Hydraulic and Pneumatic Systems — FAA A&P Test Questions (ACS AM.II.F)

What ACS AM.II.F covers

AM.II.F comes out of the hydraulic and pneumatic material in FAA-H-8083-31. The opening idea is why aircraft use hydraulics at all: a fluid system transmits force with almost no lost motion, multiplies that force, weighs little for the work it does, and holds a component exactly where it is placed. Pascal's law is the principle the test names, since pressure applied to a confined fluid acts equally in all directions, so a small force on a small piston becomes a large force on a large one at the cost of travel. Then the components. The pump supplies flow, and a pressure regulator or unloading valve keeps a constant-delivery pump from working against a full system. A pressure relief valve is the safety net that opens only when pressure climbs past a set limit. A check valve permits flow one way and blocks the other. Sequence, priority and shuttle valves control order of operation, share limited pressure, and switch between normal and emergency sources. Filters sit at several points, including pressure, return and case drain lines, because contamination is what destroys close-tolerance components. An accumulator stores pressurized fluid against an air preload, damps surges and supplements the pump, and that preload has to be released before disassembly. Actuators are single-acting, where pressure works in one direction and a spring or gravity returns the piston, or double-acting, where pressure works both ways. Fluids matter too: mineral-base and phosphate-ester fluids are not interchangeable and each requires its own seal material. High-pressure pneumatic systems add a moisture separator and a desiccant, because compressing air condenses water that would freeze in valves and corrode the plumbing.

Where this sits on the test

ACS AM.II.F is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

• FAA-H-8083-31

Three traps candidates fall into

1. The pressure relief valve gets described as the thing that controls system pressure. It is not. A pressure regulator or unloading valve does that during normal operation, and the relief valve opens only when the regulator fails or thermal expansion spikes pressure. A relief valve that operates routinely means something upstream is broken.
2. Force equals pressure times area, and candidates plug in the piston diameter instead. Area varies with the square of the diameter, so a piston twice as wide develops four times the force, not twice. The other half of the trade gets forgotten as well, because the larger piston moves a shorter distance.
3. Hydraulic fluids get treated as interchangeable because they are all called hydraulic fluid. Mineral-base and phosphate-ester fluids are not. They require different seal materials, and fitting a seal built for one into a system that uses the other lets the seal swell and fail. Check the placard before servicing.

7 free sample questions from ACS AM.II.F

1. AM.II.FTap an answer

   What is the primary advantage that makes hydraulic systems well suited for operating aircraft components?

   • AThey allow fluid to be compressed to store large amounts of usable energy
   • BThey transmit large forces with relatively little weight and low fluid friction loss
   • CThey eliminate the need for any reservoir, pump, or pressure relief device
2. AM.II.FTap an answer

   Which basic physical principle explains how a hydraulic system multiplies an applied force?

   • APascal's law, stating pressure is transmitted equally throughout a confined fluid
   • BBoyle's law, stating gas volume varies inversely with applied pressure
   • CBernoulli's principle, stating fluid pressure rises as fluid velocity increases
3. AM.II.FTap an answer

   What is the primary function of a pressure relief valve in a hydraulic system?

   • ATo prevent reverse flow of fluid back toward the system pump
   • BTo maintain a constant flow rate to each actuator in the system
   • CTo limit system pressure and protect components from excessive pressure
4. AM.II.FTap an answer

   Before disassembling a hydraulic accumulator, what must the technician do first?

   • ARelease the compressed air or nitrogen preload charge from the accumulator
   • BApply full system hydraulic pressure to seat the internal piston
   • CHeat the unit until the fluid reaches normal operating temperature
5. AM.II.FTap an answer

   What is the purpose of a check valve in a hydraulic system?

   • AIt permits fluid flow in one direction and blocks flow in the opposite direction
   • BIt limits maximum system pressure by venting fluid to the reservoir
   • CIt divides pump output evenly between two separate actuators
6. AM.II.FTap an answer

   What is the main reason filters are installed at several points in a hydraulic system?

   • ATo remove contaminants and foreign particles from the hydraulic fluid
   • BTo increase the pressure of the fluid as it moves toward the actuators
   • CTo add air to the fluid so it can absorb operating pressure surges
7. AM.II.FTap an answer

   In which type of hydraulic actuator does fluid pressure produce force in only one direction, with the opposite movement provided by another means?

   • AA double-acting actuating cylinder
   • BA single-acting actuating cylinder
   • CA balanced-area actuating cylinder

Frequently asked questions

What is the difference between a pressure regulator and a pressure relief valve?

The regulator, often called an unloading valve, is a normal operating control. It unloads a constant-delivery pump back to the reservoir once the system reaches operating pressure, then reloads the pump when pressure drops. The relief valve is a safety device. It stays closed in normal operation and opens only to dump fluid when pressure exceeds a preset limit, protecting lines, seals and components from damage.

What does a hydraulic accumulator do?

An accumulator holds fluid under pressure against a compressed air or nitrogen preload, separated from the fluid by a diaphragm, bladder or piston. It damps the pressure surges that pumps and valves create, stores energy so the system can meet a demand larger than the pump alone can supply, and holds enough pressure for limited operation of a system such as the brakes after the pump stops.

Why must an accumulator be depressurized before disassembly?

Because it is a charged pressure vessel. The air side commonly holds a preload of roughly a third of system pressure, so it can still contain a thousand psi or more with the hydraulic system shut down and the system gauge reading zero. A component that comes apart under that load becomes a projectile. Bleed the preload at the charging valve and confirm zero pressure before loosening anything.

Why does a high-pressure pneumatic system need a moisture separator?

Compressing air raises its temperature and forces out water vapor, which condenses as the air cools downstream of the compressor. Left in the system, that water corrodes lines and components and can freeze in a valve or an orifice at altitude and lock the system up. The separator removes condensed moisture, and a desiccant stage dries whatever vapor makes it past the separator.

What is the difference between a single-acting and a double-acting actuator?

In a single-acting actuator, fluid pressure moves the piston in one direction only, and a spring, gravity or an air load returns it. A double-acting actuator has a port at each end, so pressure can move it in both directions, and the selector valve decides which port sees pressure and which is connected to return. Most aircraft actuators, including gear, flaps and brakes, are double-acting.

All 40 ACS areas

Primary sources

• FAA Mechanic Airman Certification Standards (FAA-S-ACS-1)
• 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers