Cessna 172 Maintenance Manual page 22

(b) If the surface of the cable is corroded, carefully force the cable open by reverse twisting
and visually inspect the interior. Corrosion on the interior strands of the cable constitutes
failure and the cable must be replaced. If no internal corrosion is detected, remove loose
external rust and corrosion with a clean, dry, coarse-weave rag or fiber brush.
NOTE:
(c) After thorough cleaning of the exterior cable surface, apply a light coat of lubricant (VV-L-
800) to the external cable surface.
4. Corrosion Detection
A. The primary means of corrosion detection is visual, but in situations where visual inspection is not
feasible, other techniques must be used. The use of liquid dye penetrants, magnetic particle, X-ray,
and ultrasonic devices can be used, but most of these sophisticated techniques are intended for the
detection of physical flaws within metal objects rather than the detection of corrosion.
(1) Visual Inspection.
(a) A visual check of the metal surface can reveal the signs of corrosive attack, the most
obvious of which is a corrosive deposit. Corrosion deposits of aluminum or magnesium
are generally a white or grayish-white powder, while the color of ferrous compounds varies
from red to dark reddish-brown.
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(2) Liquid Dye Penetrant Inspection.
(a) Inspection for large stress-corrosion or corrosion fatigue cracks on nonporous or
nonferrous metals may be accomplished using dye penetrant processes. The dye applied
to a clean metallic surface will enter small openings or cracks by capillary action. After
the dye has an opportunity to be absorbed by any surface discontinuities, the excess dye
is removed and a developer is applied to the surface. The developer acts like a blotter to
draw the dye from cracks or fissures back to the surface, giving visible indication of any
fault that is present on the surface. The magnitude of the fault is indicated by the quantity
of dye brought back to the surface by the developer.
5. Corrosion Damage Limits
A. Following cleaning and inspection of the corroded area, the actual extent of the damage may be
evaluated using the following general guidelines and sound maintenance judgement.
(1) Determine the degree of corrosion damage (light, moderate, or severe) with a dial-type depth
gage, if accessibility permits. If the area is inaccessible, clay impressions, or any other means
which will give accurate results, should be used. In the event the corrosion damage is severe
or worse, contact Cessna Propeller Aircraft Product Support, P.O. Box 7706, Wichita, KS 67277
USA, for assistance.
(2) Light Corrosion.
(a) Characterized by discoloration or pitting to a depth of approximately 0.001 inch maximum.
(3) Moderate Corrosion.
(a) Appears similar to light corrosion except there may be blistering or some evidence of
scaling or flaking. Pitting depths may be as deep as 10 percent of the material thickness.
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
Do not use metallic wools or solvents to clean installed cables. Use of metallic
wool will embed dissimilar metal particles in the cables and create further
corrosion. Solvents will remove internal cable lubricant, allowing cable strands
to abrade and further corrode.
The indications of corrosive attack are small localized discoloration of the metal
surface. Surfaces protected by paint or plating may only exhibit indications of more
advanced corrosive attack by the presence of blisters or bulges in the protective film.
Bulges in lap joints are indications of corrosive buildup which is well advanced.
In may cases, because the inspection area is obscured by structural members,
equipment installations, or for other reasons, it is awkward to check visually. In such
cases, mirrors, boroscopes, or like devices must be used to inspect the obscured
areas. Any means which allows a thorough inspection can be used. Magnifying
glasses are valuable aids for determining whether or not all corrosion products have
been removed during cleanup operations.
© Cessna Aircraft Company
51-11-00
Page 4
Jun 1/2005