This case study describes an investigation into failures of fuel float switches in aircraft. Over the course of the investigation, four potential failure scenarios were identified but ultimately proved to be "red herrings" or misleading. Through a process of elimination and further testing, the true root cause was determined to be solder creep in the electrical connections of some switches. Solder creep had led to failure of the joints through incremental deformation over many load cycles. This root cause was only identified after reconsidering assumptions made early in the investigation.

1. This article is based on Four Red
Herrings, Float Switch Failure, and
a Case of Multiple Failure
Scenarios, a case study presented at
MS&T’08. It covers investigation of
fuel float switches in which evidence
was found to support several proposed
failure scenarios that were ultimately
misleading.
Paul DeVries*
Boeing Corporation
Huntington Beach, California
T
his case history summarizes findings of four
separate investigations involving the failure
of a certain model of electrical fuel float
switch in which several plausible but ulti-
mately “red herring” failure scenarios were pursued.
It underscores the danger of assumptions and the
value of both expert knowledge and broad-based ex-
perience. The true root cause was solder creep, but
this was determined only after a re-examination of
preliminary failure mode assumptions.
The investigation began after an air carrier com-
plained of unacceptably short service life for float
switches from a particular manufacturer. Switch
failure was typically identified by an open circuit
when the switch should have been closed. The
switches, which should have lasted many years and
millions of cycles, were failing after as little as twenty-
one months in service. The part cost of switches
was low, but failed switches could ground an air-
craft and require the affected fuel tank to be emp-
tied and vented before the switch could be replaced.
The float switch mechanism consisted of a glass
bead containing two stationary switch contacts and
a flexible magnetic reed (Fig. 1). The glass switch
bead was encased in a metallic tubular housing that
ran through the center of the float and was con-
nected to the system via electrical leads soldered to
the contacts by a lap-style solder joint. Magnets
mounted in the float forced the switch closed (Fig. 2)
when the float was raised to a predetermined height.
Red Herring #1: Excessive contact current
The original set of three failed switches was eval-
uated by the manufacturer, who found they were
mechanically sound but concluded that contacts
had been “burned” due to current overloads in ex-
cess of their electrical rating. Proposed causes for
*Member of ASM International
current overload included a short circuit in the air-
craft electrical system or accidental high current im-
posed during electrical system maintenance.
Following the manufacturer’s evaluation, the
switch contacts (Fig. 3) were forwarded to the
Boeing Metallurgical Laboratory for corroborative
evaluation. Due to the manufacturer’s extensive ex-
perience with these switches, the reported failure
mode was taken at face value and attempts were
made to identify evidence of electrical overload or
excessive contact resistance.
All contacts had been plated with rhodium, which
typically exhibited a dull finish with a light to
medium-brown-yellow patina, and small burnished
ADVANCED MATERIALS & PROCESSES/DECEMBER 2008 23
Fig. 1— a) Photo of float switch; b) X-ray image showing float switch design.
SShheellll 00..001100 iinncchh
00..000055 iinncchh
SSttaattiioonnaarryy
FFllooaatt ccoonnttaaccttss
((nnoott vviissiibbllee))
FFllooaatt
ccoouunntteerrwweeiigghhtt RReeeedd
ccoonnttaacctt
TToopp wwiirree GGllaassss
sswwiittcchh
TToopp wwiirree bbeeaadd
ssoollddeerr jjooiinntt
HHoouussiinngg RReeeedd
ssoollddeerr jjooiinntt
FAILURE ANALYSIS
“RED HERRINGS”
aa bb
Fig. 2 — Close-up of the switch contacts in the “closed”
position.
Fail analysis.qxp 11/18/2008 2:31 PM Page 1

2. areas of metal-to-metal contact. None of the con-
tacts displayed arc burns; however, tinting on some
samples suggested possible exposure to elevated
temperatures.
Metallographic analysis revealed an equiaxed
microstructure with no apparent grain growth,
suggesting that excessive currents had not been
encountered.
Apart from reliance on the manufacturer’s state-
ment of “burned contacts,” the slight discoloration
noted on some contacts was the only thing that sug-
gested possible overheating. No solid evidence of
excessive contact resistance, mechanical failure, or
excessive current was found.
Two fortunate but wholly coincidental events con-
clusively netted this “red herring.” The first occurred
during contact resistance testing when an inadver-
tent capacitor discharge momentarily routed a cur-
rent spike through a new switch, resulting in local-
ized arcing and subsequent welding of the switch
contacts.As arc-induced welding and pitting were
completely absent from all failed switch contacts,
momentary excessive over-current resulting from
improper system testing was dismissed as a pos-
sible failure mode.
