17 Wood, Fabric & Composite Structures Practice Questions & Answers

1. 1. AC 43.13-1 lists several species that may be substituted for Sitka spruce in aircraft structure. What maximum slope of grain does that table permit for the substitute species?

   • A.1:20, a limit tightened for substitute species
   • B.1:10, a limit relaxed for substitute species
   • C.1:15, the same limit as for Sitka spruce✓ Answer
   • D.1:12 for softwoods and 1:15 for hardwoods

   The wood table assigns one maximum permissible grain deviation, 1:15, to every listed species. Substitution changes a species' strength rating and its working and gluing qualities, not the allowable grain slope, so a Douglas fir or Port Orford cedar member is held to the same 1:15 as spruce. The slope must be checked on all four faces of the piece, because a diagonal or spiral grain may show on only one face.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 1, wood selection table (maximum permissible grain deviation 1:15)Report a problem with this question

2. 2. During a receiving inspection of wood intended for a spar, a mechanic finds four defects in the stock. Which one may be accepted?

   • A.Compression wood, provided its color contrast with the surrounding wood is slight
   • B.A check, provided it does not cross more than one annual ring
   • C.A spike knot, provided it lies in the center third of the beam
   • D.A mineral streak, provided careful inspection reveals no decay✓ Answer

   Mineral streaks are conditionally acceptable, and the condition is that a careful inspection finds no decay associated with them. Spike knots, checks, shakes, splits, compression wood, compression failures, and decay in any form are rejected outright regardless of size or location; only hard knots within their diameter limit, pitch pockets within their size and spacing limits, and grain deviation within 1:15 are conditionally acceptable. Slight color contrast is in fact a warning sign of compression wood, not a reason to accept it.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 1, defects permitted and defects not permitted in aircraft woodReport a problem with this question

3. 3. A wooden wing rib joint originally bonded with casein adhesive is being re-bonded. What must be done with the old adhesive, and why?

   • A.Sand the casein lightly and leave it in place, because it acts as a primer for the new adhesive
   • B.Scrape away all the old casein, because residual alkalinity stops the new adhesive curing✓ Answer
   • C.Seal the casein with varnish before bonding, because the varnish isolates the new adhesive from moisture
   • D.Soften the casein with warm water and spread it thin, because rewetted casein bonds to fresh resin

   Casein is alkaline, and alkali left on the joint faces attacks and spoils a new bond, so every trace must be scraped back to clean, sound wood before re-bonding. Casein also deteriorates with age and moisture and is incompatible with many modern finishes and preservatives, which is why resorcinol-formaldehyde resin is the adhesive identified as approved for wooden aircraft structure. A sound joint also depends on close-fitting surfaces and correct clamping pressure to give a thin, uniform bond line.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 1, adhesives for wood structureReport a problem with this question

4. 4. A wood spar is being spliced and the manufacturer's data gives no instruction on splice location. What minimum spacing is required between the new splice and an existing splice or reinforcement plate?

   • A.Not less than the combined length of both splices
   • B.Not less than twice the length of the shorter splice
   • C.Not less than three times the longer splice length✓ Answer
   • D.Not less than ten times the depth of the spar

   The spacing between two splices, or between a splice and a reinforcement plate, must be at least three times the length of the longer splice, so that highly stressed glue joints and stiffness changes are not concentrated in one region of the spar. A splice may be made almost anywhere along the spar except under wing attachment, landing gear, engine mount, or lift and interplane strut fittings, and no fitting may overlap any part of the splice. Reinforcement plates are glued to both sides of the splice with their grain running in the same direction as the spar.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 1, spar splice and reinforcement plate requirementsReport a problem with this question

5. 5. Grade A cotton is installed on an airplane whose wing loading and never-exceed speed require only intermediate-grade fabric. New Grade A tests at 80 lb per inch and new intermediate fabric at 65 lb per inch. At what breaking strength does the installed covering become unairworthy?

