In 1982, when British Airways 9 flew through a volcanic ash cloud and flamed out all four engines, the dangers of volcanic ash were undocumented.
Seven years later, when KLM 867 repeated the incident, it was clear more work was needed to detect and avoid volcanic ash. The ash is dry so does not show up on radar. It is also fine, so when it comes in contact with the airplane the static electricity makes radio communications difficult. It is abrasive, so it makes damages windshields to the point visibility is impaired. Once the particles go through the hot section of the engine they become molten and finally they recool and collect on engine turbine blades, solidifying to the point where air flow is fouled and the engines shut down. Fortunately, in every recorded case once the aircraft descends and the engines cool, the solidified ash tends to break away and the engines can be relit.
I like the Alaska Airlines Procedures for Operating in Volcanic Ash Conditions:
Gulfstream adds this cheerful note: "Has significant financial implications, may not be known until major inspection, usually considered an Act of God and is not covered by insurance."
The best plan is to avoid it in the first place and the aviation world is getting better at that. You need to stay plugged into the news and either subscribe to all the weather services that track volcanic activity or ensure your international flight planning service does. It also helps to be knowledgeable about the issue. A related post: Reading Minds.
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: Alaska volcano, from Alaska Volcano Plan, cover.
It seems the majority of cases involve the Boeing 747. Why is that? Before we got smart about detecting volcano eruptions and getting the word out to the aviation community, the Boeing 747 was the world's primary long distance traveller in areas where volcanic ash was likely. Any aircraft is at risk.
[Aeronautical Information Manual, ¶7-6-9]
The USGS report acknowledges that this list comprises a minimum number, since these reports are not made consistently.
There have been nine encounters with Volcanic Ash of transport category aircraft between 1953 and 2009, according to the USGS:
[USGS, Table 1]
The USGS estimates there have been 94 known ash encounters of severity 1 to 4. Three were severe enough to involve the loss of all engines and 26 created severe damage.
[USGS, pg. 3] Melting and resolidification of ash within jet turbine engines have been identified as the primary mechanisms responsible for engine failure in an ash encounter. The melting temperature of the magmatic silicate glass in ash is lower than the operating temperatures of modern turbine engines; consequently, ingested ash particles can melt in hot sections and then accumulate as re-solidified deposits in cooler parts of the engine, causing ignition flame-out and engine shutdown. In two encounters ranked as class 4, climb out from the cloud at maximum thrust was identified as a key operational condition for engine shutdown. When engine power increased, more ash-laden air was ingested by the jet turbines and combustion temperatures were raised; thus, conditions favorable for substantial melting of ash particles were met. This lesson has been incorporated into guidance to pilots about recommended actions to take during an encounter.
[USGS, pg. 5] [The 17 June 1991 incident] is notable in that prolonged exposure to dilute ash may have contributed to significant damage. That incident involved the same aircraft (a B747–200B) as incident 1991–14; both flights were operating between South Africa and Southeast Asia in the aftermath of the June 1991 eruption of Mount Pinatubo. In both encounters, the aircraft was operating in airspace at distances in excess of 500 km from the volcano. In [the 15 June 1991 incident], the crew noted static discharge lasting approximately an hour, but all engine parameters were normal during flight, and no problems were experienced with any of the aircraft systems. Two days later, the aircraft flew for several hours through an area coincident with remote-sensing evidence of an ash cloud before one engine lost power and a second engine was shut down by the crew. During descent, the engine that had been shut down was restarted, and the aircraft made a successful landing. These events raise the possibility of significant damage from cumulative exposure to dilute ash.
[USGS, pg. 5] The 2006 Gulfstream II incident is notable for involving a different mechanism for engine shutdown than melted ash. The aircraft was flying over Papua New Guinea at 11.9 km (39,000 ft) in apparently clear air with no ash or sulfurous odors noted by the cockpit crew. Between 39,000 ft and descent to 24,000, both engines failed and then were restarted; the aircraft landed safely. The operator and engine manufacturer conducted a thorough investigation, including borescope analysis of the engine and fuel analysis. The manufacturer concluded that a cylindrical filter in each fuel-flow regulator may have become blocked by volcanic ash, which at that altitude could have caused loss of fuel flow and thus engine shutdown; on descent, the increasing pressure would have substantially cleared the filter and allowed the engine to restart. The likely source of the ash was an eruption of Manam Volcano in Papua New Guinea.
