There are about 1,500 active volcanoes worldwide today. This makes knowing your aircraft’s procedure for inadvertently flying through an ash cloud very important. Flight through an ash cloud can produce engine flameout, windshield destruction and toxic fumes in the aircraft cabin.
Have you ever thought what might happen if you flew your TPE331-powered Twin Commander through the plume of an erupting volcano? It’s not such a far-fetched possibility.
The United States, particularly Alaska, California, Hawaii, and Washington state, has experienced more than two-dozen volcanic eruptions in the past 100 years, or an average of one eruption every four years. There are no signs that these statistics will change in the future.
Moreover, historical data suggest that about two-thirds of all volcanic eruptions on earth occur in the Northern Hemisphere. This is the area where most of the world’s air traffic takes place.
What is even more significant is the fact that about half of the predicted eruptions will take place at previously inactive sites. In other words, long-range forecasts, and thus planned avoidance of future eruptions, are nearly impossible. This means that, unless pilots submit timely and accurate reports (Pireps) about newly detected eruptions, air traffic control (ATC) is often unable to warn other flight crews.
Nevertheless, crews are being advised to review Notices to Airmen (Notams) on volcanic activities along their planned route. The web site http://www.ospo.noaa.gov/Products/atmosphere/vaac/index.html is another excellent source for current volcanic activities.
Obviously, you would never penetrate a volcanic cloud on purpose, but events could transpire that would put you in such a situation. You may not be aware that an active volcano is located near your intended route of flight. Also, an ash cloud is similar in appearance to an ordinary weather cloud, the plume may be imbedded in conventional clouds, or it may be a dark night. The result—you may not realize that you are in the middle of an abrasive and dense cloud of volcanic ash.
Let’s discuss how an encounter with volcanic ash could affect the airplane’s turbine engines.
VOLCANIC ASH PROPERTIES
Volcanic ash is similar to talcum powder in size and texture. However, it is more abrasive. The majority of ash particles are under 5 microns (0.005 mm or less than 0.0002 inches) in diameter, and the largest measure only 50 microns. Also, as they travel through the compressor section, the ash particles can be pulverized from 38 microns to about 5 microns by the time they reach the combustor and turbine section. In comparison, fine Arizona dust can be as large as 200 microns.
Along with being small, ash particles have been measured to have a hardness of 6 on the Mohs’ Hardness Scale. That compares to quartz, which can cause scratches on both glass and aluminum.
Another nasty feature is that volcanic ash particles have been found to be electrostatically charged while suspended in the plume. Consequently, they will be attracted to and cling to surfaces with which they come in contact.
Typical volcanic ash has acidic properties that range from mildly acidic with a pH factor of about 5.5, to highly acidic with a pH factor of 2.0. (pH 7.0 is considered neutral.) High acidic values imply that volcanic ash particles will have the harmful effect of corroding aircraft components at an accelerated rate if left unchecked (untreated).
Finally and most significantly, volcanic ash has a melting point of about 1150-degrees C. That temperature falls well within the operating range of most turbine engines.
In daylight visual meteorological conditions, an encounter with an active eruption should be obvious and in most cases can be avoided, especially when the volcanic plume reaches heights of close to 100,000 feet. Even large eruptions like Mount St. Helens in May 1980, which reportedly ejected four-hundred-million tons of debris within a nine-hour period in a cloud that ascended at 3,000 feet per minute, should give any alert flight crew enough time to avoid direct encounter. The June 1991 eruption of Mount Pinatubo in the Philippines, on the other hand, was ten times larger than Mount St. Helens and presented a much larger avoidance challenge.
Eruptions at night and/or in instrument meteorological conditions are extremely difficult to detect, for several reasons. First, airborne weather radar is designed to “see” water droplets only; it cannot paint volcanic ash. Second, when flying in IMC it is visually difficult to discern volcanic ash from water. Photos taken of the Mount Redoubt, Alaska, eruption in 1989/90 clearly show that the volcanic plume looked like fog or weather clouds.
When an aircraft encounters a volcanic plume and the pilot is not able to escape contact with the ash particles, a number of problems may develop. Within a few minutes and often without warning, the crew will most likely experience multiple engine malfunctions. These problems show up as an increase in turbine temperature (EGT/ITT), often exceeding maximum turbine temperature limits.
At night the crew may see a glow in the inlet, tailpipe torching, and non-recoverable engine surges. The engines may even flame out. On a flight from Amsterdam to Anchorage, a three-month-old 747-400 encountered an ash cloud from the erupting Mt. Redoubt. All four engines ingested ash and flamed out. The crew successfully restarted the engines and landed safely at Anchorage.
All four engines were replaced and many airplane systems also had to be repaired or replaced. The airplane environmental control system was replaced, the fuel tanks were cleaned, and the hydraulic systems were repaired. Several other airplanes encountered ash from this eruption, but most damage was minor because operators had been notified of the eruption.
In another incident the pilot decided not to restart one engine because the hot combustor gases had molten the volcanic ash, forming large amounts of glass on the first-stage turbine nozzle and wheel. This led to an imbalance of the center rotating group, causing it to vibrate beyond acceptable limits. In his report, the pilot stated that he elected not to restart the engine because he thought it might be ripped off the wing.
Once inside a volcanic plume, the pilot must follow certain guidelines to improve the chances for a successful outcome. First, fly out of the volcanic cloud by making an immediate 180-deree turn. Don’t try to out-climb the cloud or maintain a constant heading.
This should be followed by reducing power to reduce turbine inlet temperature (TIT). A reduction in TIT helps prevent glassification and the deposit of ash particles on static hot-section airfoils and tiny cooling holes in the hot section.
Next, switch the auto-ignition or continuous ignition system to On. Due to decreased engine-surge margin, limit the number and rate of power lever movements to prevent a compressor stall and subsequent flameout. In general, try to maximize the extraction of bleed air to further improve engine surge margins, which assures an unimpeded airflow through the compressor.
However, it should be understood that turning on engine anti-ice systems or any other bleed-air system means taking away some hot-section cooling air. The immediate result would be an increase in TIT. Therefore, the decision to use bleed air during a volcanic encounter is made by the pilot-in-command after weighing the pros and cons of using bleed air versus power settings.
Finally, time and altitude permitting, the crew should conduct a real-time damage assessment to determine the degree of engine damage and the troublefree range of operation. This should be done one engine at a time.
After successfully exiting a volcanic plume, the aircraft should be landed at the nearest suitable airport. Upon touchdown, limit the use of reverse thrust, and do not taxi too fast because it may encourage the ingestion of additional ash.
After shutting down the engines the aircraft must be considered unairworthy. It must not be flown again until a complete and thorough inspection is done in accordance with all inspection criteria outlined in the FAA-approved maintenance manual.
External inspections should be concerned with ash deposits on surfaces and in ports, vents, and linkages. Ash will cling to exposed, lubricated surfaces and may penetrate any conventional seal. All drains and ports (for example, the P2T2 sensor) must be free of contamination.
The engine inlet should be inspected for erosion and any acoustic liners that may be broken. Power lever and condition lever cables and linkages as well as valve actuators need to be checked for possible ash contamination.
For additional information on the subject of a volcanic encounter or any other topics related to TPE331 engine operations please send an e-mail message to me at Helmuth.Eggeling@Honeywell.com