Originally Posted by
Colibri49
I know that there is a chip detector spring-loaded toggle switch above the pilots' heads. Moving it one way tests all chip detectors, both for the engines and gearbox. While holding the switch against its spring you see in sequence "Chip1" for engine 1, "Chip2" for engine 2 and "Chip" for the gearbox, all showing on the VMS display panel. This test gets done before every flight. Moving it the other way in flight would put a high voltage electric pulse through the very smallest (hair fine) metal particles on the detectors and burn them off.
With the removal of the magnets from the gearbox after the REDL disaster, I would hope that the slightest particles from wear or spalling in the epicyclic gears at the top of the gearbox would be detected long before any risk could arise. But this doesn't seem to have been the case in Norway. Yet photographic evidence and some opinions suggest that there was wear. In which case why weren't particles being detected during several hours of flight before the rotor head came off? As for missing roller bearings in one of the planet gears, is the metal of the gear-wheels elastic enough to allow rollers to escape during violent deformation of all components?
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Well just to be picky, not "high voltage pulse" since the "chip" will be making a short circuit, across which it is very difficult to generate a "high voltage". In fact is a measured dose of charge (set current for a set time) such that a small chip will be vapourised, a larger chip won't be.
I presume that the body of the chip detector has a high-ish value resistor across it and the test procedure checks that this resistance is in circuit. Open circuit (wires disconnected) would therefore not give a good test, and of course a short circuit (chip present) would leave the light permanently on. A bit like the fire detection system in reverse!