Investigation Summary
The encounter occurred as the aircraft entered what appeared to be a normal-looking cloud at 25,000 feet. Unbeknownst to the crew, the cloud contained high concentrations of abrasive silicate ash from Mount Redoubt, which had erupted earlier that day. As the crew increased power to climb out of the cloud, the engines ingested the ash. The silicates melted in the hot turbine sections and fused into a glass coating on the nozzle guide vanes, disrupting the airflow and causing compressor stalls in all four engines. The resulting engine failures led to a loss of main electrical power, causing flight instruments to momentarily blank as they transitioned to standby battery power. The crew attempted numerous restarts during a rapid descent. Engines 1 and 2 were successfully relit at approximately 13,000 feet, followed by engines 3 and 4 at 11,000 feet. The aircraft landed safely, but the ash had sandblasted the windshields to the point of near-opacity and caused extensive internal damage to the avionics and engines.
Final Conclusions
The NTSB determined the probable cause was the inadvertent encounter with a volcanic ash cloud, which resulted in foreign material damage and subsequent compressor stalling of all engines. A significant contributing factor was the lack of available information regarding the ash cloud's location provided to the flight crew and operational personnel. The investigation was completed in mid-1992 (Report ANC90FA020).
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
General Electric CF6-80C2B1F Engines
High-bypass turbofan engines that failed when ingested volcanic ash melted and solidified on turbine components, causing compressor stalls.
Standby Electrical System
Provided backup power via batteries and inverters to critical flight instruments when the four main engine-driven generators failed.
FADEC
Full-authority digital electronic controls managed engine parameters; recorded data showed EGT and core speed fluctuations during the restart attempts.
Interesting Facts
- 01The aircraft was named 'City of Calgary'.
- 02The aircraft was nearly new, having been in service for only six months with 705 airframe hours.
- 03All four engines had to be replaced due to internal glassification and erosion damage.
- 04The total repair cost exceeded $80 million, one of the most expensive non-hull-loss incidents at the time.
- 05The windshields were so badly abraded by the ash that the pilots had difficulty seeing the runway during landing.
- 06A shipment of animals in the cargo hold suffered fatalities due to delays and mislabeling following the emergency landing.
Safety Actions & Advisories
Volcanic Ash Advisory Centers (VAAC)
The incident accelerated the development and coordination of international VAACs to provide real-time tracking and warnings of volcanic plumes to aviation.
Pilot Training for Ash Encounters
Procedures were updated to advise pilots to reduce thrust to idle if ash is encountered, as lower operating temperatures reduce the risk of ash melting and glassifying.