From interviews with avionic equipment design engineers it is evident that the large majority have assumed that their equipment, when mounted within the cockpit, cabin or equipment bay, would be continuously protected from water intrusion and, therefore, free from significant corrosion. Table 4-2 lists those areas of the typical airframe which are sources of moisture and fluid intrusion. Fleet experience has indicated that, especially in the cases of high speed, high performance aircraft and modern high lift capability helicopters, the inherent airframe flexibilities combined with inevitable in-service seal deterioration have precluded the condition of a water-tight airframe. There are a number of typical water entry points, and the causes for the lack of sealing integrity are discussed below.
TABLE 4-2 DESIGN CONSIDERATIONS TO COMBAT AIRFRAME VULNERABILITIES
|Gasket on access
form-in-place polysulfide gaskets at least 0.040" thick with appropriate
spacers. Preformed elastometric gaskets at least 0.030" thick.|
|Vetns, ducts and
static pressure sensors
pressure sensing components external to avionic equipment enclosure.
Improved equipment integrity.|
door EMI gasket
intrusion past EMI gasket
||Use of improved
environmental gasket. Application of Water-Displacing Corrosion
Preventive Compound, MIL-C-81309, Type III on exposed aluminum
of aircraft skin surrounding the step. Improved avionic equipment
treatment of magnesium parts through application of state-of-the-art
GASKETS ON ACCESS DOORS AND PANELS
A fixed wing carrier aircraft may have over 200 removable doors and panels. Doors on areas to which frequent access is required are sealed with a permanently mounted gasket that provides a seal between the door and the airframe. The effectiveness of a door seal in minimizing water intrusion depends upon the:
a. Structural stiffness of the door and the surrounding mating airframe doubler.
b. Thickness, compressibility, permeability and freedom from voids of the gasket.
c. Spacing of the latches or fasteners.
Increasing structural stiffness, seal thickness, and the number of fasteners all act to provide better sealing, but this also adds weight and increases maintenance manhours to open and close the door. These potential weight and maintainability penalties have led to doors with little design margin for less than optimum conditions. Also, the seal or gasket material selected for a design frequently involves a compromise between performance and cost. The initially "adequate" seal may become brittle, crack, be cut, gouged, shrink or otherwise deteriorate when placed in service.
During aircraft development the airframe sealing integrity is demonstrated on a new, parked (static) aircraft with seals that are in as near a perfect condition as possible for the test. The sealing effectiveness displayed during such tests has been found to be very non-representative of that which is experienced on aircraft in actual fleet service. While structural design guidelines related to structural stiffness and fastener spacing are fairly realistic, the gasket material thickness has been an area of frequent under-design. It is recommended that the form-in-place polysulfide type door and panel gaskets should be at least 0.040" thick with appropriate spacers. Preformed elastomeric gaskets, with their more uniform dimensional and material consistency, should be kept at least 0.030" thick with compressibility similar to that of cured polysulfide sealant.
The flight loads, pressure and temperature cycling that occur with maneuvering over the wide range of operational altitudes compound the problem of attaining watertight doors. In order to achieve the high performance characteristics of modern aircraft, flight loads are carried in thin section (skin) diagonal tension fields as well as by the primary structure. This results in a rather flexible airframe. This flexibility makes it extremely difficult, perhaps impossible, to seal the doors, panels, seams, fastener skin penetrations, etc., of a high performance airframe, whether it be a high speed fixed wing aircraft or a high lift helicopter. Not only does the wide range of flight loads make true sealing of an airframe improbable, but it has been found that panels which were sealed against water penetration on a parked aircraft would leak after the aircraft was merely towed or taxied a short distance. Thus, while there are continuing efforts to improve the resistance of naval airframes to fluid intrusion, judicious avionic equipment design should be based on the assumption that fluids will penetrate, and be present, inside a naval airframe.
VENTS, DUCTS AND AMBIENT PRESSURE SENSORS
There are a variety of skin penetrations through which ambient air for cooling is inducted or exhausted from an airframe. Also, ambient air is introduced into certain instruments and equipment for reference purposes. As the ambient air is brought into the aircraft, it can contain a significant amount of moisture which later separates or condenses out as free water within a system or assembly. When a sensor requiring ambient air is located internal to a WRA, the condensation of moisture becomes a source of corrosion within the WRA. Ideally, a sensor should measure the ambient air characteristics at a location that is external to the WRA, with only the electrical output of the sensor being introduced into the WRA for signal processing.
Vents or ducts that remain open during nonoperating periods can permit additional moisture intrusion into the airframe as moisture is condensed daily from the damp air that circulates into the aircraft.
EQUIPMENT BAY DOOR AND WRA ENCLOSURE INTEGRITY IN EMI GASKETING
Normally, equipment bay doors must maintain r.f. and d.c. electrical continuity between the door and the surrounding airframe to prevent EMI (electromagnetic interference) both from entering installed equipment as well as radiating externally to the aircraft from the installed equipment. Because of the large size of most equipment bay doors, very close spacing of fasteners as a means of controlling EMI becomes impractical due to maintainability penalties. Indeed, the space between fasteners can act as a slot antenna greatly increasing the EMI problem.
