TECH TIP: Inspecting Wood – Part Two
By Ron Alexander, VAA 27150
Inspecting and Repairing Older Wood
In my last article I discussed the basics regarding what to look for when inspecting wood. Common wood defects were presented along with how to detect them. Inspection techniques during the restoration process are somewhat different. The basics still apply but other problems may be present. The inspection procedures for older wood are basically the same as for new wood with a few additions. Understand that just because the wood was milled in 1929 does not necessarily mean it should be replaced. It is not uncommon to find wood over 80 years old that is still serviceable. However, defects in older wood are often more difficult to detect. The wood is usually covered with layers of varnish that will hide problem areas.
Basically, you will inspect the wood in your restoration project much the same way as you do when you are examining new wood. There are, of course, some differences that will be outlined. Older wood will often hide defects that will be readily apparent in new wood. This is due to the discoloration of the varnish as it ages along with normal wear. Cracks and compression failures are sometimes difficult to find. You should carefully examine the wood at the lowest points on the aircraft where moisture often collects. If the airplane has a tailwheel look at the aft section of the wings and fuselage. Dirt will collect in these areas and when it becomes wet it will hold moisture against the structure. This can result in the protective varnish being penetrated allowing moisture to get into the fibers of the wood. The following items should be inspected with regard to structural wood:
- Inspect for evidence of mildew—Mildew may result from excessive humidity and heat. If it is extensive it can result in dry rot developing.
- Inspect glue joints—all glue joints should be closely examined for evidence of cracking or opening up. This can indicate a loss of adhesion. Also, if a glue joint is discolored it may be an indication of an adverse chemical reaction that could cause loss of adhesion. (Note: if Resorcinol glue has been used it will be a dark color—this is normal). If you find evidence of failing glue joints, scrape away all of the varnish around the area. This will allow you to better examine the wood and more specifically, the glue joints. You can also use a small knife to see if you can penetrate the crack by gently pushing the blade into the wood. Use a magnifying glass to assist in your inspection of glue joints.
- Loosening of nails—nails in wooden structures are placed there to hold component parts in position until the glue dries. For all practical purposes they can be removed once the glue is dry. However, if you see any nails loosening it may be evidence that there is adverse movement of the spar that should be investigated. If there is any evidence of corrosion or water stains around the nailed area further inspection is necessary. These same inspection items apply to wood screws.
- Wood shrinkage—particularly around fittings. If the wood shrinks the bolts or screws holding fittings in place may loosen.
- Excessive moisture or wetness of the wood—be sure moisture is not being trapped in the wooden structure. Moisture will be easy to detect.
- Inspect for cracks—this item is very important. You should inspect the spar for any developing cracks. Use a small magnifying glass in suspect areas. Cracks are often difficult to see with the naked eye. Cracks may develop anywhere on the spar but are more likely to develop around fittings. The wing attach fittings that attach the wings to the fuselage should be thoroughly inspected. Also, carefully inspect strut attach fittings. Excessive stress on the entire wing structure can cause cracks. Impacts that have occurred or bolts that are too tight on the wood can cause problems. Be sure to look over the entire spar for cracks.
- Looseness of fittings—you should shake the wings in a rapid manner from the wing tips to see if you detect any loose fittings. This would be found where the wing struts are attached to the spar. If you detect any loose bolts you should investigate further for indication of wear of the bolt or wood deterioration.
- Inspection of reinforcement plates—most wooden spars have plywood reinforcement plates glued to their faces where wing and strut fittings are attached. These plates may also be found under a spar splice. Be sure these plates are not separating from the spar itself. If the glue is failing or other problems are developing, the plate itself may have to be removed and replaced.
- Loss of finish—if the varnish is being eroded from other chemicals or simply is deteriorating, mildew or a fungus can develop within the wood fibers.
- Stains—be very suspicious of any stains you observe. Stains are often accompanied by rot.
- Fungus—you should inspect for fungus. It will usually develop under hot, moist conditions when exposed to spores released by fungus in storage areas.
- Dry rot—dry rot can be caused by loss of finish, mildew, fungus, excessive shrinkage and cracks. Wood that is developing dry rot can be extremely weakened.
- Compression failure—You must be aware of this problem in aircraft that are completed and have flown. If the airplane has been subjected to unusual loads or stress, the development of a compression failure is possible. It will usually appear as a small, fine line running across the grain of the spar. It is an indication that the fibers in the wood have been strained and actually ruptured. Anytime you detect wood that has evidence of a compression failure it must be replaced.
If you suspect an area of wood is rotten or decaying, you can use a small knife to scrape and pick at the wood. If the wood splinters it should be good but if it is soft and can be easily cut into small chunks it is probably rotten and should be replaced. If any area is rotting or decaying it should be replaced with sound wood.
When you are inspecting older wood you assume that most of the major defects presented in our discussion in the last article will not be present. That is a fairly safe assumption but you must still be on the lookout for these problems. You would hope that the person who installed the wood and finished it also inspected it for major defects. If you are restoring an airplane, you will not have the opportunity to inspect these areas for a very long period of time. The airplane will more than likely be covered with fabric and not be open again for a number of years. When the covering is off is the time to find problems.
A bigger challenge occurs during a routine annual inspection (production aircraft). Removing inspection plates to look for spar defects must be accomplished during this yearly examination. Attempting to view a spar through a small opening looking into a dark area is a task. Lights and inspection mirrors are necessary to facilitate this inspection. Obviously, finding problems with wood in your aircraft is much easier when you are rebuilding or restoring the aircraft than during the annual inspection. Extreme care must be taken to ensure the yearly inspection for wood problems can be adequately accomplished.
A spar crack will often develop in the vicinity of the plywood reinforcement plates. These cracks often occur as a result of the spar shrinking when excessive drying takes place. These cracks often start under the plates themselves at the bolt hole and spread in each direction. The presence of a crack does not mean the spar must be replaced. FAA Advisory Circular 43-13 states “If the crack is not too long or too close to either edge and can be reinforced properly, it will probably be more economical and satisfactory to effect a repair than to install a new spar or section.” This type of repair consists of the cracked area being reinforced by gluing plates of plywood or spruce to both sides of the spar. These plates must be of sufficient thickness to develop the shear strength to both sides of the spar. The plates should also extend beyond the cracks as recommended in AC 43-13. The advisory circular details how to accomplish this repair. Again, no fittings are allowed within the cracked area.
What if you do find a crack in a spar or some other indication of a major problem? Can you repair the spar or other component part? The short answer is “yes”. Repairs of wood are often not simple and I would suggest you enlist the assistance of a qualified mechanic to help with this task. Obviously, in today’s environment it may be difficult to find a mechanic who is knowledgeable concerning wood repairs. Of course, when restoring a production airplane you must have an airframe mechanic do the repair or supervise you while you do it (assuming you are not an A & P mechanic). Wooden spars can be spliced according to procedures found in AC 43-13. Basically, a spar may be spliced at any point except under wing attachment fittings, strut fittings, etc. The fittings themselves must not overlap any part of a spar splice. Again, splicing a spar should only be undertaken with the help of someone who has experience with the process.
To summarize, restoring an older airplane will usually involve working with wood in one form or another. The average restorer can usually accomplish routine inspection and repair, especially with the assistance of a good A & P mechanic. More complex repairs, such as a wing splice, present a challenge. The challenge lies in making the determination as to whether to splice the spar or replace it. Subsequently, trying to locate the person with the skills needed to safely do the repair may be difficult. If in doubt concerning your ability or the ability of the mechanic to do the repair then I would opt to completely replace the spar.Top of Form