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Fella used to build airplanes is selling some Sitka. Anyone know what they’re take on runout is? You’d think they’d be against it but I don’t know…

In general, 1:12, but as they say, it's complicated...

MIL-S-6073 speaks to slope of grain as follows:

"In pieces comprising at least 70% of the footage in any lot or shipment, the slope of grain shall not be steeper than one in 15 on one face combined with straight grain on the adjacent face or equivalent combinations, In the remaining pieces the slope of grain shall not be steeper than one in 12 on one face combined with straight grain on the adjacent face or equivalent combinations."

There is also a note that establishes combinations of flat-grained and edge-grain slope of grain...too much to pull from PDF image, but please feel free to do so.

http://bowersflybaby.com/tech/Wood/MIL_6073.pdf

I believe you will find that the de facto standard for top wood is stricter than the military specification for aircraft structural wood. I also suspect that if the wood has grading stamps or other origin and inspection documents, the owner would do better selling it to homebuilt aircraft builders through the fora associated with that hobby than to a luthier. I also recognize that the owner of the wood may be selling the material as instrument material to avoid the unfortunate liability tail which may attach to aircraft use.

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For the times they are a changin'

- Bob Dylan

I used to buy spruce cutoffs from Aircraft Spruce in their bargain bundle. The quality was good sometimes, but sometimes the runout was all over the place. Out of three orders, I got one that was suitable for bracewood. I quit ordering.

For general aircraft maintenance there's a FAA-produced 'bible' of acceptable practices, AC4313-1b, that explains what wood is suitable for civil aircraft repair. buried among a zillion topics. I don't have an electronic copy, and it takes some digging, but the information is published.

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Peter Havriluk

The spruce offcuts from Aircraft Spruce may have been cut off and sold because they didn't meet the standard needed for aircraft. Likewise the military spec. for shipments of wood may include a certain amount of wood that will be rejected, or used in a less critical application by the aircraft manufacturer.
Wood being a highly variable material sometimes "less than perfect" has to be included, and used where appropriate. I think this is also true in lutherie. Many fine instruments have been built using spruce with visible runout. Although it is not a strictly cosmetic issue it is often given more weight than it deserves.
If you are buying it for brace wood, you may be as well off picking through the SPF stuff at the local lumberyard and splitting out some 2-by's and seasoning it in an attic. Personally, I look for salvage material that is well aged and dry. Wood that appears to only be useful as kindling often can have some good brace material split out of it (and quite a bit of kindling too! )
Who was it that said "Good wood is where you find it"?

Sounds good enough for Gibson but I'll take a pass...

Run out in the top is not solely a cosmetic issue. It does reduce the long-grain Young's modulus of the wood some. More importantly, if there is much run out where the bridge is glued it causes problems. The bridge tends to peel up first on the side of the top where the run out goes 'up' toward the head. Once the loose joint along the back edge of the bridge works its way in to the center where the run out changes to 'down' the wood in the top starts to split, rather then the glue line coming loose. Bad.

Often runout is caused by the fact that the tree grew with spiral grain. Usually they start off with the fibers aligned with the axis of the tree, but start to show some spiral as they get bigger. It's impossible to cut a top from wood like that with no run out anywhere; generally it's greater on the 'bark' side, depending on how the top was cut. In a case like that it's best to simply join the top on the 'heart' side to there's minimal run out in the center where the bridge goes.

Zero run out everywhere in the top is quite rare, so you have to deal with it to some degree. Figure such as curl and bear claw is caused by changes in run out and those are the extreme cases. Often a maker can join the top and establish the outline in such a way as to put the bridge on a spot with little or no run out, even on tops like that. In more normal tops you can try to get the worst run out into the upper bout, where it will have the least detrimental effect.

I do wonder if buyers are a bit fussier about run out than they need to be, and reject some tops that would work well because of minimal run out. OTOH, when it's your nickel...

Because of the narrowness of braces, runout can be minimized by splitting the wood and cutting parallel to the split, even when there is spiral growth.
The practice of cutting tops from split billets has its advantages........not the least of which is the generation of large amounts of runout- free bracewood.
It is usually beneficial to seek out those tonewood cutters who go the extra mile.

Note that you generally don't see runout directly in a quartersawn top.....what you see due to the bookmatch is the reversal in direction of the runout. So 2 degrees of runout at the joint means that the difference in reflection of the two halves is doubled, or 4 degrees. In other words, minimal runout is more of a cosmetic issue than it is structural or sonic.

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John