Turbines and Ash Clouds:
Asking for Trouble?


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Passengers may think worldwide flight cancellations and reroutings were unnecessary after Iceland’s Eyjafjallajökull’s recent eruption, but clouds of volcanic ash and dust pose real and substantial dangers for aircraft jet engines. Flying through ash clouds can cause millions of dollars worth of damage to the engines—or even shut them down entirely.

Volcanic eruptions can send ash clouds 40,000 feet into the atmosphere, higher than the cruise altitudes of large commercial aircraft. Not the soft and powdery form from a wood fire, this pulverized rock, ranging from millimeter-sized particles (like sand) to micrometer-sized particles (like clay), can remain suspended in the atmosphere for days. Ash clouds are invisible to weather and aircraft radar and can be difficult for pilots to see in daylight.

In the last 30 years, more than 90 jet-powered commercial airplanes have encountered ash clouds and have suffered damage as a result, according to Boeing. A range of consequences have been documented and experimentally determined, from total engine shutdown to greatly shortened engine component life.

 
engine combustorA test engine combustor has ash deposits on its liner and swirlers.

BA Flight 9

One of the most famous of such encounters was in 1982 when a British Airways 747-200 at 37,600 feet flew through an ash cloud from the erupting Indonesian volcano Gulunggung. The flight deck noticed "fireflies" coming towards the cockpit’s windshield and a spectacular display of St. Elmo’s fire (glow) in the engine inlets. 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 minutes, all four RB-211 engines failed. The 747 began to lose altitude, turning 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 land at Jakarta, with Captain Moody standing to peer out of a two-inch strip that remained clear in the sandblasted windshield.

KLM Flight 867

In the late 1980’s, the danger posed by volcanic eruptions was taken seriously enough that a warning system was implemented notifying all airlines flying near Anchorage, Alaska of ash cloud activity from nearby Redoubt Volcano. Despite the warnings, a new KLM 747-400, at an altitude of 24,600, feet flew into the ash plume.

All four CF6 engines ingested volcanic ash and shut down. The 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, but did cause over $80 million in aircraft damage.

fan engine faceA close-up of a fan engine face shows details of St. Elmo’s fire, and ash and dust injectors. While the image has a yellow cast, the actual glow is white. Note bright glow-ring at fan tips.

NASA Research DC8

Somewhat less dramatically, a NASA research aircraft encountered an ash cloud over the Atlantic Ocean between Greenland and Norway in 2000. Flight plans had routed the aircraft around 200 miles north of the projected position of the ash plume from Iceland’s Mt. Mekla, but the sensitive atmospheric research equipment onboard showed that they inadvertently flew through a diffuse volcanic ash cloud for seven minutes.

The flight crew noted no change in engine cockpit instrument readings, and no St. Elmo’s fire, haze, or odor. After safely landing at its destination, a visual inspection showed no apparent damage, but after returning to Edwards Air Force Base, further inspection showed ash damage to the high-pressure turbines in all four engines, resulting in $3.2 million refurbishment costs.

Developing Procedures

Michael G. Dunn and colleagues ran 16 years of laboratory experiments at Calspan Corporation testing effects of two types of ash on the jet engines and components. They found ash becomes glass-like above 2,000 °F turbine inlet temperature, and deposits on downstream airfoils causing blockages and a pressure increase that can lead to a compressor surge—and a possible engine flame out. If a pilot quickly reduces the throttle when encountering ash clouds to drop the turbine inlet temperature to below 2,000 °F, ash particles will pass through the engine. This may erode blades or vanes, but it won’t trigger a possibly disastrous turbine blockage. This, of course, leads to a loss in aircraft altitude and speed.

Based on this work, Boeing has produced a video that goes through a set of procedures that a flight crew can take when encountering ash clouds.

According to Dunn, research should be done on how much ash from the engine enters the cabin air (the haze reported on flight BA 9) and its effect on sensitive electronic components and humans. But most important, regulatory guidelines seem to be lacking on the measurement and acceptable levels of ash in which jet powered aircraft could safely fly.

The best option would be to avoid volcanic clouds, but without advance warning, this isn't always going to be possible.

[Adapted from "Asking for Trouble," by Lee S. Langston, ASME Fellow, for Mechanical Engineering.]

In the last 30 years, more than 90 jet-powered commercial airplanes have encountered ash clouds and have suffered damage as a result, according to Boeing.

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March 2011

Lee Langston

by Lee S. Langston