Cessna 172 Maintenance Manual page 130

1. General
A. Any repair that breaks the fuel bay seal will necessitate resealing that bay area. Repair parts that
need sealing must be installed during the sealing operations. All joints within the boundary of the
bay, but which do not provide a direct fuel path out of the bay (such as fuel spar flanges and rib
flanges), must be fay-surface-sealed and fillet sealed on the fuel side. Fay surface sealing is applying
sealant to one mating part before assembly. Enough sealant must be applied so it will squeeze out
completely around joint when the parts are fastened together. The fillet seal is applied after the joint
is fay-surface-sealed and fastened. Sealer is (fillet) applied to the edge of all riveted joints, joggles,
bend reliefs, voids, rivets, or fasteners. All boundaries and any other place that could become a fuel
leak are sealed. The fay sealant need not be cured before applying the fillet sealer; however, the fay
sealant must be free of dirt or other contaminants before applying fillet seal. Fillets laid on intersecting
joints shall be joined together to produce a continuous seal. Sealant must be pressed into the joint to
displace any entrapped air bubbles. Use an extrusion gun to lay a bead along joint, and work out all
entrapped air with a small paddle to eliminate bubbles.
2. Integral Fuel Bay Sealant
A. Two types of sealants are used, one to seal the bay and the other to seal access doors, fuel quantity
transmitters, fuel inlet assemblies, and fuel test receptacle. The access door sealant is more pliable,
and will not adhere to metal as firmly as the bay sealant. This permits access doors, fuel quantity
transmitter, etc., to be removed without damage. Service Kit SK210-56, available from Cessna Parts
Distribution, contains Type I Class B-2 and Type VIII Class B-2 (access) sealants with Cessna Parts
Distribution, contains Type I Class B-1/2 and Type VIII Class B-12 (access) sealants with the proper
quantity of accelerator for each sealant.
WARNING: The accelerators contain heavy metal peroxides. Keep them away
3. Mixing Sealant
A. Use all the accelerator and sealant in the container when mixing to ensure the proper ratio of
accelerator to sealant. Stir the accelerator to absorb all floating liquid before it is mixed with the
sealant. The accelerator can then be poured into the container of sealant for mixing; otherwise, a
wax-free container must be used. Stir accelerator and sealant until they become a uniform mixture.
Do not stir air into mixture so it forms bubbles; if bubbles appear, they must be removed.
CAUTION: Protect drain holes and fuel outlet screens when applying sealants.
NOTE: Work life of sealants contained in SK210-56 is 2 hours from the start of mixing. Work life
of sealants contained in SK210-101 is one-half hour from the start of mixing. This is based
on a standard condition of 77°F (25°C) and 50 percent relative humidi ty. An increase in
either temperature or humidity will shorten the work life of the sealants.
4. Applying Sealant
A. Use the following procedures as the best method for applying sealant.
(1) Apply fay surface sealant to one mating part, and install rivets or fasteners while sealant is still
within its work life.
NOTE:
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
FUEL BAY SEALING DURING STRUCTURAL REPAIR
from heat and flame. Use only in well-ventilated areas. Avoid skin
and eye contact. Wear eye shields. In case of eye contact, flush
generously with water and get prompt medical attention.
During sealing, the supply of mixed sealant must be monitored to be certain it has
not exceeded the normal work life. To check, use a small wooden paddle, or tongue
depressor, to gather a small amount of sealant. Touch this sealant to a piece of clean
sheet metal. If it adheres, sealant can still be used, if it doesn't adhere, then the
sealant has exceeded the allowable work life, and must not be used.
© Cessna Aircraft Company
57-12-00
Page 801
Jun 1/2005