Compressors get contaminated with the very polluted air they work on. If a turboprop, they are sucking in air right behind an propeller that uses oil to change its pitch. These occasionally leak. Then too, turboshaft engines in helicopters are generally smaller than the big guys pushing the heavy iron. Small compressors suffer more from contamination than big ones, although all are hurt by it.
The Lockheed Electra had an engine that was up to 4000 horsepower. It was relatively small in diameter. After some time in service, you could pull a bleed valve and scrape a layer of crud from it and see the blades with buildups. A regular part of its maintenance was to wash the compressor. Wash consisted of spraying water into the compressor while motoring the engine. A little special cleaner was added to cut oily stuff, followed by a straight water rinse. The power recovery was usually very impressive.
Sometimes, though, the buildup inside was a hardier stuff, in which case you could use walnut shells. These came, already cracked and ground into granules. A special rig was fitted to the intake and with the engine running, you allowed it to suck in the walnut shell granules. This produced a wonderful smell of roasting nuts but did clean off the crud. All well and good, but there was a limit on the number of times you could walnut blast an engine. They came up with a chemical wash. It utilized some horrid gunk that was an excellent paint stripper and would eat your skin, too, if left there for awhile. It was applied liker the water wash. The engine had to be prepared, certain bleed air lines capped off, etc. Then the gunk was sprayed in through a fitting mounted in the inlet while the engine was motored. After a short wait, the engine was water-washed and, boy, did some tar-like black stuff run back out of the tailpipe.
This cleaner was very effective, too effective in fact, as the mist from the tailpipe while it was being applied while the engine was motored would eat the paint off the rear fuselage and tail. So a couple of us had to stand there with wash-water hoses and spray the fuselage and empennage while the gunk was being applied to the engine. You did your best to avoid the fog coming out the back of the engine, as it stung very badly. It was not good to breathe in and softened the plastic frames on my eyeglasses. It did work, though, and I can testify that the compressor was clean as brand new. After fuming out part of the hangar ramp next door, though, and a lot of complaints by us about its noxious qualities, they stopped using it.
Honeywell will analyze the effects of ash ingestion in two of its TPE331 turboprop engines.
Light-Sport Aircraft Employed to Assess Problems for Airliners.