Having discussed in previous articles the process of finding an airplane to restore and what to look for prior to purchasing, it is now time to actually begin work on the project. Since many antique airplanes are made of wooden parts, let’s begin by discussing aircraft wood.
Inspecting Aircraft Wood
Inspecting and repairing wooden surfaces and parts is becoming a lost art. Fewer and fewer experts exist within this area. That makes it even more important for you, as the restorer of an antique or classic airplane, to understand as much about wood as possible. Chances are pretty good that the mechanic you are working with may not have a lot of knowledge in this area. So, where do you go to find information? Honestly speaking you are going to have to dig for good information. A series of articles appeared in Sport Aviation beginning with the December, 1998 issue through April, 1999. These articles will be helpful in gaining additional information on aircraft wood. In addition, you should obtain a copy of Military Specification 6073. This is probably the most comprehensive document available on inspecting wood. Another publication available is a government bulletin titled ANC-19. I do not believe it has been printed for quite some time but it is available through the EAA Information Services. A government subcommittee on military and civilian aircraft design along with the Forest Products Laboratory issued this bulletin. It was first published in April 1951. It provides an excellent guide to inspecting wood.
Ideally you should try to find a mechanic or another restorer/builder who has a lot of experience with aircraft wood to inspect your project. If you cannot find such a person then educate yourself and your mechanic with as much information as possible. You will want to thoroughly inspect the wood on your project for problem areas that may need repair. Before looking at these areas lets review what types of wood are used in aircraft.
Types of Aircraft Wood
Spruce has long been recognized as the best type of wood to use in the construction of aircraft. It is the standard against which all other woods are judged. It has several characteristics that make it the best type of wood for an airplane. Other types of wood are approved for aircraft use. FAA Advisory Circular 43-13 lists a number of different types of wood that may be used in an aircraft structure. Some of these are Douglas Fir, Noble Fir, Western Hemlock, White Pine, White Cedar, and Yellow Poplar. Douglas Fir is a very acceptable alternative for spruce. Its strength exceeds spruce by roughly 23%. Advisory Circular 43-13 states that it may be used as a substitute for spruce in same sizes or slightly reduced sizes providing the reduction in size is substantiated.
If you have to make a repair to a wooden surface you should do so using the same type of wood that is in place. Wood used on aircraft must be graded according to Mil Spec 6073. Production aircraft must verify a source for all materials that are used as a replacement for original parts. That means very simply that you should be able to track the origin of the wood that you will be using to replace a spar or other repair in a production airplane. Since wood is not stamped “certified for aircraft use” what does the FAA want in the form of paperwork to verify you are placing the proper type of wood on your airplane? Most of the inspectors I have contacted agree that a copy of the grading certificate stating that the shipment of wood that included your spar material meets Mil-Spec-6073. As far as actually certifying the wood as being legal to be placed upon your airplane that is the responsibility of the A & P mechanic and ultimately the IA who returns the aircraft to service after the repair. Some companies will actually manufacture a spar for replacement on a certain type of airplane. This should be done using a PMA number (Parts Manufacturing Authority). This satisfies the requirement for origin of the part but it must still be inspected and authorized by the IA after placement on the airplane.
So, the bottom line is this. When you purchase replacement wood for your restoration project, do so from a reliable aircraft supply company. Get a copy of the grading certificate. The wood should have been inspected prior to shipment but you and your mechanic should thoroughly inspect it again prior to installation on the airplane. It is difficult to find a perfect piece of wood. Understand that we are discussing the replacement of wood that will be used in a major structural component part, such as a wing spar. Non-critical components do not have to meet as stringent specifications. ANC-19 makes the following statement concerning the quality of aircraft wood “The influence of certain blemishes or imperfections is frequently overemphasized, causing unnecessary rejection of suitable material. Furthermore, since the effects of defects depend not only on their character and size, but also on their location in the piece and on the kind and magnitude of stress to which the piece is subjected, it is both possible and practical to admit some defects…”
The Inspection Process
As you begin the restoration process pertaining to wooden parts, the first step is to inspect them to ensure integrity and safety. You cannot assume that just because this airplane has been flying for years that the wooden structures were properly built using wood that meets Mil Spec 6073. When you are inspecting older wood you assume that most of the major defects 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. Usually that is true, but you still should do a complete inspection.