   • A.65 lb per inch, the full new strength of the required intermediate fabric
   • B.46 lb per inch, 70 percent of the intermediate fabric the airplane requires✓ Answer
   • C.56 lb per inch, 70 percent of the Grade A fabric actually installed
   • D.35 lb per inch, the general minimum accepted for any fabric covering

   A covering remains airworthy until it deteriorates below 70 percent of the strength of new fabric required for that aircraft, not 70 percent of whatever grade happens to be installed. Because this airplane only requires intermediate fabric, the reference is 70 percent of 65 lb per inch, about 46 lb per inch, and installing a stronger grade does not raise the condemnation point. Covering condition must be assessed at each 100-hour and annual inspection, and a laboratory tensile test of a cut sample is the definitive measurement.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 2, fabric strength criteria (70 percent of the strength of new fabric required for the aircraft)Report a problem with this question

6. 6. The doped finish on a cotton covering has become brittle and is cracking. What does an application of rejuvenator accomplish?

   • A.It seals the surface cracks so the covering need not be strength tested at the next inspection
   • B.It restores flexibility and tautness to the dope film but adds no breaking strength✓ Answer
   • C.It replaces only the ultraviolet protection lost from the aluminum-pigmented coats
   • D.It restores the fabric's breaking strength to that of new fabric of the same grade

   Rejuvenator is a solvent-rich material that re-plasticizes the existing dope film, so it restores the flexibility and tautness of the finish only. The fibers underneath have already lost strength to ultraviolet light, exhaust, and age, and no coating can put that strength back, so the covering must still be judged by testing. Mechanical punch testers are not FAA-approved and support only an opinion of general condition; a laboratory tensile test on a cut sample decides the question.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 2, rejuvenation of dope film and fabric testingReport a problem with this question

7. 7. On an airplane with a never-exceed speed above 200 mph, V notches are cut in both edges of the surface tape covering spanwise seams on control-surface trailing edges. What is the purpose of the notches?

   • A.They mark the rib lacing spacing so the stitching can be inspected without removing tape
   • B.They give a released tape a place to tear off rather than peel along the seam✓ Answer
   • C.They relieve tension in the tape so the control surface remains in static balance
   • D.They let trapped solvent escape so the dope under the tape cures at an even rate

   The notches are deliberate weak points: if the tape starts to lift at high speed it tears at a notch instead of stripping progressively down the whole seam. That matters because a length of loose tape on a movable surface acts like a trim tab and on a fixed surface like a spoiler, either of which can upset control. Related high-speed provisions include anti-tear strips under the reinforcing tape when the never-exceed speed is higher still, applied over the top wing surface and the lower surface within the propeller slipstream.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 2, surface tape V notching and anti-tear stripsReport a problem with this question

8. 8. Why is rib lacing spaced more closely on the part of a wing inside the propeller slipstream than outside it?

   • A.Air loads are higher there, so closer stitches limit lifting of the fabric between the ribs✓ Answer
   • B.The dope cools faster there, so closer stitches keep the fabric from shrinking unevenly
   • C.The fabric runs hotter there, so closer stitches offset the reduced adhesion of the dope
   • D.Exhaust residue is deposited there, so closer stitches hold the reinforcing tape against seepage

   Inside the slipstream the covering sees higher dynamic pressure and more turbulence, so the fabric tries harder to balloon away from the structure between ribs; closer stitch spacing shortens the unsupported span and keeps the covering against the rib. For this purpose the slipstream area is taken as the propeller diameter plus one rib outboard of it. The stitches are tied with the standard external modified seine knot, drawn to the inside so it lies under the surface tape.

   Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 2, rib lacing spacing and propeller slipstream areaReport a problem with this question

9. 9. Aramid (Kevlar) fabric is naturally yellow and very tough. Which property most limits where this fiber can be used in a structure?

   • A.It is stiff in bending and melts at low temperature, so it is poor in parts near the engine
   • B.It is brittle in tension and conducts electricity, so it is poor in lightning-prone areas
   • C.It is heavy and low in impact resistance, so it is poor in leading edge parts
   • D.It is weak in compression and absorbs moisture, so it is poor in compression-loaded parts✓ Answer

   Aramid is outstanding in tension and impact but carries little compressive load, and it is hygroscopic, with some parts absorbing a substantial fraction of their weight in water, which adds weight and can degrade the bond. Those two traits keep it out of compression-critical structure and make sealing and moisture control important where it is used. It is also awkward to machine, since the fibers fuzz rather than cut and special shears and drills are required.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 7, fiber reinforcement materialsReport a problem with this question

10. 10. A honeycomb panel is being built with carbon fiber face sheets. Why must aluminum honeycomb core not be used with those face sheets?

