This article focuses on the mechanical limitations of commercial jetliners while flying through volcanic ash clouds. Flying through thick volcanic ash cloud can cause millions of dollars’ worth of damage to the engines—or even shut them down entirely. In order to find out what actually happens in a jet engine ingesting volcanic ash, there is a large body of laboratory data and analysis carried out from 1980 to 1996 by Michael G. Dunn and his colleagues at the Calspan Corp. in Buffalo. Based on the work of Dunn and his co-authors, Boeing has produced a video that goes through a set of procedures that a flight crew can take when encountering ash clouds. The rather extensive research work done by the Dunn group details the effects of volcanic ash on jet engines themselves. According to Dunn, research yet needs to be done on the extent that unfilterable tiny ash particles contained in the ash-laden engine compressor enter the cabin air (the haze reported on flight BA 9) and its effect on sensitive aircraft electronic components and humans.

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In April, my wife Liz and I and our fellow passengers on an Indian Ocean cruise disembarked at Cape Town, South Africa. Given that our cruise ship had successfully steered clear of any eminently dangerous encounters with Somali pirates along the east coast of Africa, we thought we were in the clear. At the airport, ready to board a flight back home to the United States via London's Heathrow Airport, we discovered that our flight from Cape Town was canceled, due to the eruption of the volcano Eyjafjallajökull in far-away Iceland. Indeed, the ash cloud from Eyjafjallajökull—a name destined for crossword puzzles—blocked airline traffic into and out of some northern European airports, which triggered cancellations worldwide.

Luckily, Liz and I were able to get a flight the next day via Johannesburg, going through Dakar, Senegal, far south of the Icelandic volcanic ash cloud. Many of our fellow cruise ship passengers weren’t as fortunate. Some had to stay many more days in Cape Town and one couple could only make airline connections back to their home in the American Midwest through Sydney, Australia—thus flying three quarters of the way around the world to avoid Iceland's volcanic ash clouds.

Some of the passengers who were stranded with us expressed disbelief on the need for worldwide flight cancellations, due to the presence of seemingly innocuous dust clouds floating in the vicinity of some European cities. But the precautions were necessary. Clouds of volcanic ash and dust pose real and substantial dangers for aircraft jet engines. Flying through such a cloud can cause millions of dollars worth of damage to the engines—or even shut them down entirely.

According to volcano experts, about 50 to 70 of the world's approximately 1,500 active volcanoes erupt each year. The size and intensity of each eruption varies considerably, with each extending from months to years. An eruption can send ash clouds up into the atmosphere at heights that exceed the cruise altitudes of large commercial aircraft, 30,000 to 40,000 feet.

This ash is not the soft and powdery form from a wood fire. Instead, it consists of bits of pulverized rock, with the largest particles of millimeter size (like sand) and the smallest in the micrometer range (like clay particles). These bits of rock can remain suspended in the atmosphere for days. The ash clouds are invisible to weather and aircraft radar and if sufficiently dispersed, can be difficult for a pilot to see in daylight.

Aircraft manufacturer Boeing reports that in the last 30 years, more than 90 jet-powered commercial airplanes have encountered clouds of volcanic ash and have suffered damage as a result.

One of the most famous of such encounters occurred on the night of June 24, 1982. British Airways Flight 9, a Boeing 747-200, with four Rolls-Royce RB-211 jet engines and 247 passengers and 16 crewmembers aboard, passed over the island of Java on its way from Kuala Lumpur, Malaysia, to Perth, Australia. While cruising at 37,600 feet, Captain Eric Moody and others of the flight deck crew noted “fireflies” coming towards the cockpit's windshield and a spectacular display of St. Elmo's fire (glow) in the engine inlets.

It turned out that the plane had entered a volcanic ash cloud from the erupting Indonesian volcano Gulunggung, and electrically charged volcanic ash particles were interacting with the engine fan blades. Some passengers saw static electricity discharging on upper wing surfaces and intermittent, brightly lit patches in the engine exhaust. There was a strange odor and haze in the cabin, probably due to ash particles too fine to be filtered out of the cabin air, which is drawn from the jet engine compressors.

