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Managing Volcanic Ash Risk to the Safety of Flights

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Category: Weather Weather
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Tag(s) Volcanic Ash

Introduction

The unprecedented disruption of air traffic over Europe in 2010 caused by the ash cloud from the Icelandic volcano Eyjafjallajoekull calls for development and adoption of an effective safety risk management strategy for volcanic ash. Such strategy should benefit from adequate information on the extent of actual contamination within the area defined as “affected airspace” and on the likely effects on aircraft encountering specific levels of contamination within it.

This article does not consider the various hazards to aircraft in flight near volcanic eruptions, notably:

  • The various hazards to aircraft in flight near to the ash plume formed overhead explosive volcanic eruptions;
  • Flight crew recognition of and response to inadvertent penetration of volcanic ash;
  • The timely detection of new volcanic eruptions;
  • The consequences of concentrated ash fall on aviation installations on the ground.

It is instead concerned purely with the operational safety implications of flight in significant concentrations of volcanic ash, which may exist in downstream ash clouds, and with the determination of the threshold for and exposure to any such hazard.

The source of all detailed content in this article on volcanic ash and its effects is the latest draft of ICAO Doc 9691, ‘Manual on Volcanic Ash, Radioactive Material and Toxic Chemical Clouds’ 2nd Edition, 2007.

Defining Volcanic Ash

Volcanic ash is a product of explosive volcanic eruptions and is the finest grade of ejected solid debris at source, defined as being of (equivalent) diameter less than 2 mm [1]. It is generally accepted that ash in clouds downwind of an eruption will always be composed of much smaller particles than this with equivalent diameters of less than 0.1 mm. Such particles can rise to the higher levels of the plume at the site of the eruption and remain in suspension at prevailing ambient air densities. The upper winds transport the particles away to eventual dispersal in an ‘ash cloud’. Ash clouds typically form above FL200, but the lower limit of the initial cloud depends on both the height of the volcanic vent and the vigour with which material is ejected from it. Visible emissions are often dominated by vast white clouds of steam, especially when a vent is beneath an ice cap, as often occurs in Iceland. Steam clouds continuously emitted during an eruption often mask ash emissions, which almost always occur in intermittent bursts.

Volcanic ash in the immediate vicinity of the eruption plume is of an entirely different particle size range and density to that found in downwind dispersal clouds. These clouds contain only very small particles of ash[2].

Volcanic ash usually contains more than 50% silica[3]. Silica is extremely hard and has a melting point in the region of 1100 °C. It is the potential for abrasion and for melting and re-solidifying within the high by-pass jet turbines which power modern jet transport aircraft which is the origin of the damaging effect of ash encounters on aircraft.

The Effects of Ash Cloud Encounters on Aircraft

Whilst there are a number of potential effects of volcanic ash encounters on aircraft, the ones that warrant serious attention are those affecting aircraft engines. These effects are attributable almost entirely to the dominant silica particles. Their extreme hardness means that they have an abrasive effect on any surface impacted at a significant relative speed and this includes the interior of aircraft engines. However, their more significant characteristic, as far as aircraft engines are concerned, is the fact that their melting point is well below the core temperatures which typically are sustained in high by-pass jet engines operating at above flight idle thrust. Ingested silicate ash melts in the hot section of the engine and then fuses onto the high pressure turbine blades and guide vanes. This drastically reduces the throat area and both static burner and compressor discharge pressures rapidly increase and cause engine surge. Transient and possibly terminal loss of thrust can occur in the most severe cases[4] with a successful engine re-start only possible if clear air can be regained.

The ash loading[5] at which this process affects normal engine operation is not established beyond the awareness that relatively high ash densities must exist. Whether this silica-melt risk remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear.

What makes this matter of extreme importance is that since this sequence may interfere with the normal function of aircraft engines and perhaps cause them to run down completely, a similar effect can be anticipated on all the engines on an aircraft. This is therefore a serious safety hazard which invites preventive risk management strategies in line with other comparable aviation risks.

