Non-metallic structures is the ACS area covering composites, honeycomb sandwich panels, plastics, and the bonded assemblies and transparencies that have replaced sheet metal across much of the modern airframe. The airframe written test treats it as a materials-and-process subject: what a given fiber does, why a core is there, how resin cures, and which inspection method finds damage you cannot see from outside. Every AMTprep question published against AM.II.B carries a worked rationale and a handbook reference, so a wrong answer tells you which idea to go back and fix.
Written by the AMTprep editorial team · Published · Last reviewed
Written against the FAA primary sources cited at the foot of this page.
AM.II.B pulls from the composite and non-metallic chapters of FAA-H-8083-31, and the questions stay close to material behavior and repair process. Expect fiber identification first: carbon and graphite are stiff and electrically conductive, aramid (Kevlar) is tough and impact resistant but weak in compression, and fiberglass is the lower-cost, non-conductive option. Because carbon conducts, questions ask what happens where it contacts aluminum, which forms a galvanic couple that corrodes the aluminum unless the two are isolated. Matrix and process questions follow. You are asked why resin and hardener must be mixed at the ratio the manufacturer publishes, why pre-impregnated (prepreg) fabric lives in a freezer, and why a heat lamp or hot bonder is part of most repairs. Those answers all come back to cure chemistry: an off-ratio mix or a cold cure leaves resin that never reaches full strength. Sandwich construction is its own cluster. The core of a honeycomb panel separates and stabilizes the thin face sheets and carries shear, which is where the panel gets its stiffness for the weight; damage that crushes the core or admits water destroys that. Ply orientation questions test whether you understand that a laminate is directional, so repair plies have to match the original angles, not just the original count. Finally, inspection. Composite damage hides under an intact surface, so the test favors tap testing for near-surface disbond in thin skins and ultrasonic inspection for delamination deeper in a laminate, with radiography used to confirm water trapped in core cells.
ACS AM.II.B 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.
When using a wet layup repair, what does the term 'ply orientation' refer to?
A nondestructive method that uses sound to detect delamination or disbond in a composite panel is the:
When storing pre-impregnated (prepreg) composite material, why is it kept in a freezer?
What is the main reason a heat lamp or hot bonder is used during many composite repairs?
In a honeycomb sandwich structure, what is the primary purpose of the core?
Which fiber type is electrically conductive and therefore can cause galvanic corrosion when in contact with aluminum?
Aramid (Kevlar) fiber is best known for its high resistance to:
During a wet layup, why must the resin and hardener be mixed in the manufacturer's specified ratio?
Fiberglass (non-metallic) structural parts are protected from lightning strikes and dangerous voltages primarily by
A nonmetallic weather radar radome is protected from lightning strikes by the use of
How is a non-metallic nose cone (radome) protected from damage during a lightning strike?
How is the surface of a composite component made electrically conductive for static and lightning protection?
How is a radome protected from static electricity build-up while remaining radio-transparent?
How is a radome protected from a lightning strike while remaining radio-transparent?
Aircraft fiberglass panels are partially protected against lightning strikes by
To protect aircraft systems from the high current of a lightning strike to a composite airframe, the strike current is discharged through
The two stages required to form a sound adhesive bond are
Aramid (Kevlar) fabric should be stored
Common synthetic resins used in aircraft finishes and transparent plastics are made from nylon, vinyl, and
Passenger cabin windows are normally manufactured from
Acrylic cabin windows must be mounted so they can flex in place because acrylic has
Windshield delamination is best described as
A pressurized-cabin passenger window has two panes with a vent between them. Under normal flight conditions, which pane carries the cabin-to-atmosphere differential pressure load?
When an acrylic transparency is joined to a sheet-metal frame, allowance should be made for
Acrylic (Plexiglas) transparencies are resistant to which of the following fluids?
Heating a transport-category windshield is intended to
After installing an acrylic window panel, protective paper and adhesive residue may be safely removed using soap and water and
If heat is applied to the vinyl interlayer of an aircraft windshield, its ability to withstand impact will
When fitting a transparent acrylic panel into a metal frame, provision for relative movement between panel and frame is required because of
Acrylic (Plexiglas) transparencies are resistant to
During an inspection you notice bubbling (delamination) in the vinyl of a cabin window. The required action is to
Passenger cabin windows on jet transport aircraft normally have
In a fiber-reinforced composite structure, what is the primary function of the matrix (resin)?
