Have you ever thought what might happen when you fly your TPE331-powered Twin Commander through the plume of an erupting volcano? Obviously, you would never penetrate the volcanic cloud on purpose! However, due to the similar appearance of an ordinary weather cloud and unless you are aware that an active volcano is located near your intended route of flight, you may not realize that you are in the middle of an abrasive and dense cloud of volcanic ashes, especially where the plume is imbedded (IMC) or during the night.
The United States, particularly Alaska, California, Hawaii and the State of Washington, on average have experienced more than two dozen volcanic eruptions in the past 100 years, or one eruption every four years. There are no signs these statistics will change in the future.
Moreover, historical data suggest that about two-thirds of all volcanic eruptions 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, it’s nearly impossible to make long-range forecasts—and thus avoidance plans—of eruptions. 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. http://www.ssd.noaa.gov/VAAC/ is another excellent source for current volcanic activities.
In 2010, the volcanic ash cloud from Iceland’s Eyjafjallajokull eruption resulted in a $1.7 billion revenue loss to airlines and remained a threat to flight operations for months as it drifted from the mid-North Atlantic to Spain and Portugal. Airline Pilots Association’s (ALPA) Executive Air Safety Vice Chair testified that the eruption demonstrated a need for improved forecasting and standardization of information about the spread of the ash clouds, so that pilots can plan exit strategies. ALPA, for example, had received conflicting reports from air crews of different airlines. One report noted extensive ash coverage in an area, while another showed no ash coverage at all. Also noted was “the disturbing fact” that radars in use by air traffic control and on board aircraft cannot detect volcanic clouds and toxic gases, which pose a health hazard to crews and passengers.
Later that year, NASA launched the Glory Satellite, to help advance the study of volcanic composition. As a result, a cloud camera system provides images that allow the APS scans along the spacecraft ground track to be put into spatial context and to facilitate determination of cloud occurrence within the Aerosol Polarimetry Sensor (APS) instantaneous field of view.
Let us discuss how an encounter with volcanic ashes could affect the turbine engine.
THE UNIQUE PROPERTIES OF VOLCANIC ASH
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, on its path through the compressor section, the ash particles can be pulverized from 38 microns to about 5 microns by the time the ash particles reach the combustor and turbine section. In comparison, fine Arizona dust can be as large as 200 microns.
In addition to being small, the ash particles have been measured to have a hardness of 6 on the Mohs’ Hardness Scale. That compares to the hardness of quartz, which can cause scratches on both glass and aluminum. Another feature is the fact that volcanic ash particles have been found to be electro-statically charged while suspended in the plume. Consequently, ash particles will be attracted and cling to surfaces, which the particles have come in contact with.
Also, typical volcanic ash has acidic properties that range from mildly acidic, having an pH factor of about 5.5 to highly acidic, or a pH factor of 2.0 (pH 7.0 is considered neutral). High acidic values imply that the volcanic ash particles possess the harmful effect of corroding aircraft components at an accelerated rate if left unchecked (un-treated). Finally, and most significantly, volcanic ash has a melting point of about 1150° C. That temperature falls well within the operating range of most turbine engines.
IN-FLIGHT OPERATIONAL PROCEDURES
A Daylight and Visual Meteorological Conditions (VMC) encounter with an active eruption should be obvious and can be avoided in most cases; especially when the volcanic plume reaches heights of close to 100,000 feet. Even large eruptions like Mt. St. Helens, reportedly having ejected four-hundred million tons of debris within a nine hour period and its cloud ascending at 3,000 feet per minute, should give any alert flight crew enough time to avoid direct encounter with the volcanic ash. Mount Pinatubo, on the other hand, was ten times larger than Mt. St. Helens and presented a much larger challenge of avoidance.
Night time and/or Instrument Meteorological Conditions (IMC) eruptions, are extremely difficult to detect. There are several reasons why it is difficult to detect a volcanic plume at night or in IMC: First, an airborne weather radar, designed to “see” water droplets only, cannot “see” volcanic ashes. Second, when flying in IMC it is visually difficult to discern volcanic ashes from water. Actual photos taken of the Mount Redoubt eruption clearly illustrate that its 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, the following problems may develop: Within a few minutes of flying in a volcanic cloud 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, non-recoverable engine surges and flameouts of all engines may occur. In fact, on a 1989 flight from Amsterdam to Anchorage, Alaska, a new 747-400 (only three months old with approximately 900 hours total flying time) encountered an ash cloud from the erupting Mt. Redoubt near Anchorage. 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’s 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 ashes forming large amounts of glass on the first stage turbine nozzle and wheel. This had caused an imbalance of the center rotating group causing it to vibrate beyond acceptable limits. In his report, the pilot stated that he had elected not to restart the engine because he thought that the engine might be ripped off the wing if he continued with the start.
What should be learned from these reports is that once inside the volcanic plume the pilot must adhere to the following guidelines to improve his/her chances for a successful outcome of the encounter. First, fly out of the volcanic cloud by making an immediate 180-degree 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 preventing glassification and deposition of ash particles on static hot-section airfoils and in its tiny cooling holes. Next, switch the auto-ignition or continuous ignition system to ON. Due to a decreased engine surge margin, the number and rate of power lever movements should be limited to prevent a compressor stall and subsequent flameout. In general, the extraction of bleed air should be maximized to further improve the surge margins, which assures an unimpeded airflow through the compressor.
However, it should be understood that by turning ON engine ant-ice systems, or any other system that extracts bleed air, air that is to be used for hot section cooling is now taken away. 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 who must weigh the pros and cons of using bleed air versus power settings.
Finally, with time and altitude permitting, the crew should conduct a real-time damage assessment to determine the degree of engine damage and the trouble-free range of operation. This should be done one engine at a time.
POST-FLIGHT INSPECTION PROCEDURES
After successfully recovering from the flight through the 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, which may encourage ingestion of additional ash. Finally, after landing and shutting the engines down, the aircraft must be considered “air-unworthy.” This means it must not be flown again until a complete and thorough inspection is completed in accordance with all inspection criteria outlined in the FAA-approved maintenance manual.
External inspections should be concerned with the deposits of ash on surfaces, in ports, vents and in 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 call me at 480-399-4007 or send an email message to [email protected].
