Volcanic ash: To fly or not to fly?

In this report we analyse the technical details behind the closures of airspace over Europe due to the eruption of Icelandic volcano Eyjafjallajökull, and suggest what needs to be done to avoid similar difficulties in the future.

Findings 

• The position and stability of the Jet Stream directly over Eyjafjallajökull carried ash over most of Europe
• Regulators adopted the standard zero-tolerance approach to ash in the atmosphere and grounded all flights
• Later revisions of ash concentrations, up to 4mg per cubic metre, allowed flights to resume once all certificates of compliance for aeroplanes were in order
• Volcanic ash has a lower melting point than the combustion occurring in a jet engine. It therefore melts and sticks to internal components of the engine, causing damage or engine failure

Key recommendations

The clear recommendation from this report is that every opportunity should be taken for the regulatory authorities to collect field data. This data should be from actual engine/volcano combinations generated by test flying and subsequent aircraft and engine inspection. In simple terms this form of 'volcano chasing' will mean that our understanding improves only for those conditions that actually exist and only at a rate that matches the actual number of times ash plumes interact with controlled airspace.

It would also be possible to call for significant investment and action in each of the three main areas of uncertainty highlighted in this review:

1. Measurement of actual volcanic eruptions for their particle size, chemistry and ash volume, together with improved modelling of the subsequent atmospheric dispersion over distance and time.
2. Extensive modelling and testing of aircraft, their systems and engines in exposure to a range of ash types and densities.
3. Establishment and review of clear procedures and responsibilities for who will balance, and by what method, all of the unknowns and probabilities to decide whether or not to allow flights in controlled airspace.

These are clearly the priorities for action, where time and resources allow. For example, computerised fluid dynamics modelling of particle behaviour in aircraft engines is a valuable engineering project that would yield a return. This should be undertaken immediately.

The reality, however, is that a definitive overall answer to the question of an aircraft’s sensitivity to volcanic ash is not possible. The question has so many unknowns that attempts to answer it fully could absorb effort and resources out of proportion to the risk it poses.