Date: April 7, 1998 

In reply refer to: A-98-34 through -39 

Honorable Jane F. Garvey 

Administrator 

Federal Aviation Administration 

Washington, D.C. 20591 

On July 17, 1996, about 2031 eastern daylight time, a Boeing 747-131, N93119, operated 

as Trans World Airlines (TWA) flight 800, crashed into the Atlantic Ocean, about 8 miles south 

of East Moriches, New York, after taking off from John F. Kennedy International Airport, 

Jamaica, New York. All 230 people aboard the airplane were killed. The airplane, which was 

operated under Title 14 Code of Federal Regulations (CFR) Part 121, was bound for Charles De 

Gaulle International Airport, Paris, France. The flight data recorder and cockpit voice recorder 

ended simultaneously, about 12 minutes after takeoff. Evidence indicates that as the airplane

was climbing near 13,800 feet mean sea level, an in-flight explosion occurred in the center wing

fuel tank (CWT), which was nearly empty. 

The source of ignition of the CWT has not been determined, and the investigation into a 

variety of potential ignition sources continues. However, the Safety Board's investigation has 

found damaged wiring in the fuel quantity indication systems (FQIS)1,2 of the accident airplane 

 

1

 The B-747 FQIS measures fuel quantity with a capacitance measurement fuel probe system in each fuel tank. 

There are seven capacitance measurement fuel probes in the B-747 CWT. Each fuel probe consists of an inner 

tubular element that is surrounded by an outer tube. Compensators, located near the low point of each fuel tank, are 

also constructed of assemblies of tubular elements. The compensators and probes have a hard plastic terminal block 

near the top of each to provide for wiring connections. Wires from each fuel probe and the compensator are routed 

within the fuel tank through nylon clips to a connector located at the rear wing spar and are exposed to fuel and 

vapor. 

2

 Most of the B-747-100, -200, and -300 series airplanes (about 700 airplanes) are equipped with FQIS 

manufactured by Honeywell Corporation; airplanes equipped with the Honeywell system are the subject of this 

letter. About 10 percent of the B-747-100, -200, and -300 series fleet has been retrofitted with FQIS manufactured 

by BFGoodrich Aerospace Corporation (formerly Simmonds Precision). The B-747-400 series airplanes are 

equipped with the BFGoodrich system equipment. No BFGoodrich FQIS were inspected during the investigation. 

E PLURIBUS UNUM

NATIONAL TRA S PORTATIO

N

B OAR D

SAFE T Y

N

National Transportation Safety Board 

Washington, D.C. 20594 

Safety Recommendation 

6788F
2

and three retired B-747s: N931053 and N931174 and a former Air France airplane, F-BPVE,5 and 

the Safety Board was informed of damaged FQIS wiring in a British Airways B-747, G-BBPU.6

These findings illustrate unsafe conditions that may exist in other B-747s and should be 

addressed by the Federal Aviation Administration (FAA). 

The potential hazardous features found inside of B-747 fuel tanks during the investigation 

include the following: 

1. FQIS wire insulation had been damaged near the attachment point of wires to 

four CWT fuel probe and compensator terminal blocks in N93105.7 Terminal 

blocks with knurled (rough) areas on the surface had Honeywell Corporation 

manufacturing dates of November 19708 and earlier and were identified as 

Series 1, 2, and 3.9 These terminal blocks had a metal strain relief clamp 

pressing the FQIS wires against the knurling. The knurled area consisted of a 

series of relatively sharp pointed cones in the hard plastic, and the edges of the 

terminal block castings transected the cones, thus creating sharp edges 

resembling saw teeth. The FQIS wire insulation had been cut by the knurled 

area, exposing the core conductors of some wires to the grounded shielding of 

others.10 

 

3 N93105 had been undergoing maintenance when it was retired from service by TWA in 1994. The airplane had

been in storage in Kansas City since that time. 

4

 N93117 had been sold by TWA in 1992, and was subsequently placed in storage in Mojave, California, after 

77,145 flight hours. 

5 F-BPVE was retired by Air France in September 1994. The airplane was subsequently used by the Safety Board

and other agencies for testing in Bruntingthorpe, England. 

