3.5 Transparent Plastics, Windows, Windshields & Cabin Interiors

Why This Topic Is on the Airframe Test

ACS Subject II.B (Non-Metallic Structures) does not end with composites, wood, and fabric. Its last seven knowledge elements cover a completely different family of material: transparent thermoplastics and the cabin items attached to them. Those elements are thermoplastic material inspection and defect types (K23), thermoplastic storage and handling (K24), thermoplastic installation procedures (K25), care and maintenance of windows (K26), window temporary and permanent repairs (K27), maintenance safety practices for composite materials and windows (K28), and inspecting restraints and upholstery (K29).

These are also live regulatory items. 14 CFR Part 43, Appendix D, paragraph (c) — the cabin and cockpit group of every annual and 100-hour inspection — requires the inspector to check "(2) Seats and safety belts — for poor condition and apparent defects" and "(3) Windows and windshields — for deterioration and breakage." Nearly every question the FAA writes here is a specific number, a specific prohibited chemical, or a specific torque practice, so learn the values rather than the concepts.

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1. Two Plastic Families and How to Tell Them Apart

Acrylic transparencies are procured to two military specifications you should be able to tell apart:

• MIL-P-5425 — regular (as-cast) acrylic.
• MIL-P-8184 — craze-resistant acrylic, commonly called stretched acrylic. Biaxially stretching the sheet before it is cut greatly improves its resistance to crazing and to crack propagation.

Older transparencies used cellulose acetate, which yellows and shrinks with age. Both acrylic and cellulose acetate are thermoplastic — which is exactly why buffing or polishing too long in one spot generates enough friction heat to soften the surface and produce permanent visual distortion.

The optical acceptance rule: scratches and other damage that obstruct the vision of the pilots are not acceptable. Some types of damage may be acceptable at the edges of the windshield, where they are outside the primary field of view. Never assume a scratch is airworthy because it is small; assume it is airworthy only because of where it is and what the manufacturer's manual allows.

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2. Storage and Handling (K24)

Transparent thermoplastic sheets soften and deform when heated, so storage is a controlled activity, not a shelf-space problem.

Two of these are classic distractor traps. The 10° bin angle and the 18-inch stack height are frequently swapped with each other or with the 120 °F temperature limit in multiple-choice options, so memorize which number belongs to which rule.

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3. Forming, Sawing and Drilling (K25)

Cold bending. Acrylic may be cold-bent into a single curvature only, only if the material is thin, and only if the bending radius is at least 180 times the thickness of the sheet. A 0.100-inch sheet therefore needs a minimum bend radius of 18 inches. Bending tighter than that imposes so much surface stress that tiny fissures called crazing form — the same defect that solvents produce chemically.

Hot forming. Use a forced-air oven capable of operating over 120–374 °F (49–190 °C) and follow the manufacturer's recommended temperature. Practices that matter:

• Remove all masking paper and adhesive before heating; wash and blot-dry a dirty sheet first.
• Wear cotton gloves so finger marks are not pressed into the softened surface.
• Never apply hot water or steam directly to the plastic — it turns the acrylic milky or cloudy.
• Overheating produces surface bubbles that permanently impair the optical qualities.
• For uniform heating, hang sheets vertically from spring clips; heat lamps are placed on 7–8 inch centers about 18 inches from the material.
• In simple-curve forming, drape the heated sheet over the form and hold or clamp it 10–30 minutes until it cools. Do not force-cool it.

Compound curves require specialized equipment and use four methods: stretch forming, male and female die forming, vacuum forming without a form (surface tension alone shapes the canopy), and vacuum forming with a female form.

Sawing. Circular saws are best for straight cuts, with hollow-ground or set-and-side-dressed teeth to prevent binding and leave a smooth edge. Band saws are used for curves or to rough-cut — band saw to within 1/16 inch of the scribed line, then sand to final size.

Drilling. Acrylic is a very poor conductor of heat, so the heat has to be carried away mechanically. Use water-soluble cutting oil as a coolant for deep holes (it will not attack the plastic). Grind the drill with a greater included angle than you would use for soft metal and with a zero rake angle so the bit scrapes rather than cuts; the bit must be free of nicks and burrs.

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4. Cleaning, Waxing and Polishing (K26, K28)

Plastic has many advantages over glass but lacks its surface hardness, so servicing damage is the dominant failure mode.

Two more rules carry their own reasoning, and the FAA tests the reasoning:

• Never rub a transparency with a dry cloth. It scratches the soft surface and builds an electrostatic charge that pulls dust particles onto the panel, guaranteeing more scratches on the next wipe.
• Polish only a clean surface, using a soft, open-type cotton or flannel buffing wheel, and keep the wheel moving. Dwelling in one place softens the thermoplastic and distorts it.

Personal safety for this work sits alongside composite safety in ACS II.B.K28: the solvents used around transparencies are flammable and their vapors are respiratory irritants, so containers stay closed, ventilation stays on, and the same eye protection, gloves, and dust control you use for grinding cured composite apply when you sand or trim acrylic.

