There are a variety of fluids that can be present in
various areas of an airframe. Table 4-1 lists the types of fluid intrusion and the possible effects. Some of these are corrosive and several are destructive to seals which leads to fluid penetration into areas that are normally considered relatively protected from fluids. Some of these fluids are from external sources, while others are present due to internal leaks or servicing spills. Design decisions involving the use of organic materials to be used for seals, gaskets, sealants, coatings, insulation or similar protective applications should include consideration of resistance to the following fluids which are potentially present in U.S. Navy aircraft:
TABLE 4-1 THE EFFECTS OF AIRFRAME FLUID INTRUSION
swelling of some polymers|
||Lack of coating
adhesion. Introduction of insulative films on electrical connector
neoprene and natural rubber|
condensation and attack on electrical wiring|
Contaminants (free water, uring, condensation, desiccants)
condensation causing "pooling" of fluid in bathtub areas. Corrosive
attacks of unprotected bimetallic couples. Introduction of insulative
films on electrical connector contact surfaces|
reduced adhesion of some organic coatings|
MIL-T-5624 (JP-5) is a petroleum distillate with principal additives to control icing of free water, a fungicide and a lubricity improver. The fuel has minor solvent action and will cause some polymers to soften and swell. Engine fuel is not generally a problem fluid.
The primary hydraulic fluid is MIL-H-83282, a fire resistant synthetic fluid that is formulated from a synthetic hydrocarbon stock. It is compatible with MIL-H-5606 hydraulic oil in all proportions, although the fire resistance is greatly diminished with over 7% MIL-H-5606. While occasional isolated use of MIL-H-5606 may remain in certain minor applications, the U.S. Navy and the U.S. Air Force have basically shifted to the MIL-H-83282 fluid. The presence of either hydraulic oil can contribute to the lack or loss of coating adhesion. These fluids also may be a source of the insulative film that forms on some surfaces, such as electrical connector male and female contacts.
MIL-L-23699 is an ester oil used in both gas turbines (jet engines) as well as transmissions (gear boxes and integrated drive generators). This synthetic ester oil attacks both neoprene and natural rubber seals.
MIL-C-47220 is a silicate ester fluid used to cool certain high power r.f. equipment. Only exceptionally stable seals, such as fluorocarbon seals, fluoronated silicone seals and polysulfide sealants can resist attack by this fluid. Inadequate moisture exclusion on equipment using this dielectric coolant can result in water contamination of the coolant, which results in the formation of "black plague" in the fluid. This black scum-like material reduces the cooling effectiveness of MIL-C-47220.
Isopropyl Alcohol, TT-1-735, is normally used in aircraft anti-icing systems. This material does not react adversely with most seals or avionic components. It does, however, attack the Electrical Varnish, MIL-V-173, that is frequently used on electrical equipment. Isopropyl alcohol has a high rate of evaporation and, therefore, localized cooling. This can cause condensation to form on surfaces in which it comes in contact.
Various forms of moisture (water) can enter an airframe or equipment through the following means:
a. Free water entry occurs during rain, wash rack or drive through rinse operations and can include spillage of fluids carried aboard the aircraft, such as coffee, soft drinks, etc.
b. Urine that is not accurately disposed of in the lavatory can constitute a problem. Urine has been known to run along the exterior of relief tubes, onto waveguides, and end up on bilge mounted antennas and equipment.
c. Condensation can be the source of considerable moisture as the temperature lowers, or is lowered, below the dew point in the predominantly humid marine environment. This process is particularly invasive into all non-hermetically sealed units.
d. A " desiccant pump" occurs when a container with desiccant in it develops a leak. The desiccant absorbs moisture from the container and the outside air until it becomes saturated. Subsequent heating of the desiccant releases water to the inside of the container, which results in the container becoming a humidity cabinet.
All of the above sources of moisture intrusion can cause "pooling" of fluid in low points and bathtub areas, corrosive attack of unprotected bimetallic junctions, and introduction of insulative films in electrical connectors.
Fleet maintenance fluids include:
a. Solvents, such as MIL-C-81302, MIL-T81533 and P-D-680
b. Detergents, such as MIL-D-16791
c. Cleaners, such as MIL-C-43616.
d. Strippers, such as MIL-R-81294.
Obviously strippers are very aggressive fluids, but even solvents and cleaners, if not properly used, can soften or reduce adhesion of some organic materials. This almost always increases susceptibility to corrosion.