flight deck.

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B. It allowed system pressure to be indicated in the cockpit without a continuous free hydraulic line running directly into the flight deck.

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C. The piston physically separates hydraulic fluid from the pressurising gas or air source.

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D. A pressure reducer actively maintains a lower downstream working pressure during normal operation, while a pressure relief valve opens only when pressure exceeds a safety limit.

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01.043.019.048 — Explain why a power-pack hydraulic system is particularly suitable for light aircraft landing gear.

A. It provides a compact, self-contained hydraulic source for an intermittent, limited-duty function.

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B. It provides a compact, self-contained pneumatic source for an intermittent, limited-duty function.

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C. It ensures that one movement begins only after the required preceding condition has been achieved.

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D. Fuel can absorb heat from hotter fluids or components before it is burned in the engine.

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01.043.019.049 — Explain why a piston-type pressurised reservoir isolates fluid better than direct air pressurisation.

A. The piston physically separates pneumatic fluid from the pressurising gas or air source.

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B. It allowed system pressure to be indicated in the cockpit without a continuous free hydraulic line running directly into the flight deck.

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C. Some high-lift systems require continuous rotary drive through transmissions and gearboxes rather than direct linear displacement.

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D. The piston physically separates hydraulic fluid from the pressurising gas or air source.

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01.043.019.050 — Explain why a restrictor valve is often installed only in one direction of actuator travel.

A. The structural or aerodynamic hazard is often more significant in one movement direction than in the other.

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B. The structural or aerodynamic hazard is often less significant in one movement direction than in the other.

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C. Some high-lift systems require continuous rotary drive through transmissions and gearboxes rather than direct linear displacement.

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D. It ensures that one movement begins only after the required preceding condition has been achieved.

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01.043.019.051 — Explain why entrained air in a hydraulic system is often more operationally serious than a small change in fluid viscosity.

A. Entrained air greatly decreases apparent compressibility and reduces system stiffness.

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B. Entrained air greatly increases apparent compressibility and reduces system stiffness.

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C. It transfers hydraulic power, not hydraulic fluid quantity.

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D. It also smooths pressure fluctuations, damps transients, and supplies peak short-duration flow demand during normal operation.

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01.043.019.052 — Explain why a PTU can preserve useful hydraulic function without solving a hydraulic leak.

A. It transfers hydraulic power, not hydraulic fluid quantity.

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B. It transfers hydraulic fluid, not hydraulic fluid quantity.

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C. Entrained air greatly increases apparent compressibility and reduces system stiffness.

Correct Incorrect Correct answer

D. It also smooths pressure fluctuations, damps transients, and supplies peak short-duration flow demand during normal operation.

Correct Incorrect Correct answer

01.043.019.053 — Explain why a pressurised reservoir becomes especially important at altitude.

A. Ambient pressure is higher at altitude, so pump inlet conditions are more vulnerable to cavitation.

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B. It also smooths pressure fluctuations, damps transients, and supplies peak short-duration flow demand during normal operation.

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C. It is the primary normal regulating device, but it remains a control element subject to failure or miscommand.

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D. Ambient pressure is lower at altitude, so pump inlet conditions are more vulnerable to cavitation.

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01.043.019.054 — Explain why an accumulator is not simply an emergency reservoir.

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. It also smooths pressure fluctuations, damps transients, and supplies peak short-duration flow demand during normal operation.

Correct Incorrect Correct answer

C. Ambient pressure is lower at altitude, so pump inlet conditions are more vulnerable to cavitation.

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D. It is the primary normal regulating device, but it remains a control element subject to failure or miscommand.

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01.043.019.055 — What would you expect if a hydraulic reservoir lost pressurisation at high altitude?

A. Pump inlet conditions would worsen and cavitation risk would increase.

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B. Pump inlet conditions would worsen and cavitation risk would decrease.

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C. Valve performance could become erratic or degraded.

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D. Cabin differential pressure would rise rapidly.

