Engines flying through the volcanic ash cloud risk impaired compressor performance or even a compressor stall, as well as damage to the high-pressure turbine blades and the turbine nozzle as a result of overheating, manufacturer CFM International has warned.
Volcanic ash is "solid, small, hard and sharp particles", says Jacques Renvier, senior vice-president systems engineering at CFM partner Snecma. "It's very abrasive and the concern is that when encountered in the high-pressure compressor, it will wear the blades and vanes, and you can reduce the length of the blade chord. You will impact the aerodynamic behaviour of the compressor, potentially leading to engine stall and, ultimately, loss of thrust."
Particles entering an engine's combustion chamber would encounter very high temperatures, of up to 2,400°C (4,352°F), and melt before reaching the high-pressure turbine's nozzle - where temperatures are lower - causing them to deposit on the vanes. This would have "a couple of effects", Renvier says.
"One is to close the ventilation holes, so you reduce the cooling of the nozzle. Those nozzles are working in a very hot environment - it can be about 1,400°C - and the cooling air is about 700°C, so this, with a little bit of time, will damage the nozzle. But the main impact is that this accretion of glass reduces the efficiency of the fuel mixing and increases the pressure in the flowpath section, which at high altitude can lead to an HP compressor surge and engine flameout. At the end, we can lose thrust because there is some kind of blockage of the engine by this accretion of glass."
The ash particles could also enter the cooling and ventilation circuit.
It has been proven that there is "a real threat" to engines from volcanic ash, concludes Renvier. In 1982, a British Airways Boeing 747-200 lost all power in a cloud of volcanic ash over South Jakarta, and, while the aircraft glided to a safe landing, the damage to the engines was irreparable. In 1989, a KLM 747-400 suffered multiple engine failure after encountering an ash plume from Alaska's Mount Redoubt volcano.