This thin 1/4" piece of Sitka Spruce has several defects:
Left: Bark Pocket
Centrer: Pitch Pocket
Right: Burl
This photo is for defect examples only and in no way remotely aircraft grade!

Side View

Opposite side same piece

Aircraft Wood Defects

I recently had another builder contact me regarding wood selection and defects. I am posting a brief selection from my mail to him below. This is an area that I find rather disappointing as the level of wood available seems to be really rather modest. I have not purchased spar stock from Wicks or Aircraft Spruce, although I have purchased decentwood in the Aircraft Spruce bargain selection for smaller parts. I have purchased Sitka Spruce from boatbuilding suppliers to cut for myself.

I can relate to having many texts regarding wood selection but the primary three are AC 43.13, ANC-19 Wood aircraftInspection and Fabrication ,( I will scan my copy at some point, but am including this link for now), and ANC-18 Design of Wood Aircraft Structures. There is also Mil-Spec 6073 which I am not discussing for now. To simplify a decision process I guess to generalize I would say reference AC 43.13-1B (Chapter 1). You might get some relief from the rather distilled basics that AC 43.13-1B supplies by cross-referencing ANC-19 Wood aircraft Inspection and Fabrication section 2.4 Requirements for Wood in Specific Parts. I have not utilized ANC-19 section 2.2 Specific Gravity much; this references tests and formula for specific gravity. I did a simple floatation test several times and thewood I purchased met requirements and, as I have not purchased wood again until recently, I haven’t revisited this much. I did basic break tests using the method on my building blog that a Fly Baby builder suggested. He did not have the calculation/formula behind his formula utilized for the test (he did at one time but as this was given to him he had misplaced it over time). I did test a number of Sitka Spruce pieces and the values obtained from his formula are consistent with expected Modulus of Rupture and Modulus of Elasticity values from ANC-18 Design of Wood Aircraft Structures. I tested wood from both Aircraft Spruce and Public Lumber and have therefore assumed this is a valid testing method.

I am including a few random photos while working this weekend cutting around defects to select a piece for a part. I have taken other photos over a period of time but not really organized or kept track of those.

Knot to cut away from stock I was working on this weekend.

The same knot area two cuts later. 

The grain slope is OK on the bottom piece and the defect has been cut away finally leaving a useable part.

Another knot defect cut away above leaving the bottom piece. Is this piece acceptable?
Read below and my final decision.

My texts say:

It should have a 1:15 slope but when I see a wave I reference ANC-19 2.424 Requirements for Solid Spar Flanges and 2.410 Slope of Grain Requirements. 2.410 states that the slope of the grain in the middle half of the depth of solid or laminated spars may be permitted to be as steep as 1 in 10.

2.4101 Local deviations of grain slope states "It is obvious that local deviations of grain involving slopes steeper than those permitted will sometimes be permissible. It is difficult to set up definite requirements for permissible local grain deviations which will be valid or applicable to all cases,…"


There is quite a lot of reading and I tend to feel this is potentially useable, but barely, depending upon the location used. Circumstances usually allow cutting out defects and that is what I did with this. I have discarded one spar flange in the last few months after cutting away the final angle revealed a hidden defect on the edge of a spar flange. Defects on spar flange edges are not allowed. I cannot describe the disappointment! In retrospect, after my experience with the large sections of wood in my center section spar, I would tend to suggest laminating.  If you cannot find exceptional wood use ¼” laminations such as those that Steen Aero Lab has tested and posted to the internet. Any defects will generally be found in a piece that thickness. Spars also must be edge grain 2/3 of each vertical face. 


So what was the decision and what did I do with this piece? I at first used it and then thought better of it. I rebuilt the entire elevator spar this was used in.


(Update: 3 May 2012. I did finally purchase spar stock from Wicks and the wood was exceptional. It was a pleasant change from 2/3 of all the wood not useable for spars. On the other hand I will have no cuttings for smaller parts, but at this stage I am no longer in need of that.)

I will post additional pictures here as updates as time allows.