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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 costly disruption of air traffic over Europe in 2010 — caused by the ash cloud from Iceland’s Eyjafjallajökull volcano — forced the international aviation community to rapidly develop a new risk-management strategy. The strategy includes regulations, safety-management programs, satellite technology and systems to effectively mitigate threats from volcanic ash. Implementation of the first phase began around 2012.

The first in-depth analyses of engine flameout risks from high-level volcanic ash clouds had been conducted after two 1982 incidents involving a British Airways B747 (B742, en-route, Mount Galunggung Indonesia, 1982) and a Singapore Airlines B747.

Since then, some fact-finding groups have disbanded, and some initiatives have been discontinued. A few formerly essential websites now only contain content for historical research, and they refer users elsewhere for current, authoritative information.

Some guidance from 2010–2012 has been superseded in the context of evolving science and technology, investigation of a few events, and reorganisation of international committees of subject matter experts.

Therefore, whether working in government or industry, aviation safety specialists must ensure that their knowledge, warning systems, contingency plans, flight planning, pilot training and air traffic controller training reflect the latest knowledge and operational best practices.

Today’s risk-management processes require accurate information on the extent of actual contamination within danger areas of “affected airspace” and on the likelihood of aircraft encountering significant concentrations and types of volcanic ash particles.

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 (i.e., the downwind horizontal movement of ash particles that begins at the umbrella region atop the initial column) formed overhead explosive volcanic eruptions;
  • Flight crew recognition of and response to inadvertent penetration of volcanic ash;
  • The timely detection of new volcanic eruptions; or,
  • 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. Readers should review the following resources for the latest, aviation-specific information about volcanic ash:

ICAO describes Doc 9691 as follows: “The main purpose of this manual is to assist States and international organizations involved in the [ICAO] International Airways Volcano Watch (IAVW) by providing guidance on the scientific aspects of volcanic eruptions and volcanic ash in the atmosphere (including volcanic ash cloud observation/detection and movement) and the effects of volcanic ash on aircraft operations, and by providing guidance regarding States’ responsibilities within the IAVW"[1].

Doc 9691 says that, although technical and procedural issues essentially have been resolved since 1982, constant attention still must be devoted to two issues. One is sustaining voluntary cooperation among the numerous affected disciplines in aerospace, aviation meteorologists, vulcanologists, seismologists and national observing sources. The other is that, at the local level over long periods, organisations find it extremely difficult to keep current on the subject and ready to activate procedures that their people may never utilise given the rarity of explosive volcanic eruptions, which tend to occur with little or no warning.

“Pilots themselves are also an important source of information on volcanic activity and volcanic ash cloud. … Considerable progress has been made in the detection of volcanic ash from meteorological satellite data, especially data in certain of the infrared wavelengths, and the forecasting of volcanic ash cloud trajectories using computer models,”[2] ICAO states.

Reports on the ICAO website show several examples of ways in which Doc 9691 recently has influenced regional aviation stakeholder groups. In March 2018, ICAO’s South Asia/Indian Ocean ATM Coordination Group and South East Asia ATS Coordination Group jointly agreed on updates to volcanic ash–related aspects of the Asia/Pacific Regional ATM Contingency Plan.

Specifically, plan language was revised to accept the Smithsonian Institution Global Volcanism Program’s List of Volcanoes of the World for VAAC Use as the definitive list for use in the Asia/Pacific Region. VAAC is the acronym for the nine international volcanic ash advisory centres. Guidance in Appendix E of Doc 9691 recommends in part that “each State should ensure that a list of volcanoes relevant to the State is maintained at all International NOTAM Offices, with volcano name, number and nominal position” ready for use in ASHTAM/NOTAM templates.

In 2015, ICAO also conducted regional briefings summarizing the outcomes of annual volcanic-ash event-response exercises in North America and Europe. The joint task force for these exercises recommended that "States should not declare a danger [area] or restricted area in respect of volcanic ash, except over and in proximity to an erupting volcano"[3].

Definitions

Volcanic ash — produced by the explosive eruptions of a volcano — is one of the most hazardous airborne contaminants encountered in international air transport. It is the finest grade of ejected, solid debris (i.e., particles) from the source. ICAO Doc 9974 defines it as “sharp-edged, hard glass particles and pulverized rock. It is very abrasive and, being largely composed of siliceous materials [silica/silicon dioxide particles], has a melting temperature [of about 1,100 degrees C] below the operating temperature of modern turbine engines at cruise thrust [about 1,400 degrees C].”

Volcanic ash damages the jet turbine engines; abrades cockpit windows, airframe and flight surfaces; clogs the pitot-static system; penetrates into air conditioning and equipment cooling systems; and contaminates electrical and avionics units, fuel and hydraulic systems, and cargo-hold smoke-detection systems (Doc 9691).

Volcanic ash is defined by its range of particle sizes — meaning that the irregularly shaped particles have diameters roughly equivalent to spheres of less than 2 mm. Most particles are not uniformly spherical.

Ash clouds — the ash forming clouds downwind of an eruption — are always composed of much smaller fine ash particles, with equivalent diameters of less than 0.1 mm. Such particles can rise to the higher levels of the volcanic plume beginning at the site of the eruption. Ash-cloud particles remain in suspension at prevailing ambient air densities.

Mass loading, the concentration of volcanic ash in the atmosphere following an eruption, is defined in terms of particle weight-per-air-volume measurements in mg/m³. For example, estimated mass loading in a 1989 B747 encounter was 2,000 mg/m³.