There are volcanoes all over the world, though the primary threat is the so-called "Ring of Fire" in the Pacific. The following map can be downloaded by clicking on the map to display a much larger version. Smaller, more specific maps follow.
Map: Volcanoes of the world, from (ICAO Doc 9691), pg. xi.
[ICAO Doc 9691, pg. I-i_1] The highest concentration of active volcanoes lies around the rim of the Pacific Ocean, the so-called "ring of fire", which stretches northwards, more or less continuously, along the western edge of South and North America, across the Aleutian and Kurile Island chains, down through Kamchatka, Japan and the Philippines and across Indonesia, Papua New Guinea and New Zealand to the islands of the South Pacific. Other active regions are to be found in Iceland, along the great rift valley in Central and East Africa, and in countries around the Mediterranean.
Map: "The Ring of Fire," from Gringer (Wikimedia Commons).
Map: Historically active volcanoes of Alaska, from Alaska Volcano Plan, pg. 43.
Map: Historically active volcanoes of Kamchatka, from Alaska Volcano Plan, pg. 45.
Map: Historically active volcanoes of the Kuriles, from Alaska Volcano Plan, pg. 47.
Figure: Three parts or regions from an eruption column, from ICAO Doc 9691, Figure 2-4.
[ICAO Doc 9691, ¶2.2.1
[ICAO Doc 9691, ¶4.1] Volcanic ash is mostly glass shards and pulverized rock, very abrasive and, being largely composed of siliceous materials, with a melting temperature below the operating temperature of jet engines at cruise thrust. The ash is accompanied by gaseous solutions of sulphur dioxide (sulphuric acid) and chlorine (hydrochloric acid). Given these stark facts, it is easy to imagine the serious hazard that volcanic ash poses to an aircraft which encounters it in the atmosphere. 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. Moreover, the first two or three days following an explosive eruption are especially critical because high concentrations of ash comprising particles up to ~10 μm diameter could be encountered at cruise levels some considerable distance from the volcano. Beyond three days, it is assumed that if the ash is still visible by eye or from satellite data, it still presents a hazard to aircraft.
[ICAO Doc 9691, ¶2.3.1]
[ICAO Doc 9691, ¶4.2]
[ICAO Doc 9974, ¶1.3 and 1.4]
The Gulfstream procedures, while intended for a specific airplane, offer a few more things for everyone to consider.
They are also available in the cockpit G450 Quick Reference Handbook, page EB-27.
[G450 Aircraft Operating Manual, §07-02-60]
NOTE: Use of speed brakes is not recommended.
CAUTION: ENGINE DAMAGE MAY RESULT IF RPM IS NOT ALLOWED TO STABILIZE AT IDLE BEFORE ADVANCING POWER LEVERS.
NOTE: Use of the Standby Electrical Power system (HMG) is not possible with both engines windmilling.
CAUTION: MANEUVERS SHOULD BE LIMITED TO MINIMUM BANK AND PITCH ANGLES NECESSARY SINCE HYDRAULIC BOOST WILL DECREASE AS ENGINE RPM DECAYS.
Following successful airstart or clearance of the volcanic ash area, slowly accelerate each engine in turn to the required thrust and verify satisfactory operation.
Aeronautical Information Manual
Alaska Interagency Operating Plan for Volcanic Ash Episodes, June 1, 2014, available at: https://www.avo.alaska.edu/downloads/reference.php?citid=3996
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013
ICAO Doc 9974, Flight Safety and Volcanic Ash, First Edition, 2012
USGS, Encounters of Aircraft with Volcanic Ash Clouds: A Compilation of Known Incidents, 1953-2009, U.S. Department of the Interior, U.S. Geological Survey, Data Series 545, Version 1.0, 2010
Wikimedia Commons, Public Domain Artwork
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