Monday, 19 April 2010

How thick?

It is a well established phenomenon that certain types of volcanic emissions can stop a jet engine by forming glassy deposits within the burners and clogging them. One thing that has bothered me since last week is how dense does a cloud have to be to pose a significant risk to a jet engine? Not having access to empirical data is a problem. Relying on computer models alone is not a good idea because these cannot accurately reflect reality without additional input from real life sources.

The image on the top left is an image from NASA's Aqua and Terra satellites. In it, the plume from Eyjafjallajökull appears to be dissipating to relative transparency several hundred miles downrange.

Given that damage to jet engines can occur when passing through relatively dense concentrations of airborne volcanic particulate matter as in the oft quoted 'Flight Nine' near disaster, the key question is how dense does a plume have to be to inflict such a catastrophic failure? Surely the answer to this conundrum should already be in the public domain for airlines and regulating authorities to refer to. Especially if the people entrusted with making critical decisions about closing airspace are to make good judgement calls in a timely and informed manner. This news item refers to the damage incurred by several Jet fighters which flew through the plume. The question is, was this a visible section of the ash cloud they flew through or invisible particulate matter? We don't know.

Maybe we're missing some key information, and it exists in an obscure Engineering text from Rolls-Royce or Pratt & Whitney who have designed Jet engines which can cope with having the equivalent of a jet propelled chicken thrown into the compressor blades at several hundred miles per hour every couple of seconds. See video below.

Then there is this 2007 promo video where sand is ingested through a jet engine during a test. Sand is mostly silica which is the basis for glass, so its behaviour within the high temperature environment of a Jet engine should not be that different from glassy volcanic particulates from a volcanic eruption, shouldn't it?

No one, short of the most rabid Jihadi wants to see aircraft falling out of the sky, so how much can a modern jet engine actually take? More to the point, how many commercial aircraft are equipped with the quality of engine which can shrug off sand like the one in the video? Just a thought.

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