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01.043.019.056 — What would be the consequence of insufficient accumulator pre-charge?

A. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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B. Pressure smoothing and stored-energy capability would both be reduced.

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C. Crew performance could degrade before obvious physical symptoms are noticed.

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D. Pressure support may be available, but the lost fluid quantity would not be restored.

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01.043.019.057 — What would you expect if contamination reached a high-precision servo valve?

A. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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B. Valve performance could become erratic or degraded.

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C. Pump inlet conditions would worsen and cavitation risk would increase.

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D. Cabin differential pressure would rise rapidly.

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01.043.019.058 — What would be the consequence of a PTU operating normally during a hydraulic-system quantity loss?

A. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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B. Bleed-air temperature downstream could rise beyond safe limits.

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C. Pressure smoothing and stored-energy capability would both be reduced.

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D. Pressure support may be available, but the lost fluid quantity would not be restored.

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01.043.019.059 — Why is hydraulic fluid treated as incompressible for system analysis even though real fluids are not perfectly incompressible?

A. Because its compressibility is small enough that pressure transmission can normally be treated as effectively direct.

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B. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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C. Because the two hydraulic systems remain fluid-isolated and are connected only by a mechanical shaft.

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D. Because it can damage pumps, block filters, and interfere with precision valves and actuators.

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01.043.019.060 — Why is contamination one of the main enemies of hydraulic reliability?

A. Because the design removes the need for the associated protection or control function and therefore has no meaningful effect on loads, flow, pressure, or controllability.

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B. Because its compressibility is small enough that pressure transmission can normally be treated as effectively direct.

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C. Because it can damage pumps, block filters, and interfere with precision valves and actuators.

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D. Because each indication describes a different aspect of the same fuel state.

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01.043.019.061 — Why does a variable-displacement pump reduce heat generation at low demand?

A. Because it increases its own output flow instead of continuously forcing excess flow through a regulating path.

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B. Because each indication describes a different aspect of the same fuel state.

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C. Because it allows one fuel source to continue feeding an engine if the normal supply path is no longer available.

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D. Because it reduces its own output flow instead of continuously forcing excess flow through a regulating path.

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01.043.019.062 — Why is a PTU described as transferring power rather than fluid?

A. Because the two hydraulic systems remain fluid-isolated and are connected only by a mechanical shaft.

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B. Because the two hydraulic systems remain fluid-isolated and are connected only by a electrical shaft.

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C. Because its compressibility is small enough that pressure transmission can normally be treated as effectively direct.

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D. Because each indication describes a different aspect of the same fuel state.

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01.043.019.063 — A hydraulic system shows frequent pressure-regulation cycling in standby. What would you suspect?

A. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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B. Because lower altitude increases ambient pressure and improves fuel-feed margin to the engine-driven pump.

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C. It suggests abnormal internal leakage and possible pump wear or impending failure.

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D. I would suspect pressure decay caused by leakage, poor accumulator support, or another abnormal internal loss path.

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01.043.019.064 — A hydraulic pump is still running, but case-drain flow becomes abnormally high. What does that suggest?

A. It suggests abnormal internal leakage and possible pump wear or impending failure.

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B. No significant operational change would be expected because the affected function is automatically compensated for without any change in system capability.

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C. Because lower altitude increases ambient pressure and improves fuel-feed margin to the engine-driven pump.

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D. The correct concern is that there may be a leak, an indication problem, or abnormal fuel consumption.

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01.043.019.065 — An examiner asks why a PTU cannot solve a loss-of-quantity problem. What is the correct reply?

A. Because it transfers hydraulic fluid, not hydraulic fluid.

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B. Because low temperature can increase viscosity and promote wax formation, which may restrict fuel flow.

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C. Because it transfers hydraulic power, not hydraulic fluid.

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D. The correct concern is that there may be a leak, an indication problem, or abnormal fuel consumption.

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01.043.019 – HYDRAULIC SYSTEMS

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