Aircraft Wood

Sport Aviation December, 1998
by Ron Alexander

From the very beginning of aviation wood has been used in aircraft construction. Early aircraft designers and builders often used ash or hickory. They were looking for a type of wood that would be relatively lightweight in addition to being very strong. Just prior to World War I, Sitka Spruce was discovered by aircraft builders and found to be very well suited to their needs. The strength to weight ratio was discovered to be very favorable for aircraft use. Several other types of wood had similar strength to weight ratios but were not as easily harvested or as plentiful. At the time, spruce proved to be the best choice, not only because of the physical characteristics, but of equal importance was the fact that spruce was readily available and easy to use as a building material. With the advantages noted, spruce became very widely accepted as the primary material to be used in building an airplane.

With the advent of World War II, spruce became even more popular. Manufacturers used the material in the construction of a large number of aircraft. Wooden spars were fabricated from spruce in many airplanes along with ribs and other structural parts. Because of the high demand for both aircraft production and for spruce to be used as a major material in manufacturing parts, forests of this popular wood were rapidly depleted. The use of Sitka Spruce was carried into post-war construction in many aircraft. The maintenance and restoration process of existing aircraft required a large supply of wood. Wood was a popular choice for aircraft construction because of its advantageous strength to weight ratio, workability, abundance, and low cost. The largest plane ever constructed—the Spruce Goose—is largely comprised of spruce. During this time in aviation history spruce was cheaper than aluminum or steel.

Today spruce and other types of wood remain somewhat popular for aircraft construction. Many aircraft designers and kit manufacturers within the sport aviation industry use wood as a primary structure. If wood is not the primary structure it is almost sure to be found in some component part. Wood is not as strong as steel or aluminum however; the construction can be so designed that the necessary strength is achieved with corresponding savings in weight. Many designers prefer to use wooden spars in acrobatic aircraft because the wood will better withstand the bending loads imposed during aerobatics. Unlike metal, wood does not weaken from fatigue. This, of course, is an advantage to the aircraft builder. Wood is used in fabricating spars, building ribs, floorboards, instrument panels, wing tip bows, longerons and stringers, leading edges, etc. Wood is easily formed into shapes making it the obvious choice for wing tip bows, leading edges, and wing walkways. Woodworking is a skill that is easily learned by the novice who usually has a basic knowledge of wood construction and some of the necessary tools. Safety hazards are very evident unlike some of the other types of aircraft construction. You should also understand that making mistakes with wood could be costly. You cannot easily repair a piece of wood cut to the wrong dimensions.

In this and subsequent articles, I will discuss the types of wood that can be used in aircraft construction, how to inspect the wood properly to insure it is safe to use, tools needed for woodworking, plywood and glues, basics of aircraft woodworking, and how to inspect wood in a completed aircraft.

SITKA SPRUCE
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. It is light in weight with a corresponding greater strength and toughness than is found in other woods. It is easily worked, uniform in texture, resistant to rotting, and has no odor. It can also be obtained in clear, straight-grained pieces having very few defects. This is possible because of the size of a mature spruce tree. Sitka Spruce is the preferred type of wood for aircraft construction. The name Sitka was derived from a town located not far from Juneau, Alaska. Sitka Spruce is found mainly along the Pacific Northwest, particularly along the Alaskan coast. (Most of the spruce forests have been depleted along the coast of the United States and Canada.) The trees grow best in a wet, moderate climate. They will rarely be found more than 50 miles from a coastline. Spruce trees typically grow close together and in so doing they must grow very tall and fast in order to obtain necessary sunlight. Because of this type of growth they usually have few, if any, branches except near the top of the tree. This facilitates the type of growth necessary to yield knot free lumber suitable for aircraft use. A spruce tree will grow to heights of 200 feet and higher with a diameter of 8 feet or more. A tree of this size will have taken 400 years or more to reach this dimension. A spruce tree will not yield usable aircraft lumber until it is at least 5 feet in diameter. Even with this size tree only 5% or less of the resulting lumber will be of the quality necessary for aircraft construction. With this in mind it is easy to understand why we often have a deficit in aircraft grade lumber. As I mentioned earlier, use of Sitka Spruce prior to and during World War II depleted large forests of the wood.