Within a few minutes, all four engines failed and the 747 began to lose altitude, turning it into a giant glider. After falling more than 20,000 feet, the crew managed after many tries to successfully restart three of the four engines, make a 180-degree turn, and eventually was able to land at Jakarta, with Captain Moody standing to peer out of a two-inch strip that remained clear in the sandblasted windshield.

One of the passengers, Betty Tootell, wrote about this volcanic ash encounter in her 1985 book, All Four Engines Have Failed, and several television documentaries have been made based on the incident.

By the late 1980s, the danger posed by volcanic eruptions was taken seriously enough that a warning system was in place in Alaska. All airlines were to be notified of ash cloud activity caused by Redoubt Volcano, located 110 miles southwest of Anchorage, a major hub for domestic and international commercial air travel. Nevertheless, on December 15, 1989, in spite of the warnings, KLM Flight 867 from Amsterdam at an altitude of 24,600 feet flew into the ash plume just before noon, east of Talkeetna, Alaska, some 149 miles northeast of Redoubt. All four of the General Electric CF6 engines on this new Boeing 747-400 ingested volcanic ash and shut down. As with the BA 9 flight, the KLM crew managed to restart engines—at some 5,000 feet above the mountaintops—and make a safe landing at Anchorage International Airport. This ash encounter caused no loss of life and no injuries to the 231 passengers and 14 crewmembers, but did cause over $80 million in damage to the KLM aircraft. Somewhat less dramatically, a NASA research aircraft encountered an ash cloud over the northern tip of the Atlantic Ocean between Greenland and Norway on February 28, 2000. During the early morning hours, in total darkness, the DC-8 airborne science research airplane, powered by four General Electric CFM56 jet engines was en route to Kiruna, Sweden. To the south, Iceland's Mt. Mekla volcano had erupted some 35 hours before and was producing a northward flowing ash plume. The DC-8 flight plan routed the aircraft around 200 miles north of the projected position of the Mekla ash plume, but the sensitive atmospheric research equipment onboard showed that for seven minutes the plane inadvertently flew through a diffuse volcanic ash cloud. The flight crew noted no change in engine cockpit instrument readings, and no St. Elmo's fire, haze, or odor. After landing in Kiruna, a visual inspection of the DC-8 showed no apparent damage. When the aircraft returned to Edwards Air Force Base, however, further inspection showed ash damage to the high-pressure turbine of each engine, resulting in refurbishment of the four engines at a cost of$3.2 million.

Those three aircraft encounters with ash clouds document a range of consequences, from total engine shutdown to greatly shortened engine component life. To find out what actually happens in a jet engine ingesting volcanic ash, there is a large body of laboratory data and analysis carried out from 1980 to 1996 by Michael G. Dunn and his colleagues at the Calspan Corp. in Buffalo, N.Y. Dunn is now a professor and director of the Gas Turbine Laboratory at Ohio State University.

Dunn and his coauthors carried out Calspan laboratory experiments involving the measurement of the effects two kinds of volcanic ash (black scoria and Mt. St. Helens ash) have on the actual performance of a number of different models of jet engines and components. Many of their results have been reported in ASME technical papers. For instance, much of the ash ingestion work is summarized in an October 1996 paper, “Operation of Gas Turbine Engines in Volcanic Ash Clouds,” by M.G. Dunn, A.J. Baran, and J. Miatech, ASME Journal of Engineering for Gas Turbines and Power, Volume 118, pp. 724-731.

The experimental work of Dunn and his associates showed that there are five dominant ash ingestion factors of immediate concern to a flight crew. These are ash material deposition occurring on the high turbine inlet guide vanes, blocking of turbine vane or blade cooling holes, erosion of the fan and compressor blades, degradation of the engine fuel control system, and deposition of carbon-like material on the fuel nozzles.

In particular the team's experimental work showed that at turbine inlet temperatures of 2,000 ̊F or greater, the ash becomes glass-like and then deposits on downstream turbine airfoils. These deposits cause blockage and a combustor pressure increase that can lead to a compressor surge—and a possible engine flame out. If the pilot is able to reduce the throttle setting to get the turbine inlet temperature below 2,000 ̊F, the ash particles will not soften to a glassy state and will tend to pass through the engine. This may cause some erosion in the blades or vanes, but it won’t trigger a possibly disastrous blockage in the turbine section.

