What is the primary advantage that makes hydraulic systems well suited for operating aircraft components?
ACS code: AM.II.F
Correct answer: They transmit large forces with relatively little weight and low fluid friction loss
Rationale: Hydraulic systems use a confined, essentially incompressible liquid to transmit force. Because liquids do not compress appreciably, applied force is transmitted nearly undiminished, giving a high mechanical advantage with light, compact components and little friction loss. Liquids are not meaningfully compressible, so energy is not stored by compressing the fluid (that describes pneumatics), and every practical hydraulic system still requires a reservoir, pump, and relief valve.
Which basic physical principle explains how a hydraulic system multiplies an applied force?
ACS code: AM.II.F
Correct answer: Pascal's law, stating pressure is transmitted equally throughout a confined fluid
Rationale: Pascal's law states that pressure applied to a confined fluid is transmitted undiminished in all directions and acts equally on all areas. By applying the same pressure to a larger piston area, a larger output force is produced (force equals pressure times area). Bernoulli's principle relates velocity and pressure in moving fluids, and Boyle's law applies to gases, not the force multiplication in hydraulics.
What is the primary function of a pressure relief valve in a hydraulic system?
ACS code: AM.II.F
Correct answer: To limit system pressure and protect components from excessive pressure
Rationale: A pressure relief valve is a safety device that opens to route fluid back to the reservoir when pressure exceeds a preset maximum, protecting lines, seals, and components from damage. Maintaining a constant flow to actuators is the role of a flow control valve, and preventing reverse flow is the role of a check valve.
Before disassembling a hydraulic accumulator, what must the technician do first?
ACS code: AM.II.F
Correct answer: Release the compressed air or nitrogen preload charge from the accumulator
Rationale: An accumulator contains a stored gas charge (air or nitrogen) at high pressure on one side of the piston, diaphragm, or bladder. This preload must be fully discharged before disassembly; otherwise the trapped gas can drive components apart violently and cause serious injury. Applying hydraulic pressure or heating the unit would not relieve the hazard and would make disassembly more dangerous.
What is the purpose of a check valve in a hydraulic system?
ACS code: AM.II.F
Correct answer: It permits fluid flow in one direction and blocks flow in the opposite direction
Rationale: A check valve allows fluid to pass freely in one direction while preventing flow in the reverse direction, which keeps fluid from backing up through the system. Limiting maximum pressure by venting to the reservoir is the function of a relief valve, and proportioning flow between actuators is performed by flow-dividing or flow-control valves.
What is the main reason filters are installed at several points in a hydraulic system?
ACS code: AM.II.F
Correct answer: To remove contaminants and foreign particles from the hydraulic fluid
Rationale: Filters trap dirt, metal particles, and other contaminants that could score pumps, scratch valve surfaces, or clog small passages. Clean fluid is essential to component life and reliable operation. Filters do not raise pressure, and introducing air into hydraulic fluid is undesirable because trapped air causes spongy, erratic operation.
In which type of hydraulic actuator does fluid pressure produce force in only one direction, with the opposite movement provided by another means?
ACS code: AM.II.F
Correct answer: A single-acting actuating cylinder
Rationale: In a single-acting actuating cylinder, fluid pressure is applied to only one side of the piston to produce motion in one direction; the return stroke is accomplished by a spring, gravity, or air load. A double-acting cylinder admits fluid to either side of the piston to power movement in both directions, so it is not the correct choice for one-direction-only pressure operation.
Why is a moisture or water separator commonly used in a high-pressure pneumatic system?
ACS code: AM.II.F
Correct answer: To remove moisture so it cannot freeze and block lines or valves
Rationale: Compressing air concentrates the moisture it contains, and that water can freeze at low operating temperatures, blocking small passages and valves or corroding components. A moisture or water separator removes this water from the compressed air to keep the system functioning reliably. Pneumatic systems are kept clean and dry rather than oil-laden, and a separator does not heat or pressurize the air.
