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Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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 Posted: Wed Mar 20, 2013 8:10 am Post subject: Cessna Switch Bulletin CE-09-42 |
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The FAA released a Special Airworthiness Information Bulletin CE-09-42
advising that the landing, beacon and taxi light switches have been
overheating in the field resulting in smoke or fire in the cockpit.
This apparently occurs after approximately 4000 operations.
The design of the switches is less than ideal, since it is just a
slider with no over-center snap action.
Since the landing light switch in my Cessna 172 has been in operation
for 43 years and 6500 flight hours, it could have as many as 10000
operations. So I decided that I would replace the switch and tear
apart the old one to investigate its condition.
It was in better shape than I expected. The contacts are large rivets
of copper, still firmly attached to the chassis. The slider plate has
very thick paths of copper - it is not just an etched circuit board.
As the plate slides, the paths make contact with the rivets and that
is where arcing occurred. However, the sliding action of the switch
cleans the contact points, so during use the current is passing
between the contacts and the plates in a different location than where
the arcing occurred.
The contact operation of the switch is very dependent on the strength
of the spring that pushes the plate against the contacts. If the
spring weakened or broke, switch operation would be dangerously
degraded. The spring in my switch was stronger than I would have
expected.
Conveniently, the spring also functions for the detent positions of
the switch. So if one of these switches seems loose and doesn't click
into position, it is time to replace the switch.
The switch I replaced is Cessna 0713035-1 and cost me $45 Canadian.
Jeff Page
Dream Aircraft Tundra #10
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sprocket(at)vx-aviation.c
Guest
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| Posted: Wed Mar 20, 2013 1:31 pm Post subject: Cessna Switch Bulletin CE-09-42 |
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These are the same circuit functions that failed using Carling switches
after less than 100 operations in my RV-9A. These failures were well
documented on this list several years ago and are in the archives. I've
eliminated switches carrying loads in my Harmon Rocket and I am using relays
controlled by low-current switches. The relays can be replaced when (not
if) they fail in less than 30 seconds, even in flight.
The Vertical Power products do this with automotive-style electronic
switches.
Any electromechanical device, including switches, relays, breakers and
contactors have a lifetime and plans should be made to provide service
access for replacement. The promise of electronic (solid state)
breaker/relays is that there is no practical wear-out mechanism. When a
failure does occur, however, it's quite a bit more complicated to repair.
There are two philosophies at play here: Design so that it never breaks,
and don't worry about serviceability; or assume that it will break and allow
easy service access and low cost maintenance. Sometimes, however, in our
amateur-builts, we get it backwards... sigh.
V
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