How do volcanoes affect a jet engine?

Why is it dangerous to fly through an ash cloud?
26 June 2018

Interview with 

Anna Young, Cambridge University, Jess Johnson, University of East Anglia


Many of us remember being stuck and grounded in 2010, very long queues at airports with no-one going anywhere. In fact, it became the largest global air traffic shut down since World War II, and it happened because of an Icelandic volcano called Eyjafjallajökull. The ash from volcanic eruptions can cause catastrophic damage to aircraft engines. But how? And what can we do about it? Chris Smith spoke to Anna Young from the Whittle Lab in Cambridge University, and later Izzie Clarke spoke to the University of East Anglia's Jessica Johnson. Firstly, Chris asked, why is sand a problem for engines?

Anna - Basically, the ash is sand and sand can cause quite a lot of damage to the engine on its own, in the first place. But, because of its larger surface area, it can melt more easily and when it turns into glass in the hot parts of the engine that can cause really really big problems.

Chris - These are jet engines that we’re dealing with here, that’s what you work on. So can you just explain for people who are not familiar just explain very briefly how a jet engine works?

Anna - Sure. When you’re walking up to your plane the part you can see is the fan -  big set of spinning blades. And the job of the fan is to pull the air into the engine and about 90 percent of the air goes through the fan and then out through the nozzle at the back where it’s pushed out quickly, and that jet going backwards creates thrust to drive the engine forward.

Now the job of the rest of the engine is to use the other 10 percent of the air to drive the fan. So the air that’s not gone through the bypass goes into the compressor where it gets squeezed to a much higher pressure. And then it goes into the combustion chamber where we burn fuel to add energy.

Next we’ve got hot, high-pressure air and that goes through the turbine, which is another set of spinning blades that takes energy out of the air and uses that energy to spin the fan and the compressor. So the turbine uses hot air to drive the fan which produces thrust to drive the engine.

Chris - I went to a talk by someone from Rolls Royce and they summarised and said it’s suck, squeeze, bang, blow. It sucks air in at the front, squeezes it very hard and gets it hot, chucks a load of fuel in which burns, and then they extract energy at the back end having blown the gas stream out to then drive that fan. But I suppose one should point out that the gas stream in there is at very high temperature isn’t it? It’s like 1500 degrees C inside the engine when the fuel’s burning?

Anna - Yeah. At the entrance to the turbine the air is, as you say, about 1500 degrees, and that is hotter than the melting point of the turbine blades.

Chris - The engine’s running at a temperature beyond its melting point, so why doesn’t it melt?

Anna - What we do is we take some air from a cooler part of the engine from part way through the compressor, and we drill tiny little holes in the turbine blades and we blow that cool air through. That cool air creates a film that keeps the turbine just under its melting point basically.

Chris - Ah. So you’re protecting the surface of the blades with a very thin cushion of slightly cooler air so any incoming gases are sort of going to ride over the blade without touching it.

Anna - Yep.

Chris - If I chuck in whole bunch of sand and volcanic dust, what does that do to the system?

Anna - It’s likely to turn to glass because as well as being hotter than the melting point of the turbine blades, that part of the engine is hotter than the melting point of the ash. So we get glass which clogs up all of those little holes and then we  won’t have any cooling air and the turbine will melt.

Chris - Then you get a sort of ‘hot spot’ on the blade, and what will that do cause it to weaken or change shape, deform, distort?

Anna - Yeah. I think it’ll happen pretty quickly at that point and the blades will start melting so then you haven’t got a turbine to drive your fan. The other thing that happens is that air has to go somewhere, and where all the air from the rest of the compressor goes is the combustion chamber. So we have the wrong mix of fuel and air, so that can make the fire in the combustion chamber go out and then you’ve basically switched your engine off.

Chris - And then you lose power catastrophically?

Anna - Yeah.

Chris - Given that we know this, what can aircraft operators do to minimise the damage to their engines - just not fly?

Anna - Essentially, yes. In my lab we spend a lot of time simulating the flow of air through the engine. You could do that and people have done it where you add in the flow of the sand as well, but the problem is that is a very complex process and a very big calculation. So, probably by the time you’ve got your answer as to where the glass is going to form your volcanoes stopped erupting because you need such a big computer and you’ve got to leave it for so long.

