By Helen Dacre and Andrew Prata
The volcanic ash clouds released into the atmosphere by Mount Agung in Indonesia late last year brought back memories of the 2010 eruption of Eyjafjallajökull in Iceland, which caused chaos for holidaymakers in Europe. Airlines operating flights to and from Bali and its neighbouring Indonesian islands were disrupted in late-November last year; research is ongoing to reduce the impact of volcanic eruptions on aviation in the future.
Mount Agung had been showing signs of increased seismic activity since mid-September and throughout October (Figure 1). A new phase began on 21 November when an eruption produced ash and gas up to 12,000 ft (3600 m) above sea level. The height of the ash column increased during 25–28 November; reaching as high as 23,000 ft (7000 m) on 28 November.
Figure 1. Time series of seismic activity for Mount Agung. The y-axis indicates frequency of earthquakes/eruptions per day. Data and graphic courtesy MAGMA Indonesia (https://magma.vsi.esdm.go.id).
On 27 November, ash was advected toward the south-south-west which eventually forced authorities to close Denpasar International Airport, where there had been reports of ash at ground level accumulating on aircraft. Satellite imagery captured glimpses of an ash-rich plume (Figure 2), but it was often obscured by meteorological clouds. Since the eruptions in November, Mount Agung has continued to produce minor puffs of steam and volcanic ash while favourable winds have allowed Denpasar Airport to remain open.
Figure 2. Himawari-8 true colour imagery on 26 November 2017. The true colour imagery was produced following the “hybrid, atmospherically corrected” (HAC) method described by Miller et al. (2016).
Due to the damaging effect of volcanic ash on jet engines – molten ash blocks engine cooling holes causing engines to overheat and shutdown – air travel is restricted in ash contaminated airspace. A prolonged eruption, such as the 2010 Eyjafjallajökull eruption in Iceland that grounded flights across Europe, will lead to inevitable economic damage to Bali and the surrounding area due to lost tourism and productivity. In fact, there are already reports of significant impacts on the tourism industry in Bali due to recent activity at Mount Agung.
The 2010 ash crisis exposed the fragility of air travel and raised questions about the resilience and vulnerability of the world’s critical airspace infrastructure. Since 2010, work in the understanding of ash damage to aircraft has developed rapidly. In particular, aircraft engine manufacturers are now in a much better position to advise on the levels of ash that their engines can safely tolerate.
New research will aid decision-making
In a report published in July 2016, Rolls Royce (the UK’s largest engine manufacturer) outlined new engine susceptibility guidelines, which describe engine tolerance limits in terms of a dosage (i.e. accumulated concentration over time). These guidelines are based on the latest field studies carried out on aircraft engines.
At the University of Reading we are working with Rolls Royce, British Airways and the Civil Aviation Authority (CAA) to develop a tool that is able to calculate the ash dosage encountered by an aircraft along its flight path, and its associated uncertainty, for the first time.
The tool demonstrates a method by which airline operators can calculate ash dosage along time-optimal flight routes during volcanic eruptions. It also provides an assessment of the uncertainty in ash concentration forecasts. In order to represent this uncertainty, we have constructed an ensemble: a set of model realisations created by perturbing various uncertain parameters used in the model. We then use “model agreement maps” to represent the percentage of ensemble members that resulted in an ash concentration above a certain peak concentration threshold. The percentages are then discretised into three categories: less likely (0–10%), likely (10–50%) and very likely (50–100%). This approach gives the stakeholder an appreciation for uncertainty in the model and encourages the use of uncertain information in operational decision-making procedures.
Figure 3 shows an annotated screenshot of the web-tool for a hypothetical eruption of Katla volcano (Iceland) in January 2017. In this example, a peak concentration of 4 mg m-3 was used to construct the model agreement maps. The tool comprises four components: (1) model agreement maps, (2) flight route information, (3) the duration of engine exposure vs. ash concentration (DEvAC) chart (see Clarkson et al. 2016 for details) and (4) the along-flight ash concentration and dosage.
Figure 3. Annotated screenshot of the ash dosage web-tool currently under development at the University of Reading.
The new knowledge developed in the project will be used by the CAA to support strategic decision-making, and will enable new regulations to be developed that are based on the latest understanding of volcanic ash risk to aircraft engines, resulting in a more resilient UK airspace infrastructure.