Inspecting Wood Structures

//Inspecting Wood Structures

Inspecting Wood Structures

An aluminum structure, when highly stressed, will wrinkle and deform as the metal reaches its elastic state and take a permanent set that the mechanic can visually detect. However wood will not take a permanent set when overstressed and degradation can begin that will eventually lead to failure. If a wing impacts the ground, in the case of a ground loop, a very careful inspection of wood spars at the hard points is a must. In some cases, fabric wood or aluminum should be removed in order to make a complete inspection. Many years ago I lost a good friend when his all-wood homebuilt airplane shed a wing. That failure could be traced back to when the ship landed long on our local runway and impacted a large ditch bank along a canal cracking some skin loose around a landing gear. Apparently, no in-depth inspection was made, rather the skin was reattached and the airplane continued to fly. The impact could have caused a compression failure of the front spar that led to wing failure. Compression failures are very difficult to find, particularly if the inspecting mechanic does not know what they look like and where they are possibly located.

Wood components are coated with a varnish for protection against the elements because varnish is both transparent and flexible, thus the inspector can look through the finish to detect cracks, particularly in wing spars. Wood has a great ability to flex under load and when that load is removed, return to its original configuration. Wood will bend under load until it cracks and these cracks will be either longitudinal (parallel to grain) or compression (across grain). In the case of a ground loop, three scenarios will happen. The best is that no damage occurred, the worst is that a spar has broken (the pilot will hear a loud crack followed by deformation of the spar). The worst scenario is when there is a compression failure that goes undetected until the wing sustains a load that will cause the spar to fail.

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Fig.1 A compression failure of a front spar from an Aeronca 7ACA. In this photo the hard point location is to the left and the bright section of wood is where the plywood plate was located. So the spar flexed until the plywood and wing strut attach point prevented any additional flexing; load forces built-up and the wood grain was forced to compress. In this example, the failure had occurred through about 25% of the spar width. This failure is almost impossible to detect because the crack is under a wing rib. The only place where this failure could be observed is on the very top of the spar.

Fig.2 Right, looking down on top edge of spar, the compression failure can plainly be seen. The only problem is that this area is under the aluminum leading edge skin.

Fig.3 Analyzing this type of fracture is interesting because it shows both compression and tension failure of a wood spar as the photo right clearly shows. This wing struck the top of a tree and the outer 6’ of spar was ripped from the wing. In this photo the front of the spar is to the right and the aft side is to the left. The spar failed aft in a compression load, thus a very smooth fracture at about 50% spar thickness on the backside and the very jagged tension tear on the front side. The remains of the bolthole attached a compression member, so the spar failed at that hard point.

Fig.4 A photo taken before the repair was begun, showing the spar failure. As one who has been around a long time, I can attest to that awful sound of wood breaking as wing spars snap like a celery stalk. The next photograph is a major ground loop accident involving a modified Boeing A75N1 that was being used to train future Ag pilots in California’s central valley. The ship belonged to my uncle, George Baldrick and was on loan to a school for agricultural aircraft pilot training and the airplane flat got away from the pilot.

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Fig.5 4768V off the rather narrow dirt runway in central California. The violent ground loop severely damaged the left wings and did minor damage to the right upper wing. The airplane was disassembled and restored to flyable condition. The wings were uncovered for a complete inspection and the spars spliced to return them to stock configuration.

Fig.6 At a different time, another of George’s 450 Stearman biplanes, but this time the ship was a total loss with the only part saved was the registration, data plate and logbooks. I was there that day and helped pull the pilot out of the cockpit. I can still hear the wing spars cracking as the ship left the dirt runway, went through a barbed wire fence and wound up in a pasture. Think I was 18-years old at the time and it made quite an impression on me to be a safe pilot. Beside longitudinal cracks and compression failures, another area of concern is spar ends, particularly at the attach fittings. Here, moisture enters the end grain and causes fibers to expand, opening cracks along and across the grain. Shakes are cracks along wood grain and checks are cracks across the grain. In accordance with the AC43.13-1B, both are cause for rejection of the spar.

Fig.7 A shake opening up into a split in a plank of Yellow poplar.

Fig.8 An 84-year old Command-Aire upper wing spar showing longitudinal cracks. Note that the openings are parallel to grain structure and the large crack emanating from a hard point. Quite possibly this spar was in an accident because wood spars don’t just fail – there must be a reason. The smaller cracks are from moisture entering into the wood grain.

Deterioration of wood structures can sometimes be located by noting wrinkling of the fabric covering, indicating that a component of the structure has moved from its original location. This is particularly true in the case of trailing edge damage due to moisture entering the structure. The trailing edge of a wood wing rib is the weakest part of the rib and it is also located at the lowest point in the structure where water can concentrate and eventually weaken the rib due to wood rot.

The FAA AC43.13-1B details four areas of wood rib repairs – the trailing edge being a most common area for repairs. The FAA specifies that the repair of any three adjacent wing ribs or the replacement of leading edge between such ribs is a Major Repair, thus requiring the initiation of FAA Form 337. Using this criteria, it would be possible for a mechanic to repair any two adjacent wing ribs, skip a rib, repair the next two adjacent ribs, etc., and that would be considered a Minor Repair requiring just a log book entry by the mechanic. The FAA AC43.13-1B is the best source for recommended repair procedures for wing ribs. These repairs date back to the 1930’s when Aeronautics Bulletin 7H was published giving guidance material for mechanics to make repairs to ribs.

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Fig.9 A typical repair to a wood rib trailing edge that has been degraded due to moisture rot. A close look at the lower left sketch shows how the upper and lower capstrips come together at the trailing edge. To make this joint stronger I add a small triangular spruce gusset wedge into that joint before gluing on the plywood plates. Normally it will be necessary to replace all rib trailing edge ends, so I make a pattern to cut all ribs to the same length and therefore make only one size of the dovetail pieces that slip into place. For best accuracy, work off the backside of the rear spar rather then the rib T.E tip. Cut replacement section out of spruce same thickness as capstrip. Do not be tempted to use plywood here because when drilled for soft rivet the veneers tend to separate when the rivet is driven.

All damage criteria and repairs are shown in AC43.1-1B, Section 1. Not many old airplanes had Structural Repair Manuals but there were Aeronca L-16 and Piper L-4 manuals that show major repairs to wood, steel tube and aluminum structure. These are very helpful when working on the civilian versions of these military ships.

By |2016-11-13T09:33:06+00:00August 18th, 2015|Flying-Wire|Comments Off on Inspecting Wood Structures

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