The second event emerged as collateral informa-
tion from an unrelated testing program evaluating
the effects of sustained high current loads on float
switches made by the same manufacturer. During
testing, one switch was subjected to 15 amps DC for
two minutes before failure in the solder joint at-
tached to the stationary contact. The microstructure
of the reed contact subjected to short-term current
overload differed little from that seen in failed
switches. Conversely, current overload (15 Amps
DC for 20 minutes) was sufficient to produce con-
tact melting and significant grain growth in adja-
cent material. Overcurrent testing had demonstrated
that switch contacts could withstand loads 20 times
greater than the switch rating and 33 times greater
than the maximum current anticipated from the
electrical system. In short, the “burned contact” hy-
pothesis was a “red herring.”
Red Herring #2: Flaking magnets
X-ray inspection of twelve additional failed
switches found that eleven were mechanically func-
tional. Thus, insufficient magnetic field strength was
dismissed as a possible failure mode.
Subsequent sectioning of the twelve float assem-
blies found eight whose magnets exhibited a scaly,
flaky topography. The flaking condition suggested
several possible float-jamming mechanisms, such
as the buildup of loose flake material jamming be-
tween the housing tube and bushing, or magnet
swelling and spalling that could effectively clamp
the magnets onto the housing tube.
To support failure analysis, fourteen fully opera-
tional switches were removed from an in-service
aircraft. The magnets in all but the newest of these
switches exhibited some type of flaking..
The float from a new off-the-shelf switch unit
was evaluated for comparison purposes. As ex-
pected, the new magnets did not exhibit the flaking
phenomena. A possible explanation for magnet
flaking emerged when Fourier Transform Infrared
(FTIR) spectroscopy determined that the phthalate
oil plasticizer found in the new magnet matrix ma-
terial was missing from most of the failed switch
magnets. The new switch magnet was immersed in
hot (140ºF) Jet ‘A’ fuel for two weeks; subsequent
FTIR testing confirmed that the fuel had leached the
phthalate oil from the magnet matrix.
While circumstantial evidence supported the
“magnet flaking/float jamming” hypothesis, sev-
eral irreconcilable observations refuted it. For ex-
ample, four of the failed switch float assemblies had
not displayed a flaky magnet topography, yet one
of these floats exhibited definite jam-like evidence.
Also of the eight failed float switches whose mag-
nets exhibited flaking, only three of the floats exhib-
ited magnetic debris on the float bushings where
jamming would likely occur. Lastly, while lab ex-
periments demonstrated that fuel could leach plas-
ticizer from magnets, this did not result in a pro-
gressive magnet flaking phenomenon.
Despite the prevalence of magnet flaking and ev-
idence of magnetic debris on some bushings, flake-
related float jamming could not be positively con-
firmed. By contrast, the float with the most definite
jam-like evidence was an assembly whose magnets
displayed no significant flaking. Given the amount
of circumstantial evidence, the hypothesis was not
entirely dismissed. However, the data inconsis-
tencies suggested the failure hypothesis was a red
herring.
Red Herring #3: Dimensional interference
During investigation of the flaking phenomenon
on the magnets, note was made of circumferential
striation-like marks that were attributed to contact
with the tubular housing. While the cause of magnet
flaking remained unresolved (it could not be repro-
duced in the lab) occasional non-concentricity noted
between the float, the magnets, and the bushings
suggested that a lack of dimensional control com-
bined with magnet flaking or swelling might pro-
duce the “clamping action” these striation-like
marks seemed to indicate. This hypothesis was
aided by the observation of occasional axial scuff
marks and worn epoxy nodules noted on some
magnets.
Drawing dimensions for the float assembly had
not been provided; however, diametric analysis
based on dimensions from the new off-the-shelf float
24 ADVANCED MATERIALS & PROCESSES/DECEMBER 2008
Metallo-
graphic
analysis
revealed an
equiaxed
micro-
structure
with no
apparent
grain
growth,
suggesting
that
excessive
currents
had not
been
encountered.
Fig. 3— Switch contacts as received from the manufacturer.
Fail analysis.qxp 11/18/2008 2:31 PM Page 2

3. assembly (i.e. a magnet with no flaking) produced
estimated diametric clearances as low as 0.003 inch.
The presence of epoxy deposits on the magnet ID
would likely exacerbate any clearance problems,
and the wear noted on some of these deposits indi-
cated contact between the epoxy deposits and the
switch housing tube.
