I had an incident in my aircraft last weekend. The issue involved a
battery cable and my lack of awareness of the real corrosive power of
electrical potential.
Paradoxically, headset noise was the first sign of trouble. A background
drone shadowed the engine frequency in flight.
In an effort to resolve the issue, I pulled the panel in a fruitless
search for broken earth cables, wiring issues and suchlike. Thereafter,
the engine was reluctant to start due to insufficient cranking power.
'What had I done ??'
On my next starting attempt, the engine once again cranked reluctantly.
This was immediately followed by the appearance of frightening amounts
of smoke from behind the panel !
A dramatic evacuation followed and the fire extinguisher was made ready.
Fortunately, the smoke cleared in less than a minute.
Cutting to the chase, this problem arose from corrosion.
Specifically, corrosion on the inside face of a battery earth cable lug
at the battery attachment point. (seems obvious with hindsight, doesn't
it !!)
On the negative battery terminal, I have:
1/ A large gauge, starter earth cable. (Oxidised, on closer inspection)
2/ A smaller gauge 'panel earth'. (Functioning normally).
During cranking, because of significantly increased electrical
resistance on starter earth, most of the negative charge was re-routed
and bourn by 'panel earth'. Consequently, this melted insulation on thin
earth wiring behind the panel, which was completely unable to bear the
electrical load of the starter.
Once repairs were completed, the headsets too, performed normally.
Indeed it's worth noting that they were responsible for 'red flagging'
the problem in the first place.
Obviously, battery earth will now receive regular close attention and
will be thoroughly corrosion protected. This is my lesson.
Alan
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