The use of Sitka Spruce is certainly not limited to aircraft construction. As a matter of fact, the aircraft industry uses a very small percentage of the total spruce that is milled. The majority of spruce harvested is used for ladders, house construction, masts of sailboats, barrels, cabinets, sounding boards for organs and pianos, and other uses. The building and restoration of sport aircraft is a very insignificant market for the wood industry. To further complicate the issue, several forests of spruce are protected from cutting by environmental issues such as preserving the spotted owl.

Lets look briefly at how a piece of aircraft grade spruce is milled and inspected prior to arriving at your front door. The trees are harvested by lumberjacks and sent to a sawmill. That mill in turn will cut the trees into smaller pieces known as "cants". These cants are usually 6-8 inches square and anywhere from 10 to 20 feet long. They are then placed on a barge for the trip from Alaska to Washington. When they arrive at the lumber mill in Washington or Oregon they are then cut down to smaller sizes for the uses discussed earlier. I am aware of only one or two mills located in the northwest that will set aside wood for aircraft use. These mills will try to find aircraft grade wood and accumulate enough over time to fill the orders of the aircraft suppliers. This can often take several months. Once they have accumulated enough wood they will kiln dry the wood and then cut it into the desired thickness and width. The wood is graded to a military specification by a certified lumber grader. That military specification is numbered 6073 and I will define it later in the article.

The pieces of wood are then shipped to the aircraft supplier. They are typically sent in lengths of between 10-20 feet at a nominal width of 6 inches. The boards will be planed smooth on the flat surfaces and rough cut on the edges. A board cut in that manner is termed a S2S board meaning it has been surfaced on 2 sides. The nominal 6-inch width is a problem for the aircraft supplier and ultimately the aircraft builder. Nominal 6-inch width means the width may be slightly less than or greater than 6 inches. When you, as the builder, need a finished 6-inch spar for your aircraft the supplier may have difficulty providing that dimension. The nominal 6-inch width may only finish to slightly over 5 inches when the edges are cut smooth. Widths over 6 inches are scarce. Lengths over 14 feet are also scarce. A thickness over 1 inch is hard to find. Why? Because the wood must be free from defects and typically the larger the piece of wood the more likelihood of discovering a disqualifying defect. The bottom line—it is very difficult for an aircraft supplier to acquire high quality spruce in the dimensions needed for aircraft construction.

When a company receives the wood from the lumber mill they in turn will cut the pieces into sizes ordered by their customers. Notice one very important point—absolutely no one has stamped this wood "aircraft quality". The mill does not certify the wood as aircraft quality nor does the aircraft supply company. Years ago some of the suppliers would stamp "aircraft certified" on pieces of wood. Not today! Forget about receiving wood that is certified for use in an airplane. The only grading that occurs at the mill is done to meet Mil-Spec-6073 but in no way will they tell you that the wood is aircraft quality. The aircraft supply company in turn will inspect the piece of wood they cut for you but they will not stamp it as aircraft certified. You are ultimately responsible to ensure that the piece of wood you are placing in your airplane is of the necessary quality to be used within the structure of your airplane. (We will discuss in detail how to inspect wood later in this series of articles.)

The sale of spruce is a nightmare for a supply company. The price they pay for shipments of spruce is very high. In addition, they have high costs in preparing the wood for shipment. The wood is very easily damaged when working with it or storing it. And finally, at least 40% of the wood they receive cannot be used for spar material. That means they must either cut the wood into smaller pieces to be sold as capstrips and longerons or burn them in their fireplace. Cutting the wood into smaller pieces is labor intensive. Even with the high price you will pay for a spruce spar the aircraft company is not making money. I was in that business for over 17 years and can personally attest to that fact.

ALTERNATIVES TO SPRUCE
If Sitka Spruce is so difficult to obtain why not find alternatives? A valid question and certainly a number of alternatives to spruce do exist. First of all, lets look at what type of wood can be used to replace spruce within an aircraft structure. Understand that written information regarding the use of wood in aircraft is limited. The primary source of information for the builder may be obtained from FAA Advisory Circular 43-13, government bulletin ANC-19, and from a copy of Mil-Spec-6073. 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. I will discuss the ones that are the most readily available—Douglas Fir, White Pine, and Western Hemlock.