Although flaking could conceivably eliminate
this clearance, it could not be proven conclusively
that it had done so. As the magnet flaking phe-
nomena had not successfully been reproduced in
the laboratory, analysis could not prove that the
flaking mechanism would actually produce forces
large enough to effectively clamp the float assembly
onto the tubular housing. While the hypothesis
could not be completely dismissed, the fact that
none of the floats had actually been stuck at the time
the failed switches arrived at the laboratory sug-
gested this failure mechanism was a “red herring.”
Red Herring #4: An inappropriate assumption
Tubular metal conduit typically carries electrical
lead wires from the fuel float switch to a circuit ter-
minal block.As a result, the analysis presumed that
vibration loads on the electrical leads would be neg-
ligible, and that the conduit could not place a ten-
sile load on the lead wires. Solder joint failures were
presumed to be caused by overload incurred during
switch removal. So firm was this conviction that a
detailed solder joint investigation had effectively
been precluded from investigation.
As the previous three failure hypotheses had
proven false, unlikely, or lacking support, and as
over half of the failed switches had been received
with some form of solder joint damage, investiga-
tors were forced to consider this assumption a self
imposed “red herring,” and initiated serious inves-
tigation into the solder joint failure.
The investigation returned to the fourteen
switches which had been removed from an in-
service aircraft. Two of these switches already con-
tained solder joint failures; the remaining twelve
switches were subjected to tensile testing.
Calculations estimated a joint strength of 26
pounds; however, actual tensile test values ranged
from 8 to 23 pounds, with an average value (17
pounds) approximately half that of the bulk elec-
trical wire itself. At the time it was not suspected
that these “baseline” solder joints might be under-
going failure by creep, which could explain such
widely varying results.
The lab-induced solder joint failure occurred by
solder shear, by tensile overload of individual wire
strands, or by a combination of both. Solder shear
failures exhibited a dimpled-rupture morphology,
while wire strand failures exhibited necking – both
typical of ductile overload. Poor solder wetting, ap-
parent on a number of wire leads, may have con-
tributed to low solder joint tensile properties. Solder
had adhered to the surface strands but had fre-
quently failed to fully penetrate the wire bundle.
This effectively created a “solder shell” around
solder-free inner strands. Failure in these joints pro-
duced necking and dimple rupture in the outer
strands, while unsoldered inner strands simply
pulled out of the “solder shell.”
Of the failed switches, six were available for solder
joint evaluation. Four of these switches exhibited
separation of the electrical leads, while two had
solder joints that remained intact.
Scanning electron microscopy of the failed joints
found that fractures typically occurred through the
solder, producing a smeared, grainy fracture sur-
face. Chatter marks were noted on two of the frac-
ture faces. These marks were closely spaced at one
end, but the spacing gradually increased as they
progressed from one end of the joint to the other.
Examination of intact joints often found evidence
of joint deformation accompanied by surface
cracking that displayed the same grainy texture seen
in the failed solder joint fractures.
The intergranular nature of the failures suggested
failure by creep rupture. Creep is known to occur
in solder at sustained loads much lower than the
solder tensile strength. Creep would be exacerbated
by the small joint size and lack of wetting noted
during tensile testing. The presence of chatter marks
supported an incremental deformation scenario (e.g.
creep), and suggested a cyclical loading element in
the failure process.
Ironically, the granular nature of solder joint
failure had been observed in passing early in the in-
vestigation. The analyst, being unfamiliar with the
appearance of creep, had noted it as interesting and
unusual, but had not viewed it as significant.As the
need for substantive solder failure analysis grew,
creep was reconsidered as a possible failure mode.
Readily available references contained limited in-
formation on the characteristics of creep.
However, consultation with a solder expert con-
firmed the intergranular nature of creep rupture and
revealed that creep often produces a “coarsened
band” microstructure in the plane of the strain (Fig.
4). The “coarsened band” phenomenon had not
been found during the literature search of common
reference materials.
Subsequent re-evaluation of available solder joints
found that six of fifteen faulty switches had failed
via solder creep, three had failed by other means
(presumably float jamming), and six could not be
evaluated (data could no longer be obtained).
For more details about the investigation, please
contact the author.
For more information: Paul DeVries, Boeing Co.,
Huntington Beach, CA 90706; tel: 714/372-9411;
paul.h.devries@boeing.com; www.boeing.com.
ADVANCED MATERIALS & PROCESSES/DECEMBER 2008 25
Fig. 4 — Deformed solder joint displays a coarsened band (arrow) as well as
probable mechanism for producing chatter marks (dashed arrow).
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