6 G-BBPU is an in-service B-747-136. At the time of its inspection on November 1, 1996, the airplane had been 

operated 89,639 hours and 17,437 cycles since new. 

7 Few terminal blocks from N93119 were recovered and most of those were fragmented or otherwise damaged. 

Although few of the fragments had attached FQIS wires, chemical traces on the exterior of damaged wire insulation 

had been deposited on and around previously damaged surfaces. Damage similar to that found in N93105 has been 

seen in some FQIS components from F-BPVE. 

8 On May 28, 1969, Boeing implemented a requirement for the wires to withstand a 50-pound pull, and on

December 29, 1969, Honeywell Engineering Change Order 69 15826 revised the design to a Series 4 terminal

block, which deleted the use of screws to fasten FQIS wires to the terminal block and introduced the use of threaded 

studs and nuts. On the Series 4 block, the metal strain relief clamp and knurling were deleted and the FQIS wires

were held within the eye of a "P"-shaped nylon clamp that held the wiring above the terminal block surface. The

change order was to be effective as soon as new terminal blocks were available. Boeing reported that a production 

change was made at Boeing that installed the Series 4 terminal blocks in [airplane] line number 65 and onward.

Since N93119 was line number 153 and was delivered on October 27, 1971, Boeing concluded that it was

improbable that it was delivered with Series 3 terminal block probes. A mixture of terminal block series that

included Series 1-3 and subsequent designs were found in each of the cited B-747 airplanes, including N93119.

9 The Honeywell Component Maintenance Manual still shows the Series 1-3 terminal blocks and metal strain relief 

clamps as "applicable" [acceptable] for use. Honeywell has reportedly supplied them as replacement parts, although 

only the updated design is now sold.

10 Wire shielding covers the inner insulation and core conductor with a layer of woven wire, which isolates the 

conductor from electromagnetic signals and provides protection to the inner insulation and core conductor from 

external mechanical damage. Additional insulation covers the wire shielding.
3

2. In addition to the knurled surfaces found in the Series 1-3 fuel probes, B-747 

fuel probe terminal blocks and compensators have squared edges that can 

damage wire insulation. A wire that had been located against the edge of a 

Series 1-3 terminal block from N93105 had a lengthwise cut in its insulation. 

(In contrast to the B-747 Series 1-3 terminal blocks, Honeywell also makes 

B-757 and B-767 fuel probes with terminal block edges that are smooth and 

rounded.) 

3. The insulation of a fuel probe wire from the CWT in N93105 was also found 

to be displaced (cold-flowed), exposing its core conductor. The wire had been 

one of several pressed under the strain relief clamp of a Series 1-3 fuel probe 

terminal block. Wire insulation was also displaced by cold-flow or chafe at 

points of tight contact between wires not under the knurled clamps and where 

wires were pressed against plastic heat-shrink material on adjacent wires, in 

some instances exposing the conductor of one wire to the shield of a second 

wire. Displaced insulation that had been damaged but not breached was 

identified at various locations where wires pressed against other wires, where 

wires were in contact with the edge of a clamp, and at the edges of nylon clips 

where the FQIS wire routing made sharp turns inside the fuel tanks. Points of 

chafing and potential chafing were also found where FQIS wires contacted 

structure in the CWT of N93117. 

4. During the accident investigation, two inappropriate repairs were found in the 

FQIS wiring in the wing tip fuel tanks of the accident airplane and another 

inappropriate repair was found by Boeing in a B-747 operated by another 

airline. The shielding of an N93119 wingtip tank FQIS probe wire had been 

previously broken and repaired. The repair of the wire consisted of splicing 

with a crimped connector and covering it with adhesive tape secured by wire 

bundle lacing tape.11 Although the repair was functional, separated wire 

strands were found at the edge of the crimped connector. The separated 

strands had flat and angled-surface features, indicative of a fatigue failure. 

