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float assemblies. External to the fuel tank, condition checks were also accomplished on fuel
5.0 Boeing Working Group Report (continued)
August 4, 2000 Industry AFFSP Report Page 51
system electrical components mounted on the outside of the tank walls which interface with
equipment located inside the tank. Typical electrical equipment inspections included boost pumps,
scavenge pumps, jettison pumps, refuel valves, transfer valves, jettison valves, temperature
sensors, float switches, and pressure switches.
Condition checks did not reveal any chronic problems. There were some reports of damaged
tubing, loose couplings, and loose or broken lockwire, none of which would affect the safety of
the airplane. It could not be determined from the inspection results at what point in time the tubing
may have been damaged. The damage may have occurred during the original manufacture of the
airplane or during a subsequent fuel tank entry to address an airline maintenance or inspection
requirement. In either case, the proposed plan is to enhance the existing zonal inspections to
identify and correct these types of conditions.
The information reported indicates that there is very little corrosion going on inside the tanks.
Most of the tubing and mechanical components generally look as they did when they were new.
External to the fuel tanks, the electrical component inspections noted occasional but generally
minor conditions of corrosion. It was expected that there would be some corrosion because of the
environment in which the equipment operates. Most of the electrical equipment that was inspected
is located either on the front or rear spars of the wing or in the wheel well area. These components
are exposed to rain, deicing fluids, cleaning fluids, salt fog, and other environmental conditions.
External wiring interfaces to the electrical equipment mounted on the fuel tank were also
inspected. Only minor discrepancies were reported for the Puget Sound–built airplanes. Inspection
of the Long Beach DC-9 models identified an instance where power wiring had been misrouted
and was installed in runs with fuel quantity indication system (FQIS) wiring. A service bulletin
was issued to correct this situation on all DC-9s, and the FAA followed up with an airworthiness
directive to make the service bulletin mandatory. Investigation determined the root cause to be a
rework service bulletin that was performed improperly.
There was one instance reported where the inspection found an incorrect bonding jumper installed
on an Override / Jettison Pump Motor Impeller Unit. The jumper appeared to have been installed
as the result of an incorrect maintenance action and was reworked appropriately.
Except for the items noted above, none of the conditions found during this phase of the inspection
affect the continued airworthiness of the airplanes on which the conditions were found. However,
consideration is being given to recommending a periodic visual / tactile inspection check for
conditions of broken or damaged hardware, loose couplings, broken or loose lockwire, and
corrosion of bonding jumpers. The proposal would be for the inspection to be accomplished at a
heavy check interval, probably in conjunction with structural inspections.
5.6.4 Fuel Quantity Indication System
Most of the FQIS installed on Boeing airplanes use a capacitance measurement system for
computing fuel quantity. These systems typically include tank units and compensator units. Newer
systems may include densitometers. These systems are designed not to provide an ignition source
even under conditions of failure.
5.0 Boeing Working Group Report (continued)
Page 52 Industry AFFSP Report August 4, 2000
The fuel quantity indication system is the only system that has electrical wiring installed internal
to the fuel tanks on Boeing-built airplanes. As part of the service bulletin program, detailed
instructions were given to inspect both the FQIS components and electrical wiring.
The component inspections included all tank units, compensators, densitometers, single-point
sensors, and terminal strips. Items that were found during the inspections included a few loose
terminal connections and sulfide contamination on the terminal blocks of components and
terminal strips. Loose terminals were tightened per instructions in the service bulletins. The FQIS
incorporates design features that tolerate the fuel tank environment factors such as corrosion and
contamination. Boeing is working with the FAA on a study to determine what issue, if any, is
posed by sulfidation. The FAA study also will provide recommendations on the prevention and
removal of sulfidation, if required. Boeing testing done in advance of this study has indicated that
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