Originally Posted by
Colibri49
I haven't looked at this topic for a few weeks, so hopefully I'm not about to repeat something which might already have been raised by others. As someone with thousands of hours flying the EC225, 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. Two such "fuzz burns" are allowed in flight, after which if it's a gearbox "Chip" warning a landing must be made as soon as possible e.g. on the nearest available helideck offshore. I've only ever had a "Chip 1" in flight and I brought engine 1 back to idle in the cruise, to minimise further possible damage. The particle was found to be negligible and the engine was returned to service. 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?
Lube oil magnetic chip detectors with fuzz suppression circuitry have been standard practice with aircraft engines/gearboxes for many years. The chip detector has magnetic sensor rings with a short gap between them. When enough ferrous metal debris accumulates to bridge the gap it closes a circuit that signals a potential fault. Some systems automatically perform one or two burn-off cycles prior to sending a warning signal.
The ability to distinguish between harmless "nuisance" debris (or fuzz) and larger debris that indicates a more serious problem is very important. Every engine and gearbox continually generates nuisance debris throughout its life from metal surface contacts in rolling element bearings/gears/splines/etc. Bearings/gears/splines tend to generate a greater amount of nuisance debris when new, and much less after they have been run-in.
The sensing end of a magnetic chip detector is commonly positioned so that it has maximum exposure to the passing lube oil flow, and ideally in a location where the denser ferrous metal debris particles naturally tend to separate from the oil flow. Chip detectors are often positioned inside (the upstream side) of a
filter screen at the oil pump pick-ups, which further helps to separate debris particles in the lube oil flow. The advantage of this filter screen and chip detector arrangement is makes things easier for techs to perform an inspection after there is an indication from the chip detector. They can first remove the chip detector to see exactly what size/shape of debris has accumulated on the magnet rings. They can also remove the filter screen to look for larger debris.