Boeing recommends that such broken FQIS wire be removed, solder-repaired, 

and covered with heat-shrink tubing. The second inappropriate repair found in 

N93119 was on a post-Series 3 compensator, where an oversized terminal 

block strain relief "P-clamp" had been used. The replacement P-clamp was 

larger than specified and unable to grip the FQIS wire harness. To provide 

strain relief, the wire harness had been looped to pass through the clamp twice 

and was still a loose fit. The third inappropriate repair was found in the CWT 

 

11 Wiring in B-747s is assembled into harnesses with lacing tape made of Dacron, fiberglass, or Nomex, as specified 

in the Boeing Standard Wiring Practices Manual, section 20-00-11, page 17, Table XX, "Tie Materials." 
4

of G-BBPU, where chafed FQIS wires had been repaired with fuel tank 

sealant.12 

The damaged wiring at the terminal blocks was found only after the wiring had been 

removed. A close visual inspection in the tank without removing the wires would have been 

insufficient to disclose damage that is concealed between wires or under wire clamps. These 

types of damage could create spark gaps that are very small and that could become latent failures 

in the wiring system. 

Boeing issued Service Bulletin (SB) 747-28-2205 on June 27, 1997, and a notice of status 

change for this SB on September 25, 1997, to address B-747 fuel tank inspection procedures. 

However, the recommended inspection procedures for FQIS wires, fuel probes, and 

compensators were not addressed in sufficient depth for operators to find wire insulation damage 

similar to that found during the TWA 800 accident investigation. Most of the damaged FQIS

wire insulation found during the accident investigation was concealed beneath strain relief

clamps or other wires and was discovered only after the wiring was removed from terminal

blocks. In some cases, the damage was not apparent until the ends of the wires were inspected 

under magnification. 

On October 27, 1997, Boeing issued a notice to B-747 operators (M-7220-97-1725) 

describing a planned SB that would provide further details on inspecting B-747 fuel probes, 

compensators, and FQIS wires. In an October 30, 1997, letter to the Safety Board, Boeing stated 

that the new SB will recommend the replacement of Series 1-3 fuel probes, the reporting of 

damage involving Series 4 and later fuel probes, the replacement of certain CWT FQIS wire 

harnesses, and the inspection for proper wire routing and existing damage; the SB will also 

establish an electrical resistance check of very low voltage and establish standards for FQIS 

repairs. 

The Safety Board appreciates Boeing's efforts to develop a new SB to improve inspection 

of B-747 CWT FQIS components. However, compliance with SBs is not mandatory. The Safety 

Board believes that the FAA should issue, as soon as possible, an airworthiness directive (AD) to 

require a detailed inspection of FQIS wiring in B-747-100, -200, and -300 series airplane fuel 

tanks for damage, and the replacement or the repair of any wires found to be damaged. Wires on 

Honeywell Series 1-3 probes and compensators should be removed for examination. 

In December 1969, Boeing reportedly discontinued using the Honeywell Corporation 

Series 1-3 fuel probes (with knurled terminal block surfaces and metal strain relief clamps) and 

began using the Series 4 (and later) fuel probes13 as a product improvement. However, the 

change was not considered mandatory and Series 1-3 fuel probes are still found in airplanes. 

This investigation has shown that the knurling and the sharp edges of the early design terminal

 

12 The Boeing Standard Wiring Practices Manual describes methods and materials that can be used for wire repairs. 

It does not list fuel tank sealant as an approved material for repair of electrical wiring.

13 See footnote 7.
5

blocks create damage to wire insulation. Changing to a Series 4 terminal block reduced the 

potential for FQIS wires to be damaged by the terminal blocks. However, the Honeywell 

overhaul manual still shows the Honeywell Series 1-3 terminal blocks as "applicable for use." 

The Safety Board believes that the FAA should issue an AD to require the earliest possible 

replacement of the Honeywell Corporation Series 1-3 terminal blocks used on B-747 fuel probes 

with terminal blocks that do not have knurled surfaces or sharp edges that may damage FQIS 

wiring. 

