Volcanic ash plume over Icelandic settlement of Grindavik, 14 January 2024 [Author: Glen Scott]
During a volcanic eruption, huge quantities of fragments of pulverised magmatic rock can be ejected into the atmosphere, reaching great height and accumulating in clouds which then drift with the wind. This material eventually settles out of the atmosphere onto the ground in a process of sedimentation (ashfall). The larger, and heavier particles, settle out of the atmosphere sooner and closer to the point of eruption, whereas the smaller lighter particles travel further and remain suspended in the atmosphere for longer.
On 15 April 2010, large sections of airspace over Northern Europe began to be affected by a large area of fine volcanic ash drifting south east from an erupting volcano in Iceland. The suspension of acceptance of IFR flight plans for flights in any part of the affected airspace because of the perceived risk effectively shut down the commercial air transport system over a wide area.
Eruption at Eyjafjallajokull April 17, 2010. (Source: Wikicommons. Author: Árni Friðriksson)
Volcanic Ash (Visible Ash or Coarse Ash) is defined as very small solid particles ejected from a volcano during an eruption which have intermediate axes measuring between 1/16 (0.625) mm and 2 mm (US Geological Survey)
Volcanic Dust (Fine Ash) is further defined by the same source as particles smaller than 1/16 (0.0625) mm across.
Volcanic Ash Clouds: Dense, definite and clearly identifiable dark clouds made up of volcanic ash, dust and fumes. Volcanic Ash Clouds are usually located within the vicinity of the volcano (within 200 nm depending on the height of the eruption column) and generally die out after one or two days.
Volcanic Dust Contamination: Widespread concentrations of volcanic dust and fumes floating in thin layers in the atmosphere. Traces of volcanic dust contamination can remain in the atmosphere for years.
Flight into a volcanic ash cloud is considered unsafe and should be avoided. Severe adverse effects such as engine flame-out, engine overheating, clogging of pitot-static probes, abrasion of external/internal components, etc. (see section on Effects below).
In concentrations of 4kg/hm3 or less, volcanic dust contamination is estimated to pose a similar risk to sand aerosols contamination and constitutes a maintenance issue more than a flight safety issue.
Vocanic ash does not show up on aircraft weather radar or ATC radars because of the small size of the particles. Ash particles carry electrical charges and, within a cloud of volcanic ash, this can give rise to Thunder and Lightning in the area immediately overhead the eruption.
At night, St Elmo's Fire, created when charged ash particles hit the aircraft, may be the first circumstantial indication to a flight crew that they are flying into dense volcanic ash. Other indications might be a sulphurous smell and dust within the cabin.
Volcanic Ash encounter can result in engine damage and malfunction:
- Engine Malfunction. The principal risk to continued safe flight, arising from flight through high concentrations of volcanic ash, is the melting within the engine of ash particles, which are predominantly composed of silicates with a melting point of 1100°C. This melting point is considerably less than the core operating temperature of high by pass turbine engines which, at normal thrust settings, is at least 1400°C; there will be a trend for the core temperature to increase as engine design focuses on improved specific fuel consumption. 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 with a successful engine re-start only possible if clear air can be regained. If present at sufficient densities, ash particles can also contribute to engine malfunction by simple deposition. In either case, the added debris clogs up the engine airflow and is likely to initially lead to engine surging and ultimately to a Flame Out. Reducing the thrust setting quickly to idle may lower the core temperature enough to prevent silicates melting.
- Long Term Engine Damage. The abrasive effect of volcanic ash particle impact can cause surface roughness inside turbine engines which, whilst it will not affect their continued normal operation, will result in a reduced specific fuel consumption. It is impossible to repair such damage, so the life of an affected engine could be considerably reduced.
- External Surface Corrosion. Ash can cause significant damage to the exposed surface of the aircraft skin and to the outer ply of windscreens. If the ash encounter is severe, the latter may become sufficiently abraded to be difficult to see through.
Engine Damage to a NASA DC-8-72 Airplane From a High-Altitude Encounter With a Diffuse Volcanic Ash Cloud
British Airways Flight 9 (24 June 1982)
Cruising at FL370, the aircraft, a Boeing 747-200, British Airways Flight 9, en-route at night from Kuala Lumpur to Perth, entered a dense cloud of volcanic ash in the vicinity of a volcanic eruption from Mount Galunggung. The crew had noticed St Elmo's fire and an acrid smell and dust had entered the cabin through the air conditioning system. All four engines failed and the aircraft started to descend. Once clear of the ash cloud, the crew managed to restart the engines in succession but because of continued malfunction of one of them, it was shut down and an en route diversion was made to Jakarta on 3 engines.
- Avoidance. Since the above event, the International Civil Aviation Organization (ICAO) has implemented a system of nine Volcanic Ash Advisory Centres (VAAC) which are tasked with issuing information on the location and flight level of volcanic ash clouds through SIGMET charts and in Volcanic Ash Advisory messages. However, this information rarely includes any information on particle size or on cloud density and so the real degree and extent of the hazard can be difficult to determine.
- Escape. Inadvertent penetration of significant volcanic ash concentrations from new explosive eruptions, which have not yet been detected and notified, can still occur and is most likely at night. Such encounters can be recognised as such and an escape manoeuvre performed taking into account terrain circumstances - see Volcanic Ash: Guidance for Flight Crews.
Accidents & Incidents involving Volcanic Ash
On 24 June 1982, a Boeing 747-200 had just passed Jakarta at FL370 in night VMC when it unknowingly entered an ash cloud from a recently begun new eruption of nearby volcano, Mount Galunggung, which the crew were unaware of. All engines failed in quick succession and a MAYDAY was declared. Involuntary descent began and a provisional diversion back to Jakarta, which would necessitate successful engine restarts to clear mountainous terrain en-route was commenced. Once clear of cloud with three successful engines restarts and level at FL120, the diversion plan was confirmed and completed with a visual approach from the overhead.
On 15 December 1989, a Boeing 747-400 positioning for a planned en-route stop at Anchorage with crew awareness of a significant volcanic eruption in progress some 150 nm upwind entered volcanic ash during descent north northeast of the airport. When an attempt to climb out of the ash using full thrust was made, all engines failed. After repeated and eventually successful engine restart attempts as a 13,000 feet loss of altitude occurred, the fight was completed. The Report of the comprehensive NTSB Investigation remains unpublished with only a brief factual report containing neither Safety Recommendations nor Safety Actions issued.
Airports near areas where Volcanic activity occurs
Boeing Flight Crew Briefing Video about Volcanic Ash Avoidance
Flight Safety Foundation
US Geological Survey