Wednesday, April 21, 2010

Airspace Realities

Europe's airways are open again, with a few exceptions. The volcano is still erupting, the weather pattern has not changed: what has changed is the definition of 'safe to fly'. The realities have forced a hard cold look at the regulations, and their urgent revision.
The airlines forced this revision. The people most experienced in safe airline operations were able eventually to influence those who have the power of decision.

The computer model used by the regulators is called NAME - Nuclear Accident Model. NAME was developed after the Chernobyl disaster. It is designed to model the spread of radioactivity after a release into the atmosphere. I understand it performed well, where it was possible to check predictions against measurements, but the information needed when assessing air safety around a volcano differs from that after a nuclear accident.

Critical parameters for an aircraft flying into dust include the mass of dust per unit air volume, the average size of the dust particles, and the speed of the aircraft and its engines.
I understand the smallest ducts in a jet engine are in the turbine blades, passing cooling air. These are sized around 1 millimetre, so may be blocked by particles of that size.

From the Icelandic volcano central England lies approximately 1000 miles south-east. So volcanic dust in a 50 mph wind from the north-west will take 20 hours to get here. In practice the dust followed a longer path, in the clockwise circulation round high pressure west of Ireland.
So what size of volcanic dust grain will still be airborne more than 20 hours after eruption?
I don't know, but it must be very small, micrometric at the most, I would guess.

Really we should talk about 'smoke', not 'ash' or 'dust'. An aircraft or engine susceptible to damage by smoke should not be flying at all.

Obviously no aircraft should fly through the erupting plume. The urgent need is to develop means to plot contours of maximum particle size and concentration around the eruption, and to determine the safe limits for aircraft. The danger zone round a volcano is likely to be much smaller than that round a nuclear release - radioactive smoke might well be more dangerous that radioactive dust.

It is now imperative to measure and model, so future airways management is evidence based. And I would restore to airlines the responsibility to decide when it is safe to operate, tasking the meteorological services to provide reliable information.

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