Cessna 172 Maintenance Manual page 128

Preparing Damaged Area In Wing Fuel Bay for Repair
1.
A. Before performing any maintenance in fuel bay area, it will be necessary to defuel and purge the fuel
bay. To defuel and purge the fuel bay, proceed as follows:
WARNING: During all fuel system servicing procedures,
WARNING: Always ground airplane prior to performing any maintenance of the
WARNING: Avoid drainage from residual fuel held in disconnected fuel lines;
WARNING: Use NS-40 (RAS-4) (Snap-On Tools Corp., Kenosha, Wisconsin),
NOTE: Covers or caps should be installed on lines and fittings to prevent entry of foreign material,
and to prevent damage to threads.
(1) Ground airplane to a suitable ground stake.
(2) Ensure airplane battery switch is in OFF position.
(3) Turn fuel selector valves to OFF position
(4) Remove fuel filler cap on bay that is to be defueled; insert defueling nozzle.
(5) Remove as much fuel as possible through filler opening.
(6) Remove drain valves from bottom side of fuel bay and drain remaining fuel into a clean, open
container. Use defueling nozzle to remove fuel from container.
(7) If necessary, repeat procedures for opposite wing.
WARNING: Purge fuel bays with an inert gas (argon or carbon dioxide)
(8) Insert inert gas supply hose into fuel filler opening.
(9) Allow gas to flow into bay for several minutes to remove all fuel vapors. Since argon or carbon
dioxide are heavier than air, these gasses will remain in bay during repair. Non-sparking tools
shall be used to make repairs (air motors, plastic scrapers, etc.).
NOTE:
NOTE:
(10) Remove all existing sealant from area to be sealed, leaving a taper to the remaining sealant.
The taper will allow a scarf bond and a continuous seal when the new sealant is applied.
NOTE:
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
WING FUEL BAY REPAIRS
equipment must be available.
fuel system.
this accumulation constitutes a fire hazard.
MIL-T-83483 (thread compound, anti- seize, graphite petrolatum),
or engine oil as a thread lubricant or to seal leaking connections.
Apply sparingly to all but first two threads of male fittings, being
careful not to allow entry of compound into fuel system.
prior to repairing fuel leaks to preclude possibility of
explosions.
Portable vapor detectors are available to determine presence of explosive mixtures
and are calibrated for leaded fuel. The detectors can be used to determine when it is
safe to make repairs.
During structural repair, parts must be predrilled, countersunk or dimpled, and
cleaned before being sealed and positioned for final installation.
The best method of removing sealant is with a chisel-like tool made of hard fiber or
plexiglass. Remaining sealant can be removed with aluminum wool. Steel wool or
sandpaper must not be used.
© Cessna Aircraft Company
fire fighting
57-11-00
Page 801
Jun 1/2005