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the oxidation does not affect the continued airworthiness of the DHC-8.
The inspection program examined the capacitance probes for damage, corrosion, and deposits. The
fuel probe wiring was examined for damage, chafing, and discoloration as well as for cracked and
degraded insulation. Bombardier also conducted a design review for FQIS wiring routed external
to the fuel tank. The external FQIS wiring is shielded and separated from high-power wiring.
No evidence was observed of damage, corrosion, or deposits associated with the fuel probes.
There were isolated cases of chafing on capacitance probe wiring associated with relatively hightime
aircraft. The chafing is attributed to the aluminum conduit. The FQIS fuel system wiring is
designed not to provide an ignition source even under fault conditions.
Bombardier will carry out a one-time visual inspection of DHC-8 FQIS wiring for damage and
chafing. Bombardier will also carry out a one-time functional check of the magnetic dipstick
electrical bonding. Bombardier is proposing that these inspections be accomplished during
scheduled maintenance activity. In addition, Bombardier is proposing a periodic inspection of
FQIS wiring integrity as well as magnetic-dipstick electrical bonding as part of the aircraft
6.0 Bombardier Working Group Report (continued)
Page 62 Industry AFFSP Report August 4, 2000
maintenance program. These periodic inspections will be carried out during scheduled fuel tank
maintenance activity.
6.6.3 Fuel Components, Fuel Lines, and Fittings
The inspection program specified a visual inspection of fuel tank tubing, components, and fittings
including the low-level float switch, auxiliary fuel pump, pilot valve, pressure relief valve and
conduit, scavenge ejector pump, refuel and transfer line, vent line, engine-feed line, waste fuel
line, low-level warning conduit, pressure switches, solenoid valves, and shutoff valves. The
tubing, fittings, and components were examined for damage, corrosion, leakage, and indications of
heating or discoloration.
In general, the fuel tank components were found to be in excellent condition. There was one
reported case of a damaged fuel line, which was replaced. During one of the initial DHC-8 aircraft
inspections, it was determined that the fuel coupler on the vent and scavenge lines at station 249 of
the right-hand wing had been installed incorrectly with the larger diameter of the coupler facing
outboard instead of inboard. This condition resulted in fouling between the coupler and the inside
surface of the wing access cover. Bombardier considered this finding to be an airworthiness item
and issued a service bulletin, mandated by Transport Canada, to inspect the fleet and rectify the
condition.
6.6.4 Electrical Bonding and Grounding
In general, fuel system components are electrically bonded by means of bonding jumpers and
mating-surface bonds (direct-interface bond between components). These bonds are designed for
electrostatic, lightning, and fault-current protection. Bombardier measured the electrical bonding
resistance for numerous fuel system components and tubing.
As outlined in section 6.6.1, a small percentage of the bonding jumpers was found to exceed the
build standard resistance. The bonding jumper resistance levels that exceeded the build-standard
resistance are well within the safety margin for electrostatic bonds. In some cases, the bonding
jumpers used for electrostatic protection may carry lightning currents. Bombardier is conducting a
test program to investigate the effect of increased bonding-jumper resistance on lightning
protection.
The fault current bonds for the 115-V AC-powered auxiliary fuel pump were within acceptable
limits. The design review indicated that two fuel tank electrical components were not designed
with fault current protection. The 28-V DC fuel shut off valve is located on the wing rear spar
external to the fuel tank. The pressure relief valve position indication switch is a 28-V DC
component located within the fuel tank. Bombardier will recommend a mandatory service bulletin
to incorporate fault current bonding for these components. In addition, Bombardier is proposing a
periodic functional check of the fault current bonding for fuel tank electrical components as part of
the aircraft maintenance program. These periodic inspections will be carried out during scheduled
fuel tank maintenance activity.
6.0 Bombardier Working Group Report (continued)
August 4, 2000 Industry AFFSP Report Page 63
6.6.5 FOD
The inspection process produced no reports of foreign object debris within the fuel tanks, which
were found to be very clean. The existing zonal FOD inspections have proven to be effective.
6.6.6 General Fuel Tank Condition
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