I am watching the scramble in Europe as an Icelandic volcano with an unspellable name has emerged from beneath a glacier and belched out a cloud of ash, dust and noxious fumes. It has been proven by several penetrations of volcanic ash plumes that jet engines find the contents of the cloud unappealing. Losing four engines on a B747 some years ago was a convincer, and it is fortunate that they were able to achieve restarts at much lower altitudes. So when the metrological services shut down flying in North Western Europe because of the presence of the ash, none of the airlines concerned let out a complaint. Apparently, a Finnish FA-18 fighter was sent up to see if it was really there and the pictures I have seen on the Internet of the turbine blades made the airlines seem very prudent indeed. We can only hope the volcano will subside before even more financial damage is inflicted on our fragile industry.
I had previously seen damage from ash in its effect of windshields and even cabin windows, The effects of running into almost microscopic ash particles at Mach 0.80 took awhile to appear but ultimately the frosting became apparent. The front windshields did better because they have a thin glass overlay, but eyebrows and side windows lacked this and became opaque. I was rather surprised that the passenger windows were also affected since they don’t face forward into the breeze. However, there were also other things affecting them. A sales and service vice president complained to me that maintenance was not keeping up the cabin windows and they were getting badly scratched, he said, by volcanic ash. He cited passenger complaints about them.
I suggested that possibly there were other sources for the scratching than volcanic ash. In particular the cleaners who were under his control were dry washing the aircraft using long-handled mops. I believed that this was scratching the windows. He vehemently denied that his boys would do that as they had been trained not to do it. All scratches on cabin windows were from volcanic ash! I became unpopular when I noted that there were indeed small horizontal scratches that could be attributed to the ash but that aircraft did not fly straight up and down and so the large vertical scratches were from his cleaners.
Of course it was so. You try working a 15-foot long mopstick with a holder and mop head on its end up in the air while making polishing movements. After a while the mop comes down while you rest for a moment and the mophead flops onto the ramp. There it picks up its fresh load of sand and grit and then up it goes for its next session of “cleaning” the airplane. One rule of management should be is to walk around and look at what is going on. The higher you get, the more the flunkies who will surround you will tell you only what you want to hear.
The volcanic ash has a different effect on the internals of an engine. Yes, there can be some eroding of compressor blades, but it is further back that the real harm takes place.
Ash has a lower melting point than, say, sand. When it gets sucked into the engine it travels back through the compressor and enters the “hot section.” It will melt and fly back where it will encounter nozzle guide vanes and turbine blades. These are hot and the sticky glob of ash will adhere to them. If Messrs GR, Pratt or Rolls had wanted ceramic-coated turbine blades they would have built them in. Suffice to say that modern engine blade designs are fine tuned for efficiency. Any modification to their shape causes a drastic drop in that.
There is also the clogging factor. These blades have air passages in them and little outlets that allow a layer of cooler air to protect the blade from the direct hot gas going through.
Clog any of those holes either by a grain of ash inside or a globule of melted ash on the outside and you will get a hot spot. Severe blade erosion can then result and maybe even loss of a blade. So all in all, it is better to avoid ash than to have to deal with the results of its ingestion.
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.