7 
DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 
of very old trees. If each annual ring is composed of approximately 
one-third or more of summer wood the piece possesses the necessary 
strength. In those pieces with very narrow annual rings, in which 
the summer wood is indicated by a mere dark line at the outer edge 
of each annual ring, the wood is very soft and weak, often having a 
specific gravity as low as 0.34 (Fig. 1). 
Sometimes the proportion of material of low specific gravity in 
Douglas fir airplane lumber is exceedingly high. The writer has seen 
several consecutive carload lots of selected wing-beam stock at one 
factory in which from 25 to 50 per cent of the pieces in each car were 
below the minimum specific gravity. The stock was cut from old 
Fic. 1.—Cross sections of wing beams of Douglas fir of average and low specific gravity. 
The large proportion of summer wood, indicated by the dark bands, in the piece of good 
specific gravity (on the right) in comparison with that in the piece with low specific 
gravity (on the left) is plainly shown. 
slow-grown trees, which yield the “ yellow fir” so much preferred by 
the trade, but which invariably contain a large percentage of material 
of low specific gravity not suitable for aircraft or any other type of 
construction where high strength is requisite. 
The same general relations hold good in Sitka spruce. Here, 
again, if the annual rings are too few or too many per inch, they in- 
dicate wood of low density. The minimum specific gravity for this 
species is established at 0.36. 2 
It is often difficult to approximate the specific gravity by visual 
examination of the proportion of summer wood per annual ring 
in the case of those pieces close to the minimum density permitted 
in softwoods. There is considerable chance for error even with 
Douglas fir, but with spruce this is increased, owing to the fact 
