LONDON

—The dark specter of volcanic eruption brews again beneath Iceland. Early in February, the Icelandic Meteorological Office warned of an increased risk of eruption in an area covering Iceland’s second largest volcano, Bárdarbunga, which dwarfs the Eyjafjallajökull volcano that closed European airspace last April. If, or when, the disruptions happen, will airlines be better prepared to handle them?

Volcanic eruptions can blast debris ranging from micron-sized particles to head-sized boulders up to five miles high, according to Roger Dinius, chief consulting engineer at GE Aviation. The small particles linger. Scientific studies* reveal that a micron-sized particle thrown to 10 km altitude can take 3.3 years to fall to ground, while 50-micron particles take 11.5 hr. to fall.

The ash that lingered generated much debate. Various agencies in the U.S. and Europe met to discuss the known problems caused by volcanic ash—where airworthiness was an issue, and where it wasn’t. The U.K. Meteorological Office wanted to publish maps of ash concentration, which differed from previous approaches to other incidents elsewhere. This required determining what concentrations of ash pose a danger. Ultimately, the large airframe and engine OEMs agreed on a consensus of 2 mg per cubic meter, the upper limit of the industry’s operational experience with sand and dust and the level at which volcanic ash becomes visible.

As affected airspace closed on April 15, 2010, operators were issued this OEM guidance: If an aircraft had flown through contaminated air and the crew hadn’t observed or noticed volcanic ash, then a walk-around inspection was advised before returning the aircraft to service. If ash was noticed inflight, then operators should follow the inspection procedures set out in the aircraft maintenance manual (AMM).

“The big difficulty for operators was that the AMM does not differentiate between exposure to light ash concentrations or dense volcanic ash, and there are no formal guidelines,” notes José Moreira, TAP fleet engineer.

Previous eruptions show that ingestion of volcanic ash by turbofan engines can erode fan blades and airfoils and block cooling holes. Additionally, in fine talcum-powder form, ash can penetrate seals, contaminating engine oil and causing sensor failures. This potential for damage prompted GE, Snecma and CFM International to revise replacement limits for changing fuel and oil filters.

A key difference between airborne sand and volcanic ash is that ash can melt in the engine combustors and stick to the high-pressure turbine nozzle, disturbing and restricting outflow and causing engines to stall and lose power. This is the biggest detriment to airworthiness.

However, ash effects on engines are not linear, Dinius explains. Ash melts at between 800C and 1,200C, so there’s greater potential for melting at higher power settings that raise the engine working temperature. Low power settings minimize this effect.

Further guidelines from major airframe OEMs focused on abrasion damage to the leading edges of wings and stabilizers, and to the intakes of fuselage air ducts.

With many airline customers and no formal maintenance guidelines for volcanic ash events, MROs moved to develop sampling inspection programs. SR Technics, for example, determined that inspecting 10% of its customer’s short-haul aircraft known to have flown through contaminated airspace would reveal any ash-cloud effects. In the event of negative findings, the program would be extended to a wider percentage of the fleet.

According to Thomas Schoch, SR Technics’ head of engine performance engineering, some aircraft were assumed to have flown through the ash area defined by the U.K. Met Office. These aircraft were inspected for abrasions on the airframe, fan blades and engine compressor, as well as for ash deposits on the engine turbine. No abnormalities were found.

A similar approach was taken by Iberia Maintenance, which also established an inspection program that could be escalated for any aircraft potentially exposed to ash.

At TAP, 20-30% of its 55-strong fleet was grounded for some of the airspace closure, but only two of its aircraft were suspected of contact with the ash cloud. Subsequent checks of these aircraft, including airframe inspections and boroscope examinations, showed no evidence of ash.

British Airways conducted a test flight through U.K. airspace three days after it closed with a Boeing 747-400 scheduled to undergo a C check at its maintenance facility at Cardiff. Its positioning flight from Heathrow was extended to test the effects of ash levels in the airspace. It flew at various altitudes during a 2-hr.-46-min. flight. Rob Crew, general manager of BA Engineering Services, says that the aircraft encountered no difficulties and subsequent inspections at Cardiff found no evidence of ash.

Air France KLM, Lufthansa and Air Berlin also conducted test flights during the closure with no conclusive results. Even the U.K. Met Office test flights, launched to map ash concentrations, only encountered ash at lower-than-expected amounts.

The unspecified length of the closure meant few airlines could take advantage of the downtime to clear defects on grounded aircraft. That said, SR Technics still attempted to do this, reducing the open work-list on some aircraft and arranging special ground times. But this activity was limited because operators did not want aircraft grounded by ad hoc maintenance.

Airspace reopened on April 21, and, in the following week, BA undertook detailed airframe and engine inspections on more than 23 aircraft, including internal engine inspections based on OEM guidance and airframe and systems analysis. Sample inspections of an additional 24 engines were also completed as part of a program developed with the engine OEMs. Still, no evidence of ash emerged.

Lessons Learned

In retrospect, aircraft encounters with ash during the April 2010 event were minimal, primarily because the closure was obeyed. A few incidents of volcanic dust on fan blades were reported—but nothing more. The best-learned lesson was how to keep aircraft airworthy in ash-contaminated environments, including covering air intakes on the ground and water washing or vacuum cleaning engines before use.

But minimal contact with the ash cloud meant little was learned about airframe or engine tolerance to volcanic ash. Since then, research has been limited to literature searches and reviews of past tests involving sand and dust ingestion carried out in other world regions. Some military data on dust and dirt ingestion at higher concentrations has been shared to improve insight into engine capabilities and associated failure modes.

“This research has just established what we know,” says GE’s Dinius. “It confirms the consensus: discernable ash concentrations should be avoided.”

To better understand this hazard, ICAO launched its year-long International Volcanic Ash Taskforce in July 2010. It will consider how aircraft might safely fly through low levels of volcanic ash instead of avoiding it.

“Right now, there’s no legitimate need to have rules or regulation on what engines should be expected to tolerate,” says Dinius. “To move limits beyond ‘visual ash’ requires a way to determine ash density levels and thereby maintain a safe operation—and safety remains the priority.”

This spring, European authorities will simulate a volcanic ash cloud to determine how best to maintain air travel. Given the recent warning from Iceland, the timing couldn’t be better.

*Source: “Removal Processes of Volcanic Ash particles from the Atmosphere,” by Gregg Bluth, Michigan Technological University, Bill Rose and Matt Watson