Non-Metallic Structures practice questions

Answer: A) With thermocouples placed in contact with the part at the repair and read by the hot bonder

Heated composite repairs are monitored with thermocouples in contact with the part. The hot bonder uses them to control the cure temperature profile and avoid under-curing or overheating.

• ACorrect. Enough thermocouples are placed on the part, at locations chosen from a thermal survey, so the controller can track and control the actual repair temperature throughout the cure.
• BIncorrect. The blanket setpoint does not show the actual part temperature. Heat sinks such as substructure can leave areas under-cured or overheated.
• CIncorrect. A single surface reading cannot track temperature through the hours-long cure cycle, and the bagging materials hide the true part temperature.
• DIncorrect. The vacuum gauge checks bag integrity and consolidation pressure, not temperature. Temperature has to be measured directly.

Answer: C) As a lightweight filler for fairing

• AIncorrect. Release agents are separate products.
• BIncorrect. Fibers carry the load, not fillers.
• CCorrect. Mixed with resin, they make a light, sandable filler.
• DIncorrect. Hardeners and catalysts are separate.

Answer: B) Wet-sand with progressively finer abrasive paper, then buff with a polishing compound

Fine scratches in acrylic are removed by wet-sanding with progressively finer abrasive paper and then buffing with compound. Solvents, coarse dry sanding and excess heat can craze or distort the plastic.

• AIncorrect. Solvents such as acetone and lacquer thinner attack acrylic and can cause crazing. Only cleaners approved for transparent plastics should be used.
• BCorrect. AC 43.13-1B says to sand with 320- or 400-grit paper on a felt or rubber pad, using soapy water as a lubricant. You then move to finer grits and finish by buffing with compound to remove all sanding marks.
• CIncorrect. Coarse dry sanding cuts deeper scratches and can overheat the surface. The procedure calls for light pressure, a lubricant and progressively finer grits.
• DIncorrect. Too much heat from buffing softens and distorts acrylic and can leave residual stress that leads to crazing. Polishing should use light pressure and avoid overheating.

Answer: B) To use atmospheric pressure to consolidate the plies and draw out trapped air

Vacuum bagging is a pressure application method. Atmospheric pressure on the evacuated bag consolidates the plies and removes air, volatiles and excess resin, while heat comes from a separate source.

• AIncorrect. Cure heat comes from heat blankets, heat lamps or ovens. The vacuum bag only holds pressure, although it is often used together with a heat blanket.
• BCorrect. Evacuating the bag lets atmospheric pressure press evenly on the repair. This compacts the plies, removes trapped air and volatiles, and helps bleed off excess resin, producing a strong, void-free bond.
• CIncorrect. The bag, breather and release films are removed after cure. Environmental protection comes from the finish system that is applied afterward.
• DIncorrect. Thermocouples and a hot bonder controller monitor and control temperature. The bag itself does not measure anything.

Answer: A) Whether the repair resin has reached an adequate degree of cure

A Barcol impressor measures surface indentation hardness, which shows whether the resin has cured properly. Disbonds and thickness need other inspection methods.

• ACorrect. The Barcol impressor measures surface hardness. Readings below the specified value suggest the resin is undercured.
• BIncorrect. Disbonds and delaminations are found by tap testing or ultrasonic inspection, not by a surface hardness reading.
• CIncorrect. Fiber volume is controlled by weighing materials and checked by lab methods such as resin burn-off. A hardness test doesn't measure it.
• DIncorrect. Thickness and ply count are checked by measurement, layup records or ultrasonic methods, not by indentation hardness.

Answer: B) The reference for ply fiber orientation

• AIncorrect. Cure is tracked with time and temperature records.
• BCorrect. Each ply's angle is laid up relative to the warp clock.
• CIncorrect. Fastener layouts are shown separately.
• DIncorrect. Mix ratios come from the resin data.

Answer: A) Expanded copper or aluminum foil or wire mesh at the outer surface

Composite structures are typically protected by a conductive layer, such as expanded metal foil or wire mesh at the outer surface, that carries lightning current. Repairs must restore this protection system.

• ACorrect. Composites conduct electricity poorly. Metal mesh, foil or wires near the outer surface give the strike current a conductive path, which limits damage to the laminate.
• BIncorrect. Fiberglass isolation plies are used to prevent galvanic corrosion between carbon fiber and aluminum. An insulating ply does not conduct strike current away, so it does not provide lightning protection.
• CIncorrect. Static wicks bleed off precipitation static to reduce radio interference. They are not designed to carry lightning strike current.
• DIncorrect. Extra resin is a non-conductor. It does not provide a current path and would not prevent strike damage.

Answer: A) An off-ratio mix cures improperly, reducing strength and heat and chemical resistance

The resin-to-hardener ratio controls how completely the resin cures. Changing it, even to speed up the cure, reduces the strength, thermal resistance and chemical resistance of the repair.

• ACorrect. The ratio sets how completely the resin cross-links. Too much or too little hardener leaves the matrix under-cured or brittle, which lowers its strength, thermal resistance and chemical resistance.
• BIncorrect. FAA guidance specifically says not to change the ratio to alter cure time. Cure time is controlled by temperature and the cure schedule.
• CIncorrect. An off-ratio mix changes the cured properties, not just the working time. Leftover unreacted resin or hardener weakens the matrix.
• DIncorrect. Bleeding removes excess volume of the mixed resin. It does not fix a chemically off-ratio mix, which will still cure improperly.

Answer: B) Shut off the pump and watch the gauge for any drop in vacuum over a set time

Before cure, the bagged repair is leak-checked. The pump is isolated and the gauge is watched for vacuum drop to confirm the bag will hold compaction pressure through the cure cycle.

• AIncorrect. Tap testing is a nondestructive inspection done after cure. Uncured plies can't give a meaningful result.
• BCorrect. FAA guidance calls for checking the bagged repair for leaks before cure. In a drop test, the pump is isolated and the gauge is watched for a specified time. A leak would reduce compaction pressure and could let air into the laminate.
• CIncorrect. The bleeder absorbs excess resin during cure, so saturation can't be judged before heat is applied. A saturated breather also blocks vacuum.
• DIncorrect. A witness sample is used to confirm cure after the cycle. Gelled resin before the bag is checked would mean the working time has already run out.

Answer: B) Mild soap and water

• AIncorrect. Solvents can craze acrylic.
• BCorrect. Use mild soap, water and a soft cloth; many solvents craze acrylic.
• CIncorrect. These scratch the surface.
• DIncorrect. This scratches the surface.