(2)
A seep is a leak where the wetted area is not over 4 inches after time interval noted above has elapsed.
(3)
A heavy seep is a leak where the wetted area is no larger than 6 inches after the time interval noted above has elapsed.
(4)
A running leak is any leak in excess of a heavy seep. Fuel will usually reappear immediately after being wiped dry and can run or drip from a surface.
B. Figure 18 gives in tabulated form the various leakage categories and the repair actions required.
WARNING: CAREFUL EXAMINATION MUST BE MADE TO ENSURE THAT THE LEAK IS NOT PROGRESSING TO A CRITICAL AREA OF THE AIRPLANE, OR THAT THERE IS NO POSSIBILITY OF FUEL BEING BLOWN INTO THE FUSELAGE AREAS.
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Fuel Leak Size Patterns 500
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10. Causes of Leaks
A. Improper Preparation of Surface
(1) Leakage exists only when a fault in the sealing provides a path through the sealant barrier (seal plane). Since jet fuel has a low surface tension and viscosity., it can leak through extremely small openings and travel for a considerable distance. Foreign material such as grease, metal chips, human hair, loose paint, wax, or oil will reduce the adhesive bond sufficiently to cause premature failure. All surfaces must be completely free of foreign material before sealant is applied.
B. Improper Mixing or Storage of Sealants
(1) Synthetic rubber forms the base for most sealing compounds. The manufacturer adds different ingredients that control strength, curing time, plasticity, work life, and fuel resistance. Sealants are supplied in two parts; base material and curing compound (accelerator). Extreme care must be taken to ensure that only the proper proportions and the curing compound (accelerator) recommended by the manufacturer axe used. Any variation from the manufacturer's instructions will change the physical properties of the mixture. This may produce a seal failure and tank leakage. Sealing compounds can be stored on the shelf before mixing with curing compound. After a specified period of time it is outdated and is tested for curing capabilities before use. Some sealants can be stored under refrigeration for short times after being mixed with curing compound After the storage time on mixed sealant has elapsed it must be discarded. Old sealant will not flow or stick properly thereby producing an unsatisfactory sealing job and a leak.
C. Improper Application of Sealants
(1) Application of sealant is important. The manufacturer's recommendations must always be followed. When a brush precoat is recommended it must be applied. Elimination of precoat or any steps in sealant application impair the sealant's ability to seal properly. Air bubbles in sealant burst in time and provide a void or pin hole for fuel to leak through. If structure gaps are bridged, instead of being filled, voids in sealant are left. Re-entrants provided by overlapping sealant and lack of good faired contact with all surfaces will allow a leak in time. Application of sealant not in accordance with the manufacturer's recommendations will cause integral tank leaks.
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D. Loose Fastener
(1) Loose fasteners may initiate a fuel leak by allowing movement in a fabricated assembly. Excess movement may crack faying surface seals which will provide a fuel path through the seal plane. Loose rivets will not maintain a self-sealing status. The integral fuel tank design incorporates interference fits in close tolerance holes for fasteners installed in tank boundaries. All replacements of these fasteners must maintain this requirement.
E. Improper Installation of Fittings
(1) Fittings and connections that are attached or pass through tank boundaries are sealed with O-rings. When O-rings are installed they can be pinched, scored, twisted, or otherwise improperly installed, causing leaks.
11. Methods of Leak Detection
A. The following methods are acceptable means of tracing a leak from where it appears on the outside of an integral fuel tank to the source on the inside. The methods appear in the order of increasing difficulty and they should be tried in that order, if possible.
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