On the visibility of airborne volcanic ash and mineral dust from the pilot’s perspective in flight

This paper is dedicated to the memory of our friend and outstanding technician Hans Rüba († 2011), who was involved in all those aircraft missions.
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Abstract

In April 2010, volcanic ash from the Eyjafjalla volcano in Iceland strongly impacted aviation in Europe. In order to prevent a similar scenario in the future, a threshold value for safe aviation based on actual mass concentrations was introduced (2 mg m−3 in Germany). This study contrasts microphysical and optical properties of volcanic ash and mineral dust and assesses the detectability of potentially dangerous ash layers (mass concentration larger than 2 mg m−3) from a pilot’s perspective during a flight. Also the possibility to distinguish between volcanic ash and other aerosols is investigated. The visual detectability of airborne volcanic ash is addressed based on idealized radiative transfer simulations and on airborne observations with the DLR Falcon gathered during the Eyjafjalla volcanic ash research flights in 2010 and during the Saharan Mineral Dust Experiments in 2006 and 2008. Mineral dust and volcanic ash aerosol both show an enhanced coarse mode (>1 μm) aerosol concentration, but volcanic ash aerosol additionally contains a significant number of Aitken mode particles (<150 nm) not present in mineral dust. Under daylight clear-sky conditions and depending on the viewing geometry, volcanic ash is visible already at mass concentrations far below what is currently considered dangerous for aircraft engines. However, it is not possible to visually distinguish volcanic ash from other aerosol layers or to determine whether a volcanic ash layer is potentially dangerous (mass concentration larger or smaller than 2 mg m−3). Different appearances due to microphysical differences of both aerosol types are not detectable by the human eye. Nonetheless, as ash concentrations can vary significantly over distances travelled by an airplane within seconds, this visual threat evaluation may contribute greatly to the short-term response of pilots in ash-contaminated air space.

Highlights

► We contrast microphysical and optical properties of volcanic ash and mineral dust. ► We assess the visibility of volcanic ash from the pilot’s perspective. ► We deduce the lowest concentration likely to be visible under clear-sky conditions. ► We show that volcanic ash cannot be distinguished from other aerosol layers just by sight.

Introduction

Every explosive volcanic eruption is producing ash. The amount of ash generated varies between subordinate to predominant as a function of internal (magma) or external parameters. As a consequence, a single eruption may affect its direct neighbourhood only or the entire planet. Volcanic eruptions may not only impact the global climate like Pinatubo which led to a tropospheric cooling of 0.5–0.8 K (Parker et al., 1996), they may also pose a hazard to aviation due to adverse effects on the aircraft’s engines or navigation equipment. Besides volcanic ash, also mineral dust is known to have potentially severe impact on aircraft. However, strong mineral dust storms mainly affect take-off and landing of aircraft by causing very poor visibility. In contrast, volcanic ash particles are considered more dangerous to aircraft engines because they commonly have a glassy groundmass that will change its material properties upon heating. This transition from solid- to liquid-like behavior will start far below the typical melting temperatures of crystals or rocks, at temperatures met in aircraft turbines (Casadevall, 1993, Dingwell, 1996, Kueppers et al., 2010, Lavallée et al., 2012). Mechnich et al. (2011) describe the results of laboratory experiments with artificial volcanic ash and dust particles. They investigated the effects of particle deposition and high-temperature interactions on the thermal barrier coatings of high-pressure turbine airfoils. They concluded that some constituents of volcanic ash can become sufficiently liquid to cause damage on the thermal barrier coatings of turbine blades at temperatures which are generally exceeded in jet engine turbines. Aside from the adverse effects on turbine airfoils other damage, such as the erosion and blinding of windows or clogging of aeronautic sensors, can occur.