The upper winds transport the particles away from the plume to eventual dispersal in an ash cloud. Ash clouds typically form above FL 200, but the lower limit of the initial ash cloud depends on both the height of the volcanic vent and the energy with which material is ejected from the vent. 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 compared with those found in downwind-dispersal ash clouds. Ash clouds contain only very small particles of ash. They have a maximum equivalent diameter no greater than 1/16 inch (0.0625 mm) or 50 μm (micrometres/microns).

Most aircraft engine failure events related to volcanic ash have occurred in the overhead plume relatively close to the eruption site — not in the more distant ash clouds.

Training for airline pilots and air traffic controllers emphasises that neither ground-based nor on-board weather radar will enable them to detect the small particles in the ash clouds. Thus, flight crews may not get any advance warning of entering the ash cloud unless they have official analyses of data from external sensors, such as satellite-based instruments. One exception is ground-based X-band weather radars that, if within 100 km of a volcano’s ash column, effectively show the height of the column — at the time when mass loading and ash-cloud loading with large ash particles are greatest, according to Doc 9691.

As noted, the circumstances are important because aircraft encountering ash plumes and ash clouds are at risk of potential damage in the forms of abrasion by ash particles and of silicate-ash particles melting and re-solidifying within high by-pass jet turbines. The following section briefly discusses how engine flame-outs occur.

Ash Cloud Effects

Whilst there are a number of potential effects of volcanic ash encounters on aircraft, the extreme hardness of silica means that these particles have an abrasive effect on any surface impacted at a sufficient relative speed, and this includes the interiors of aircraft engines.

However, the more significant characteristic of silica particles is what occurs 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 nozzle guide vanes. “This drastically reduces the high-pressure turbine-inlet guide-vane throat area, causing the static burner pressure and compressor discharge pressure to increase rapidly, which, in turn, causes engine surge,” Doc 9691 says. “This effect alone can cause immediate thrust loss and possible engine flame-out.” In that scenario, successful engine re-start only may be possible by re-entering airspace with clear air.

The degree of mass loading at which the above process affects normal engine operation may not be established for a given engine type, beyond the awareness that relatively high ash densities must exist. Whether the silica-melt risk exists at the much lower ash densities characteristic of downstream ash clouds has been unclear.

What makes this extremely important is that — because this process may interfere with the normal function of aircraft engines, perhaps causing 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, requiring commensurate risk-management strategies.

The abrasion damage to aircraft engines caused by ash impact, whilst not affecting their continued normal function, cannot be repaired and permanently reduces their operating efficiency thereafter.

The precise risk to engine function from ash ingestion and/or silicate-particle 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 evolution has been limited only by the availability of heat-resistant metallic alloys that can cope with the temperatures now routinely achieved, as noted.

The situation for other generic engine types is likely to be different. Piston engines may be capable of continued normal operation in relatively high mass 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.

Ash Cloud Detection

No official definition of the hazardous level of mass loading in ash clouds or the significance of particle size at any given mass loading has been introduced. The prevailing theory involves the huge spatial variation of the overall mass loading within dissipating ash-cloud zones.

However, the capability to observe an ash cloud “zone’s” lateral extent by remote sensing (i.e., satellite-based instruments, ground-based light detection and ranging (LIDAR), etc.) has greatly improved in recent years. Some of these platforms, however, have not carried sensors specifically designed for volcanic ash detection.

Data from U.S. National Oceanic and Atmospheric Administration (NOAA) satellites, however, in recent years have been used to create volcanic-ash detection and ash-property products for the world’s VAACs. The VAACs, in turn, provide advisory data to air navigation service providers, helping air traffic controllers to determine when to divert or to ground flights.

According to a joint satellite program of the U.S. Geological Survey (USGS) and the U.S. National Aeronautics and Space Administration (NASA), “Advanced analysis of data from polar orbiting and geostationary satellites reduces the probability of a disastrous and/or costly aircraft encounter with volcanic ash and helps to minimize the cost associated with avoiding volcanic ash"[4].

“Only 10 percent of the world’s volcanoes are routinely monitored from the ground, making satellites the only frequently available tool that can reliably identify volcanic eruptions anywhere in the world. … [Academic researchers using accurate, high-resolution imaging instruments] have designed a series of operational products utilizing satellite data that derive the height and particle size [and extent] of volcanic ash, while also accurately showing its spread”

Establishing the details of ash cloud composition and ash cloud vertical extent — other than by X-band ground radar, as noted — has proven to be much more challenging.

Definitions of the exact extent and density of ash clouds — as they disperse both laterally and vertically — are now possible by use of remotely sensed lateral ash-particle concentrations. Nevertheless, meaningful vertical profiling can be almost impossible. If data sampling is undertaken by suitable instrumentation, including instrumented research aircraft, the status of dissipating ash clouds is so volatile that such measurements rapidly become unusable.

Ash Cloud Movement

The processes of forecasting ash-cloud transport and dispersion have been even more difficult. The processes are based on a combination of a relatively well-understood and well-modelled meteorological process for forecasting wind, temperature and stability of the atmosphere along with 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 have been restricted in their ability to provide the much desired, exact answers as to how ash clouds propagate. This limitation is mainly due to absence of real-time input data on the ash-particle mass and/or concentration. Also problematic is insufficient validation and calibration of the output by actual downwind ash-cloud conditions.

Any ash cloud inevitably grows laterally in the zone of transport, with resultant dissipation and eventual dispersal. Understanding the natural process apparently requires far more real-time data on ash presence than has been available.

Doc 9691 says, “Work is currently under way to combine the predictive capability of the atmospheric transport and dispersion [VATD] models with the actual position of volcanic ash as identified by satellite in data assimilation mode. … Verification of [VATD] models, as well as the underlying forecast meteorological models, is an ongoing task”[5].

References

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