3.5 Fabric Covering: Inspection, Defects & Repair
Key Takeaways
- Fabric is airworthy until its breaking strength falls below 70 per cent of the strength of new fabric required for the aircraft, not of the fabric actually installed.
- Grade A cotton is 80 lb per inch new and unairworthy below 56 lb per inch; intermediate grade is 65 lb per inch new and unairworthy below 46 lb per inch; gliders below 8 lb/ft2 and 135 mph use a 35 lb per inch limit.
- Mechanical punch testers such as the Maule and Seyboth are not FAA approved, depend on coating thickness and calibration, and must not be used on glass fabric.
- The field breaking-strength test uses a 1 1/4 inch by 4 inch sample cut from a top exposed surface, coatings removed and edges ravelled to a 1 inch width.
- Doped-on patch repairs are limited to openings not over 8 inches with a 2-inch overlap; larger openings need increased overlap, wider finishing tape or a sewn-in panel.
What 6.3.3 Requires
Sub-module 6.3.3 Fabric covering is examined at level 1 for B1 and B3 only — it is not applicable to category A or to B2. Its four bullets are:
- Characteristics, properties and types of fabrics used in aeroplanes
- Inspection methods for fabrics
- Types of defects in fabrics
- Repair of fabric covering
Section 3.3 dealt with bullet 1 — polyester versus cotton and linen, heat-shrinking, rib attachment and dope systems. This section covers the inspection, defect and repair bullets, which is where the examinable numbers live. The reference throughout is FAA AC 43.13-1B, Chapter 2, plus the Supplemental Type Certificate (STC) manual for whichever proprietary covering system is installed.
Fabric Grades and Their Numbers
| Material | Specification | Min. new tensile strength (undoped) | Min. new tearing strength | Deteriorated (unairworthy) limit | Thread count |
|---|---|---|---|---|---|
| Grade A mercerised cotton airplane cloth | TSO-C15d (references AMS 3806D / MIL-C-5646) | 80 lb per inch warp and fill | 5 lb warp and fill | 56 lb per inch | 80 min., 84 max. |
| Intermediate grade mercerised cotton | TSO-C14b (references AMS 3804C) | 65 lb per inch warp and fill | 4 lb warp and fill | 46 lb per inch | 80 min., 94 max. |
Grade A is required on aircraft with a wing loading of 9 lb/ft² or greater, or a placarded never-exceed speed of 160 mph or greater. Intermediate grade is permitted below both thresholds.
Modern aircraft are almost always covered in polyester (Ceconite, Poly-Fiber, Superflite and similar) under an STC rather than under a TSO. Polyester does not rot, is far more durable, and its condition is governed by the STC holder's manual, which must be followed for materials, process and repair. The fabric source identification stamps inside the wings and fuselage identify what is installed.
The Airworthiness Criterion
This is the single most examinable fact in 6.3.3:
Fabric is considered airworthy until it deteriorates to a breaking strength of less than 70 % of the strength of new fabric required for that aircraft.
Note the wording carefully — it is 70 % of the strength required for the aircraft, not 70 % of whatever was installed. If Grade A cotton (80 lb/in new) has been used on an aeroplane that only requires intermediate fabric (65 lb/in new), it may deteriorate to 46 lb/in (70 % of 65) before replacement, not to 56.
The condition of the fabric covering must be determined at every 100-hour and annual inspection, because fabric strength is a direct airworthiness factor.
| Aircraft category | Unairworthy when breaking strength falls below |
|---|---|
| Wing loading < 9 lb/ft² and Vne < 160 mph | 46 lb per inch width, regardless of fabric grade |
| Wing loading ≥ 9 lb/ft² or Vne ≥ 160 mph | 56 lb per inch width |
| Glider/sailplane, wing loading ≤ 8 lb/ft² and Vne ≤ 135 mph | 35 lb per inch width, regardless of fabric grade |
Inspection Methods
1. Visual and tactile
Start with the areas the environment attacks hardest: upper surfaces (ultraviolet), around exhaust outlets, under the wing walk, at drain grommets, along the leading edges, and anywhere a fairing or a control cable can chafe. Look for cracked, peeling or deteriorated coatings, loose or broken rib stitching, damaged finishing tapes, and fabric that has gone slack or is "drumming" in the slipstream. Press firmly with the thumb: sound fabric resists, degraded fabric feels papery and may split.
2. Mechanical punch testers
Punch (or "prick") testers such as the Maule and the Seyboth press a calibrated point against the finished fabric. The examinable point is their status:
Mechanical devices used to test fabric by pressing against or piercing the finished fabric are not FAA approved. They are used at the mechanic's discretion to form an opinion about general fabric condition.
Their accuracy depends on the individual device's calibration, the total coating thickness, coating brittleness, and the types of coating and fabric. They are not applicable to glass-fibre fabric, which shears easily and will read very low regardless of its true breaking strength.
3. Field breaking-strength test
If the punch tester reads in the lower range, or the overall condition looks poor, a more accurate field test is made:
- Cut a 1¼-inch wide × 4-inch long sample from a top exposed surface (the worst-case area).
- Remove all coatings and ravel the edges to a 1-inch width.
- Clamp each end, anchor one clamp to a support structure, and apply load by adding sand to a container suspended a few inches above the floor.
- Compare the breaking load with the table values above.
4. Laboratory test
If the result is still in question, send a sample to a qualified testing laboratory for breaking-strength tests in accordance with ASTM D5035.