As noted in the comparison chart in Figure 1, 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. Fir does have a tendency to split making it somewhat more difficult to work. It is also heavier than spruce—about 26% heavier in fact. Remember, you are allowed to use a smaller dimension due to the increased strength. Years ago, a number of aircraft manufacturers would route out a portion of a fir spar to save weight. Boeing used that method on a number of PT-13 and PT-17 trainers. Can you purchase good Douglas Fir? Some lumberyards will have a good supply at a reasonable price. Most aircraft supply companies find it just as difficult to obtain Douglas Fir as they do Sitka Spruce. The cost from an aircraft supply company will be about the same. You may find Douglas Fir cheaper at a lumberyard. What about White Pine? As you can see from the chart White Pine is 85-96% as strong as spruce. It is easy to work with and is somewhat available. A number of kit manufacturers are using White Pine successfully within their designs. It is low in hardness and shock resisting capability. It cannot be used as a direct substitute for spruce without an increase in size to compensate for the lesser strength.

Western Hemlock has been used in the construction of aircraft for a number of years. The popular Pietenpol airplane used Western Hemlock in constructing spars for a number of years. The strength properties slightly exceed spruce and the wood may be used as a direct substitute for spruce. FAA Advisory Circular 43-13 notes that it is less uniform in texture than spruce.

Concerning the comparison chart in Figure 1, two definitions are in order. A ring per inch is a measure of the rate of diameter growth of a tree. These rings correspond closely to yearly increments of growth of the tree. See Figure 2. They are not necessarily definite criteria for strength. Mil-Spec-6073 defines the number of rings per inch needed for spruce that will be used in an aircraft structure. According to government bulletin ANC-19 – Wood Aircraft Inspection and Fabrication—it states "Rejection of material on the basis of the number of rings per inch is somewhat arbitrary, because it does not always reflect the strength of the piece." Maximum grain slope is the deviation of annual growth rings from parallelism with the longitudinal axis of a piece of wood. See Figure 3. Looking at the face of a board the growth rings should not slope upward or downward more than the specified amount, usually 1 inch in a 15-inch length of the wood. This slope of grain is usually termed diagonal grain. Both of these will be discussed in detail in the section pertaining to inspection of wood.

CHART OF WOODS FOR AIRCRAFT USE

TYPE OF WOOD       STRENGTH                            MINIMUM RINGS PER INCH                       MAXIMUM GRAIN SLOPE          
Sitka Spruce                100%                                  6                                                               1:15                                          
Douglas Fir                   Exceeds Spruce                 8                                                               1:15                                          
White Pine                    85-96%                              6                                                               1:15                                          
Western Hemlock         Slightly exceeds spruce       6                                                               1:15                                          

INSPECTING WOOD FOR AIRCRAFT USE
If I cannot obtain spruce or any other wood that is certified for use on aircraft how do I know I have the correct quality? The answer is you don’t. You must find someone familiar with inspecting wood or learn more about it yourself. Now, there is certainly a difference between production aircraft and experimental aircraft concerning legalities. There is, however, no difference in concerns for high quality.

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 in a production airplane, as an example. 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). That satisfies the requirement for origin of the part but it must still be inspected and authorized by the IA.

As you are probably aware, the materials used in experimental aircraft do not have to meet any legal requirements. That does not exempt the builder from using common sense and good judgement. In other words, even though I do not have to verify the origin of the wood used in an experimental aircraft I will want to do so. I would strongly encourage the builder to thoroughly inspect the final piece of wood for defects prior to installation. Now, the builder of an experimental aircraft can go to a local lumber yard and purchase spruce, fir, or white pine. If you are purchasing spruce make sure it is Sitka Spruce. No grading certificate is required and usually cannot be obtained because an expert does not grade that wood. There is nothing wrong with purchasing wood from a lumberyard as long as you know what you are buying. In other words, you must become familiar with allowable defects and tolerances regarding the use of wood in aircraft. This will be accomplished by acquiring information through AC43-13, ANC-19, Mil-Spec-6073, etc. to ensure you know what is safe in your airplane. If you do not feel confident in performing this inspection, find someone who is knowledgeable concerning aircraft wood.

I will outline the proper procedure for identifying and inspecting wood for use in aircraft. This will help you familiarize yourself enough with the major defects to be able to identify them and know that the wood you are using is safe. I will be quoting largely from Mil-Spec-6073, a document that can be purchased through Aircraft Spruce & Specialty. The Forest Products Laboratory also has a wealth of information concerning wood in general. They have been very helpful to the EAA and to custom builders in general. Ben Owen and his group of knowledgeable people in Information Services at EAA Headquarters also have a lot of good information regarding aircraft wood.