So, what do you look for during this inspection? Lets begin by going over a few of the specifications found in Mil Spec 6073. These criteria should be used to inspect the existing wood and any new wood that you purchase for repairs.
Grain–Flat or Edge Grained Material
You have no doubt heard the term quarter sawn or vertical grain wood. When inspecting wood you will encounter two types of cuts—quarter sawed and plain sawed. You will want the piece of wood to be quarter sawn. An edge grained (quarter sawn) piece of wood is defined as one in which the annual growth rings make an angle of 45 degrees or more with the wider surface of the board. A flat grained board is defined as one in which the annual growth rings make an angle of less than 45 degrees with the wider surface. You want edge grained wood because it shrinks and swells less than flat grained when the moisture content changes. In other words, a wing spar will change dimension less with a change in moisture content if the wood is vertical face edge grained.
Slope of Grain
This is often referred to as diagonal grain. It is simply a deviation of the annual growth rings from parallel when viewing the face of the board. Mil Spec 6073 allows the maximum slope to be 1 unit in 15. Simply, within a 15-inch section of the spar the slope of the growth rings should not exceed 1 inch. This is very important within the outer sections of a spar—in particular the outer 1/8 of the depth. You will often find local deviations that may exceed this requirement. It is very difficult to set up specific requirements for local deviations. You may observe steeper slopes such as a wave in the grain. As long as it is isolated it should not affect the strength. The general slope, however, must not exceed the 1 in 15. Curly or uneven grain is fairly common and is allowed as long as it does not extend over more than one-quarter of the width of the face of the board at not more than one place every 4 feet of length.
Rings per Inch
The annual growth rings per inch should be measured and compared to the maximum allowed. On Sitka spruce, there should not be fewer than 6 annual growth rings in any one-inch. These rings are measured in the radial direction on each end of the board. Measurement on the longitudinal surface will be accurate as long as the piece of wood is truly edge grained. Western hemlock and white pine also should have a minimum of 6 per inch; however, Douglas Fir must have a minimum of 8 annual growth rings per inch.
Pitch Pockets and Bark Pockets
Pitch pockets are small, lens shaped openings found within an annual growth ring. They are usually longer than they are wide. Sometimes they are several inches in length. They occasionally will contain resin. Pitch pockets are common in spruce and fir. A bark pocket is a patch of bark that is enclosed in the wood. They resemble pitch pockets but are usually smaller. Both pitch and bark pockets may weaken the wood. Mil Spec 6073 states that a pitch or bark pocket should not be less than 12 inches apart longitudinally and the product of the width and length of a pocket shall not exceed one-quarter square inch. If more than one is found in any square foot of surface, the sum of the products shall not exceed one square inch.
To be well within limits you can use the following guidelines: Pitch pockets should not exceed 1/8 inch in depth, be wider than one-quarter inch, and not longer than 2 inches. They should be at least 12 inches apart. These restrictions will work as a general rule of thumb. If you need to calculate more than this you should take a second look at the board to see if you want to use it.
A pitch streak should not exceed one-half inch in width. This may resemble a dark streak on the wood. If more than one streak is found the total width of the streaks should not exceed 10% of the width of the face on which they appear. Compression Wood
Compression wood is often referred to as hard grain. Mil Spec 6073 states that compression wood in streaks wider than one-half inch shall not be permitted. Compression wood usually has relatively wide annual growth rings and has a yellowish or slightly brownish color when the wood is dry. It will usually be found as streaks of darker wood interspersed with streaks of normal wood. The normal wood will usually have more narrow growth rings. The strength properties of compression wood are lower than that of normal wood, particularly with regard to stiffness and shock resistance. Compression wood will break with a brittle fracture instead of splintering.