    • A.Aluminum core conducts lightning current into the carbon plies and burns the resin
    • B.Carbon and aluminum expand at very different rates, so the bond line fatigues in service
    • C.Aluminum core cannot be bonded with the epoxy film adhesives used for carbon skins
    • D.Carbon is cathodic to aluminum, so galvanic action corrodes the aluminum core✓ Answer

    Carbon is strongly cathodic to aluminum, so with any moisture present the two form a galvanic cell and the aluminum core corrodes away behind an apparently sound skin. The same rule governs fasteners: titanium or corrosion-resistant steel is used in carbon structure rather than aluminum. Aramid paper honeycomb is the common core for such panels, and boron is the fiber that can be bonded to aluminum because it has no galvanic potential against it.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 7, core materials and galvanic corrosion of carbon fiber structureReport a problem with this question

11. 11. During a coin tap inspection of a bonded composite structure, a mechanic hears a dull sound in one area. Which statement about that result is correct?

    • A.The area means nothing unless the laminate is more than about four plies thick
    • B.The area indicates excess resin, which is the condition the tap test is designed to find
    • C.The area is certainly a disbond, since internal design changes produce a sharp ringing sound
    • D.The area may be a disbond or simply an internal design change such as a doubler✓ Answer

    A tap test compares sounds: a clear, sharp ring indicates well-bonded material and a dull thud indicates a discrepancy, but a legitimate internal feature such as a doubler or a core-to-laminate transition also dulls the sound. That is why the inspector must know the internal configuration, work in as quiet an area as possible, and confirm findings by ultrasonic or another method. The technique is limited to near-surface flaws and is not reliable in laminates more than about four plies thick.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 7, audible sonic (coin tap) testingReport a problem with this question

12. 12. Which composite condition can a careful visual inspection be expected to detect without further nondestructive inspection?

    • A.Matrix crazing distributed through the thickness of a laminate
    • B.Resin starvation and ply bridging at the surface of a laminate✓ Answer
    • C.Disbond between a skin and the honeycomb core beneath it
    • D.Delamination between plies in the interior of a laminate

    Visual inspection is the primary in-service method and reliably finds conditions that reach the surface: resin starvation or richness, wrinkles, ply bridging, blisters, discoloration from overheating or a lightning strike, impact marks, dents, and abrasion. Internal flaws such as delamination, skin-to-core disbond, and matrix crazing give no dependable surface evidence and require tap testing, ultrasonic inspection, thermography, or radiography. Treating a clean visual as proof of internal soundness is the error to avoid.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 7, visual inspection and nondestructive inspection of compositesReport a problem with this question

13. 13. A laminate repair calls for plies with the fabric warp at 0 degrees, but the roll carries no warp clock marking. How is the 0-degree direction taken?

    • A.Along the length of the roll, since orientation defaults to zero off the roll✓ Answer
    • B.At 45 degrees to the selvage, since a bias cut spreads the load evenly in a repair
    • C.Along the longest dimension of the damage, since the repair must carry that load first
    • D.Across the width of the roll, since the fill fibers are the straightest and strongest

    Warp is the longitudinal direction of the fabric, and because those fibers run straightest it is the high-strength direction; when no warp clock is marked, orientation defaults to zero as the material comes off the roll, so 90 degrees is across the width. Repair plies must reproduce the orientation and stacking sequence of the plies they replace, within the alignment tolerance in the manufacturer's structural repair manual, because a laminate's strength comes from where its fibers point. A repair laid up at the wrong angles will look correct and still fail to restore strength.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 7, fiber orientation and the warp clockReport a problem with this question

14. 14. What is the correct way to clean an acrylic windshield?

