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2
Philippine Institute of Volcanology and Seismology, Quezon City, Philippines.
3
Michigan Technological University, Department of Geological Engineering, Houghton, MI 49931.

ABSTRACT
The explosive eruptions of Mount Pinatubo in June 1991 injected enormous clouds of volcanic ash and acid gases into the stratosphere to altitudes in excess of 100,000 feet. The largest ash cloud, from the June 15 eruption, was carried by upper level winds to the west and circled the globe in 22 days. The June 15 cloud spread laterally to cover a broad equatorial band from about 10°S. to 20° N. latitude and contaminated some of the world's busiest air traffic corridors. Sixteen damaging encounters were
reported between jet aircraft and the drifting ash clouds from the June 12 and 15, 1991, eruptions. Three encounters occurred within 200 kilometers from the volcano with ash clouds less than 3 hours
old. Twelve encounters occurred over Southeast Asia at distances of 720 to 1,740 kilometers west from the volcano when the ash cloud was between 12 and 24 hours old. Encounters with the Pinatubo ash cloud caused in-flight loss of power to one engine on each of two different aircraft. A total of 10 engines were damaged and replaced, including all four engines on a single jumbo jet.

Following the 1991 eruptions, longer term damage to aircraft and engines related to volcanogenic SO2 gas has been documented including crazing of acrylic airplane windows, premature fading of polyurethane paint on jetliners, and accumulation of sulfate deposits in engines. Ash fall in the Philippines damaged aircraft on the ground and caused seven airports to close.

Restoration of airport operations presented unique challenges, which were successfully met by officials at Manila International Airport and at Cubi Point Naval Air Station, Subic Bay. Lessons
learned in these clean-up operations have broad applicability worldwide.

Between April 12 and June 9, 1991, Philippine aviation authorities issued at least eight aeronautical information notices about the preeruption restless state of Mount Pinatubo. The large number of
aircraft affected by the Pinatubo ash clouds indicates that this information either did not reach appropriate officials or that the pilots, air traffic controllers, and flight dispatchers who received this information were not sufficiently educated about the volcanic ash hazard to know what to do with the information.

INTRODUCTION
Jet aircraft are damaged when they fly through clouds containing finely fragmented rock debris and acid gases produced by explosive volcanic eruptions (Casadevall, 1992). Clouds of volcanic ash and corrosive gases cannot be detected by weather radar currently carried aboard airplanes, and such clouds are difficult to distinguish visually from meteorological clouds. In the past 15 years, there have been more than 80 in-flight encounters between volcanic ash clouds and commercial jet aircraft.

The explosive eruptions of Mount Pinatubo volcano in the Philippines in June 1991 injected enormous clouds of volcanic ash and gases into the stratosphere to altitudes in excess of 100,000 ft. Within several days of the June eruptions, at least 16 commercial jet airplanes had been damaged by in-flight encounters with the drifting ash clouds from Pinatubo. Closer to the volcano, ash fall in the Philippines damaged about two dozen aircraft on the ground and affected seven airports. This report describes the effects of the 1991 Pinatubo eruptions on aircraft and airports, seeks to understand why so many encounters occurred, and reviews the solutions to the ash-cloud hazard reached by Philippine
authorities.

Incident number Date Time (G.m.t.)1 Location Latitude Longitude Altitude (feet) Aircraft type Comments
91-01 6/12/91 0420 170 km from volcano; 60 nautical miles from LUBANG along air route B460.
14°00' 119°30' 37,000 747-300

During a 3-min encounter with volcanic ash, crew experienced thin haze inside aircraft that smelled like a burning electrical wire. Aircraft landed safely at Manila Airport. Aircraft and engines were inspected and serviced at Manila in accordance with recommended procedures. When aircraft attempted to depart, its four engines had a strong vibration, and aircraft was grounded at Manila for detailed maintenance and replacement of all four engines.

91-02 6/12/91 uk 720 km west of volcano on route from Singapore to Tokyo.
13°50' 113°50' 37,000 747-400

No significant damage to aircraft when inspected
on ground in Tokyo.

91-03 6/12/91 1630 Approx. 1,000 km from volcano; between way points
ADPIM and 11°10' 112°10' 33,000 DC-10
series 40 Flight from Kuala Lumpur to Tokyo;

observed a discharge phenomena on windshield for 20 min. Ground inspection at
Narita revealed no LAVEN. damage. Encounters 3 and 11 involved same aircraft.