Abrasion damage to aircraft engines caused by ash impact, whilst not affecting their continued normal function, cannot be repaired and will permanently reduce their operating efficiency thereafter. However, this is an economic issue rather than a safety one and establishing what knowledge would better inform the management of that economic risk is not the primary focus of this article.

The Basis of Current Risk Management Guidance

The ICAO International Airways Volcano Watch (IAVW)[6] system and the current ICAO guidance have mainly been developed in response to:

  1. the series of inadvertent aircraft encounters with volcanic ash in or near eruption plumes at night beginning in the 1980’s
  2. a developing awareness of the increased risk of malfunction of aircraft engines which have core temperatures which exceed the melting points of the silicate components of ash - the high by-pass fan jet engines found on most jet transport aircraft.

It assumes that volcanic hazards to aircraft can be identified and that significantly affected airspace can then be notified accordingly for avoidance. The system has been shown to work (reasonably) well in respect of the avoidance of flight in the areas close to eruptions because here, the density of ash is such that the need for exclusion is obvious. Downstream ash clouds are a much more challenging matter.

The guidance is further predicated on the risks to the continued operation of the high by-pass fan jet engines which power almost all current generation jet transport aircraft and takes no account of the potentially different vulnerability of aircraft powered by other engine types.

Detection and Definition of Ash Clouds

There is currently no accepted definition of a hazardous level of ash loading in ash clouds or on the significance of particle size at any given loading. This could be explained by the huge spatial variation of the overall ash loading within dissipating ash cloud ‘zones’.

However, the ability to observe the ash cloud ‘zone’ lateral extent by remote sensing (satellite, ground based Light Detection And Ranging (LIDAR), etc.) has greatly improved in recent years despite the fact that those platforms from which data can be rapidly accessed still do not usually carry sensors specifically designed for ash detection.

Establishing the detail of ash cloud composition and its vertical extent has proved much more challenging. The problem is made more complex because of the dynamic balance between dispersal of ‘old’ ash and the episodic contributions of ‘new’ ash load in established downstream ash cloud ‘zones’. Definition of the extent and density of ash clouds, as they disperse both laterally and vertically, is possible by use of remotely sensed lateral concentrations. However meaningful vertical profiling can be challenging, and is almost impossible. If data sampling by suitable instrumentation, including instrumented research aircraft is undertaken, the status of dissipating ash clouds is so volatile that such measurements can rapidly become history.

Forecasting Ash Cloud Movement

The process of forecasting ash cloud transport and dispersion has been even more difficult as it is based on a combination of a relatively well understood and modelled meteorological process for forecasting of wind, temperature and stability of the atmosphere and the absence of adequate real time data to use in the parallel modelling of particulate dispersion. The combined models, which are called Volcanic Ash Transport and Dispersion (VATD) models, require various source data to describe the ash column which include eruption cloud height and vertical distribution, particle size distribution, and the period(s) of activity. The modelling of downwind ash concentrations also requires the rate of eruption mass and / or the absolute volume of source ash. As a consequence the VATD models are restricted in their ability to provide the much desired exact answers to how a ash clouds propagates, mainly by an absence of real time input data when it comes to the ash mass and/or concentration but also by insufficient validation and calibration of the output by actual downwind ash cloud conditions.

Modelling the spatial complexity created by the combination of episodic bursts of new explosive activity, which will feed the ash cloud intermittently as long as active eruption continues, and the fact that the extent of any ash cloud inevitably grows laterally in the zone of transport, with resultant dissipation and eventual dispersal, requires far more real time data on ash presence than is presently available.

Generic Engine Type and Ash Hazard

The precise risk to engine function from ash ingestion and/or melting varies according to engine design but more significantly according to engine generic type.