In the past 60 years, at least 129 planes inadvertently flew through the ash plumes originating from explosive volcanic eruptions from, for example Galunggung (1982, Java, Indonesia), Redoubt (1989/1990, Alaska, USA) or Pinatubo (1991, Philippines) volcanoes. For 79 of those encounters various degrees of airframe and engine damage were reported (Casadevall, 1993, Guffanti et al., 2010). In nine cases one or more engines temporarily failed due to the melting and resolidification of ash in the jet engine turbine, but it was possible to restart the engines in-flight (Guffanti et al., 2010). Although most of the incidents involving ash-related aircraft damage occurred at distances smaller than 1000 km downwind of the volcano and within the first day after the onset of the volcanic eruption, there have been exceptions with incidents at even larger distances (Casadevall, 1994, Guffanti et al., 2010). The severity of effects observed among the different volcanic ash incidents is highly variable, and the International Civil Aviation Organization (ICAO, 2001, ICAO, 2007) classifies ash encounters on the basis of a severity index ranging from 0 to 5: in the case of a class 0 encounter, only acrid smell, or electrostatic discharge (St. Elmo’s fire) is noted, whereas, for example, a class 4 encounter causes temporary engine failure, but it is possible to restart the engines in-flight. So far no class 5 encounters (engine failure or other damage leading to crash) have been reported. Besides short-term effects such as engine failure, also long-term damages are observed which are primarily related to the exposure to the acidic gas sulfur dioxide (Casadevall, 1993). Although aircraft have been capable of flying safely in regions with low ash concentrations, critical concentration threshold values could not easily be defined, and the analysis of previous accidents after ash encounters did not lead to conclusive limits. Therefore, the lesson learned from these ash encounters in the 1980s and 1990s was that the only way to ensure maximum flight safety, is the complete avoidance of ash-laden air (Casadevall, 1993). The procedure recommended by ICAO, 2001, ICAO, 2007 was “(…) regardless of ash concentration – AVOID AVOID AVOID”. In particular, it was recommended to avoid any “visible ash”.

This “zero tolerance” rule lead to the closure of large parts of the European airspace for several days after the eruption of Iceland’s Eyjafjalla volcano1 (63.63°N, 19.62°W, 1666 m above sea level (a.s.l.)) in April 2010 causing the most extensive restrictions to the airspace over Europe since the end of World War II. The explosive summit eruption on 14 April 2010 followed after a phase of effusive flank eruptions (Sigmundsson et al., 2010), and the prevailing meteorological conditions subsequently led to the fast transport of volcanic ash to central Europe. The total eruption period of the Eyjafjalla (including days with almost no activity) lasted 39 days, which by far exceeded the duration of any explosive eruption phase in Iceland in the past 30 years (Petersen et al., 2012). The optically strongest ash layer (aerosol optical depth (AOD) 0.7–1.2 at 500 nm) ever measured over Europe reached Germany on 16 April 2010, and peak ash mass concentrations retrieved from the lidar measurements in Leipzig and Munich showed values of 1.0 and 1.1 mg m−3, respectively, with an uncertainty range of 0.65–1.8 mg m−3 (Ansmann et al., 2010, Gasteiger et al., 2011). Airborne in situ measurements on 19 April 2010 still revealed values between 0.03 and 0.11 mg m−3 over Leipzig and Munich (Schumann et al., 2011). Between 14 and 24 April 2010, up to 14% of the European atmosphere (between 10°W–30°E and 36°N–60°N) showed volcanic ash concentrations of more than 0.2 mg m−3 and in 1.5% of the area 2 mg m−3 was exceeded (Stohl et al., 2011). For mineral dust, concentrations of 0.2 mg m−3 and higher are not uncommon in regions adjacent to deserts like the Saharan desert (Schütz, 1980, Weinzierl et al., 2009), but as aforementioned volcanic ash is more dangerous to aircraft engines.

