Landing Lights CB tripped

T

Todd LeClair

Filing Flight Plan
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Todd LeClair
While doing night training the CB tripped for the landing lights. It re set on the ground, then tripped again. Prior to annual, one of the landing lights was not working. The bulb was replaced.
I think this is the first time it has been flown at night since.
Any ideas or recommendations on where or how to start trouble shooting this?
'65 182 H Skylane
 
I'd start with a visual inspection. Pay special attention to the area around where the bulbs are and then where the landing light switch is. Look for frayed insulation or any way the terminals could possibly touch the metal aircraft structure. The problem with these sort of things is often the fault is intermittent and doing things like taking the bulbs out may move things around enough to (temporarily) clear the fault.

Take the bulbs out, landing light switch off. At the fixture, one pin should be connected to the airframe ground. The resistance there should be close to 0. The other pin should be close to infinite. If zero you've definitely got a problem. Now you're in a divide and conquer mode. If there are any junctions in the wire from the switch (including the switch), disconnect those. Test each again to see if you are looking at the faulted segment. If you find it, examine the entire segment carefully, or just replace it.
 
Is the light on the wing or front cowl? Either way, you should be able to inspect a significant portion of the wiring to determine its age/condition.
 
1965? Very likely the breaker's internal contacts are oxidized from age and the small amount of heating caused by the bimetal stip in the breaker. Oxidation causes resistance and extra heating. Another source of heat is oxidized crimps between the wires and terminals at the breaker. Any extra heating of the breaker will cause it to trip far sooner than it should. Some of Cessna's airplanes of that vintage had breakers whose ratings were much too close to the actual amperage being drawn by the light, so age and oxidation eventually caused them to trip unnecessarily. A really good example was the single 4522 lamp that drew 19 amps at the 14-volt charge level, on a 20-amp breaker.
 
Since it just had an annual,I would start with the breake,is it original equipment? Had a problem on a 172 changed out the breaker no further problems.
 
I'd also check the actual bulb installed if it started happening when the bulb was replaced. Someone put a 250W in there would cause a problem.
 
Wouldn’t the smartest thing (after a decent visual inspection) be to simply measure the current draw?

Don’t make this harder than it is. Either the breaker is tripping because it’s faulty, or it’s tripping because there’s an over-current. Figure out which before proceeding.

Only happen in the air or also on the ground?
 
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