nuckolls.bob(at)aeroelect
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 Posted: Fri Nov 05, 2010 9:08 am Post subject: Toggle Switch Failures in Canadian C-152 |
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The C-152 switches cited in the Canadian publication
have nothing in common with the failures we've been
discussing here on the List in recent years . . .
The failure modes we've considered have to do
with a riveted construction of the Carling switches
that have been used on light airplanes for about
50 years. We've also considered some poor joining
(crimps) in the wire grips of terminals.
http://www.aeroelectric.com/Mfgr_Data/Switches/Carling/Carling_F-series.pdf
Over time, something has changed in materials,
design or process that make these devices more vulnerable
to loss of contact between the various metallic
joints in the current carrying path. All of the
numbered features in this figure . . .
http://aeroelectric.com/Pictures/Switches/Toggle_Switch_with_Fast-On_Tabs.jpg
A number of readers have reported failures in
Carling switches with most occurring in the one
circuit that generally operates at significant
current flow all the time for day and night
operations . . . strobes.
The failures have generally demonstrated
a rise in joint resistance followed by heating
exacerbated precipitation of the failure . . .
http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html
The C-152 switches referenced in the article are
are smaller and a cut below the Carling, F-series
switches we've been discussing. I'm familiar with
them but do not know who made them. They ARE a much
less expensive switch but I don't recall that they
used the riveted construction common to the F-series
Carlings. I believe the construction of these switches
looks more like this:
http://aeroelectric.com/Pictures/Switches/Miniature_Rocker_Cutaway.jpg
No rivets, only a pair of moving contacts that
get the benefit of some sliding motion that
tends to improve contact performance compared
to contacts that are simply mashed together. It's
interesting that this design REDUCES number of
metallic joints in current path from 10 down to
5. Pretty slick little switch.
I captured the Canadian failure analysis
and started to sift if for salient data and
conclusions.
http://www.aeroelectric.com/articles/C152_landing_light_switch_failures.pdf
This document is only a small cut above the typical
dark-n-stormy night story found in legacy aviation
rags. These guys did a electron microscope and
spectrometer analysis of the gooey stuff on the
switch to decide that it was probably dust and
drippings from a fitting on the oil pressure gage above.
Availability and applications of whippy tools
does not automatically translate into better
conclusions and remedial actions. As soon as
the writer mentioned "no DC ratings", commented
on circuit breaker performance, and iced the
cake with recommendations for a 70A rated panel
mounted switch, the value of their subsequent analysis
stepped off into the weeds. See:
http://www.aeroelectric.com/articles/Switch_Ratings.pdf
The effects have rippled throughout the aviation
community.
http://www.cessna.org/public/saib/docs/saib-ce-09-42.pdf
http://www.cessna170.org/forums/download/file.php?id=1199&sid=8c7275731a2108c3bdd961d763760192
http://n631s.blogspot.com/2009/09/landing-light-switches.html
Bottom line was that the switches used in these
applications (whether the C-150/152 mini rockers
or the larger Carling F series) have a SERVICE LIFE
that is driven more by CHRONOLOGICAL AGE and ENVIRONMENTAL
CONDITIONS (dripping oil, dust, moisture, etc) combined
with a LACK OF ACTIVITY that tends to burn away corrosion
and contaminants.
Would the failure have been prevented by installation
of a superswitch?
http://aeroelectric.com/Pictures/Switches/Toggle_Switch_with_Mold-Captured_Terminals.jpg
It might have taken longer . . . but the switch would eventually
wear out and need replacement. The mode of failure would
probably have been less exciting. But these switches
are 10x the price of the devices used in tens of thousands
of production airplanes with a very low rate of spectacular
failures.
Light GA aircraft place unusual demands on switches. I've
been suggesting for nearly 20 years that the OBAM aircraft
owner/operator simply shotgun a new set of switches into
the airplane every 5 years or so. Costs $50 in parts
and a couple hours work. Can't do that on a TC aircraft
for less than an indentured servant contract on your
first born. Nonetheless, the spectacular failure of this 20+
year old switch covered with greasy dust was determined
to be "inadequate to the design".
For every one that HAS failed, there are thousands
of switches that have NOT failed. I personally witnessed
extensive "life cycle testing" of new switches in
Cessna's lab . . . a landing light was switched on/off
for thousands of times before any new switch was considered
"adequate to the design". But like all laboratory
testing adventures, stresses on the device during
testing had little relevancy to stresses experienced
in service.
Just how did these investigators EXPECT a switch
to fail? Some switches may fail passive but quite
often, they get hot and smell bad. This report
suggests that a device "adequate to the design"
would last until the airframe was scrapped or at
least not get hot and smell bad. Sorry my friend
but it just doesn't work that way.
Bottom line is that the vast majority of switch
failures being reported to those-who-know-more-about-
airplanes-than-we-do have reached end of life never
fully explored and recognized by the appropriate
recommendations in a service manual. The report was
also noteworthy in citing lack of maintenance
that did not at least clean up and fix leaks of
oil above the switch that most certainly exacerbated
an otherwise "normal" failure.
Well guess what? Cessna responded in the only way
they COULD. Limit service life to the high current
switches to FOUR years . . . not flight hours but
calendar service life. Something that could and
should have happened 30 years ago.
How should WE react to this? Periodic replacement
of highly stressed switches is a good idea. Cheap,
easy, effective. Also, fix oil drips and clean up the
mess . . . but YOU guys do that anyhow . . . right?
Use relays to buffer the current demands on panel
mounted switches? Good . . . but it drives up
parts count and cost of ownership. The switch
WILL probably run to life of the airframe while
you still do periodic replacement of relays.
A simple, low cost preventative maintenance program
based on understanding is the best way to keep
these things from happening in your airplane . . .
like periodic replacement of highly stressed switches.
Having said all that, know that the failures we've
discussed recently about switches and terminals used
to wire those switches are a completely separate
set of events and solutions unrelated to the Canadian
activity that seeded this thread.
Bob . . .
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