When laying up a fiberglass repair, what term describes the orientation of the cloth weave relative to the direction of the applied load?
Which method is commonly used to detect disbonds or delaminations in a honeycomb sandwich panel during a field inspection?
What is the purpose of a peel ply used during a wet layup composite repair?
When a composite repair is cured under a vacuum bag, what is the primary reason for applying vacuum to the layup?
Which reinforcing fiber is electrically conductive and therefore can create a galvanic corrosion concern when placed in direct contact with aluminum structure?
What is a recognized disadvantage of aramid (Kevlar) fibers when machining or repairing a composite part?
When mixing a two-part resin system for a composite repair, why is the correct resin-to-hardener ratio critical?
What does the term "pot life" refer to when working with a mixed composite resin system?
In a typical scarf or tapered repair to a solid laminate, why are the damaged plies removed in a stepped or tapered fashion?
Why must moisture be removed from a honeycomb core or composite laminate before performing a heat-cured bonded repair?
What is the function of a bleeder ply in a vacuum-bagged composite layup?
Why is transparent acrylic (Plexiglas) aircraft window material cut, drilled, and trimmed with care to avoid sharp notches or scratches?
ACS code: AM.II.B
Correct answer: The direction of the reinforcing fibers in each layer
Rationale: Ply orientation describes the direction in which the reinforcing fibers run in each individual layer (for example 0, 45, or 90 degrees). Maintaining the original orientation is critical because composite strength is directional. The count of layers is the ply count, and cure temperature is a separate process parameter.
ACS code: AM.II.B
Correct answer: Coin tap test
Rationale: The coin-tap (ring) test relies on the difference in sound between a solid bonded area (sharp ring) and a delaminated or disbonded area (dull thud). Dye penetrant is used for surface cracks in nonporous metals, and magnetic particle inspection works only on ferromagnetic materials, so neither is suitable for composites.
ACS code: AM.II.B
Correct answer: Low temperature slows curing and extends shelf life
Rationale: Prepreg already contains the resin in a partially uncured state, so the chemical cure reaction is continuously progressing. Freezer storage slows that reaction, preserving the material's working life and tack. The freezer does not stiffen the fibers for cutting or dry out the resin; in fact, frozen prepreg must be thawed sealed to avoid moisture condensation.
ACS code: AM.II.B
Correct answer: To cure the resin properly and develop full strength
Rationale: Most repair resins require elevated temperature to complete the cross-linking (cure) reaction and reach full design strength; a hot bonder or heat lamp supplies and controls that heat. Heat is not applied to soften the parent skin for shaping, nor to burn off contaminants, which are removed by cleaning before layup.
ACS code: AM.II.B
Correct answer: To separate the face sheets and resist shear loads
Rationale: In a sandwich structure the thin, stiff face sheets carry the bending tension and compression loads, while the lightweight core separates them (increasing stiffness like an I-beam web) and carries the shear between them. The core does not carry the in-plane tension or form the outer aerodynamic surface.
ACS code: AM.II.B
Correct answer: Carbon (graphite) fiber
Rationale: Carbon/graphite fiber is electrically conductive and cathodic relative to aluminum, so direct contact in the presence of moisture sets up a galvanic cell that corrodes the aluminum; a barrier ply of fiberglass or a sealant is used to isolate them. Fiberglass and aramid fibers are nonconductive and do not create this galvanic problem.
ACS code: AM.II.B
Correct answer: Impact and tensile loads
Rationale: Aramid fiber has excellent tensile strength and outstanding impact and damage tolerance, which is why it is used where toughness matters. It performs comparatively poorly in compression, and it is not noted for high-temperature resistance, so those choices are incorrect.
ACS code: AM.II.B
Correct answer: An incorrect ratio results in a weak or improper cure
Rationale: Two-part resin systems cure through a chemical reaction that depends on the correct proportion of resin to hardener; an off-ratio mix leaves unreacted material and produces a soft, weak, or incompletely cured repair. The ratio is a strength/chemistry requirement, not a means of setting color or arbitrarily speeding the cure.
