Volcanic ash engine damage is less of a risk in piston engines than turbines, because the combustion air is normally filtered, according to the UK Civil Aviation Authority.

The CAA advises piston operators to restrict the use of carburettor hot air in flight because, when they select it, it bypasses the filter. The authority adds, however, that "the risk of ash ingestion must be balanced against the risk of [carburettor] icing".

A CAA flight operations communication tells light aircraft pilots that "if any ash damage becomes apparent in flight, possibly by windscreen or leading edge impact, pilots should attempt to leave the area either horizontally or vertically or both".

When ash has been reported in the region but is not visible, CAA advice is not to fly in or close to cloud, especially cumuliform cloud, because it can be an indicator of where ash is most concentrated: "Volcanic ash particles provide ideal cloud (and ice) nuclei."

A high concentration of ash may show as increased haze, and the CAA points out that just below the top of the haze layer, "dust concentration levels are usually high and discrimination between normal dust and ash will be extremely difficult".

Meanwhile, the extent of any airframe, propeller, rotor and intake damage depends on the impact speed of the ash, so the advice is to consider the option of flying more slowly.

The ultimate warning is that "it may be prudent to avoid flight over areas where the choice of landing areas in the event of engine failure is limited, even in twin-engined aircraft".

The agency also warns pilots to be aware that "ash may also find its way into pitot/static systems, or affect the lubrication of moving parts such as rotor heads, gearboxes and other bearings".

Source: Flight International