The volcano in Eyjafjallajokul, Iceland

Lightning streaks across the sky as lava flows from a volcano in Eyjafjallajokul, Iceland.

Friction between the ash particles in the volcanic cloud creates enormous amounts of electricity that sparks and discharges around the crater.

The same electrical build-up is a danger to aircraft flying through volcanic ash and can cause the aircraft fuselage to glow. This effect is similar to St Elmo's fire.

Danger to Aircraft Engines
The other danger to aircraft engines is the adverse effect that volcanic ash has inside the modern aero engine, primarily in jet engines. The high temperatures and compression that is essential for propulsion in jet engines, melts the silica in the ash and converts into a sticky goo that clogs essential fuel jets, inlets and outlets.

The struggling engines can generate excess amounts of unburnt fuel, which escapes and ignites as it leaves the engine. This gives the impression of an engine fire but it is really a fire in the exhaust jet.

The next stage occurs when the jet engine becomes so clogged with sticky volcanic ash that it 'flares out' or extinguishes and shuts down. This can happen to all onboard aircraft engines within a very short space of time.

Re-starting a Jet Engine
Re-starting a jet engine in mid-flight is a standard procedure for aircraft pilots but when all your engines are out, it becomes much more stressful. Especially, if the engines are so clogged with sticky goo that the fuel cannot get through to the combustion chambers.

The problem with this scenario is that the sticky ash goo that clogged and stalled the engines is still present and it will continue to prevent the engine from re-igniting.

BA Flight
In the, now infamous, BA flight in which this scenario happened, the passengers and crew were fortunately saved by a number of lucky coincidences.

Once all the engines 'flared out', the outside temperature at high altitude quickly cooled the inside structure and the sticky goo quickly became brittle, causing pieces of it to break off, leaving a sufficient number of fuel jets clear to re-start the engines.

Once the aircraft was clear of the volcanic ash cloud and with the engines restarted, they were able to make a safe landing.