To maximize the non-restricted airspace in which aircraft could operate under a limited presence of volcanic ash, aviation experts agreed on preliminary threshold values for volcanic ash based on empirical assumptions at the end of April 2010 (Schumann et al., 2011). As a result, less frequent closures of airspaces were necessary in the later eruption phase of the Eyjafjalla volcano. On 23 May 2011, after the eruption of Iceland’s Grimsvötn volcano, the German Government (http://www.bmvbs.de) regulated that aircraft may fly in volcanic ash without special attention below a concentration limit of 0.2 mg m−3. Between 0.2 and 2 mg m−3 enhanced procedures apply. Areas with concentrations above 2 mg m−3 are still to be avoided entirely although exceptions are possible at concentrations between 2 and 4 mg m−3 for special flights such as search and rescue or research flights. Although, maximum ash concentrations in the vicinity of past aircraft encounters of class 4 were analyzed to exceed at least 4 mg m−3 with far higher concentration in the vicinity (Witham et al., 2012), the hazard of volcanic ash to aviation likely depends on more parameters than just the airborne ash mass concentration including the time spent in an ash layer, the engine type and the power setting. It may also vary between different eruptions (or even within a single eruption) because ash properties are not uniform.

Having in mind the safety criterion of “no flight in visible ash,” a key question is whether a pilot has the means to avoid flying through potentially dangerous ash layers just by visual observation of the atmospheric situation out of the flight deck. The goal of this study is to assess whether it is possible from the pilot’s perspective in flight to detect the presence of volcanic ash and to distinguish between volcanic ash and other aerosols2 by sight. We focus on volcanic ash and mineral dust, the two aerosol types which potentially impact aircraft operation. In this paper we contrast the occurrence of mineral dust and volcanic ash aerosol and discuss their respective microphysical and optical properties. We show how the appearance of volcanic ash plumes changes with distance from the emitting source and use radiative transfer simulations to investigate the visibility of such aerosol layers for clear-sky conditions. Based on those simulations we discuss the lowest concentrations at which a volcanic ash aerosol layer is still visible under idealized conditions and if a pilot en route can distinguish airborne volcanic ash with a concentration of 2 mg m−3 from mineral dust layers of the same concentration.

Section snippets

Data basis

To contrast the properties of volcanic ash and mineral dust in detail, we use data from three field experiments with the DLR research aircraft Falcon (a Dassault Falcon 20E twin engine jet aircraft). Table 1 gives an overview of the data used in this study: the mineral dust data were gathered within the framework of the Saharan Mineral Dust Experiment (SAMUM, Ansmann et al., 2011a, Heintzenberg, 2009). Within SAMUM, two field experiments were performed: SAMUM-1 (summer 2006, Morocco) focused on

Occurrence of mineral dust and volcanic ash particles

The atmospheric occurrence of volcanic ash and mineral dust differs due to different source locations and different mobilization mechanisms. The characteristics of volcanic ash and mineral dust layers are outlined below and summarized in Table 3.

Visibility of airborne volcanic ash

In general, the question of whether a pilot can see an ash layer (or any other prominent aerosol layer4), that means, the visibility of airborne volcanic ash, depends on many parameters. The visibility of an object depends in particular on the brightness and color contrast between the object and the background (Koschmieder, 1925). A perfectly black object can be seen

Summary and conclusions

Several incidents in the past have demonstrated that volcanic ash can have severe consequences on aviation. With the safety criterion “no flight in visible ash” in mind, we investigated whether a pilot has the ability to visually detect the presence of potentially dangerous volcanic ash (i.e., with mass concentration larger than 2 mg m−3) and safely avoid affected regions. The visual detectability of airborne volcanic ash at various distances from the volcano was discussed based on observations

Acknowledgements

This work was funded by the Helmholtz Association under Grant number VH-NG-606 (Helmholtz-Hochschul-Nachwuchsforschergruppe AerCARE), and by the Deutsche Forschungsgemeinschaft (DFG) under Grant number FOR 539 (Forschergruppe SAMUM). The volcanic ash research flights with the DLR Falcon were funded by the Bundesministerium für Verkehr, Bau und Stadtentwicklung (BMVBS, Federal Ministry of Transport, Building and Urban Development). We thank the Falcon flight crew, DLR flight operations, and all

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