Wanted: hard facts on how volcanic ash affects jets

By Paul Marks How much volcanic ash can an aircraft safely fly through? If you listen to the organisations dealing with flight safety, you may believe it’s a known quantity, backed by scientific data. Far from it. The fact is, no one has rigorously tested how different jet engines cope with different concentrations of volcanic ash. That has implications for air safety if tolerance levels are set too high, and for the economics of aviation (and the wider economy) if they are set too low. This week the UK’s Civil Aviation Authority (CAA) – which has been in the vanguard of Europe’s efforts to cope with ash from Iceland’s Eyjafjallajökull volcano – upped the ash tolerance limits for some aircraft to 4 milligrams of ash per cubic metre of air. Just under a month ago, however, it said 2 milligrams per cubic metre was the limit. And before that, the International Civil Aviation Organization had decreed aircraft were not permitted to fly through volcanic ash at all. “The justifications for increasing the ash tolerance levels don’t seem to be based on any real scientific measurements. It’s alarming,” says Fred Prata, an atmospheric physicist at the Norwegian Institute for Air Research in Kjeller, near Oslo. So why is the ash tolerance level so movable a feast? One factor is that “safe” concentrations are being calculated based on previous encounters with ash. “The regulator is starting with a very small tolerance concentration and then either tightening it or relaxing it after engines flying through those ash concentrations have been inspected,” says Riti Singh, an aero engine expert at Cranfield University, UK. The CAA took its decision based on its knowledge of past incidents – with very high ash concentrations around 2 grams per cubic metre – and inspections or 83 aircraft that have encountered ash, either visibly or by smelling sulphur in the cockpit, at altitude between 18 April and 18 May. It used UK Met Office models to estimate the levels of ash the craft were exposed to. No ash damage was found on any of the aircraft, says CAA spokesman Jonathan Nicholson. The new limit currently applies only to a certain type of airliner made by Bombardier of Canada, but the CAA says it will extend it to others once it has sufficient data from aircraft and engine makers to be sure they are safe. But this doesn’t establish the upper safety limit for ash density. One way to achieve that, Prata believes, may be through a programme of ground-based lab tests, in which increasing concentrations of ash are fed into engines. But this is not straightforward: ground-based tests cannot mimic atmospheric flying conditions, while the handful of altitude test chambers that exist are not designed to cope with ash, says Singh. He advocates running simpler tests using only those components that are likely to be affected by ash – such as turbine blades. “You’d then set up test conditions that mimic the real conditions at altitude. That might work,” he says. Setting safe levels will be a challenge. But somewhere between full engine tests at flight air pressures, and more selective tests on critical engine components, there’s an evidence base waiting to get airborne. More on these topics:
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