A volcanic ash cloud produced by Chile’s Puyehue volcanic eruption has circumnavigated the globe and floated over Australia twice, disrupting flights and leaving over 120,000 passengers stranded.

Dr John Olsen from the University of NSW’s School of Mechanical and Manufacturing Engineering, explains why airlines won’t let their pilots fly through the volcanic ash cloud.

Why can’t planes fly through the volcanic ash cloud?

They can, but the trouble is, there are a lot of long term consequences. If you have a low density cloud of ash, you can get erosion of gas turbine blades and you can get build ups on some of the components in the engine.

Whilst this may not stop the engine then and there on the spot, it could have consequences in terms of the long term operation and safety of the engine.

So it’s not necessarily the case that they would drop out of the sky?

No. If you were to fly through a relatively dense cloud of ash, like straight over the volcano itself, you could be in trouble. But that’s not what the problem is here. In Australia, we are nowhere near the volcano, but nonetheless the ash is still scattered around in the atmosphere.

The fan blades at the front of the engine might be affected but it’s not just that. Deeper within the core of the engine itself, you could end up with deposits coating the turbine blades. It could get into the seals and the bearings and that could have some nasty effects in the long run.

There have been cases of aircraft with engine outage situations but that is because the ash density has been quite high in those areas.

It’s not just the engine. A very dense cloud of volcanic ash could affect the ability of the pilot to see and if you are flying through the plume, it could have an effect on the wind screen. They can land aircraft on instruments but it is nice to be able to see.

Does the chemical make up of the ash make a difference?

I believe so. It depends on what type of stuff is coming out of the volcano. You can get glassy material that has a melting point lower than the temperatures you get within the engine itself.

You get temperatures of less than 690K kelvin before you get into the combustion chamber but downstream of the combustion chamber you start to get very high temperatures approaching about 1700K under certain operating conditions.

So the turbine blades will be most affected. It’s there that this glassy substance can start to melt and then, as it cools, coat the blades. It’s not so much the compressor blades or fan blades – there would be some erosion of them but it is deeper into the engine that you will end up with further problems.

Are the airlines grounding flights because they want to save money on repairing the planes or because it is actually a threat to passenger safety?

It’s probably both. It’s expensive and it could be dangerous in the long run. There’s maintenance done on these aircraft but they don’t want to pull the engines out to service them if they can help it. It would be a costly exercise to replace those things.

Why can’t planes just fly under the ash cloud or over it?

They tend to fly in certain corridors where it’s going to be most efficient to fly.

So why have some airlines made the decision to fly earlier than others?

I would think that it depends on the risk assessment by the individual airline. Some airlines may have more experience with this than others, with the Iceland volcano for example.