Features of the fuel probes and wiring installation used in B-747s are similar to those of 

Honeywell fuel probes used in other airplanes, including the B-707, Lockheed C-130, B-757, and

B-767. The B-707 and C-130 terminal blocks have a different shape but have some features

similar to the B-747 design, including sharp edges. The B-757 and B-767 fuel probe terminal 

blocks have rounded edges and cast wire relief areas that are not used in the B-747 terminal 

blocks; the FQIS wires are retained in the cast wire relief areas by a flat metal bar. Wiring 

attached to the terminal blocks in airplanes other than the B-747 has not been examined by the 

Safety Board staff during the TWA 800 investigation. However, because of the similarities

found during a review of fuel probe designs, the Safety Board is concerned that FQIS wiring

problems discovered in this investigation may also exist in other airplanes with similar designs.

Therefore, the Safety Board believes that the FAA should conduct a survey of FQIS probes and

wires in B-747s equipped with systems other than Honeywell Series 1-3 probes and

compensators and in other model airplanes that are used in 14 CFR Part 121 service to determine 

whether potential fuel tank ignition sources exist that are similar to those found in the B-747.

The survey should include removing wires from fuel probes and examining the wires for damage.

Repair or replacement procedures for any damaged wires that are found should be developed.

14 

Dark deposits were found around the wiring connections of fuel probes that had been 

removed from various fuel tanks in N93105, N93117, N93119, and F-BPVE. The deposits were 

found on wire insulation and on numerous plastic sleeves of crimped wire splices. A scanning 

electron microscope revealed that the dark deposits on N93119 and N93105 fuel probes 

contained copper, silver (silver-plated copper wiring is used in fuel tanks), and sulfur 

(a contaminant in jet fuel). The deposits on an N93119 FQIS compensator fragment were further 

examined at a U.S. Air Force research laboratory (Wright Laboratory) and were determined to be 

similar to copper sulfide deposits found in previous examinations of fuel probes from military 

aircraft. The laboratory had previously found that the deposits gradually reduced resistance 

between electrical connections of the military airplane fuel probes. 

Wright Laboratory staff received a fuel probe that had been removed from a military 

trainer and tested at a maintenance depot while the probe was still wet with fuel. The test 

involved voltage and current levels greater than those that would be available from the FQIS. 

According to the Wright Laboratory staff, disassembly of the probe revealed soot and carbonized 

copper-sulfide deposits, apparently from the ignition of fuel vapors. A report by the Wright

 

14 Boeing is currently conducting a survey of Honeywell Series 4 probes and compensators.
6

 Laboratory15 states that a subsequent visual inspection found "discoloration and possible arcing 

on the bottom" of the probe. The report stated further, "It appears the internal probe wires were 

damaged by a fire. Evidence of an electrical arc was evident on the nylon cap which would have 

provided the required energy needed to ignite residual fuel." Another fuel probe documented by 

the same set of reports had evidence of an arc-track16 with deposits composed of copper sulfide 

and carbon. Unburned deposits that were photographed by a scanning electron microscope had 

the appearance of flaking paint. Electronic testing for the resistance value of similar deposits on

a third fuel probe revealed "small scintillating arcs" between the flakes, as current was increased

to 5 milliamperes (voltage unknown) between a set of probes located 10 millimeters apart. When

drops of JP-4 fuel were placed on the arcing deposit, the report said, "heat generated by the 

[electric] current rapidly evaporated the fuel. Resistance increased from 13,200 ohms to an open 

circuit (>20M)17 after a few seconds." The flaking copper sulfide deposits were found to be a 

brittle substance that clung tenaciously to plastic materials and could only be cleaned by

mechanical abrasion. The report concluded the following: 

The residues act as a thin film resistor that will rupture and open if significant 

current is passed through the material. Residue formation is most likely the result 

of a long-term degradation or corrosion process. Exposed silver plated copper 

wiring and other silver containing surfaces (electrodes) are apparently reacting 

with the sulfur in the fuel. This deterioration process is most likely time 

dependent and, as the probes age, more probe [calibration] failures can be 

expected. 

Copper sulfide deposits were found inside the FQIS wire insulation of N93105 and 

N93119, where the wires had damaged insulation. The Safety Board is concerned that copper 

sulfide deposits on FQIS wires could become ignition sources in B-747 and similarly designed 

fuel tanks. The Safety Board believes that the FAA should require research into copper-sulfide 

deposits on FQIS parts in fuel tanks to determine the levels of deposits that may be hazardous, 

how to inspect and clean the deposits, and when to replace the components. 

