The real danger to air passengers is not the ash cloud ? it's these men

The two forthright Irishmen at the helm of Ryanair and British Airways have been outspoken in their criticism of the Met Office and aviation authorities over their assessment of the risk posed to aircraft by the plume of volcanic ash drifting in and out of UK airspace.

Willie Walsh, chief executive of International Airlines Group, formed from the merger of BA and Iberia, has joined his business rival Michael O'Leary, who heads Ryanair, in condemning the forecasts and risk assessments from the Civil Aviation Authority (CAA) and the Met Office.

Mr Walsh said yesterday that he had sent one of his aircraft through an area designated a "red zone" by the CAA and found "absolutely nothing" in terms of visible damage to the cockpit windows, aircraft surfaces or engines.

He aligned himself with Mr O'Leary, who had done something similar the day before with a Ryanair test flight at 41,000ft in Scotland, using some very colourful language to describe the "mythical" idea of red zones established by the CAA and the Met Office.

The chief executives have a track record of decrying the attempts by aviation authorities to bring some science to the assessment of the risks posed by volcanic ash. Last year, Mr Walsh said that the closure of UK airspace in the aftermath of the Eyjafjallajokull eruption was "a gross overreaction to a very minor risk", while Mr O'Leary said that the closure was "completely unnecessary".

But the men are guilty of playing to the gallery of public opinion in their criticism of the measures taken to minimise the risk to air travellers. They know their stock will rise by condemning those whose job it is to ensure airline safety - even if it means flight cancellations.

A single satellite snapshot apparently showing no volcanic ash in an area designated a red zone does not disprove the methodology used by the CAA and the Met Office, as Mr Walsh implied. And a single test flight at one altitude does not show that the entire concept of a red zone is a myth, as Mr O'Leary suggested on Tuesday.

The idea of a red zone, where the concentrations of volcanic ash exceed 4,000mcg per cubic metre, was instigated to get around the need for the blanket closure of airspace applied last year. This is the concentration of ash that is recognised as posing a significant risk to aircraft.

There is little scientific dispute over the risk to any aircraft that flies though a cloud of volcanic ash, which acts like a harsh, abrasive powder that can enter the smallest nooks and crannies of a plane's engine. The ash is composed of tiny fragments of pulverised rock and glass and it can grind away an aircraft's exterior, such as the windows of the cockpit and the blades of a turbine. But it also melts at the high temperatures inside a jet engine and sticks to the moving turbine blades, which alters their aerodynamics and causes the engines to stall.

This happened to a British Airways Boeing 747 in 1982, when at 36,000ft it flew too close to an ash cloud from the Galunggung volcano in Indonesia.

All four engines failed and the pilot only just managed to restart them after descending to 12,000ft, at which point the engines had cooled down enough for the melted volcanic glass to splinter from the turbine blades.

A study published earlier this year by Icelandic and Danish scientists supported the measures taken in 2010 to minimise the risk to aircraft from the Eyjafjallajokull eruption.

Tests on volcanic ash indicated that it retains its highly abrasive nature for several weeks after it is sent into the air.

The difficulty for the Met Office and the CAA is predicting where the current plume of ash from the Grimsvotn volcano will drift. They use a plethora of scientific instruments, from optical sensing machines on the ground to satellites in space, to gauge the thickness and whereabouts of the ash. They also model its future drift and altitude using sophisticated computer models of changes to wind speed and direction. When Mr O'Leary denounced the red-zone concept on Tuesday after a single flight at 41,000ft, he failed to mention that the Met Office had predicted that the ash cloud would not be at this altitude at this point in time.

Equally, when Mr Walsh said his own test had found absolutely nothing, he failed to say that the red-zone idea is based on a probabilistic notion, where dangerous concentrations of ash are likely, rather than definitely, found.

The next time Mr Walsh holds up a small vial of volcanic ash to emphasise just how small and insignificant the concentrations in a red zone are, he should be made aware that he, along with the CAA, may be held to account if one of his aircraft falls out of the sky after flying through an ash plume.