On the ground with the hydraulic system depressurized, the unbalanced flight control surfaces of a transport airplane will typically
ACS code: AM.II.F
Correct answer: droop from their faired position
Rationale: With the hydraulic system depressurized, the powered actuators no longer hold the surfaces against gravity, so unbalanced surfaces such as ailerons and elevators hang down — a normal condition called droop. They return to commanded positions once hydraulic pressure is restored.
What is the typical operating pressure of a transport-category aircraft hydraulic system?
ACS code: AM.II.F
Correct answer: 3,000 psi
Rationale: Per FAA-H-8083-31, the long-standing typical operating pressure for transport-aircraft hydraulic systems is 3,000 psi, which balances component size against the force needed to operate flight controls, landing gear, and brakes. Pressures of 300 or 1,000 psi are far too low; some newer aircraft use 5,000 psi, but 3,000 psi remains the typical value.
A pneumatic system pressure indicator takes its indication from the
ACS code: AM.II.F
Correct answer: storage reservoir.
Rationale: A pneumatic (air-pressure) indicator displays the stored air supply available, so it senses pressure in the air storage reservoir/bottle rather than transient compressor pressures. Reading at the compressor inlet or outlet would show varying delivery pressure, not the usable stored quantity the crew needs for services such as braking or door operation.
What is the primary purpose of an accumulator in an aircraft hydraulic system?
ACS code: AM.II.F
Correct answer: To store fluid under pressure, dampen pressure surges, and supplement pump output during peak demand
Rationale: Per FAA-H-8083-31, an accumulator stores a reserve of fluid under pressure, dampens pressure surges, supplements pump output during peak demand, and provides limited emergency operation if the pump fails. It cushions the system against the rapid pressure fluctuations that occur during operation.
Which property of a hydraulic fluid is its tendency to resist flow, and why is it important?
ACS code: AM.II.F
Correct answer: Viscosity; it must be high enough to seal yet low enough to flow without sluggishness
Rationale: Per FAA-H-8083-31, viscosity is a fluid's resistance to flow. A satisfactory hydraulic fluid must have proper viscosity: if too low it will leak past seals; if too high it will offer excessive resistance to flow and cause sluggish operation. Selecting the proper viscosity is essential to system performance.
What color is MIL-PRF-5606 mineral-base hydraulic fluid, and what is one of its principal hazards?
ACS code: AM.II.F
Correct answer: Red; it is flammable and has a relatively low flash point
Rationale: Per FAA-H-8083-31, MIL-PRF-5606 (mineral-base) hydraulic fluid is dyed red. It has a relatively low flash point and is flammable, which is one of its principal disadvantages compared to phosphate-ester (Skydrol) fluids that are more fire resistant.
What type of hydraulic fluid is used in most large transport-category aircraft because of its fire resistance?
ACS code: AM.II.F
Correct answer: Phosphate-ester-base fluid (Skydrol)
Rationale: Per FAA-H-8083-31, phosphate-ester-base fluids (commonly known by the trade name Skydrol) are used in most commercial transport aircraft because of their fire-resistant properties. These fluids are purple in color and require seals and components compatible with the fluid type.
Why is it critical that hydraulic fluids of different types not be intermixed in a system?
ACS code: AM.II.F
Correct answer: The seals and components are compatible with only one fluid type and the wrong fluid can damage them
Rationale: Per FAA-H-8083-31, the seals, hoses, and components of a hydraulic system are made of materials compatible with only one type of fluid. Mixing fluid types, or using the wrong fluid, can cause seals to swell, deteriorate, or fail and can damage system components. Each system is placarded for the correct fluid.
What is the main function of the reservoir in a hydraulic system?
ACS code: AM.II.F
Correct answer: To store a supply of fluid and allow space for expansion and the escape of trapped air
Rationale: Per FAA-H-8083-31, the reservoir is a tank that holds a supply of fluid for the system, provides space for fluid expansion and the escape of trapped air, and supplies fluid to the pump. It also allows fluid returning from the system to give up entrained air and contaminants.
