Cessna 172 Maintenance Manual page 131

(2) Apply a fillet seal to the repaired area on the inside of the bay.
(3) Apply a fay surface seal to access doors, fuel quantity transmitters, etc., if removed, and install.
Allow sealant to cure; refer to Curing Time, for time requirements.
(4)
(5) Clean stains on outer surface.
(6) Test fuel bay for leaks as described in Testing Integral Fuel Bay.
5. Sealing Fuel Leaks
A. First determine the source of the fuel leak. Fuel can flow along a seam or structure of the wing for
several inches, making the leak source difficult to find. A stained area is an indication of the leak
source. Fuel leaks can be found by testing the complete bay as described in Testing Integral Fuel
Bay. Another method of detecting the source of a fuel leak is to remove access doors and blow with
an air nozzle from the inside of the bay in the area of the leak while soap bubble solution is applied to
the outside of the bay. After the leak source has been found, proceed as follows:
(1) Remove existing sealant in the area of the leak as described in Chapter 57, Wing Fuel Bay
Repairs.
(2) Clean the area and apply a fillet seal. Press sealant into leaking area with a small paddle,
working out all air bubbles.
(3) If leakage occurs around a rivet or bolt, restrike the rivet or loosen bolt, retorque, and reseal
around nutplate.
(4) Apply fay surface door sealant to access doors, fuel quantity transmitters, etc., if removed, and
install.
(5) Test fuel bay for leakage as outlined in Testing Integral Fuel Bay.
6. Curing Time
A. Class B-2 sealant has a maximum tack free time of 40 hours and a maximum cure time of 72 hours.
These values are based on a standard condition of 77° F (25° C) and 50 percent relative humidity.
B. Class B-1/2 sealant has a maximum tack free time of 10 hours and a maximum cure time of 30 hours.
These values are based on a standard condition of 77° F (25° C) and 50 percent relative humidity.
C. The cure of sealants can be accelerated by an increase in temperature and/or relative humidity. Warm
circulating air at a temperature not to exceed 140° F (60° C) may be used to ac celerate cure. Heat
lamps may be used if the surface temperature of the sealant does not exceed 140° F (60° C). At
temperatures above 120° F (49°C), the relative humidity wil l normally be so low (below 40 percent)
that sealant curing will be retarded. If necessary, the relative humidity may be increased by the use of
water containing less than 100 parts per million total solids and less the 10 parts per million chlorides.
7. Testing Integral Fuel Bay
A. The fuel system consists of two vented, integral fuel tanks (one in each wing). The following
procedures are for testing integral fuel bay.
(1) Remove vent line from vent fitting and cap fitting.
(2) Disconnect fuel lines from bay.
(3) To one of the bay fittings, attach a water manometer capable of measuring 20 inches of water.
(4) To the other bay fitting, connect a well-regulated supply of air (1/2 psi maximum, or 13.8 inches
of water). Nitrogen may be used where the bay might be exposed to temperature changes while
testing.
Make sure filler cap is installed and sealed.
(5)
CAUTION: Do not attempt to apply pressure to the bay without a good regulator
(6) Apply pressure slowly until one-half psi is obtained.
(7) Apply a soap solution as required.
(8) Allow 15 to 30 minutes for pressure to stabilize.
(9) If bay holds for 15 minutes, without pressure loss, bay is acceptable.
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
and a positive shutoff in the supply line. Do not pressurize the fuel
bay to more than one-half psi or damage may occur.
© Cessna Aircraft Company
57-12-00
Page 802
Jun 1/2005