TASK 53-11-37-600-812
3. Lower Lobe Structure - Corrosion Prevention
Figure 202
A. General
(1) The fuselage is of semimonocoque construction utilizing aluminum skins, circumferential
frames and longitudinal stringers. The fuselage skin is installed with circumferential butt joints
and longitudinal lap joints. The floor beams act as tension ties across the frames. In the lower
lobe area, shear ties from the skin to the frame are used between stringers with an inner angle
on the frame.
(2) The lower lobe structure including stringers, frames, shear ties, faying surfaces at doublers and
straps, etc., are susceptible to corrosion due to moisture accumulation, moisture laden
insulation blankets, cargo spillage, toilet effluent leakage and environmental contaminants. The
lower lobe areas described herein include the cargo compartments, bilge areas and the
electronic compartment.
(3) To help clean out contamination in the lower lobe, dam divide the bilge into compartments for
optional hose out operations (Figure 202 (Sheet 3)). But on some airplanes, these dams have
foam blocks which can become soaked with moisture and cause corrosion.
(4) Some lower lobe doublers came apart from the skin, and corrosion and cracks occurred on
airplanes with 1400 flight hours or more. The damaged areas were between Stringers 26L and
26R under the forward cargo compartment, BS 360-540 and between Stringers 25L and 25R
under the aft cargo compartment, BS 727-1016.
(5) At the nose wheel well, the three hinge bracket supports for the wheel well doors on the left
and right side have pockets that can catch moisture.
(6) Insulation blankets are provided on cabin interiors for passenger comfort and to minimize the
condensation of warm cabin air on cold skins and stringers. Corrosion has been experienced in
areas where the blankets are not installed taut and wrap around stringers or lay on the skins.
Reports of water soaked blankets have been common in these instances.
(7) Treatment of the areas under galleys and lavatories is described in Figure 203.
(8) Much corrosion and separation of doublers has been reported in the lower lobe. Areas where
corrosion is of particular concern is from BS 260 to BS 360, stringers S-19 left to S-19 right.
(9) Refer to CORROSION PREVENTION, SECTION 51-00 of this manual for a discussion of the
Aging Airplane Corrosion Prevention and Control Program and related documentation.
Structural items within this section are subject to the unique requirements of the mandatory
Corrosion Prevention and Control Program.
B. References
Reference Title
51-00 CORROSION PREVENTION
51-00-51 INSPECTION AND DETECTION
51-00-59 STANDARD PREVENTIVE MAINTENANCE PROCEDURES
SRM 737-678 Structural Repair Manual
C. Consumable Materials
Reference Description Specification
A00247 Sealant - Pressure And Environmental -
Chromate Type
BMS5-95
C00032 Coating - Protective Enamel, General Use BMS10-60 Type I
737-600/700/800/900
AIRCRAFT MAINTENANCE MANUAL
53-11-37
Page 206
D633A101-ILF Oct 15/2015
ECCN 9E991 BOEING PROPRIETARY - Copyright © Unpublished Work - See title page for details
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