What is the purpose of pressurizing the hydraulic reservoir on a high-altitude aircraft?
ACS code: AM.II.F
Correct answer: To ensure a positive flow of fluid to the pump inlet and prevent pump cavitation at high altitude
Rationale: Per FAA-H-8083-31, the reservoir is pressurized to provide a positive flow of fluid to the pump inlet at high altitudes where atmospheric pressure is low. Without pressurization, the reduced ambient pressure could allow the pump to cavitate due to insufficient fluid supply.
In a hydraulic system, what is the function of a check valve?
ACS code: AM.II.F
Correct answer: It permits fluid to flow in one direction but prevents flow in the opposite direction
Rationale: Per FAA-H-8083-31, a check valve allows fluid to flow freely in one direction but blocks flow in the opposite direction. It is one of the most common valves in a hydraulic system and is used to prevent reverse flow that could cause loss of pressure or unintended actuator movement.
What is the primary purpose of a system relief valve in a hydraulic system?
ACS code: AM.II.F
Correct answer: To limit maximum system pressure by relieving excess fluid back to the reservoir
Rationale: Per FAA-H-8083-31, a system pressure relief valve limits the maximum pressure in the system by relieving excess pressure back to the reservoir. It serves as a safety device, typically as a backup to the pressure regulator or unloading valve, to protect the system from overpressure.
What is the purpose of a pressure regulator (unloading valve) in a hydraulic system that uses a constant-delivery pump?
ACS code: AM.II.F
Correct answer: To maintain system pressure within a set range and unload the pump when there is no demand
Rationale: Per FAA-H-8083-31, with a constant-delivery (fixed-displacement) pump, the pressure regulator or unloading valve maintains system pressure within a predetermined range and unloads the pump (routes its output back to the reservoir at low pressure) when no demand exists, reducing pump wear and heat generation.
How does a variable-displacement (constant-pressure) hydraulic pump control system pressure?
ACS code: AM.II.F
Correct answer: It automatically varies its fluid output volume to maintain a constant system pressure
Rationale: Per FAA-H-8083-31, a variable-displacement pump automatically varies its output volume to maintain constant system pressure. As demand decreases and pressure rises, an internal compensator reduces the pump's displacement to near zero flow, so a separate pressure regulator/unloading valve is not required.
What is the difference between a single-action and a double-action hydraulic actuating cylinder?
ACS code: AM.II.F
Correct answer: A single-action cylinder is powered in one direction only; a double-action cylinder is powered in both directions
Rationale: Per FAA-H-8083-31, a single-action (single-acting) cylinder receives fluid pressure to move the piston in only one direction, with another force (spring or gravity) returning it. A double-action (double-acting) cylinder receives fluid pressure to move the piston in both directions, giving powered movement each way.
What is the purpose of a hydraulic fuse in a hydraulic system?
ACS code: AM.II.F
Correct answer: To shut off flow to a line that develops a leak or rupture, preventing total fluid loss
Rationale: Per FAA-H-8083-31, a hydraulic fuse is a safety device that seals off (shuts off flow to) a portion of the system if it detects an abnormally high flow rate or a large volume of fluid loss, such as from a ruptured line. This prevents complete loss of system fluid through a break.
When servicing a gas-charged (air or nitrogen) accumulator, what must be done before the unit is disassembled?
ACS code: AM.II.F
Correct answer: The compressed air or nitrogen preload must be completely released
Rationale: Per FAA-H-8083-31, before disassembling an accumulator the technician must release all of the air/nitrogen preload (gas charge). Failure to relieve the high-pressure gas can cause the accumulator to come apart violently and cause serious injury or death. Discharging the preload is a critical safety step.
What does a hydraulic system's filter bypass (relief) valve do when the filter element becomes clogged?