ACS code: AM.II.B
Correct answer: bonded metallic grounding conductors embedded in the part
Rationale: Non-conductive fiberglass parts cannot carry a lightning strike to ground by themselves, so they are protected by bonded grounding conductors such as embedded metal foils, meshes, or strips. These conductors provide a low-resistance path that safely carries the strike current and dangerous voltages away from the structure.
ACS code: AM.II.B
Correct answer: bonding (diverter) strips installed across the dome
Rationale: A radome is nonmetallic and transparent to radar energy, so it cannot itself carry a lightning strike. Metallic bonding (diverter) strips fitted across the dome provide a low-resistance path that intercepts strike current and conducts it safely to the airframe, protecting both the radome and the antenna behind it.
ACS code: AM.II.B
Correct answer: Metallic bonding (diverter) strips across the surface
Rationale: A radome is non-conductive, so metallic bonding (diverter) strips are bonded across it to intercept lightning energy and carry it safely to the metal airframe, protecting the structure. Special paint alone or an internal grounding strap does not provide this external discharge path (FAA-H-8083-31, Non-Metallic Structures).
ACS code: AM.II.B
Correct answer: By applying a conductive coating or paint to the surface
Rationale: Composite materials are non-metallic and poor conductors, so they are made conductive by applying a conductive coating (special paint) or an embedded conductive mesh to provide a discharge path for static and lightning currents to the metal structure. Plain copper or aluminum wires are bonding leads, not the means of making the composite surface itself conductive (FAA-H-8083-31, Non-Metallic Structures).
ACS code: AM.II.B
Correct answer: Anti-static conductive paint applied to the surface
Rationale: A radome must remain radio-transparent, so it cannot be covered with metal mesh. Static electricity build-up is bled away by anti-static conductive paint, which provides a controlled discharge path to the airframe without blocking the radar signal (FAA-H-8083-31, Non-Metallic Structures).
ACS code: AM.II.B
Correct answer: Metallic diverter strips bonded across the surface
Rationale: A radome must stay radio-transparent, so it is protected from lightning by metallic diverter strips bonded across its surface that intercept the strike and conduct it to the airframe. Special paint or grease would not provide the conductive discharge path that diverter strips give (FAA-H-8083-31, Non-Metallic Structures).
ACS code: AM.II.B
Correct answer: a coat of conductive paint
Rationale: Fiberglass panels are non-conductive, so a lightning strike cannot dissipate through them as it would on metal skin. A coat of conductive paint (or embedded conductive mesh) provides an electrically continuous path that carries strike current away to the metallic structure and discharge points, reducing localized burn-through. Non-conductive paint offers no such protection.
ACS code: AM.II.B
Correct answer: electrically connected primary conductors
Rationale: Composite airframes lack the natural conductivity of metal, so a network of electrically connected conductors (such as embedded metal mesh, foil, or bonding strips) is built into the structure to carry lightning current safely to static-discharge points. A coat of paint alone cannot handle the high strike currents.
ACS code: AM.II.B
Correct answer: wetting and setting.
Rationale: Per FAA-H-8083-31, an adhesive bond first requires wetting, in which the liquid adhesive flows over and intimately contacts the surfaces, followed by setting, in which it cures into a solid bond. Without adequate wetting, the cured joint would be weak.
ACS code: AM.II.B
Correct answer: in sealed moisture-proof bags.
Rationale: Aramid (Kevlar) fiber is hygroscopic and readily absorbs moisture, which degrades the resin bond and the cured laminate properties. It is therefore kept in sealed, moisture-proof bags, often with desiccant, until use. Orientation alone does not protect against the principal hazard, which is moisture.
ACS code: AM.II.B
Correct answer: acrylics.
Rationale: Synthetic resins used in aircraft finishes and plastics are man-made polymers, and the recognized group includes nylon, vinyl, and acrylics. Cellulose is derived from natural plant fiber and asphalt is a bituminous material, so neither belongs with the synthetic family. The Aviation Maintenance Technician Handbook identifies acrylics among the standard synthetic transparent plastics.
ACS code: AM.II.B
Correct answer: stretched acrylic.