    • A.Wipe with a household window spray and paper towels, because the spray lifts grit without pressure
    • B.Wipe with an alcohol-dampened cloth and buff dry, because alcohol removes oil films without scratching
    • C.Flush with plenty of water and mild soap using a soft cloth, because solvents and dry rubbing craze it✓ Answer
    • D.Rub with a dry shop cloth and then wax it, because water left on acrylic clouds the surface

    Plenty of water with mild soap floats the grit off the surface instead of dragging it across the plastic, and a soft grit-free cloth, sponge, or the bare hands are the only acceptable applicators. Solvents such as alcohol, acetone, benzene, lacquer thinner, deicing and extinguisher fluids, and ordinary window-cleaning sprays attack the thermoplastic and craze it, and rubbing a dry surface scratches it while building a static charge that pulls dust back onto the panel. Wax is applied only after the panel is clean, in a thin even coat polished with a soft cloth.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 3, care and cleaning of transparent plasticsReport a problem with this question

15. 15. How should flat acrylic sheet stock be stored in a shop?

    • A.With the masking paper in place, in bins tilted about ten degrees from vertical, away from heat✓ Answer
    • B.With the masking paper removed, stacked flat with the largest sheets on top of the smaller ones
    • C.With the masking paper removed, in bins tilted about ten degrees from vertical, away from heat
    • D.With the masking paper in place, standing fully vertical in direct sunlight to keep it dry

    The masking paper is left on as long as possible because it is the sheet's protection against scratches, and the bins are tilted roughly ten degrees from the vertical so the sheets lean against support and do not buckle. Heat sources and paint-shop fumes must be kept away because acrylic is a thermoplastic that softens and distorts, and direct sunlight is avoided because it degrades the masking adhesive until it bonds to the plastic. If sheets must lie flat, the stack is kept low with the smallest sheets on top so no sheet overhangs unsupported, and formed parts are supported to shape and kept below the temperature limit for the material.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 3, storage and handling of transparent plastic sheetReport a problem with this question

16. 16. A crack has been found in an acrylic cabin side window. What is done first, and why?

    • A.Stop-drill each end of the crack, to remove the stress concentration that makes it grow✓ Answer
    • B.Bevel the edges of the crack, to relieve the surface tension that opened it
    • C.Fill the crack with cement, to bond the faces before the crack ends can move
    • D.Apply tape across the crack, to hold the faces in register until a replacement arrives

    A crack tip is an extremely sharp notch, and the stress concentration there is what drives the crack across the panel; drilling a small round hole at each end blunts the tip and arrests the growth, so stop-drilling always comes before any other step. A temporary repair then bolts through holes drilled along the crack using standard screws, nuts, and washers with a washer on both sides of the plastic, while a permanent repair for a minor crack injects a polymerizable cement by capillary action. Because no sanding or repair that distorts the pilot's line of vision is allowed and replacement is usually more economical, damaged acrylic panels are generally replaced with material equivalent to the original.

    Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices, Ch. 3, repair of cracks in transparent plasticsReport a problem with this question

17. 17. During inspection of an installed seat belt, which finding condemns the belt?

    • A.Webbing with cut or frayed fibers, since the strap can no longer carry its rated load✓ Answer
    • B.A buckle with light surface scratches, since any buckle damage voids its approval
    • C.Webbing that is faded but intact, since loss of color proves the strength is gone
    • D.A belt more than ten years in service, since approved belts carry a fixed service life

    The webbing is the load path, so cuts, fraying, abrasion, and chemical or ultraviolet degradation of the fibers reduce its breaking strength below the rating the belt was approved to and require replacement with an approved unit. Fading alone or light scratching of a buckle that still latches and releases correctly does not condemn a restraint, and approved belts are not assigned a blanket calendar life. The inspection also confirms that the identification and technical standard order markings are present and legible, and that the attach hardware, anchorage, and surrounding structure are sound.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), seats, belts and harnesses; 14 CFR 43.13Report a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Airframe (FAA-H-8083-31). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the AIRFRAME written test only; the General and Powerplant written tests and the oral and practical tests are separate. Torque values, cable tensions, inflation pressures, servicing quantities, wear limits, and repair criteria always come from the manufacturer's current maintenance data, the structural repair manual, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →