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Starter Contactor location on a Long-EZ

 
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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 28, 2018 10:46 am    Post subject: Starter Contactor location on a Long-EZ Reply with quote

At 05:43 AM 1/27/2018, you wrote:
Quote:
Bob,
There is an account of the incident on Brian Deford’s website located at http://deford.com/cozy/fire.html.

I am also considering installing the starter solenoid on the inside (cabin side) of the firewall. I also plan on protecting alternator B leads with fusible links sized to protect the wire ( larger than the alternator capacity). In this way, any wire crossing the firewall will be protected.

"The close ups of the engine/firewall show where the fire originated, but have not yielded any clues to me on the exact component that failed. I'm more convinced as time has gone on and I've had a chance to think about the failure modes that a failure of one of the contactors, particularly the starter contactor, is the most likely cause of the electrical fire and not a short circuit of the cable to the firewall. However, it can likely never be proved."

It's unfortunate that the remains around the fire's origin
were not examined more closely. I cannot imagine how a contactor
would fail and release energy to ignite adjacent flammables.

Correct me if in error. It seems that airplanes of these
materials have a structurally composite firewall with a
thin, steel sheet on the engine side. If point of origin
was the fat-wire's firewall penetration, arc damage to the
wire's stranding at the edge of the firewall sheet would be
a tell.

He said the initial smoke was light colored which does
suggest burning insulation . . . but changed to black
no doubt when structural plastics began to burn. It
is VERY difficult to burn the insulation on a fat wire,
that's why light aircraft generally do not 'protect'
fat wires . . . they don't develop the kind of hard
fault necessary conduct current that would heat the
wire to levels that damage the insulation.

Fat wires that rub grounded airframe are 'soft' faults
that arc away and generally erodes the airframe with
only localized heating. In this case, the localized
heating may have been sandwiched tightly with the
composite structure of the firewall which became
initiation-fuel for what followed.

Beech failure analysis files include an incident in
New Mexico on a King Air that experienced sudden
disconnect of elevator cables while on final. The
pilot did a go around and managed to fly the airplane
using only pitch trim and accomplished a safe landing.

Inspection revealed that a windshield de-ice inverter's
6AWG DC power wires were misplaced during a maintenance
event and had been rubbing on the elevator cable. Motion
of the cable eventually wore through the insulation and
intermittent arcing ensued. I forget how many flight
hours between the service and the incident . . . but it
was a lot! It took a long time to eat through the strands
of the control cable . . . but the copper wire was hardly
damaged, nor was there a lot of damage to the wire's
insulation. No smoke was detected by crew.

This but one example of a 'soft' fault that did not
open the breaker protecting the wire but ultimately
parted the elevator control cable.

I'm unable to hypothesize a chain of events that
migrated from a stable condition at the time of battery
removal and a high-energy release of battery energy
within minutes of battery replacement. If the point
of origin was at the edge of the firewall sheet, this
may well have been another example of soft-fault that
had been going on for some time but, like those elevator
cables, too small to be noticed.

Brian's story is certainly sad and distressing to contemplate.
But I don't think there is enough data to suggest that
any re-arrangement of hardware will make this already rare
event even less probable. No process provides more confidence
in a design than on-the-fly-failure-mode- effects-analysis
. . . as every part is installed, be constantly aware of
ways that part can be compromised with unhappy if not dangerous
consequences.

Proximity of combustibles (firewall honeycomb?) with
potential arc generators (firewall steel sheet) are
combinations worthy of extra attention.

Being a low risk pilot calls for situational awareness
and staying ahead of the airplane. Low risk fabrication
is no different.




Bob . . .


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Airdog77



Joined: 24 Nov 2013
Posts: 76
Location: Northern Virginia

PostPosted: Mon Jan 29, 2018 10:25 am    Post subject: Re: Starter Contactor location on a Long-EZ Reply with quote

Bob,

Thanks for your analysis of this incident. I always learn a ton through you and others on this forum!

Just for clarification, our firewalls are 1/4" Finnish birch plywood with a ply or two each side of fiberglass. Then the hot side gets a layer of fiberfrax covered with a thin layer (0.016-0.025") of either stainless steel or aluminum.

As with researching any subject, there are invariably other characteristics that appear and reveal themselves and add to the original topic being researched. I am strongly leaning towards placing the starter contactor on the cold side of the firewall, but not in response to Brian's incident, but more so as a matter of logistical ease for my configuration that became clear to me the more I researched this issue. To be certain, by doing this I'm not stating an overriding concern for getting a high current cable through the firewall, it is again merely one of logistics and space.

I will say --and not that this is my reasoning for a go/no-go decision-- that I've learned over these past few days that placing the starter contactor on the cold side of the firewall is a VERY common practice in the canard community --again, for whatever reasons individual builders' have. Actually, surprisingly common in fact.

Perhaps some of that has to do with the inherent design of canards, wiring and space logistics (my overarching reason for going this route), tribal culture, or just individual preference.

As you have alluded to many times, this flexibility is one of the outstanding characteristics of the OBAM world... to adjust when necessary to optimize one's build.

Regards,
Wade


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Airdog
Wade Parton
Building Long-EZ N916WP
www.longezpush.com
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