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A large number of air carriers (airlines and other operators) participated in this voluntary industry
effort. In particular instances, inspections of center wing fuel tanks and main wing tanks were
performed on the same airplanes. Figure 5-5 identifies the 94 operators who participated in the
Boeing Working Group inspection program.
The commitment by the airlines and Boeing has been significant in accomplishment of these
inspections. Based on Boeing’s estimate of the time required to conduct a particular fuel tank
inspection, it is estimated that more than 110,000 labor-hours have been expended in actual
performance of the inspections. This estimate is likely conservative as feedback from the
participating airlines indicate the Boeing estimate is low. In addition, many hours have been spent
by the airlines and Boeing in preparation and planning for the inspections, in resolving questions
about findings in the inspections, in restoring discrepancies to production levels, and in supporting
working group meetings that are not included in the above estimate. The level of voluntary
participation and effort expended by the airlines and Boeing in supporting all of the working
groups is unprecedented and attests to the industry’s ongoing commitment to continuously
enhancing the safety of air travel.
5.6 Results
As stated, the program established by the Boeing Working Group included requirements reviews,
drawing reviews, manufacturing process reviews, and in-service inspections. This program
provided a comprehensive look at fuel system design, manufacture, and maintenance. The
information, facts, and data collected from all of this activity are being used to identify areas of
possible improvement and enhancement.
The inspection portion of the program provided a significant amount of data and information as to
the overall condition of airplane fuel systems. Collectively, the data gathered provided ample
information regarding the overall integrity of the design and maintenance of the Boeing in-service
fleet. This data has been collected, analyzed, and reviewed for disposition. Design change
(corrective action) and / or scheduled maintenance activity change recommendations are in work.
5.6.1 Electrical Bonding
All tubing, mechanical components (e.g., fittings, pump housings, valve bodies), and electrical
components (e.g., pumps, valves, actuators, pressure switches) are typically bonded through the
use of bonding jumpers or a mating surface (fay surface) bond on all Boeing airplanes. Every bond
path between a piece of equipment and the primary structure of the airplane is installed to a
predetermined resistance value. The inspection program measured these bond paths.
5.0 Boeing Working Group Report (continued)
August 4, 2000 Industry AFFSP Report Page 49
Development of the detailed inspection instructions used in the service bulletins and inspection
documents was accomplished by performing a comprehensive review of the fuel system design for
each model. An inspection value was assigned accordingly. If a bond exceeded the inspectionvalue,
rework was specified to bring the bond back to the original manufacturing limits even if the
measured value was still within safe limits. This manufacturing limit was chosen for rework to
collect data on the causes of higher resistance values found.
The types of bonds measured were for static electricity (electrostatics), fault current handling, and
lightning protection.
Findings were consistent across all models. Static bonds, used for bonding tubing and
nonelectrical components, did not exhibit any aging problems. The small percentage of static
bonds that did require rework to bring them back to the “as new” condition were well within the
margin required to eliminate static charge buildup.
A condition was found on some Long Beach models where a bond path through a foil wrap on
tube clamps was found to be delaminating. Airworthiness was maintained because redundant
bonding is provided for each tube, and some of the foil typically remained in the clamp. An
enhancement to the clamp is being studied with clamp suppliers.
Fault current bonds, installed external to the fuel tank on fuel system electrical components
(pumps, valves, and so on), exhibited some increase in bonding resistance on the in-service
airplanes. Those installations that required the most rework characteristically had an extremely
stringent bonding resistance requirement of less than one milliohm (<0.001 ohm). In reviewing the
designs of these bond paths and discussing the installations with manufacturing, it was determined
that the design did not allow sufficient margin to ensure that once the bond was installed, it would
stay within the established limits.
　
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