Rationale: Passenger cabin windows are normally made from stretched acrylic, which is light, shatter-resistant, and able to flex with the pressurized structure. Glass would be heavy and prone to cracking, so acrylic is the standard cabin-window material, with heavier laminates reserved for windshields.
ACS code: AM.II.B
Correct answer: a larger coefficient of thermal expansion than the surrounding structure.
Rationale: Acrylic has a much larger coefficient of thermal expansion than the surrounding aluminum structure, so over the wide temperature range of flight it expands and contracts more than its frame. The window must therefore be mounted so it can flex and move freely in place, avoiding being stressed or cracked.
ACS code: AM.II.B
Correct answer: separation of the glass from the vinyl interlayer.
Rationale: Laminated windshields are built from glass or acrylic plies bonded to a vinyl interlayer. Delamination is the separation of these bonded layers — the glass or acrylic parting from the vinyl interlayer — often appearing as cloudy or bubbled patches. It is not melting of the glass or the entire windshield peeling away from the airframe.
ACS code: AM.II.B
Correct answer: The outer pane.
Rationale: The cabin window assembly normally has two structural panes with a vent between them. Under normal pressurization the outer (primary) pane carries the full cabin-to-atmosphere differential pressure load, while the inner pane is a fail-safe backup that takes the load only if the outer pane fails.
ACS code: AM.II.B
Correct answer: expansion of the acrylic due to heat.
Rationale: Acrylic has a much higher coefficient of thermal expansion than sheet metal, so when the two are joined, allowance — clearance, slotted holes, or flexible mounting — must be made for the acrylic expanding and contracting more than the metal with temperature change. Failing to allow for the acrylic's greater movement can crack the transparency.
ACS code: AM.II.B
Correct answer: Kerosene.
Rationale: Acrylic is attacked and crazed by strong organic solvents such as acetone and MEK, which dissolve or stress-craze the plastic, so these must never be used near it. It is, however, resistant to kerosene, making kerosene the fluid it can tolerate among the options given.
ACS code: AM.II.B
Correct answer: demist, anti-ice, and maintain strength and toughness.
Rationale: Windshield heating serves several purposes at once: it demists the inner surface, prevents ice forming on the outer surface, and warms the vinyl interlayer to keep the transparency in its tough, impact-resistant state for bird-strike protection. Heating for any single one of these alone understates its full purpose.
ACS code: AM.II.B
Correct answer: denatured alcohol.
Rationale: Acrylic transparencies are attacked and crazed by strong solvents, so only mild cleaners are used; soap and water followed by denatured alcohol safely removes adhesive residue. Lacquer thinner and MEK are aggressive solvents that would craze and damage the acrylic.
ACS code: AM.II.B
Correct answer: increase.
Rationale: The vinyl interlayer absorbs impact most effectively when it is warm and at its toughest, so applying heat raises its temperature into the ductile range and its impact resistance increases. This is why windshields are kept warmed by their heater film in flight.
ACS code: AM.II.B
Correct answer: the higher coefficient of expansion of the panel compared with the frame.
Rationale: Acrylic expands and contracts much more than the surrounding metal frame for a given temperature change because it has a higher coefficient of thermal expansion. Provision for relative movement prevents the panel from binding or cracking as the two materials change size at different rates.
ACS code: AM.II.B
Correct answer: kerosene.
Rationale: Acrylic tolerates mild hydrocarbons such as kerosene without crazing, which is why it is handled and cleaned around such fluids. Acetone and carbon tetrachloride are aggressive solvents that attack and craze acrylic, so it is not resistant to them.
ACS code: AM.II.B
Correct answer: replace the window only if the bubbling affects vision.
Rationale: Minor vinyl bubbling is a cosmetic delamination that is acceptable while it stays within limits, so the window is replaced only once the bubbling encroaches on the crew or passenger field of vision. Replacing on sight of any bubble is unnecessary, but ignoring it entirely is not acceptable.
ACS code: AM.II.B
Correct answer: two structural panes.
Rationale: Passenger cabin windows on jet transports use two structural panes, each able to carry the full pressure differential, giving fail-safe redundancy. A separate inner non-structural scratch pane is often fitted as well, but the load-bearing transparency consists of two panes, not one or three.