Compression wood should not be confused with compression failure. Compression failure is a more serious defect and will be discussed next. Compression Failure
Compression failure is not allowed in aircraft quality wood. What makes it more serious is the fact that it is often difficult to detect. Compression failure is defined as deformations or buckling of the wood fibers resulting from severe stress in compression along the grain. It can be recognized as wrinkles across the face of a board. These wrinkles vary from being well-defined wrinkles that are visible to the eye to a very slight crinkling that would only be visible with a microscope. Compression failures develop when trees are bent severely by the wind or snow or, more commonly, when the tree is cut. If the tree is felled over another log or any other object a compression failure may result. Rough handling of the log may also cause this defect.
Compression failures seriously reduce the shock resisting capacity and bending strength of the wood. When under even a low stress, a compression failure will exhibit itself as a complete fracture across the grain of the wood. For this reason wood suspected of having compression failure should never be used in the primary structure of an aircraft.
If you suspect compression failure, you should use a concentrated source of light to view the area. This will enable you to better see the suspected defect. View it at an angle of 45-90 degrees to the grain. A failure will appear as an irregular line extending across the grain of the board.
A knot in a piece of wood creates obvious problems. They actually cause a distortion and discontinuity of the wood grain. This has a weakening affect. When lumber dries the knots will shrink more than the surrounding wood with a resulting loosening affect. When they are loose they are likely to fall out of the board. Of course, knots are also cosmetically undesirable.
Concerning the inspection of our spar material, knots are allowed but must meet certain criteria. Any knot on the wide surface of wood that is somewhat flat grained will usually be a round knot. Military Specification 6073 states these knots shall not exceed one-half inch in diameter and the sum of the diameters in any square foot of surface shall not exceed one inch. Edge grained material is what we will probably be using in aircraft construction. Knots on the wide surface of an edge grained piece of wood are usually spike knots. These knots shall not exceed one-quarter inch in diameter. A knot on the narrow surface of a piece of wood (the edge) is not permitted.
A valid question is “How do I measure a knot”? Is the knot itself measured or is the distorted grain surrounding the knot included? There will certainly be a difference and the method used to measure the knot will determine whether or not the piece of wood is acceptable. ANC-19 says that the correct method is to measure only the knot itself. It is very difficult to accurately measure the surrounding area because you will be unsure as to where to begin and end that measurement. The wood around the knot will usually slope away from the knot itself. The knot is easy to measure so use its dimensions to determine whether or not the spar is acceptable.
The wood being subjected to very extreme temperatures causes brashness. Brash wood is often termed “brittle” wood. Brash wood will break suddenly and completely with a small deflection. When it breaks it produces no splinters. It also has very low resistance to shock. Of course, brash wood is not acceptable to place within an aircraft structure. Brashness is also hard to observe. It should not be a problem for the aircraft restorer or builder because it should be discovered at the mill and not shipped to a kit manufacturer or aircraft supplier.
Mil Spec 6073 requires the professional inspector at the mill to test random samples of wood for brashness. They are required to test 1 out of every 20 boards in a shipment for brashness. If any of these samples fail to meet the brashness requirement, each piece of the shipment will then be examined for this problem and accepted or rejected on the basis of this test.
Splits are the result of rough handling of the wood or an induced stress. A split will usually extend completely through the entire thickness of the wood. They may be found anywhere on the piece of wood and are cause for rejection.
The above criteria may be found in Mil Spec 6073. Our discussion provides a brief review and is general in nature. In subsequent articles I will present specific items to look for in older wood that will be in place on your restoration project.
Inspecting wood is a challenging endeavor. Its importance cannot be overemphasized. As I am sure you can see, purchasing wood to be used for repairs or reconstruction from a source other than an aircraft supplier or kit manufacturer should be done with caution. The aircraft supply company will receive wood that meets Mil Spec 6073 and they in turn will inspect it for obvious defects. Again, you and/or your mechanic should accomplish the final inspection.