The investigation has also found that although the design for the B-747 CWT FQIS 

provides for limited electrical power in the fuel tank,18 the FQIS wires are routed in bundles with 

nearly 400 other wires, some of which carry up to 350 volts.19 The FQIS harness routed between 

 

15 Wright Laboratory Report "Analysis of Trainer Aircraft Fuel Probes I," dated March 1990, by George Slenski, 

Materials Integrity Branch, Systems Support Division, Materials Directorate.

16 Arc-tracking is an insulation failure leading to flashover. Tracks develop along the discharge path on the surface 

of the insulation. The tracks are generally more conductive than the virgin insulation. These tracks carbonize quickly 

into significant conducting paths.

17 Mega-ohms are one million ohms of electrical resistance.

18 Power to the FQIS is limited by Boeing to 0.02 millijoules, or less than 10 percent of minimum ignition energy 

(MIE) required to ignite Jet A fuel under laboratory conditions, according to the American Petroleum Institute 

Recommended Practice 2003, Fifth Edition, December 1991, entitled "Protection Against Ignitions Arising out of 

Static, Lightning, and Stray Currents."

19 Zone A ceiling light wire W-1306-L1892-22 carries up to 350 volts. Numerous other wires carry 115 volts 

alternating current (VAC) and 28 volts direct current and are routed in bundles with FQIS system wires. Boeing
7

the CWT and the flight engineer's panel in the cockpit contains one shielded wire and two 

unshielded wires in a woven fiberglass sleeve. Boeing noted in an October 27, 1997, letter that 

this is a common design for capacitive FQIS systems. Behind the flight engineer's panel, the 

sleeved set of Teflon-insulated FQIS wires was connected to unprotected20 general airplane 

wiring21 that was routed to the fuel totalizer indicator and to the electrical/equipment (E/E) 

compartment located beneath the forward cabin and behind the nose landing gear. Additionally, 

unshielded Teflon wiring from the right wing fuel tanks was attached to a terminal strip located 

on spanwise beam No. 2 in the CWT, then was routed through the left wing fuel tanks to the 

ground refueling panel gauges located between the Nos. 1 and 2 engines. At the ground refueling 

panel, the fuel tank wiring was routed with other aircraft wiring for the refueling indicators and 

controls. 

Electrical short circuits can introduce high voltage into low voltage conductors. For 

example, it was determined that a military C-130 fuel tank exploded in the 1970s after improper 

maintenance had created a short circuit within a fuel gauge electrical connector.22 Maintenance 

work on the connector was not finished before the flight, and the investigation found that 115 

VAC power was inadvertently allowed to enter the fuel tank through the shielding of FQIS wires. 

In the investigation of a May 11, 1990, Philippine Airlines B-737-300 CWT explosion at 

Ninoy Aquino International Airport, Manila, Philippines, the exact source of ignition was never 

established. However, the Safety Board later concluded, "It is possible that the combination of a 

faulty float switch and damaged wires providing a continuous power supply to the float switch 

may have caused an electrical arc or overheating of the switch leading to the ignition of the 

center fuel tank vapor." 23

An Air Force study24 of data from 1986 to 1989 mishaps25 caused by electrical failures 

found 652 records, of which 326 were examined in detail. Of the 326 reports, 49 involved

"conductors" (typically aircraft wiring) and 51 involved "connectors" of numerous types. The 

study concluded the following: 

 

RA164 Center Wing Tank Wire Bundle Analysis Report, December 17, 1996, indicates bundle No. W186 contains

12 192-volt wires for the flight engineer panel lighting. 

20 Wires that were not isolated or shielded and that were routed in bundles with other wires, some of which carried

power for other airplane systems.

21 Wire markings identified the general N93119 aircraft wiring as (Boeing Specification) BMS13-42A, marketed by 

Raychem. The wire was sold commercially under the trade name "Poly-X." Other types of wire were also used in 

the construction of B-747 airplanes.

