Aircraft Wood Components

8/8/2019 Aircraft Wood Components

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Aircraft Wood Components

Inspection For Deterioration

1/12/2003 Ref FAA Inspection Procedures 1

Fadi Ghorayeb


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Purpose.This material provides guidelines for theinspection of wooden components of aircraftprimary structures.Components of primary aircraft structuresinclude, but are not limited to:

1. Spars2. Ribs3. Formers4. Stringers5. Longerons6. Stiffeners7. Beams

8. Bulkheads


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Background and Discussion.Experience has shown that in addition to

the normal routine maintenanceinspections, all aircraft which have woodencomponents in their primary structure

require very thorough repetitiveinspections, especially of the glued joints,to determine continuing structuralsoundness.


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Excessive Moisture While excessive moisture hasbeen the cause of both glued

joint failures and delamination of

plywood, another factor to beconsidered is the deterioration of

the structure with time.


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TestsTests have shown that even in well

maintained and properly storedcomponents,1. the loss of linear strength of a glued joint can

amount to 60% in ten years' time.2. Fungi may, attack the wood resulting in a

condition designated as decay.3. Decay can occur at temperatures ideal for

growth of plant life.4. Serious decay occurs only when the moisture

content of the wood is above the fiber

saturation point (average 30 percent).


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Previously dried woodwhen previously dried wood is contacted bywater, such as provided by;

1. rain,2. condensation,3. contact with wet ground,

will the fiber saturation point be reached.The water vapor in humid air alone will not wetwood sufficiently to support significant decay,but it will permit development of some mould.If excessive moisture is not allowed to enter thewood fiber, there is virtually no limit to thecomponents structural life expectancy.


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Acceptable Methods.To ensure the structural integrity and

continued airworthiness of the wood, thefollowing inspection procedures aresuggested:

1. Exterior Surface Inspection.2. Internal Inspection.3. Moisture Test And Probing Inspection.


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Exterior Surface Inspection.Inspect the entire exterior surface of thecomponent (Wing, fuselage, tail, etc.) forthe following characteristics:

1. Indications2. Visible evidence3. Visible cracks4. Exterior damage

Mark the areas which have thecharacteristics described above forfurther detailed procedures described at

a later stage


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Indicationsthe wood immediately below the fabricis:

1. soft2. contains excessive moisture (i.e. swollen).

Soft wood is located by depressing thecomponents surface in the vicinity of thearea in question with a rounded, blunt

instrument and comparing its hardnesswith that of good wood.Note that the areas being compared

must have identical substructure.


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Visible cracks in the paintThis is important as water is

prevented from entering thecomponent by the fabric/paintbarrier.Any cracks in this barrier, no matterhow small, may comprise theprevention of water from enteringthe wood.


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Exterior damage would allow water to penetrate the

fabric/paint barrier and enter thewood. The surface features describedin slides(8), (9), and (10) may beaccentuated by illuminating thesurface with a light source placed at

a shallow angle.


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Technique used by experienced and qualified

person 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 directlybelow the surface MAY have decay


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Internal Inspection. Remove all inspection/access covers.(b) Using a strong bright flashlight and a mirror, inspect the entire interior of thecomponent 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; and5. Detectable moisture.

Make note of any areas which have the characteristics described in slide 12 and

refer to slide 14 for additional inspection procedures.Be certain that all drain holes are completely open and free of burrs and/or piecesof fabric which would cause water to be retained.


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Moisture Test And Probing Inspection. If the inspection described in slides 8 thru 13identify any questionable areas, continue theprogressive 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 InstrumentCompany, Boonton, New Jersey, or equivalent).


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Moisture Test And Probing Inspection. The probing inspection is designed toidentify wood by penetrating it with asharp object such as an awl or sharppocket knife.

You may wish to "calibrate" yourself andyour probe instrument by testing knowngood wood of a quality equal to that usedin the component.Note that the airframe is constructed withseveral different kinds of woods, each of which have noticeably different hardness.


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The inspection of a component If during the inspection of acomponent you suspect that thestructure has decay close to the

surface,Remove a small plug of the wing skin

(1/16 inch thick or 1/8 inch thick) toprobe inspect the structure materialdirectly.


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The inspection of a component Sharpen a inch drill bit so that its point

angle is very flat and provide it with astop which prevents it from penetrating toa depth greater than the thickness of theskin.Test the drill bit on a separate piece of plywood to ensure that it cuts clean andpenetrates the proper amount.


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Probing inspectionIf the probing inspection indicates good wood, the

plug must be replaced using standard repair procedures such as those specified in FAA AdvisoryCircular 43.13-1B,

ACCEPTABLE METHODS, TECHNIQUES, ANDPRACTICES - AIRCRAFT INSPECTION AND REPAIR,

September 8, 1998.


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Tank removal Condition If the inspection described inslides 8 thru 13 gives you reasonto suspect that there may be

decay in a fuel tank area, a morethorough inspection may be

conducted by removing fuel tankcovers.


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Moisture content If moisture content is below 15% and the wood issolid as determined by probing, the structure canbe considered airworthy.If moisture content is 15% or above and thewood is solid as determined by probing, thestructure can still be considered airworthy butrepetitive inspections of suspected areas arerequired every 15 days until moisture content is

below 15%.Moisture content will decrease provided noadditional water is allowed to enter wood fibers.


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The drying process The drying process may be assisted by

directing warm, dry air over the entiresuspected area, taking moisture readingsdaily; do not allow the moisture content to

go below 10%.All deficiencies which would allow water tocome in contact with wood fibers MUST becorrected prior to exposing the aircraft tohigh moisture conditions.


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Probing Indicator If probing indicates soft or decay wood,

the affected structural members must bereplaced.


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Accomplishment of repairs The repairs may be accomplished with

reference to the following documents:FAA AC 43.13-1B:, Acceptable Methods,Techniques and P ractices AI RCRAFTI NSP ECTION AND REP AI R

Federal Aviation Administration 1999; available from:Superintendent of DocumentsU.S. Government Printing Office

Washington, D.C.U.S.A. 20402


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Accomplishment of repairs ANC-18: Design of W ood AircraftStructures, Chapter 4 Detail StructuralDesign, Munitions Board Aircraft Committee,June 1951; copies of this document may beobtained from: USA Naval Depot

5801 Tabor AvenuePhiladelphia,