The air transport industry was caught off guard in April when huge plumes of ash from Iceland’s Eyjafjallajökull volcano effectively shut down flying in Europe for a week, stranding thousands of passengers and draining at least $1 billion from air carriers. Now, several months later the fact that engineers are still having a hard time understanding precisely how volcanic ash damages an aircraft and its engines lends weight to the view that the industry was woefully ill-prepared for this problem.
The general principles are well known–particles cause abrasion everywhere and can create layers of sediment on engine blades, not forgetting corrosion. But understanding the physics of a phenomenon calls for complex equations and number-crunching.
“If the industry had spent on R&D 10 percent of what it lost during the volcano crisis, we would have a much better knowledge of aircraft behavior in small ash concentrations–they could even be certified to fly in such small concentrations,” Jean-Pierre Mesure, deputy technical director for aircraft airworthiness and operations with French civil aviation authority DGAC, told AIN. He explained that ash particles that stay in the atmosphere are the smallest ones–the bigger, heavier ones fall.
The size of these airborne ash particles is in the order of a few micrometers. Most of them are made of silica (SiO2), a very hard material, and have sharp edges. “They are very abrasive,” Mesure said.
Like Rubbing with Sandpaper
Flying into silica-contaminated air can damage air intakes, turbofan blades, propellers, wings, the windshield and antennas–like rubbing them with sandpaper.
In addition, these particles are electrostatic and discharge on to
the aircraft itself, causing electromagnetic interference, possibly affecting radio communications.
In an engine, the main damage caused by the ash is erosion.
Fan and compressor blades are especially affected. Compressor performance is degraded, Michel de Gliniasty, the general scientific director of French aerospace research office Onera, told AIN. As a consequence, the combustor is not fed properly with air and the surge margin decreases.
Silica’s fusion point is 1,400-degrees Celsius–it melts in the combustor. It can re-solidify into glass on high-pressure turbine blades, creating unbalance. It can simultaneously obstruct the tiny holes that are part of the blades’ cooling system. As blade surface temperature increases, the crew has to pull the throttle back. In some cases, glass can have disappeared from the turbine by the time the aircraft is on the ground so technicians cannot see the problem immediately. Or, flowing down into the low-pressure turbine, silica can re-solidify there and erode the blades.
Silica can even re-solidify in the combustor, causing a power surge. The engine can flame out and relight successively several times in a short period. This can have a favorable effect–breaking the layer of solidified silica, de Gliniasty noted. Another consequence can be the contamination of bleed air, which itself is used in the air pressurization system.
Some other particles are made of silicate–a family of compounds that includes silicon, oxygen and, for example, metal. “Silicate can melt at lower temperatures. This can be right at the end of the high-pressure compressor, when temperature reaches 700-degrees Celsius. This leads to the risk of partly blocking air injection into the combustor,” de Gliniasty explained.
Ash can cause a variety of problems. Should an aircraft suddenly enter a dense ash cloud, engines could choke because of a lack of oxygen. Corrosion can be a subsequent effect due to gases from the eruption reconstituting themselves into the very corrosive compound H2S4.
There is great uncertainty as to the exact connection between a certain level of air contamination by volcanic ash and the resulting level of damage to an aircraft. Sometimes entering the ash cloud causes no more damage than leaving black sediment on fan blades, wing leading edges and the nose. However, corrosion can appear later.
Inspections made after the recent event involved engines that had flown into slightly contaminated air. Each inspection takes six hours using a borescope while slowly spinning the engine rotors. A lot of engines from different operators were monitored, Mesure said, but the technicians found nothing–just a small amount of dust but no sediment or damage. However, despite the time spent on the inspections, some areas at the root of the blades were not examined. This meant that the precise ash concentration and size of particles into which the aircraft had flown remained unknown.
In the U.S, Honeywell has borescoped the TPE331-5 turboprop engines removed from a Dornier Do 228 that flew 10 hours in the heart of the Icelandic ash cloud and 22 hours in the outer zone. The aircraft was operated by the UK’s National Environment Research Council. One of the engines was run in a test cell late in June, while the other was torn down for inspection. Honeywell plans to release a full report on the condition of the engines by the end of this month.
In mid-April, the authorities asked the engine makers, “What can your engines tolerate?” The answer was simple, according to Mesure: “We do not know; we have never tried.” In fact, there is no airworthiness specification for volcanic ash.
Nevertheless, a working group was formed, representing airworthiness authorities, navigation service providers, weather experts and airlines–60 organizations in total. The assignment was to define when flights could resume.
The group came back quickly with a few numbers. The main one was a threshold of two milligrams per cubic meter. Below that, it was estimated that neither hazard nor dissuasive economic impact, such as too many inspections or too much wear, could be feared.
Concentration models were run again and scientific aircraft used lidars (laser-based instruments that can measure concentrations), ran particle counters and took air samples. Airliners flew without passengers and were inspected after. “We saw that a lot of areas were below the two-milligram threshold,” Mesure said. Therefore, the authorities decided to allow flying in these areas and a fleet monitoring program was created.
Two milligrams per cubic meter level of contamination looks like a tiny concentration, but you need to consider the air flow through the engine. A CFM56 engine, for example, sucks in 300 cubic meters (10,000 cu ft) of air per second, according to Snecma. Optimizing turbofans in terms of noise and fuel burn requires high-bypass ratios. This means a lot of air, and therefore a lot of particles, go through the engine. In addition, cooled high-pressure turbine blades withstand higher temperatures, which is good for fuel burn, but makes them more vulnerable.
Could particle separators be useful? All engines have a centrifugal separator, de Gliniasty said. On the ground, gills (located just downstream the low-pressure compressor) open to blow particles from the primary airstream to the bypass airstream. Ideally, this principle could be used in flight but the Onera expert pointed out that this would drastically degrade engine efficiency. Mesure was negative, too. “This works with sand but not with such small particles, which have too little inertia,” he said.
Volcanic ash is a rare encounter in the sky but air transport cannot ignore it or say it is too unusual to take it into account. In late May and early June, air traffic in New Caledonia was disrupted by a volcano located in Vanuatu. Records show 90 to 100 aircraft have suffered from encountering ash in the last 30 years; these small incidents caused variable damage and some diversions.
In 1982 and 1989, two Boeing 747s underwent flameouts of all four engines before their crews managed to relight them. The 1982 serious incident took place in Indonesia as a consequence of the Galunggung volcanic eruption. The 1989 incident happened near Anchorage, Alaska, during an eruption of Mount Redoubt. A Snecma engineer suggested that air transport will have to suffer an actual accident due to volcanic ash if action is to be taken.