Ash does not blast.

Nearly a week into the volcanic ash crisis plaguing swaths of Europe, passengers and airlines alike are starting to tire of the restricted airspace. The haunting cloud drifting thousands of feet above Earth’s surface is often invisible to the naked eye both at ground level and high into the reaches of the troposphere, causing many to wonder how this material could impact a flight. Could all of these microscopic particles of ash really be that big of problem?

Yes, and in many ways.

Large volumes of volcanic ash have an obvious effect on flight performance. Any particulate getting into cooling holes will cause the engine pressure and temperature to increase, dropping efficiency and potentially causing serious issues inside of the engine. This failure mechanisms poses an immediate and large threat to aircraft safety and is the primary situation that airlines are trying to avoid.

But even small volumes — parts per million of the material — can have a long-term detrimental effect on engine performance.

Typical engine combustors operate at extremely high temperatures — hot enough to melt most metals — and the materials used in each component are specially designed to withstand this heat. The single-crystal turbine blades used in the fabrication of commercial engines often are exposed to temperatures well over 2500°F, and because of this are coated with a special Thermal Barrier Coating (TBC) to prevent overheating. In short, the TBCs prevent the turbine blades from melting.

Part of what helps the TBCs do their job is their microstructure. Instead of being fully crystalline, solid materials like the compressor blades, most coatings are porous and less dense, preventing them from transferring too much heat. But this also subjects them to infiltration by foreign particles like calcium magnesium alumino silicate (aka CMAS, formed in and near sand particles) or volcanic ash.

Over time, these embedded particles fill in the pores of the TBC, and they remain in the microstructure as the engine gets hot and cold over and over again. Each time the engine heats and cools, this thermal cycling creates strain between the two materials, and like a sealed bottle of water in the freezer, the container eventually will burst. And once the TBC breaks down, heat can flow freely to the compressor blades, potentially melting a section and causing a catastrophic failure.

Depending on the volume of ash or particle ingested, this can happen quickly over several engine cycles or over a long term of repeated use. But the result is the same: failure during operation.

TBC degradation is only one mechanism for long term failure. Engineers also need to consider abrasion, creep and a host of other materials problems that can result from interaction between volcanic ash and highly specialized engine components.

As you can probably guess, this is partially why the European Aviation Safety Agency is being so cautious with easing restrictions on airspace — many of the long term effects of volcanic ash (which varies in composition by geographic location) on engine components are unknown. Only with time, testing and weeks of analysis will the full impact of these materials be know. Until then, we’re going to have to wait for the skies to clear.

Read more about the short term effects of volcanic ash at popsci.

Check out Alaska Airlines’ operating procedure near ash here.

Boeing’s comprehensive study on engine performance in ash clouds can be found here.

via Gadling