DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. Bo 
again become suitable. Laminations with incipient decay should 
not be used in propellers. In this place the reduction in strength 
need not be so carefully considered as the variation from the normal 
shrinking and swelling that would result. Unequal and particularly 
unusual strains and stresses must be avoided above all things in 
propellers. 
Incipient decay usually appears as a discoloration, in some cases 
pronounced, in others so faint as to be practically invisible. Most 
of this decay in airplane lumber was actually in the tree when it was 
cut or in the logs when they left the woods. It is rare that any 
serious effort is made in the woods or at the mills to cut out incipient 
decay. When the logs are bucked and sawed the typical decay is 
usually trimmed off, leavi ing the less apparent incipient decay in the 
lumber. After sawing, the upper grades of lumber, which include 
airplane stock, are usually handled carefully enough at the larger 
mills to prevent further damage. 
When decay commences in a living tree, it spreads upward in the 
heartwood if the infection entered at the butt, or in both directions 
if it occurred higher on the trunk. Very rarely do the decays in the 
heartwood of living trees attack the sapwood. Beyond the typical 
decay, that is, where the wood is decidedly rotted, extend the incipi- 
ent stages of decay, which become less and less apparent as the dis- 
tance from the typical decay increases. Finally, the incipient decay 
ends entirely. The wood beyond is then sound. The incipient decay 
rarely ends abruptly or evenly, but usually fades out in one or more 
irregular streaks, which may be short or long. It usually extends 
only 3 or 4 feet longitudinally beyond the typical decay, but with 
certain wood-destroying fungi on some hosts the incipient decay 
may extend 15 feet or more in advance of the typical decay. Fur- 
thermore, the latter is always bounded radially by incipient decay, 
and this boundary is often irregular. Boards sawed from diseased 
trees may contain all stages of decay or incipient decay, occupying 
part or all of the board. The fact that the fungi causing decay in 
standing trees may continue their work of destruction in logs in the 
woods, or even in sawed lumber if conditions are favorable, indicates 
the necessity for having logs removed from the woods, sawed, and 
the lumber dried with reasonable promptness. 
When lumber is green the discolorations indicating incipient decay 
are more intense than when the wood has seasoned for some time. 
During the drying process the discolorations fade in varying de- 
grees. Furthermore, if a new discoloration appears within one or 
two weeks after the lumber comes from the saw it is practically 
certain that it is not caused by one of the wood-destroying fungi 
attacking the piled lumber, since the latter work more slowly. A 
sap-staining fungus or a chemical reaction is the most likely agent 
in such a case. 
Incipient decay should be detected and eliminated before the 
lumber is worked into individual parts. If the entire piece is not 
defective the sound portion can be sawed out and utilized. In 
marking a piece for cutting, however, it must be remembered that 
decay extends more rapidly with the grain in a tree or piece of wood 
than it does across the grain; thus, to be perfectly safe, an allow- 
ance of 2 feet should be made in the direction of the grain beyond 