To prevent EMI leakage, conductive gaskets are used to provide the continuity needed to preclude the passage (in or out) of r.f. or other forms of radiated energy. The conductive EMI gaskets achieve their conductivity by metallic particle or mesh embedment in the gasket or seal. Generally, this embedment is a dissimilar metal that is very cathodic to the door and airframe skin. Typically, particles of silver, monel, a copper alloy or graphite are in contact with aluminum. In the presence of moisture (an electrolyte), galvanic corrosion of the aluminum results. The products of this corrosion are insulative and severely degrade the electrical effectiveness of the EMI gasket. The wire mesh embedment type of gasket also can be subject to wicking of moisture along the embedded strands with resultant corrosion. The inclusion of the conductive wires in the elastomeric gasket degrades the capability of the gasket to perform the sealing function, and the mating of the highly conductive metals to the aluminum housing, doors, aircraft skin, etc., creates bimetallic couples that will severely corrode (and destroy the EM I function) if the seal is less than perfect. In short, the two functions being attempted with an EMI gasket appear to be mutually incompatible.
A possible solution to this problem is the application of Water-Displacing Corrosion Preventive Compounds such as MIL-C-8 1309, Type 111, on the exposed aluminum surfaces where the EMI gasket metallic particles can penetrate the compound so as to maintain the integrity of the system. This method requires reapplication of the compound each time the integrity of the seal is broken. This puts the burden of continued reliability on the repair technician. A gasket configuration with separate provisions for the EMI and the environmental protection requirements is the best technology available at the present time. This requires an environmental gasket on both sides of the EMI gasket. Assure the outside protective surface finishes go around the corners and under the environmental gasket.
POOR RADOME INTEGRITY
Sealing of the large nose radomes has been particularly difficult to maintain due to the relatively long spacing between the hinge line and the latches. The repeated opening and closing of the nose radome for maintenance leads to considerable seal damage. The atmosphere inside of the nose section during static periods is essentially ambient and subject to condensation with the diurnal temperature excursions. In addition to the difficulty of sealing the radome from moisture intrusion, it is noted that extended periods of maintenance during which the radome is open are common on most aircraft. Especially when deployed, such exposure allows a high saline content layer of moisture to be present on all equipment mounted in this area.
OPEN WINDOWS AND DOORS
Helicopter cabins are particularly vulnerable to fluid intrusion due to the cabin doors and windows frequently being open, both while on the ground and when airborne. During missions such as SAR (Search and Rescue), the importance of the best possible visibility for all persons on board dictates that flight be performed with doors and windows open. The open water pickups result in the introduction of considerable salt water into the cabin and subsequently into the bilges and onto the lower mounted antennas and equipment. Occasionally, helicopter cabins are hosed down with fresh water as a cleaning process, introducing major water intrusion. One engineering investigation reported evidence of 12 inches of standing water in the bilge of a helicopter.13 Those helicopter types designed for open water landings obviously cannot have drain holes in the bilge. Helicopters often are flown with the side windows open at night, as wind coming in on one side lets the pilot know he is flying in a skid. Because moveable (non-sealed) windows in this type of aircraft are so frequently open, or so leaky when closed, any equipment mounted directly below such windows will be subject to moisturevintrusion.
The design of equipment for installation in a helicopter obviously should recognize the presence of saline moisture throughout the aircraft, especially the gross water intrusion characteristic of areas near doors, including clamshell doors. The areas below moveable windows also require exceptional shielding protection for equipment mounted in such areas.
OPENINGS FOR STEPS
In some airframes the backing area behind the external step is not sealed to the skin surrounding the step. In such designs, fluid penetrating the step area (especially during aircraft washdown) also can penetrate into the airframe interior, causing moisture and/or fluid intrusion of exposed avionic equipment.
MAGNESIUM RADAR COMPONENTS
Because of the weight savings that can be achieved by the use of magnesium for aircraft parts, there have been repeated attempts over the years to utilize this metal in various applications on naval aircraft. Generally these attempts have been unsuccessful due to the very active (corrosive) nature of magnesium in a marine environment. Recently, however, a radar pedestal and antenna fabricated from magnesium alloys have successfully survived in the fleet environment. The treatment of the magnesium included the following:
a. Anodize per MIL-M-45202.
b. All possible parts are painted, using Epoxy Primer, MIL-P-23377, followed by a topcoat of Polyurethane, MIL-C-83286 (preferred), or Epoxy, MIL-C-22750 (alternate).
c. At any dissimilar metal to magnesium joint, an intermediate aluminum loaded adhesive is interposed between the alien metal and the magnesium.
d. On surfaces that are not painted for electrical reasons, a polyimide film or a mylar is glued to the magnesium.
e. All fasteners (if electrically allowable) are insulated from the magnesium using a sealant. Also, the fastener is coated with Epoxy Primer MIL-P-23377, which is allowed to cure prior to installation.