91-04 6/15/91 1740 Approx. 1,150 km from volcano; between way points
SUKAR and CAVOI. 13°10' 110°50' 29,000 747-400

Aircraft encountered ash cloud at 29,000 ft at approximately 600 nm west of volcano. Crew observed St. Elmo's fire on the windshield and a scent similar to an
electrical fire in the cockpit for 6 to 8 min as they went through the ash. There was no abnormal indication in the cockpit. The crew observed a green echo, which seemed to be ash on weather radar, but it disappeared when they
were clear of the ash. Flight attendants reported thin (whitish) fog in the cabin, most dense in the upper deck compartment, followed by the forward cabin. The flight was continued to Tokyo, where engine inspection revealed that all four
engines were damaged and were replaced. First- stage nozzle guide vane cooling air holes were 70-80% blocked. Other damage occurred to the cockpit windows, cabin windows, Pitot static probes, landing light covers, navigation lights,
and all leading edge areas.

91-05 6/15/91 1547 Over Vietnam on route from Hong Kong
to Singapore; in Bangkok FIR.
13°00' 108°00' uk 747-SP
Ash and sulfur odor, electrostatic discharge, blue-green light over
Vietnam. Ground inspection revealed no significant damage, and
aircraft continued in service.


91-06 6/15/91 uk uk uk uk uk 747-200

freighter Aircraft flew through "heavy volcanic ash." Cockpit and cabin areas
were contaminated with volcanic ash. No additional information
available.

91-07 6/15/91 uk Route between Tokyo and Singapore.
uk uk 35,000 747-251

Flight from Narita to Singapore was rerouted to Manila due to weather
in Singapore area. En route to Manila, encountered volcanic ash
cloud at 35,000 ft for approximately 12 min and was then diverted to
Taipei. Engines set at cruise. Sparks were noted coming from windows
and Crew reported hearing ash hit the aircraft. EGT for all four engines rose 40-50°C and started to fluctuate. One hour later all EGTs were
back to normal. Ground inspection in Taipei revealed no significant
damage to exterior or to engines. Aircraft continued in service.

91-08 6/15/91 uk <200 km from volcano; on
approach to Manila from south.
uk uk uk DC-10
series 30

Flight from Sydney to Manila encountered ash on approach to Manila from south. Engines set at low power but found to contain "lots of ash" when inspected after landing. Exterior abrasion visible, including engine cowls.

91-09 6/15/91 uk Route between Singapore
and Osaka. uk uk uk 747-300

Aircraft was in ash cloud for 29 min while en route from Singapore to Osaka. Date of encounter uncertain, probably 6/15; one report indicates 6/19.
Inspection of aircraft exterior showed no significant damage.
Engines #1 and #4 were replaced; "90% of the first-stage turbine blades
have bullseyes on the airfoil's mid-span pressure side and some first-stage vane leading edge ash buildup at 3 o'clock position."

91-11 6/15/91 1730 Approx. 1,050
km from 15°15' 110°30' 29,000 DC-10 series 40

Flight from Kuala Lumpur to Tokyo; volcano; between way points SUKAR
and CAVOI, 120 nautical miles from CAVOI. observed a discharge
phenomena on windshield for 25 min. Ground inspection at Narita revealed no damage. Encounters 3 and 11 involved same aircraft.

91-12 6/15/91 1910 Approx. 1,050
km from volcano; between way points SUKAR and CAVOI,
120 nautical miles from CAVOI.
15°15' 110°30' 29,000 DC-10
series 40

Flight from Singapore to Osaka; crew
observed a discharge phenomena on
windshield for 30 min. Ground inspection at
Narita revealed no damage.

91-13 6/15/91 0910 Approx. 100
km from volcano; flight from Manila to
Hong Kong. uk uk uk 747-428

After takeoff from Manila, airplane skirted a volcanic ash cloud.
On the ground in Hong Kong, black marks were noted on the exterior of the left wing. Engines were borescoped and no discrepancies were found. Airplane continued to Delhi. Preparing to leave Delhi, unable to start engine #1. Fuel pump was replaced and additional inspections of airplane revealed no damage. Airplane continued to Paris.


91-14 6/16/91 uk Route between Kuala Lumpur and Kota Kinabalu.
uk uk uk 737-200

freighter Indications that aircraft flew through volcanic ash cloud were apparent only after aircraft underwent ground inspection in Kuala Lumpur, which revealed abrasion of plexiglass landing light covers and navigation lights, which were totally opaque. Cowling intakes were abraded and rough to the touch, while compressor blades were remarkably clean. Landing gear
bays were covered in ash with ash sticking to oily surfaces. No apparent damage to windshields.

91-15 6/17/91
(?) uk Flight likely on Tokyo to
Singapore uk uk uk DC-10

Airplane reportedly encountered ash from
Pinatubo on June 17. #3 route. engine was reported to have been shut down in flight; ash encounter may have caused in- flight shutdown. Inspection of engines revealed "heavy deposits" of what was presumed to be volcanic ash. No information about flight route, encounter duration, and such.

