How much volcanic ash can an aircraft safely fly through? If you listen to the organisations dealing with flight safety, you may believe it鈥檚 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鈥檚 (CAA) 鈥� which has been in the vanguard of Europe鈥檚 efforts to cope with ash from Iceland鈥檚 Eyjafjallaj枚kull volcano 鈥� upped the ash tolerance limits for some aircraft to 4聽milligrams of ash per cubic metre of air. Just under a month ago, however, it said 2聽milligrams per cubic metre was the limit. And before that, the had decreed aircraft were not permitted to fly through volcanic ash at all.
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鈥淭he justifications for increasing the ash tolerance levels don鈥檛 seem to be based on any real scientific measurements. It鈥檚 alarming,鈥� says , an atmospheric physicist at the Norwegian Institute for Air Research in Kjeller, near Oslo.
Fly it and see
So why is the ash tolerance level so movable a feast? One factor is that 鈥渟afe鈥� concentrations are being calculated based on previous encounters with ash.
鈥淭he 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,鈥� says , an aero engine expert at Cranfield University, UK.
The CAA took its decision based on its knowledge of past incidents 鈥� with very high ash concentrations around 2聽grams per cubic metre 鈥� 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.
Where鈥檚 the limit?
But this doesn鈥檛 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 鈥� such as turbine blades. 鈥淵ou鈥檇 then set up test conditions that mimic the real conditions at altitude. That might work,鈥� 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鈥檚 an evidence base waiting to get airborne.
