B. All non-aluminum fasteners which penetrate the exterior skin are installed
with wet sealant. A coat of sprayable sealant is applied to the inboard
flange of stringer 10 and above. A coat of BMS 3-23, type II corrosion
inhibiting compound is applied to all structure within 2 of inches of the
body skin, and the body skin itself, from stations 1609 to 1720. On air-planes line number 241 and on, per PRR 54133, this compound extends the full
length of Section 46, and also includes the upper surface of the passenger
floor.
C. The stringers, frames and skins have been found susceptible to corrosion due
to moisture entrapment between the skin and insulation blankets. Corrosion
can readily start where protective finishes have been broken or deterio-rated. Refer to 53-60-02, Fig. 1 for lower lobe structure.
D. External drains are either leaf spring or plunger type drain valves which
close when the airplane is pressurized. Drain paths through the internal
structure lead to the external drain valves. Refer to 53-60-03, Fig. 1 for
valve location.
E. The corrosion protective finish for the structure surrounding the No. 1
cargo and No. 2 passenger door opening consists of a chemical conversion
coating (alodine) on clad aluminum parts, anodize surface treatment for
non-clad parts and a coat of BMS 10-11, type I, primer. The doubters around
the doors are installed with fay surface sealant. All non-aluminum fasteners
which penetrate the exterior skin are installed with wet sealant. After
assembly, a touchup coat of BMS 10-11, type I, primer is applied to bare
fasteners and a coat of BMS 10-11, type II, white enamel is applied around
the doors. A coat of BMS 3-23 is applied except on the door assembly and
door opening (Ref 53-60-04, Fig. 1).
F. The primary corrosion area is under the door sill, floor panels, floor beams
and doubters or triplers at door openings. Contaminants are tracked in by
passengers, crew members, cargo and service personnel or by driven rain/snow
when door is opened.
G. Insulation blankets are provided on cabin interiors for passenger comfort
and to minimize the condensation of warm cabin air on cold skins and string-ers. Corrosion has been experienced in areas where the blankets are not
installed taut and wrap around stringers or lay on the skins.
H. Corrosion can occur along the upper chord of Body Station 1681.8 floor beam
at the nutplates where the floor panel is attached.
Volume 2-757
53 -60-01
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