22 The Safety Board was permitted to review a report regarding a military C-130 fuel tank explosion that occurred 

after improper maintenance created a short circuit that created an ignition source in the fuel tank. The airplane 

identification and the date and location of the incident have not been released.

23 National Transportation Safety Board. August 1, 1990. Safety Recommendations A-90-100 through -103.

24 Contract F33615-89-C-5647, completed January 1989, to develop a handbook for the evaluation of electrical 

components in aircraft accident investigations.

25 According to the Air Force, there are four classes of mishaps in the Norton database [of USAF mishaps]. Classes 

A, B, and C generally represent in-flight conditions that result in some damage to the aircraft. The fourth class 

includes potential mishaps, which may be the result of unusual conditions observed during maintenance or preflight 

checks.
8

The majority of aircraft mishaps involving electronics are related to 

interconnection problems. Interconnection problems are primarily due to wiring 

and connector failures. Chafing, which results in electrical arcing of wiring, and 

corrosion, which results in the electrical breakdown in connectors, appear to be 

the dominate failure mechanisms. 

Such findings are not unique to military mishaps. For example, on July 19, 1997, a 

Lufthansa B-747 freighter (D-ABZC) was on final approach to JFK International Airport, New 

York, when seven circuit breakers popped in the cockpit. Afterward, maintenance personnel 

found 47 (non-FQIS) wires burned in more than 8 inches of the affected wire bundle; the wires 

were located beneath the oxygen bottles in the "cheek" area to the right of the forward cargo 

compartment. The wires led to the leading and trailing edge flaps, landing gear circuitry, and the 

anticollision lights. Circuitry for the wing flap asymmetry detection and a flap electrical drive 

motor led to the burned area, and each of those components needed replacement. The airplane 

had been purchased from another carrier and, in April 1993, was modified by a third company to 

the freighter configuration. Lufthansa found that this airplane and five others that were modified 

by the same company had metal drill shavings and other debris in that area of the wire bundle. 

The incident demonstrated the danger of allowing metal shavings to remain on wiring and the 

possibility of introducing enough electrical energy into unrelated circuits to damage electrical 

components. 

In addition to investigating the potential for introducing energy into FQIS wires from 

direct short circuits, tests were conducted to determine the energy that can be induced into 

unshielded FQIS wires by electromagnetic inductance (EMI). Laboratory tests26 have shown that 

EMI can introduce elevated levels of energy into FQIS wiring, and sparks can be induced by 

adding foreign material to the fuel probes, thus creating spark gaps. This amount of energy was 

only found during tests in which a spark gap was artificially created between the Lo-Z (outer fuel 

probe electrode or terminal) and ground. To date, testing has not duplicated those results on an 

airplane. The investigation of this issue is continuing. 

Wire shielding and physical separation each provide EMI and chafe protection for the 

inner conductor and a path to ground for short circuits from other wires and are widely used in 

airplanes. However, two of the three recovered FQIS wires from N93119 that had been routed 

between the CWT and the cockpit in a woven sleeve were not protected from EMI or chafing by 

shielding or separation from other wires. Also, BMS13-42A wires that were found routed from 

the cockpit end of the FQIS harness to the E/E compartment were not shielded or separated. In 

1974, Boeing incorporated an overall shield around all three CWT FQIS wires routed between 

the CWT and flight engineer panel; in 1980, Boeing added further shielding to FQIS wires 

behind the flight engineer panel. However, these wiring changes were not required for 

previously manufactured airplanes, such as N93119. In its October 27, 1997, letter, Boeing

 

26 Tests were conducted to Boeing specification to create transient voltages and sparks by switching electrical

power on and off in wires that had been laid parallel to the CWT bundle. Tests induced up to 0.060 millijoules of

energy in the CWT harness, exceeding the API practice 2003 reference for an MIE requirement of 0.025 millijoules.
9

acknowledged the additional benefits of shielding, but wrote that the shielded wire was used to 

correct for electrical noise in the FQIS wires (not for EMI or chafe protection). 