91-16 6/17/91 0412 930 km from volcano; 50 nautical miles
east of way point IDOSI on route A901.
19°30' 112°40' 37,000 747-200B

Flight from Johannesburg to Taipei via Mauritius.
Encounter occurred at 37,000 ft 50 nm east of way point IDOSI on
route A901; entered a cloud at 0412 G.m.t.; temperature increased
from -48°C to -37°C in 2 min; aircraft descended to 29,000 ft and landed at 0540 G.m.t.; engine #1 surged and was shut down; engine #4 lost power; descended to 29,000 ft to restart #1. Aircraft landed safely at Taipei. Service
terminated. Engine #1 replaced and aircraft returned to South Africa on 6/21 for further inspection.


91-17 6/15/91 na Aircraft on ground at
Manila International Airport. 14°30' 121°00'

On ground L-1011 Maintenance crew attempted to remove
volcanic ash from window by using wiper blades. Resulted in abrasion of windows, which required replacement.




DAMAGE
When a jetliner flying in excess of 400 knots (740 km/h) enters a cloud of finely fragmented rock particles, the principal damage will be abrasion of the exterior, forward-facing surfaces and
accumulation of ash into surface openings (Casadevall, 1992). An example of the exterior damage to one jumbo jet after an encounter with a Pinatubo ash cloud is shown schematically in figure 7. Ingestion of ash into the engines will cause abrasion damage, especially to compressor fan blades. Because jet engines operate at temperatures in excess of 700°C, melting of ash and accumulation of this ash in the turbine section is an important problem as well (Przedpelski and Casadevall, 1994).
Remelted ash may block the passage of air through the engines and cause the engine to stop. In an least
one airplane (incident 91-04 in table 1), first-stage nozzle guide vane cooling holes were 70 to 80 percent blocked.
Figure 7. Damage to exterior surfaces of a 747-400 jumbo jet following an encounter with the June 15,
1991, ash cloud from Mount Pinatubo.

The majority of the Pinatubo encounters occurred at distances of up to 2,000 km from the volcano with an ash cloud that was at least 12 h old. The aging of the ash cloud allowed the coarser ash to settle from the cloud and prevented some of the more severe damage such as that which occurred to jumbo-
jet aircraft from earlier encounters with volcanic ash (Smith, 1983; Tootell, 1985; and Casadevall,
1994). In the Pinatubo case, there were few reports of abrasion of forward-facing cabin windows, so it is suggested that particles larger than about 30 m in diameter had already settled from the cloud. Particles smaller than this diameter are efficiently swept over the window surface by the slipstream and do not impact the window surface (Pieri and Oeding, 1991).

Longer term damage related primarily to the SO2 gas and sulfuric acid aerosols produced by the eruption (Self and others, this volume) did not become apparent until months after the eruption. Some Asian-based carriers noted that jet engines on their airplanes have accumulated deposits of sulfate minerals such as anhydrite and gypsum in the turbine. This material blocked cooling holes in the first- stage nozzle guide vane at the inlet to the turbine section of the engine and thereby interfered with the cooling of the turbine. As a result, engines overheated. The sulfate deposits found in the turbine section appear to be related to ingestion and oxidation of SO2 and sulfuric acid aerosols that originated
in the Pinatubo eruption clouds of June 15 (Casadevall and Rye, 1994).

Additional problems related to the acidic aerosols include the increased incidence of crazing of acrylic windows (Berner, 1993) and fading of polyurethane paint on jetliners (T.M. Murray, Boeing, written commun., 1993). Unlike the circumstances involving in-flight encounters with the ash clouds, which
were largely restricted to the region west of the volcano, the gas cloud from Pinatubo has been widely dispersed throughout the Northern Hemisphere and has thereby affected aircraft that fly in this airspace. A similar increase in the incidence of window crazing was observed for several years following the
eruptions of El Chichón Volcano in 1982 (Rogers, 1984; 1985; Bernard and Rose, 1990). Pinatubo erupted nearly 3 times more SO2 than did El Chichón (Bluth and others, 1992). Thus, the types of problems related to volcanogenic sulfur gas and sulfuric acid aerosols may be expected to persist longer following the Pinatubo activity than after El Chichón.


LONG-TERM DAMAGE
In addition to the aircraft damage that was immediately evident in the days following the June 15 eruption, damage related primarily to SO2 gas has been reported by some airline companies and
manufacturers. One year after the eruption, in June 1992, there was an incident involving loss of engine power on a jumbo jet owing to accumulation of sulfate deposits in jet engines. Isotopic studies of these deposits suggest that the sulfate is derived from the ingestion and oxidation of SO2 and
sulfuric acid aerosols that originated in the Pinatubo eruption cloud of June 15 (Casadevall and Rye, 1994). Related problems recognized in 1992 such as the increased incidence of crazing of acrylic windows (Berner, 1993) and fading of polyurethane paint on jetliners are also due to volcanogenic sulfuric acid droplets in the atmosphere. Frequent inspections of aircraft should reveal any corrosion problems due to volcanogenic sulfur gases.

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