ACS code: AM.II.F
Correct answer: It opens to allow fluid to bypass the clogged element so flow continues, though unfiltered
Rationale: Per FAA-H-8083-31, a filter assembly contains a bypass relief valve that opens to allow fluid to flow around a clogged element. This ensures continued fluid flow (unfiltered) rather than starving the system, though it means contaminants are no longer being removed until the element is replaced.
Why are most aircraft hydraulic systems designed to operate at high pressures (such as 3,000 psi)?
ACS code: AM.II.F
Correct answer: Higher pressure allows large forces to be produced with smaller, lighter components, saving weight
Rationale: Per FAA-H-8083-31, high operating pressure allows the system to transmit large forces using smaller, lighter actuators and lines, saving weight and space, an important advantage in aircraft. Higher pressure means a given force can be produced with a smaller piston area.
What type of hydraulic pump is most commonly used as the engine-driven primary pump in modern high-pressure aircraft systems?
ACS code: AM.II.F
Correct answer: Axial-piston pump
Rationale: Per FAA-H-8083-31, the axial-piston pump (often variable-displacement) is widely used as the primary engine-driven pump in modern high-pressure systems because of its high efficiency, durability, and ability to operate at the high pressures required. Gear and vane pumps are also used, but piston pumps dominate high-pressure applications.
What is the primary difference between a hydraulic system and a pneumatic system?
ACS code: AM.II.F
Correct answer: A hydraulic system uses a nearly incompressible liquid while a pneumatic system uses a compressible gas
Rationale: Per FAA-H-8083-31, both systems transmit force through a fluid, but a hydraulic system uses a relatively incompressible liquid while a pneumatic system uses a compressible gas (usually air or nitrogen). Because gas is compressible, pneumatic systems store energy and behave differently from hydraulic systems.
What advantage does a pneumatic system have that makes it well suited as an emergency backup for landing gear or brakes?
ACS code: AM.II.F
Correct answer: Stored compressed gas can provide emergency actuation without a running pump or return system
Rationale: Per FAA-H-8083-31, because air is compressible, a charged pneumatic system stores energy and can deliver it without a pump running. A stored high-pressure gas bottle can provide a one-time emergency actuation of gear or brakes if the normal hydraulic system fails, with no return lines or reservoir needed.
Why must moisture be removed from a pneumatic system's air supply?
ACS code: AM.II.F
Correct answer: Moisture can freeze at altitude and block lines, orifices, and valves, and can cause corrosion
Rationale: Per FAA-H-8083-31, moisture in compressed air can freeze at the low temperatures encountered at altitude, blocking small passages, orifices, and valves and preventing proper operation. It can also cause internal corrosion. Therefore moisture separators and dehydrators are used to keep the air dry.
When a high-pressure pneumatic storage bottle is to be charged, which gas is typically used and why?
ACS code: AM.II.F
Correct answer: Nitrogen, because it is dry and inert and will not support combustion
Rationale: Per FAA-H-8083-31, dry compressed air or, more commonly, nitrogen is used to charge high-pressure pneumatic systems and accumulators. Nitrogen is preferred because it is inert (will not support combustion) and is dry, which avoids the moisture and fire hazards associated with using oxygen or ordinary air.
What is indicated when a hydraulic actuator operates in a spongy or jerky manner rather than smoothly?
ACS code: AM.II.F
Correct answer: Air is trapped in the system and must be bled out
Rationale: Per FAA-H-8083-31, spongy or erratic actuator operation is a classic symptom of air trapped in the system. Because air is compressible, trapped air causes the system to feel spongy and operate erratically. The remedy is to bleed (purge) the air from the system.
What is the purpose of a pressure gauge snubber installed in a hydraulic pressure indicating line?
ACS code: AM.II.F
Correct answer: To dampen pressure surges so the gauge reads steadily and is not damaged
Rationale: Per FAA-H-8083-31, a snubber (restrictor/orifice in the gauge line) dampens pressure pulsations and surges so that the gauge gives a steady, readable indication. Without it, rapid pressure fluctuations would cause the needle to flutter and could damage the gauge mechanism.