The development of gas turbine engine design has been accompanied by steadily increasing core temperatures to achieve increasingly better specific fuel consumption. The process has been limited only by the availability of heat resistant metallic alloys which can cope with the temperatures now routinely achieved which, at cruise thrust, exceed 1400 °C. Turbine engine development has resulted in core temperatures which now significantly exceed the silica melting point and the most serious hazard to aircraft engine function has become an automatic consequence of operation of these engines in sufficiently dense ash concentrations.

The situation with other generic engine types is likely to be different. Piston engines may be capable of continued normal operation in relatively high ash loading, although there is no data on this. The air intake and core temperature characteristics of turboprop and turboshaft engines represent yet another engine type group. At present, there is no generic guidance for either of these engine type groups on the effect of volcanic ash at ash cloud densities with normal engine operation.

Technical Enablers of Proportionate Risk Management

An effective and proportionate risk management strategy would require, but not be limited to:

  1. A better understanding of the threshold of tolerance of different aircraft engine types to the ingestion of volcanic ash at the loading found in downstream ash clouds.
  2. Improved measurement and / or sensing of the variation of actual ash loading within laterally identified downstream ash clouds.
  3. Improved real time data on the new particulate loading at the eruption source of ash clouds, so that the load dispersion modelling contribution to VATD can more effectively contribute on a par with the currently more accurate modelling mechanisms for the wind velocity contribution to ash cloud dispersal.

This should all be considered against the procedures available to flight crew to maintain safety following unexpected encounters with ash loadings which are always possible whatever proactive restrictions are in place.

The overall effect of the above would be to permit more informed definition of airspace within which specific aircraft type operations should be temporarily suspended.

EASA Safety Information Bulletin

On 22 April 2010 EASA published a Safety Information Bulletin (SIB) related to flight in airspace with a low contamination of Volcanic Ash. It informed users that aircraft and engine TC-holders are being requested by EASA to develop instructions necessary for continued safe flight, such as specific pre- and post-flight inspections, and those for continued airworthiness, taking into account the effects of operation of aircraft in airspace with low contamination of volcanic ash.

Owners and operators of turbine powered aeroplanes and helicopters are recommended to take the following actions:

  • Accomplish daily inspections when operating in an area of low volcanic ash contamination, to detect any erosion, accumulation of volcanic ash, or aircraft- and/or engine damage or system degradation;
  • Report any encounter with volcanic ash, or any other relevant findings, to the engine- and aircraft TC holders, the National State of Registry of the aircraft, to EASA and to the National Authority of the State through which flight was conducted.

Reports can be submitted to EASA by email at volcano@easa.europa.eu. Further information can be obtained from the EASA Safety Information Bulletin.

EUROCONTROL Volcanic Ash Safety Data Collection Function

The unprecedented disruption of air traffic over Europe (15 - 20 April 2010) caused by the ash cloud from the Icelandic volcano Eyjafjallajoekull triggered the initiation of dedicated reporting arrangements in a number of European States.

Following coordination and agreement with authorities of Western European States EUROCONTROL established a central volcanic ash safety data collection function to collect and analyse post flight reported data, and share lessons learned to support the desired improvement in safety management standards, guidance materials and practices related to volcanic ash risk management.

Aircraft operators and service providers should forward reports about volcanic ash associated anomalies and occurrences affecting flight safety to the following address: evair@eurocontrol.int.

Related Articles

Further Reading

ICAO

UK CAA

References

  1. ^ Most volcanic ash particles are not uniformly spherical
  2. ^ These particles are referred to as ‘Fine Ash’ which is usually defined as containing particles with a maximum equivalent diameter of not greater than either 1/16 inch (0.0625mm) or 50μm (micrometres / microns)
  3. ^ ‘Silica’ is equivalent to silicon dioxide Si O2
  4. ^ Most documented engine failure events have occurred not in downstream ash clouds, but in the overhead plume relatively close to the eruption.
  5. ^ Ash Loading is defined in terms of mg/m³ which means that no discrimination in respect of the balance between ash particle size and ash particle density is available.
  6. ^ The IAVW includes State Volcano Observatories and Volcanic Ash Advisory Centres