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? 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?
I still feel on balance that the cause was detachment of one of the gearbox suspension rods, for as yet unknown reasons. Sudden shattering of gearbox internal components without any prior signs of wear seems beyond belief.
what we do know is this:
TURØY - Helikopteret avbrøt flygning tre dager før ulykken
There where some "indication lamp" that was providing several messages
in the days before the accident causing the flight to be canceled and the
helicopter to return to base immediately.
After that it was also occuring during test flights, after which
a "component" was changed
if this is coupled to detecting metal particles is too early to say
and purely speculation without having the first hand info.
In which case why weren't particles being detected during several hours of flight before the rotor head came off?
It could be days before, hours or it could be minutes.
Until we know the facts it is impossible to answer that question.
Still to me the last conclusion in your post is a bit too soon.
My personal opinion is that 3 rollers missing from the planetary gear isnt
enough to kill the gearbox even if the roller would get stuck between the carrier and the sun gear.
but it would probably cause surface deterioration that would significantly accelerate the wear on all components in the gearbox.
some signs of wear can be seen on the components. some other components look to be damaged upon impact. The rollers are inserted by squeezing
the outer ring on normal bearings.
if you squeeze a ring gear it will break, so probably there are other methods
of installing the rollers without squeezing the outer ring, and thus
it must also be possible that the rollers can escape.
but honestly i find that as well not very probable.
I guess a part of the analysis is to predict/simulate if there was shearing of the gearbox during rotation. There are many methods to do this
could be a reason that AIBN was doing extensive search for parts in the area
after the accident.
I guess as well there are pilot voice recorder readings to say
how eventual indications lamps or error messages was interpreted during the flight before the accident