The Safety Board recognizes the difficulty and expense associated with physically 

separating FQIS wires from other wires and adding shielding to FQIS wires on in-service air 

carrier airplanes. Access is limited behind avionic racks and at bulkhead electrical connectors, 

and rewiring is labor intensive. However, the separation of the FQIS from other power sources 

by shielding and separation can protect fuel tank wires from power sources that can potentially 

ignite an explosive vapor in a fuel tank. The Safety Board believes that the FAA should require 

in B-747 airplanes, and in other airplanes with FQIS wire installations that are corouted with 

wires that may be powered, the physical separation and electrical shielding of FQIS wires to the 

maximum extent possible. 

Because of the variety of latent potential ignition sources found in B-747 fuel tanks, and 

the variety of means by which energy can be introduced into FQIS wires, the Safety Board does 

not believe that correcting wiring deficiencies and addressing system failures would fully protect 

the B-747 CWT and other fuel tanks against all potential ignition sources. Total FQIS wire 

shielding or separation from other wires would be very difficult to change in airplanes already in 

service and would not address failures within system components, such as fuel gauges, ground 

refueling volumetric shutoffs, and data acquisition units. Unless the volatility of fuel tank vapors 

can be eliminated as a potential hazard, electrical power surge suppressers may be the most 

effective method of preventing the FQIS from becoming an ignition source. On December 1, 

1997, the FAA issued a notice of proposed rulemaking applicable to B-747-100, -200, and -300 

series airplanes that agreed with this premise and would require either the installation of 

components for the suppression of electrical transients by electromagnetic interference, or the 

shielding and separation of the electrical wiring of the FQIS. 

Surge suppressors installed where FQIS wires enter fuel tanks could provide added 

protection against excessive power surges in the FQIS system, regardless of origin. Surge 

protection systems are used in a range of devices, from autopilots to personal computers. Boeing 

has successfully used electrical surge suppression in other applications, but has noted that 

extreme care would have to be used in an FQIS application to account for possible influences on 

system operation and failure modes. Because the basic concepts of most capacitance FQIS 

systems are similar, the Safety Board believes that the FAA should require, in all applicable 

transport airplane fuel tanks, surge protection systems to prevent electrical power surges from 

entering fuel tanks through FQIS wires. 

Therefore, the National Transportation Safety Board recommends that the Federal 

Aviation Administration: 

Issue, as soon as possible, an airworthiness directive to require a detailed 

inspection of fuel quantity indication system wiring in Boeing 747-100, -200, and 

-300 series airplane fuel tanks for damage, and the replacement or the repair of
10

any wires found to be damaged. Wires on Honeywell Series 1-3 probes and 

compensators should be removed for examination. (A-98-34) 

Issue an airworthiness directive to require the earliest possible replacement of the 

Honeywell Corporation Series 1-3 terminal blocks used on Boeing 747 fuel 

probes with terminal blocks that do not have knurled surfaces or sharp edges that 

may damage fuel quantity indication system wiring. (A-98-35) 

Conduct a survey of fuel quantity indication system probes and wires in Boeing 

747s equipped with systems other than Honeywell Series 1-3 probes and 

compensators and in other model airplanes that are used in Title 14 Code of 

Federal Regulations Part 121 service to determine whether potential fuel tank 

ignition sources exist that are similar to those found in the Boeing 747. The 

survey should include removing wires from fuel probes and examining the wires 

for damage. Repair or replacement procedures for any damaged wires that are 

found should be developed. (A-98-36) 

Require research into copper-sulfide deposits on fuel quantity indication system 

parts in fuel tanks to determine the levels of deposits that may be hazardous, how 

to inspect and clean the deposits, and when to replace the components. (A-98-37) 

Require in Boeing 747 airplanes, and in other airplanes with fuel quantity 

indication system (FQIS) wire installations that are corouted with wires that may 

be powered, the physical separation and electrical shielding of FQIS wires to the 

maximum extent possible. (A-98-38) 

Require, in all applicable transport airplane fuel tanks, surge protection systems to 

prevent electrical power surges from entering fuel tanks though fuel quantity 

indication system wires. (A-98-39) 

Chairman HALL, Vice Chairman FRANCIS, and Members HAMMERSCHMIDT, 

GOGLIA, and BLACK concurred in these recommendations. 

[original signed] 

By: Jim Hall 

 Chairman