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Section 6.3.1(b) / 6.3.2 / 6.3.3

Composite Repair, Wood & Fabric

Detecting and repairing composite damage, wooden structure, and doped fabric covering. Category B2 does not sit these sub-sections.

Notes

Composite repair, wood & fabric, summary notes

Main ideas
  • Composite damage is often hidden. An impact can cause internal delamination or disbond while leaving barely visible impact damage (BVID) on the surface, and water can enter a honeycomb core through a crack and freeze. A visual check alone is not enough: composites are inspected by tap test, ultrasonic, thermography and X-ray, and the result is assessed against the structural repair manual before any airworthiness decision.
  • Repair restores the load path. Damage within limits may be sealed or filled; beyond that the plies are scarfed or stepped back and new plies laid up to the same orientation and count as the original, then cured under heat and pressure. A repair that ignores fibre orientation puts the strength in the wrong direction, and a bolted patch on a laminate concentrates load where the original design carried it through the fibres.
  • Wooden structure is graded on grain, moisture and glue. Spruce is the classic spar material for its strength-to-weight and straight grain; the grain slope, knots and shakes decide whether a piece is airworthy. Wood is inspected for rot, compression failures across the grain, and glue joints that have starved, gaps or has crept. Moisture content matters: too wet and the joint fails, too dry and the wood splits, so it is stored in controlled humidity.
  • Fabric covering is doped, tensioned and inspected for strength. The covering is attached, then coated with dope, which tautens it and protects it from ultraviolet light. It is checked for tears, chafe at ribs and stringers, and loss of strength with age, tested against the minimum breaking strength for the aircraft. Once the fabric falls below that figure it is recovered, not patched.
  • ⚠ Exam trap: composite impact damage can be extensive internally while almost invisible on the surface. NDT is required, not just a visual check.
Solved examples

  1. A CFRP control surface is struck by a ground vehicle. The surface shows only a faint mark. Why can it not simply be signed off as serviceable?

    Carbon-fibre laminates can suffer extensive sub-surface delamination from an impact that leaves barely visible damage (BVID) on the outside. The load-carrying plies may be broken internally. It must be assessed by NDT (tap test/ultrasonic/thermography) against the SRM limits before any airworthiness decision.

Quiz

Composite repair, wood & fabric quiz

Composite Repair, Wood & Fabric, quiz

Ref 6.3.1(b) / 6.3.2 / 6.3.3Pass 75%

  1. 1. 'Barely visible impact damage' (BVID) is a concern because:

    Internal delamination can be severe with little surface sign
    It is always obvious on the surface, so a visual inspection is sufficient to find it
    It affects only the paint and surface ply, leaving the load-carrying laminate untouched

  2. 2. Composite internal damage is best found by:

    NDT such as tap test, ultrasonic or thermography
    A visual check alone
    Weighing the part

  3. 3. A CFRP control surface is struck by a vehicle and shows only a faint surface mark. It must be

    Assessed by NDT for hidden delamination (BVID) before release
    Signed off if it still looks smooth
    Painted over

  4. 4. Spruce is the classic aircraft spar timber because it offers:

    A high strength-to-weight ratio with straight, even grain
    The greatest absolute strength of any timber used in aircraft
    Complete immunity to moisture change and fungal attack

  5. 5. A compression failure in a wooden member runs:

    Across the grain, as a line of buckled fibres
    Along the grain, as a clean split from end to end
    Around a knot, following the disturbed grain

  6. 6. Wood used in aircraft structure is stored at controlled humidity because:

    Too wet fails the glue joints and too dry splits the timber
    Dry wood loses its resistance to fungal attack and rot
    Humidity changes the grain slope that decides airworthiness

  7. 7. Dope is applied to a fabric covering mainly to:

    Tauten the fabric and protect it from ultraviolet light
    Bond the fabric to the ribs and stringers underneath
    Add the structural strength the covering needs to carry load

  8. 8. A fabric covering that has fallen below its minimum breaking strength must be:

    Recovered, as patching does not restore an aged covering
    Patched over the weakest areas found during the strength test
    Re-doped, which restores the strength lost to ultraviolet light

  9. 9. When a damaged composite laminate is repaired, the replacement plies must match the original in:

    Fibre orientation and the number of plies
    Resin colour and the surface finish applied
    Overall patch thickness, whatever the ply directions

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