wetted area may become larger. A slow seep, when
The wing mating surfaces are built to extremely
wiped dry, will not reappear in a short period of time.
close tolerances to allow for proper sealing. The sealing
of these mating surfaces is attained by using gaskets or
· SEEP. A seep is a fuel leak that reappears in less
sealants, or a combination of both. In most cases, the
than an hour (approximately) after it has been wiped
perimeter of the cell is sealed by using a nonhardening
sealant that is injected into a groove machined in one
· HEAVY SEEP. A heavy seep is a fuel leak that
structural member along the mating surface. The
attachment screws and bolts are sealed by placing
reappears immediately after it has been wiped dry.
O-ring seals under the heads. Protruding bolt heads are
· RUNNING LEAK. A running leak is a fuel leak
sealed by special seals that consist of an O-ring
that flows steadily.
embedded in a metal washer. Figure 4-76 shows the
sealing of integral fuel cell screws and bolts.
Most aircraft structural repair manuals do not
classify a slow seep or seep in an open area (the
surfaces of the aircraft exposed to the airstream) as a
flight hazard. A slow seep or seep in an open area need
The inspection of integral fuel cells consists mainly
not be repaired before flight if structural integrity exists
of a check for external leakage around skin joints,
and there is no danger of an increase in leak intensity
rivets, screws, and bolts on every preflight inspection.
during flight. Slow seeps and seeps considered
The fuel cell fittings and connections should also be
acceptable for flight should be frequently inspected to
inspected for evidence of leakage. Fuel cell leaks are
ensure the leak intensity does not increase prior to
classified in the following categories: slow seep, seep,
heavy seep, and running leak.
Heavy seeps and running leaks are classified as
· SLOW SEEP. The least severe leak classifica-
flight hazards, regardless of their location in the
tion is the slow seep. This is a very slow fuel seepage
aircraft. Any leak classified as a flight hazard must be
that wets a small area. Over a period of hours, the
corrected before flight.
Figure 4-76.--Sealing integral fuel cell screws and bolts.