Note that removing a sample creates a hole that must then be repaired — which is why the punch tester survives as a screening tool despite not being approved.
Types of Defect
| Defect | Cause | Significance |
|---|---|---|
| Ultraviolet degradation | Sunlight reaching the fibres through a failed or thin pigmented coating | The dominant failure mode. Cotton, linen and polyester filaments all deteriorate under UV; glass fabric does not, but glass is degraded by other mechanisms |
| Rot and mildew | Moisture plus fungal attack — organic fibres only | Cotton and linen only; polyester and glass do not rot. A coloured tint on the back side of cotton or linen indicates a fungicide was present in the dope |
| Dope cracking and coating failure | Ageing, embrittlement, excessive dope build, vibration | Cracks open a UV and moisture path straight to the fibre |
| Loss of tautness / drumming | Coating breakdown, incorrect original tensioning, heat | Aerodynamic flutter risk and accelerating fatigue of the covering |
| Chafing and abrasion | Contact with structure, fairings, cables, boots | Localised, usually repairable by patch |
| Tears and punctures | Impact, ground handling, bird strike, hangar rash | Sized against the repair rules below |
| Broken rib stitching / lifted tapes | Vibration, thread deterioration, poor original workmanship | The stitching holds the fabric to the ribs against aerodynamic suction — broken stitching is airworthiness-critical |
| Bent or warped structure showing through | Underlying damage, not a fabric defect | Investigate the structure, not the covering |
Repair of Fabric Covering
Repairs fall into two families: doped-on patches for smaller openings and sewn (stitched-in) panels for larger ones. All of the thresholds turn on two variables: the length of the opening in any direction and the aircraft's Vne.
Doped-on patch repair
| Opening size (any direction) | Vne | Repair |
|---|---|---|
| Not over 8 in | < 150 mph | 2-inch overlapped doped patch |
| Less than 8 in | > 150 mph | 2-inch overlap patch plus 2-inch wide finishing tape on all sides, centred on the edge of the overlap |
| More than 8 in, less than 16 in | < 150 mph | Doped patch overlapped by ¼ of the opening's maximum dimension, with the overlap not exceeding 4 inches |
| More than 8 in, less than 16 in, on a wing top surface | > 150 mph | 4-inch patch overlap plus 2-inch finishing tape on all sides |
| More than 16 in | < 150 mph | 4-inch patch overlap plus 4-inch wide finishing tape centred on the patch edge |
| More than 16 in | > 150 mph | 4-inch patch overlap plus 6-inch wide finishing tape centred on the patch edge |
Method: trim the opening to remove irregular edges, remove the old pigmented coatings from the overlap area, apply a coat of clear dope around the opening, lay the patch, brush out voids and wrinkles while saturating only the overlap area, allow one hour to dry at 70 °F, wet the patch to pull out creases, then build up clear and pigmented dope over the whole patch. For a small opening, the loose fabric edges can be temporarily supported by threads stretched across the hole, clipped out afterwards.
Sewn-patch repair
Openings that cannot be closed with stitches are repaired by sewing in a new fabric section:
- Trim the opening straight on four sides so that straight finishing tape can lie over the stitches.
- Size the new section so both edges can be folded back ½ inch to increase stitch-tear resistance.
- Temporarily attach the four corners with thread, start with a double thread and a square knot, and hand-stitch, securing with a half hitch at a maximum of every 10 stitches.
- Wet the new section to remove creases, dry, apply one coat of clear dope, and after one hour at 70 °F apply finishing tape centred over the stitching.
Finishing tape width follows the same size rule as the patches: 3 inches normally, 4 inches if the opening is more than 8 but less than 16 inches, and 6 inches if the opening is over 16 inches, is on a wing top surface, or Vne exceeds 150 mph.
Polyester systems
Under an STC the repair scheme is the STC holder's, not the AC's. Typically the patch is bonded with the system's fabric cement, heat-shrunk with a calibrated iron, and finished with the system's approved coatings. Coatings other than those authorised by the STC may only be used with prior authority approval, and this must be recorded in the aircraft records. A burn test on a sample will reveal whether dope has been applied over polyester.
Exam Traps
- 70 % is the airworthiness threshold; 56 lb/in and 46 lb/in are the numbers it produces. A question giving Grade A cotton on an aircraft requiring only intermediate fabric is testing whether you apply 70 % of the required strength (46), not of the installed strength (56).
- Punch testers are not approved; they inform an opinion. A question asking for the "approved" or "accurate" method wants the cut-sample field test or the ASTM D5035 laboratory test.
- 8 inches and 16 inches are the patch thresholds; 150 mph is the Vne threshold. 160 mph and 9 lb/ft² are the fabric grade thresholds. Different numbers, different purpose.
- Polyester does not rot, but it does degrade under ultraviolet light — "synthetic therefore immortal" is the trap.
- Fabric condition is assessed at every 100-hour and annual inspection, not only at recover.
Grade A cotton fabric (80 lb per inch new) has been installed on an aeroplane whose type design only requires intermediate-grade fabric (65 lb per inch new). At what breaking strength does the covering become unairworthy?
What is the correct status of a Maule or Seyboth mechanical punch tester used on doped aircraft fabric?
How is the field breaking-strength test sample prepared when a punch tester gives a low reading?
A 6 inch tear is found in the fabric covering of an aeroplane with a Vne of 130 mph. Which repair is appropriate?