In EASA land the Cessna landing / taxi light switches have to be replaced every x years which is nonsense
I have had a landing light switch go up in flames on a C172, so changing them from time to time may not be a bad idea. It does startle you when you see smoke starting to pour out from underneath your instrument panel.
Peter I checked for mis-wiring – nothing goes poof when I pull it.
This could be still a redundant power supply for example. Have you measured it with a DVM to be sure it stays on? This is highly uncommon, but if it does you really should have it replaced ASAP. What system is it connected to according to the labeling?
Achim I reckon it shouldn’t be too hard to dismantle and clean oder?
A circuit breaker is typical not a part which can be opened up. Cleaning corroded or pitted contacts is NEVER a good idea for a permanent fix. If this is done of a switch or relay, it should be renewed as soon as possible. As the protective coating will be damaged by sandpaper for example.
Circuit breakers have a limited life span. I would replace, they are not expensive. I bet that a large percentage of GA circuit breakers would never trigger even if the avionics were to go up in flames.
Circuit breakers can easily handle the double output for a few seconds. On a real short these will go out. It is not common problem that CB’s wont trip.
There is one common problem with circuit breakers which is caused by corrosion / high resistance. After a while the resistance gets to high and the circuit breaker becomes a voltage divider. This is a problem that is often overlooked, especially when protecting bulbs. One takes out bulb, bulb seems ok, measure voltage on contact (with bulb removed). 12 or 24 Volt available (no voltage drop over circuitbreaker as there is no load). So bulb gets renewed and systems still inoperative. Measurement on these should always be with load attached.
In EASA land the Cessna landing / taxi light switches have to be replaced every x years which is nonsense
This is not true, this is also an FAA SAIB based on a bulletin from Cessna: Link to bulletin
I can confirm it is quite common for these switches to melt the plastic housing. Actually these switches are overloaded. Again if their resistance increases due to arcing more heat will be generated inside the switch, leading to melt the plastic housing.
but the circuit breakers can remain there for 50 years without triggering once or being tested. I plead guilty to having a bunch of 1979 circuit breakers that never triggered once
During maintenance pull-able circuit breakers get regulary pulled. This is also suggest (without load) as standard practice of wiring inspections, this keeps the contacts clean.
According to labeling it’s connected to the EHSI. Haven’t taken a voltage meter to it yet.
During maintenance pull-able circuit breakers get regulary pulled. This is also suggest (without load) as standard practice of wiring inspections, this keeps the contacts clean.
In the 1970s aircraft, most CBs were of the non pullable type. Hard to test them without causing a short circuit. I thought about replacing them all with pullable CBs but that is a lot of work and Euros…
According to labeling it’s connected to the EHSI. Haven’t taken a voltage meter to it yet.
Nothing too important, eh ?
I would get this sorted out earliest.
In the “classical” slaved compass system like the KCS-55, the CB labelled “HSI” powers the entire system i.e. the KG102A directional gyro and that in turn powers the KI525 HSI, the KMT112 fluxgate, the KA55 slaving test switch and 1 or 2 other bits. If you have an RMI that might have a separate CB.
If you have an EHSI like a Sandel then you should have a separate CB just for that – example (search for “5A circuit breaker”)
Loads of GA planes are mis-wired somewhere. It is not a trivial thing to avoid because most mods are done to old planes (average GA piston age is 25-30 years) and the installer doesn’t get an opportunity to rip out old wiring and re-do it all. Most users never know because most never use the full functionality, and most maintenance companies never check the CBs.
