Flushing the box is not a good way to get rid of crud. An immediate scrupulous cleaning and inspection of the box might have been a good idea. Here is why: Bearing and gear debris is usually high hardness. Bearing debris is then imbedded in soft surfaces and then wears away the hard surface running against it. Bearing and gear debris does not imbed in other bearings or gear teeth, but it microscopically dents these rolling contact surfaces. This looks like etching of the surfaces, which accelerates wear and shortens life of the surfaces by increasing the rate of surface fatigue (wears the hard layer down faster), opening clearances and making more hard debris which accelerates the process. If the dust you describe is aluminum, a view under a microscope with show these tiny chips to be either bright or dull. The bright ones are new and likely quite soft, which means the box is still making these chips. Dull means it has oxidized, and it is now a nice hard polishing abrasive, which will embed in soft surfaces and/or wear other surfaces it rubs on.
From the sounds of things, this box ran more than 10 hours with debris circulating and maybe making more debris. Even if you can clean the box, replace the parts that appear to be damaged, and run it some more, the hard debris most likely shortened the life of the gears, bearings, seals, etc in the box, governor, and prop. Any loosening of bearings takes the gears out of conjugate action, and then shortens gear, bearing, and shaft lives. Trying to fly it fixed pitch sounds to this transmission engineer like a seriously short term end game.
Given my view of the gearbox, if someone I know just had to fly it, I would advise flights to remain within the glide cone to an airport.
If it were mine, I would do four things:
- Remove the existing gearbox and any plumbing or hoses, install a new gearbox with new plumbing;
- Send the prop and governor to a prop shop, and have them take samples of oil, contamination, and silt, then clean and IRAN both;
- Find a way to filter the oil as it is run - 25 micron is not too fine, we filter log splitter hydraulics at 5 micron;
- Inspect oil frequently. That includes looking at the dipstick, doing oil analyses, inspecting your filter, you-name-it.
Why replace the plumbing? In the automotive transmission world, we learned the hard way that significant tranny damage deposits debris in the cooler and lines that is almost impossible to flush out, but that goes back into circulation later and precipitates a repeat tranny failure. Starting in the mid 1990's, my employer instituted cooler and line replacement if ANY metallic fines were found in a failed gearbox - repeat failures just about vanished once that became standard. So, yeah, replace any plumbing in gearbox, governor, and prop circuit.
I would also not be able to resist tearing down the removed gear box. Take samples of the silt in the box, governor, and prop for analysis. Gear faces, bearing surfaces, splines, and seals would get the look with magnification and the sun or strong light over my shoulder on to the parts, and maybe under a binocular microscope too. Metal surfaces imbedded or damaged by hard debris will look etched or roughened and may sparkle in the sun or a single point light source. Seals or journal bearings may have debris imbedded or a lot of wear. If the particles are very fine and left over from earlier issues, the parts may look fine, but I would still be very skeptical of their acceptability.
I would also make note of the materials in all damaged surfaces and then send oil and contamination off for analysis. Wear metals will reflect the surfaces damaged. On top of steel and aluminum, if there are journal bearings, expect aluminum, tin, zinc, copper, lead. Other materials might be dust from cleaning abrasives. If you can reassemble with all known good parts, go ahead. I dearly hope you will not need it.
Billski