ACS code: AM.II.B
Correct answer: To transfer loads between fibers and hold them in alignment
Rationale: Per FAA-H-8083-31, in a composite laminate the reinforcing fibers carry the structural loads, while the matrix (resin) supports and transfers loads between the fibers, holds them in alignment, and protects them from the environment and abrasion.
ACS code: AM.II.B
Correct answer: Fiber orientation
Rationale: FAA-H-8083-31 explains that fiber orientation is critical to the strength of a composite repair. The warp direction (lengthwise threads of the fabric) must be oriented to match the original ply's fiber direction so the repair carries load the same way as the parent structure.
ACS code: AM.II.B
Correct answer: Coin-tap (ring) test
Rationale: According to FAA-H-8083-31, the coin-tap (ring) test is a simple, widely used field method for detecting disbonds and delaminations in composite and honeycomb structures; a dull or dead thud indicates a defect, while a sharp clear ring indicates a solid bond.
ACS code: AM.II.B
Correct answer: To leave a clean, textured surface for later bonding after it is removed
Rationale: FAA-H-8083-31 describes the peel ply as a sacrificial layer placed on the laminate surface and removed after cure to leave a clean, textured surface that is ready for secondary bonding or painting without additional sanding.
ACS code: AM.II.B
Correct answer: To consolidate the plies and remove entrapped air and excess resin
Rationale: Per FAA-H-8083-31, vacuum bagging applies atmospheric pressure to consolidate the plies, remove entrapped air and volatiles, and pull out excess resin, producing a higher-quality laminate with a proper fiber-to-resin ratio.
ACS code: AM.II.B
Correct answer: Carbon (graphite) fiber
Rationale: FAA-H-8083-31 notes that carbon/graphite fibers are electrically conductive and cathodic to aluminum; direct contact can cause galvanic corrosion of the aluminum, so a barrier (such as a fiberglass ply) is used to isolate carbon from aluminum.
ACS code: AM.II.B
Correct answer: They are difficult to cut cleanly and tend to absorb moisture
Rationale: FAA-H-8083-31 states that aramid fibers are difficult to cut and tend to fuzz when machined, requiring special sharp tooling, and they readily absorb moisture, which must be controlled before repair and cure.
ACS code: AM.II.B
Correct answer: An incorrect ratio causes an improper cure and reduced repair strength
Rationale: Per FAA-H-8083-31, the resin and hardener (catalyst) must be measured and mixed in the exact ratio specified by the manufacturer; an incorrect ratio results in an improper cure, reducing the strength and durability of the finished repair.
ACS code: AM.II.B
Correct answer: The time after mixing during which the resin remains usable
Rationale: FAA-H-8083-31 defines pot life (working life) as the period of time after the resin and hardener are mixed during which the mixture remains workable before it begins to gel and can no longer be properly applied.
ACS code: AM.II.B
Correct answer: To increase bonding area and gradually transfer load across the repair
Rationale: FAA-H-8083-31 explains that tapering or stepping the plies in a scarf repair increases the bonding area for each replacement ply and distributes the load gradually across the joint, avoiding an abrupt stress concentration at the bond line.
ACS code: AM.II.B
Correct answer: Trapped moisture turns to steam during cure and can cause disbonds or voids
Rationale: According to FAA-H-8083-31, trapped moisture turns to steam at cure temperatures, which can cause additional delamination, disbonds, voids, or blown core. The repair area must be dried before cure to prevent these defects.
ACS code: AM.II.B
Correct answer: It absorbs excess resin to help achieve the proper fiber-to-resin ratio
Rationale: FAA-H-8083-31 describes the bleeder ply as an absorbent layer that soaks up excess resin bled from the laminate during cure, helping to achieve the correct fiber-to-resin ratio; it also provides a path for air and volatiles to escape.
ACS code: AM.II.B
Correct answer: Notches and scratches act as stress risers that can cause cracking or crazing
Rationale: FAA-H-8083-31 notes that scratches, nicks, and sharp edges in transparent plastics act as stress risers (stress concentrations) that can lead to crazing and cracking; edges are smoothed and holes are drilled oversize to relieve stress.
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