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5. Window Repairs, Temporary and Permanent (K27)

The governing principle: whenever possible, replace rather than repair extensively damaged transparent plastic. A carefully patched part is not the equal of a new section — neither optically nor structurally.

Temporary repair — stop drilling. At the first sign of crack development, drill a small hole with a #30 or 1/8-inch drill at the extreme ends of the crack. This localizes the crack and prevents further splitting by distributing the strain over a larger area. Each crack occurring at any hole or tear is drilled in the same manner. If the cracks are small, stop drilling usually suffices until replacement or a more permanent repair can be made.

Permanent repair — cementing. Acrylic is joined with solvent or polymerizable cements, and the shrinkage behavior is what gets tested:

• Polymerizable cements shrink as they harden, so the freshly poured bead is deliberately left above the surfaces being cemented to compensate; the bead is machined off afterward if appearance requires it.
• Bubbles float to the top of a cement bead in a gap joint and cause no problem if the bead is machined off; disturb the joint as little as possible while lifting them out.
• Cemented joints may be machined 4 hours after assembly, but waiting 24 hours is better.

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6. Windshield and Window Installation (K25)

Use material equivalent to that originally used by the manufacturer. Transparency properties vary greatly between products — expansion characteristics, brittleness at low temperature, resistance to sunlight discoloration, and surface checking — and MIL-HDBK-17 Part II, Transparent Glazing Materials, is the reference. Substituting a transparency with different characteristics is a known source of later trouble.

The six installation principles, with the numbers the exam asks for:

1. Never force a plastic panel out of shape to make it fit a frame. If it does not fit easily, obtain a new panel or heat and re-form the whole panel. Cut and fit at ordinary room temperature when possible.
2. Do not place the plastic under excessive compressive stress. It is easy to develop more than 1,000 psi by overtorquing a nut and bolt. Tighten each nut to a firm fit, then back the nut off one full turn — snug, but still rotatable with the fingers.
3. Use spacers, collars, shoulders, or stop-nuts to prevent excessive tightening, and retain every such device the manufacturer used. Use the original number of bolts, complete with washers and spacers. Riveted frames need the same anti-crush provisions.
4. Mount panels between rubber, cork, or other gasket material — this waterproofs the installation, reduces vibration, and helps distribute compressive stress into the plastic.
5. Plastics expand and contract considerably more than the metal channels holding them. Mount windshield panels to a minimum channel depth of 1-1/8 inches, with 1/8-inch clearance between the plastic and the bottom of the channel, so the panel cannot fall out when it contracts at low temperature or deforms under load.
6. Make bolt and rivet holes through the plastic 1/8 inch oversize and centered so the plastic cannot bind or crack at the hole edge. Slotted holes are also recommended.

Number discipline for this section: 180 × thickness is the cold-bend radius, 1-1/8 inch is the channel depth, and 1/8 inch is both the channel clearance and the hole oversize. Three different values, three different jobs.

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7. Restraints, Upholstery and Interior Fire Protection (K29)

Approval basis. Safety belts must be of an approved type conforming to FAA standards: TSO-C22 for seat belts and TSO-C114 for restraint (shoulder harness) systems. Belts eligible for installation are identified by the proper TSO marking on the belt itself, and each safety belt must be equipped with an approved metal-to-metal latching device.

Condition inspection. The webbing of safety belts, even when mildew-proofed, deteriorates from constant use, cleaning, and the effects of aging. Fraying is the wear indicator — a frayed belt is likely unairworthy because it can no longer hold the minimum required tensile load. Airworthy type-certificated belts may be removed for cleaning and reinstalled, but when a TSO belt is found unairworthy, replacement with a new TSO-approved belt or harness is required. Belts may be repaired only in accordance with specifications approved by the responsible FAA Aircraft Certification Office.

The washing trap. AC 43.13-1B paragraph 9-64 is the source of one of the most-missed facts in this subject: upholstery and/or belts that have been washed may lose some or all of their fire-resistant qualities, and unless the soap is completely removed from the cloth, the strength of the material may be significantly reduced. The manufacturer must be consulted to determine how to restore the fire-resistant qualities.

Interior burn testing. If an annual inspection is performed on a Part 23 airplane in which a new interior was installed, and the records show no burn test by an FAA-approved repair station, no STC, and no FAA Form 337, then a burn test meeting Part 23, Appendix F must be accomplished before the aircraft is approved for return to service. Where the fire protection qualities of existing cabin interior materials are unknown, the request for that information goes to the aircraft manufacturer or the local FAA regional office, citing the model, manufacturer, serial number or manufacture date, and the operating rule (Part 91, Part 121, and so on).

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Section Traps to Rehearse

• Fraying condemns webbing — not stretching, not fading of the TSO label.
• Washing belts and upholstery attacks fire resistance and strength, not the latch approval.
• A #30 or 1/8-inch drill stops a crack; acetone does not weld one shut.
• The nut is backed off one full turn, not one flat and not one quarter turn.
• Deicing fluid and fire extinguisher fluid are on the prohibited-cleaner list; candidates routinely assume anything approved for use on the airframe is safe on a transparency.