CFM56: engines to withstand the atmospheric conditions
"There has never been an accident linked to a cloud of volcanic ash," says Jacques Renvier, senior vice-president engineering, Snecma (Safran group). There have just been a few rare incidents following passage through highly concentrated clouds of ash, since the particles could infiltrate into the combustion chamber and then be deposited in the high-pressure turbine, reducing the air passage cross-section and causing a loss of thrust. Also, the ash in suspension in the air causes wear and tear on the compressor blades, which again might lead to a loss of thrust.
Design and certification
There is no certification standard for engines as far as ash is concerned. "Too many factors are involved for defining a limit," Jacques Renvier explains: "The nature of the ash, its concentration, the exposure duration, the engine characteristics... There are no technical solutions regarding the ingestion of heavy concentrations. Improving the current capacity of the engines to ingest ash would adversely affect the engine in terms of weight, fuel consumption and CO2 emissions, while the best remedy consists in avoiding passage through a visible ash cloud." That said, CFM56 engines are certified for many other meteorological hazards: icy conditions, chunks of ice, hailstones, ingestion of water and hail, etc. "The design takes account of these risks," Jacques Renvier points out. "The most exposed components - fan blades, compressor blades - are reinforced. We seek to limit the quantity of ice build-up through the geometry (conical form of the nose cowl) or else by means of thermal anti-icing, and the quantity of water and hail penetrating into the high-pressure compressor by optimizing the discharge valves upstream in order to evacuate as much of it as possible." During certification testing, we project hailstones, large quantities of water and hail and sheets of ice into the engine in order to verify the resistance of the engines across their operating range.
The essential role of the pilot
As far as volcanic ash is concerned, "The instruction remains to avoid flying through visible clouds of it," Jacques Renvier goes on to say. "If this occurs, the instruction is to throttle back and get out of the cloud." The risk disappears at a distance of beyond 800 to 1000 km from the volcano. The large particles will have fallen from the sky and the smaller particles will only have the effect of causing premature wear and tear on the engines, which will be detectable during maintenance operations. The role of the pilot remains essential in managing flight under these conditions and it is therefore vital that he or she has reliable information about the location of the volcanic cloud and its characteristics.