I have been asked many times about the longevity of a wooden aircraft. When properly constructed, sealed and finished, a wood aircraft is as durable as a metal aircraft. The information in this article was provided to me by the Canadian Owners and Pilots Association (COPA)and is part of the COPA Guide to Amateur-Builts.
Remember that this information is 20 years old and glues have improved considerably since then but the techniques for testing a wooden structure is applicable today.
AMA: 571.101 /5
Date: May 26, 1988
Subject: AIRCRAFT WOODEN COMPONENTS - INSPECTION FOR DETERIORATION
This advisory material provides guidelines for the inspection of wooden components of aircraft primary structures.
2. REFERENCE AIRWORTHINESS STANDARDS.
Chapter 549, Amateur-Built Aircraft Section 549.19, paragraph (b); Chapter 571, Maintenance of Aeronautical Products - Appendix A.
3. BACKGROUND AND DISCUSSION.
Experience has shown that in addition to the normal routine maintenance inspections, all aircraft which have wooden components in their primary structure require very thorough repetitive inspections, especially of the glued joints, to determine continuing structural soundness.
While excessive moisture has been the cause of both glued joint failures and delamination of plywood, another factor to be considered is the deterioration of the structure with time. Tests have shown that even in well maintained and properly stored components, the loss of linear strength of a glued joint can amount to 60% in ten years' time. Fungi may, under conditions that favour their growth, attack the wood resulting in a condition designated as decay. Decay can occur at temperatures that favour growth of plant life in general. Serious decay occurs only when the moisture content of the wood is above the fibre saturation point (average 30 percent). These conditions are particularly prevalent in the South-eastern United States but may also be encountered in Canada. Only when previously dried wood is contacted by water, such as provided by rain, condensation, or contact with wet ground, will the fibre saturation point be reached. The water vapour in humid air alone will not wet wood sufficiently to support significant decay, but it will permit development of some mould. If excessive moisture is not allowed to enter the wood fibres, there is virtually no limit to the components structural life expectancy.
4. ACCEPTABLE METHODS.
To ensure the structural integrity of the wood, the following inspection procedures are suggested:
4.1 Exterior Surface Inspection
(a) Inspect the entire exterior surface of the component (Wing, fuselage, tail, etc.) for the following characteristics:
(l)Signs which indicate that the wood immediately below the fabric is soft or contains excessive moisture (i.e. swollen). Soft wood may be located and/ or confirmed by depressing the components surface in the vicinity of the area in question with a rounded, blunt instrument and comparing its hardness with that of good wood. Note that the areas being compared must have identical substructure.
(2)Signs which indicate that the fabric / paint is delaminating from the wood surface (bubbles, discoloration, boils, soft spots and other surface flaws).
(3)Cracks or breaks in the paint.
Water is prevented from entering the component by the fabric/ paint barrier. Any cracks in this barrier, no matter how small, may comprise its ability to prevent water from entering the wood.
(4)Exterior damage which would allow water to penetrate the fabric / paint barrier and enter the wood. The surface features described in (1), (2), and (4) may be accentuated by illuminating the surface with a light source placed at a shallow angle.
The following technique may be used by an experienced person to detect soft and/or decayed wood in the wing spars. Tap the wing directly above and below both spars with a small rounded, blunt instrument, approximately the size of a small pocket knife. Start at the outboard end and work inboard, listening to the sound generated by the wing. The sound quality will change slowly. If the change in sound is abrupt, the wood directly below the surface may have decay.
The above method may also be adapted to check other components for decay.
(b)Mark the areas which have the characteristics described in paragraph 4.1(a) and refer to paragraph 4.3 for additional inspection procedures.
4.2 Internal Inspection.
(a) Remove all inspection/ access covers.
(b)Using a flashlight and a mirror, inspect the entire interior of the component for the following characteristics:
(1) Wood decay;
(2) Water stains on wood or covering;
(3)Pooled dust/ dirt which may indicate evidence of previous standing water;
(4) Rust or corrosion on metallic surfaces; and
(5) Detectable moisture.
(c)Make note of any areas which have the characteristics described in paragraph 4.2(b) and refer to paragraph 4.3 for additional inspection procedures.
(d)Be certain that all drain holes are completely open and free of burrs and/ or pieces of fabric which would cause water to be retained.
4.3 Moisture Test and Probing Inspection.
(a)If the inspection described in paragraphs 4.1 and 4.2 identify any questionable areas, continue the progressive inspection by testing these areas per the following procedures:
(1) Test for soft/ decayed wood with sharp probe.
(2)Test for moisture content using suitable resistance type moisture meter (model G-2, Delmhorst Instrument Company, Boonton, New Jersey, or equivalent).
The probing inspection is designed to identify wood by penetrating it with a sharp object such as an awl or sharp pocket knife. You may wish to "calibrate" yourself and your probe instrument by testing known good wood of a quality equal to that used in the component. Note that the airframe is constructed with several different kinds of woods, each of which have noticeably different hardness.
(b)If during the inspection of a component you suspect that the structure has decay close to the surface, you may remove a small plug of the wing skin (1/16 inch thick or 1/8 inch thick) to probe inspect the structure material directly. Sharpen a 1/4 inch drill bit so that its point angle is very flat and provide it with a stop which prevents it from penetrating to a depth greater than the thickness of the skin; test the drill bit on a separate piece of plywood to ensure that it cuts clean and penetrates the proper amount. If the probing inspection indicates good wood, the plug must be replaced using standard repair procedures such as those specified in FAA AC 43.13-1A.
(c)If the inspection described in paragraphs 4.1(a)(3) gives you reason to suspect that there may be decay in a fuel tank area, a more thorough inspection may be conducted by removing fuel tank covers.
(d)If moisture content is below 15% and the wood is solid as determined by probing, the structure can be considered airworthy. If moisture content is 15% or above and the wood is solid as determined by probing, the structure can still be considered airworthy but repetitive inspections of suspected areas are required every 15 days until moisture content is below 15%. Moisture content will decrease provided no additional water is allowed to enter wood fibres. The drying process may be assisted by directing warm, dry air over the entire suspected area, taking moisture readings daily; do not allow the moisture content to go below 10%. All deficiencies which would allow water to come in contact with wood fibres MUST be corrected prior to exposing the aircraft to high moisture conditions.
(e)If probing indicates soft or decay wood, the affected structural members must be replaced. The repairs may be accomplished with reference to the following documents:
(1)FAA AC 43.13-1A: Acceptable Methods, Techniques and Practices AIRCRAFT INSPECTION AND REPAIR' Department of Transportation, Federal Aviation Agency 1972; available from:
Superintendent of Documents
U.S. Government Printing Office
(2)ANC-18: Design of Wood Aircraft Structures, Chapter 4 Detail Structural Design, Munitions Board Aircraft Committee, June 1951; copies of this document may be obtained from:
USA Naval Depot
5801 Tabor Avenue
(3) Designer or Kit Manufacturer drawings and repair schemes.
(4)Modifications and repairs recommended by the Regional Manager Airworthiness.
Chief, Airworthiness Standards