The turbine inlet temperatures at high altitude cruise for older engines, such as those on flights BA 9 and KLM 867, may be in the 2,200 ̊F to 2,400 ̊F range, and can be even higher in newer engines. So it is important for a pilot to reduce the throttle quickly when encountering ash clouds. This, of course, leads to a loss in aircraft altitude and speed.

Based on the work of Dunn and his co-authors, Boeing has produced a video that goes through a set of procedures that a flight crew can take when encountering ash clouds. The rather extensive research work done by the Dunn group details the effects of volcanic ash on jet engines themselves. According to Dunn, research yet needs to be done on the extent that unfilterable tiny ash particles contained in the ash-laden engine compressor enter the cabin air (the haze reported on flight BA 9) and its effect on sensitive aircraft electronic components and humans.

But most important, what currently seems to be lacking are regulatory guidelines on the measurement and on the acceptable levels of ash in which jet powered aircraft could safely fly. Right now the safest approach is one of avoidance.

I certainly found avoidance to be the answer in my own encounter with volcanic ash. In November 1972 I was a member of a mountain climbing group in Ecuador. Four of us, paired up on two ropes, were climbing one of the world's highest active volcanoes, snow-covered Cotopaxi, 19,812 feet high. As we came within about 500 feet of the volcano's summit rim, a storm came up, blowing snow pellets, ash, and a strong sulfur smell down upon us. Suddenly my hair stood on end, sparks were jumping from my eyebrows to the metal rim of my goggles and my ice ax took on a strange glow. Not wanting to turn into lightning bolts, the four of us quickly retreated to a safer elevation, to climb again another day.

1.
Operation of Gas Turbine Engines in Volcanic Ash Clouds
,
M.G.
Dunn
,
A.J.
Baran
, and
J.
Miatech
,
1996
,
ASME Journal of Engineering for Gas Turbines and Power
, Vol.
118
, pp.
724
731
2.
Deposition of Volcanic Materials in the Hot Sections of Two Gas Turbine Engines
,
J.
Kim
,
M.G.
Dunn
,
A.J.
Baran
,
D.P.
, and
E.L.
Tremba
,
1993
,
ASME Journal of Engineering for Gas Turbines and Power
, Vol.
115
, pp.
641
651
.
3.
Experimental Determination of the Influence of Foreign Particle Ingestion on the Behavior of Hot-Section Components Including Lamilloy,
M.M.
Weaver
,
M.G.
Dunn
, and
T.
Heffernan
,
1996
, presented at the International Gas Turbine and Aeroengine Congress & Exhibition, Birmingham, U.K.
4.
Indicators of Incipient Surge for Three Turbofan Engines Using Standard Equipment and Instrumentation,
J.A.
Baran
and
M.G.
Dunn
,
1996
, presented at the International Gas Turbine and Aeroengine Congress & Exhibition, Birmingham, U.K.
5.
Response of Large Turbofan and Turbojet Engines to a Short-Duration Overpressure
,
M.G.
Dunn
,
R.M.
, and
V.S.
Oxford
,
1989
,
ASME Journal of Engineering for Gas Turbines and Power
, Vol.
111
, pp.
740
747
.
6.
Performance Deterioration of a Turbofan and a Turbojet Engine Upon Exposure to a Dust Environment
,
M.G.
Dunn
,
C.
,
J.E.
Moller
, and
R.M.
,
1987
,
ASME Journal of Engineering for Gas Turbines and Power
, Vol.
109
, pp.
336
343
.
7.
Interpretation of Gas Turbine Response Due to Dust Ingestion
,
P.F.
Batcho
,
J.C.
Moller
,
C.
, and
M.G.
Dunn
,
1987
,
ASME Journal of Engineering for Gas Turbines and Power
, Vol.
109
, pp.
344
352
.
8.
Nuclear Blast Response of Airbreathing Propulsion Systems: Laboratory Measurements with an Operational J-85-5 Turbojet Engine
,
M.G.
Dunn
and
J.M.
Rafferty
,
1982
,
ASME Journal of Engineering for Power
, Vol.
104
, pp.
624
632
.