42 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 
mercuric chlorid, is unfortunately a deadly poison, corrodes metal, 
and is very difficult to handle. The subject of preservative treat- 
ment is one about which little is known as applied to airplanes. 
Little information is available as to what fungi actually cause 
decay in finished airplanes or as to the types of decay found. Un- 
doubtedly the fungi most concerned are those commonly attacking 
the manufactured product, such as the dry-rot fungus, the brown 
Lenzites, or the rose-colored Fomes. Fungi decaying the heartwood 
of living trees are not commonly found. When they do appear, this 
is practically proof positive that the manufacturer used wood with 
incipient decay in the fabrication of the wooden parts. 
SUMMARY. 
Among the softwoods or conifers the most valuable for airplane 
construction are red, white, and Sitka spruce, the last being most 
important on account of its large size and the consequently greater 
proportion of clear lumber that can be obtained. A. splendid substi- 
tute for spruce, and its superior where durability must be consid- 
ered, is Port Orford cedar. However, the supply of this wood is 
limited. Douglas fir, which is much heavier than spruce and there- 
fore not so desirable, is also extensively used. In those parts of an 
airplane frame requiring great strength and toughness, hardwoods 
are used. White ash is best, but white oak, hard maple, and rock 
elm may be substituted. Hickory is principally used for tail skids. 
Black walnut and true mahogany are unsurpassed for propellers, 
but yellow birch, sweet birch, African mahogany, black cherry, hard 
maple, and white oak are acceptable substitutes. As the supply of 
timber diminishes in the future, a wider variety of woods will be 
acceptable for airplane construction. 
All wood is subject to defects, of which one of the most serious is 
decay; but other defects which reduce the strength of timber must 
be recognized. Among these can be mentioned spiral and diagonal 
erain, specific gravity that is too low or too high, brashness caused 
by excessive temperatures during steaming or kiln drying, com- 
pression failures, shakes, pitch pockets, and insect galleries. 
Decay in its incipient stage is often not readily recognized; but 
wood with incipient decay must not be used in airplane construction, 
since infected wood may be reduced in strength. Furthermore, the 
decay may continue if suitable conditions arise. The first indi- 
cation of decay is usually a discoloration of the infected wood, but 
not all discolorations result from decay. Marked discoloration of 
the wood, particularly the sapwood, usually accompanies pith-ray 
flecks and wounds made by lghtning and sapsuckers. Conditions 
favorable for decay also promote sap stains. These discolorations of 
the green sapwood of various softwoods and hardwoods occur in two 
ways: (1) By an oxidation of the organic compounds in the cells 
of the sapwood when exposed to the air and (2) by the attack of 
sap-staining fungi, the hyphe of which feed on the organic com- 
pounds in the cells of the sapwood without attacking the cell walls 
except to a negligible extent. The discolorations are confined to the 
sapwood as a rule, but occasionally the sap-staining fungi, may dis- 
color the heartwood slightly. For practical purposes wood so dis- 
colored is not reduced in strength. 
