01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
smoke, smog, ozone, and corrosive fumes associated
with heavy industry are also present.
3-13.8.3. Tropics. The greatest challenge to the aircraft
industry is the design of equipment that is protected
from corrosion and deterioration in the heat and humidity
that prevails in the tropics. Even though they
encompass only a small portion of the earth's land
area, the tropics demand the greatest amount of
consideration from the standpoint of corrosion treatment
and control. The tropical environment is sustained by
long periods of heavy rainfall, during which as many as
100 inches of rain may fall. Relative humidities of up to
100% at ambient air temperatures of 85F (29C) pose
formidable threats of corrosion. When high humidity
and temperature conditions are combined with salt-
Figure 3-30. Biological Growth on Helicopter Wall
laden air, the corrosive environment becomes extremely
severe. Extended periods of high heat and humidity
3-14.2. MICROORGANISMS. Microbial attack, as the
contribute to rapid corrosion of metals, cracking and
flaking of rubber and plastic materials, and deterioration
term is used in this manual, includes the action of
of seals. Equipment, whether stored or in use, requires
bacteria, fungi, or molds. Microorganisms are nearly
special protective containers and frequent preventive
everywhere and outnumber all other types of living
maintenance. Many items become covered with fungi in
organisms. Those organisms causing the greatest
a matter of hours. For effective operation of electronic
corrosion problems are bacteria and fungi (see
equipment in the tropics, special efforts must be made.
Figure 3-30). Damage resulting from microbial growth
The critical combination of high temperatures,
can be caused by: (1) the tendency of the growth to hold
condensation, high relative humidity, and contaminants
moisture, which then causes corrosion; (2) digestion of
such as salt and sand may cause catastrophic failure of
the substrate as food for the microorganism; or (3)
equipment. Intensive preventive maintenance and the
corrosion of the surface beneath the growth by secreted
best possible protective techniques are necessary for
corrosive fluids. Modern avionic equipment, because
of complexity, dense packaging, and higher sensitivity,
aircraft and their components.
is more susceptible to damage from microbes than
earlier systems. Condensed moisture can provide
3-14. BIOLOGICAL CORROSION.
conditions that promote the growth of molds, bacteria,
and fungi. Once established, these growths continue
3-14.1. ANIMAL AND INSECT DAMAGE. Damage to
to absorb and hold moisture. Acid secretions from the
aircraft and aircraft subsystems may be caused by
insects, birds, and various small animals, especially in
microorganisms are strong electrolytes. These
tropical environments. Equipment in storage is most
electrolytes corrode the underlying metal. Some
susceptible to this type of attack. Insects and small
nonmetals provide nutrients that can accelerate growth.
animals may enter through vent holes or tears in
The presence of bacteria and fungi can readily be
packaging and sometimes build nests. They like to feed
identified by damp, slimy, and bad smelling growths.
on various organic materials, such as polyethylene,
These vary in color from black, bluegreen, green, to
insulation, and wire coatings, which can result in system
yellow. Table 3-3 lists some common materials and
or equipment failure. Another type of damage can occur
the corresponding effect of moisture and fungi.
when electrical insulation, varnishes, and circuit board
coatings are eaten by insects. When bare wires or
3-14.2.1. Bacteria. Bacteria may be either aerobic or
circuit components are exposed, more areas become
anaerobic. Aerobic bacteria require oxygen to live.
available for corrosion and shorting to occur. Moisture
They can accelerate corrosion by oxidizing sulfur to
absorbed by nests plus excretions from animals may
produce sulfuric acid or by oxidizing ammonia to
cause corrosion and deterioration that goes unnoticed
produce nitric acid. Bacteria living adjacent to metals
until equipment is put to use and fails. Damage may also
may promote corrosion by depleting the oxygen supply
occur when organic materials, such as upholstery, are
or by releasing metabolic products. Anaerobic bacteria,
shredded for nests or consumed as food.
on the other hand, can survive only when oxygen is not