Chris - But engineers can say “oh, don’t fly” when there’s a volcano, but pilots may not be able to avoid one if one suddenly goes off. So, under those circumstances, obviously they could try to avoid the ash cloud but there are potentially going to be particles going into the engine so what does that do, just shorten the lifetime of the engine?

Anna - Yeah. If it’s just a case of more straightforward sand which isn’t going to melt then the front part of the engine bears the brunt of that. If you’ve ever been on the beach on a windy day and the sand gets whipped up into your face and it stings, if you imagine doing that at 600 miles an hour that's what happens to the fan and the compressor, so the sand particles blast the blades.

We design the blades very carefully. The tolerance on them is around the width of a hair and then you’re blasting random bits off with sand, suddenly you don’t have nice, smooth shapes that you designed and the air won’t pass as smoothly through. So you start losing efficiency, you start using more fuel. If that’s a commercial plane that means you’ve got to pay more for your ticket, and you’ve probably got to start replacing bits.

Chris - Presumably, if you do end up with this sort of damage to the engine, this can be monitored and you could replace the blades that are worn?

Anna - Yes, yes you can. And one of the things that we do in our lab is we look at different forms of damage and see when is it bad enough that it needs to be replaced.

Chris - Yeah. So you basically know what the ‘safe’ threshold to operate on is?

Anna - Exactly.

Chris - Right then, better just stay on the ground in that case! That was Anna Young from Cambridge University.

Izzie - Jess Johnson is also here from University of East Anglia. What about other technologies? Can volcanic eruptions affect what’s going on at home as well?

Jess - They can, yes. People who live near volcanoes probably do have to deal with some of the issues associated with them on a day to day basis. Particularly if there is ash, it could be drawn into their electronic equipment. Your computer usually has a fan to cool in down in the same way that it could damage the jet engines, but not quite the same way. But the ash can clog up the computers, damage the computers, clog up air conditioning units and things like that.

Ash also acts in a very strange way when it gets wet. It gets quite heavy and almost cement-like and so if you have an accumulation of ash on your roof or on your gutters and then it gets wet then it can be very very heavy. There have been cases of people that have flat roofs, those roofs collapsing under the weight of the ash.

Izzie - My goodness. Obviously, if we have an eruption all of that gas and that ash can get redistributed over the globe so can that also then move onto other areas and perhaps impact people’s lives in other ways as well?

Jess - Yeah, absolutely. Depending on the size of the eruption the ash column can get into different layers of the atmosphere and be distributed. It can affect the climate and it can affect crops as well.

Chris - While we’re talking about gases and things, Jess, there have been reports in the past of volcanoes that are belching up gases which then, being heavier than air, flow down into valleys and settle in low points and asphyxiate people. Is that just apocryphal or does that happen?

Jess - That does happen, yeah. I think the famous example was in Cameroon where there was a bubble of CO2 that got released from a volcano that had a water lake inside it. And, yeah, because CO2 is one of the big gases that is released from volcanoes, and it is heavier than air, it does just settle in a valley and, yeah, people died because there wasn’t any air to breathe, there wasn’t any oxygen.

Chris - When you talked about mud just now I wondered if you were also going to talk about the manifestation that’s happened in Guatemala with mud flows?

Jess - Exactly.

Chris  Because one of the things that Izzie was talking about was the Iceland eruption and the fact that there was a lot of snow. And so you’ve got a lot of heat with a lot of snow which creates a lot of water so you get this toxic combination of mud formed because you’ve got ash with water and the whole thing turns into a big catastrophe?

Jess - Yeah. We call the “Lahars” and they’re a very common phenomenon. What happens is it can happen during the eruption if there is an eruption during a heavy rain or a monsoon. You can have water and the ash mixed together and cause a very fast flowing mud flow which can be very damaging. It can happen during the eruption, or it can happen at a later date if the ash has accumulated on the volcano and then there is an addition of water. So, ice melting or a rainy season or something like that, the ash can be remobilised, can cause big landslides and mudflows. And, as I said before, this stuff is like cement so it can be really really damaging.

Izzie - Absolutely. Now what about some of the gases that get into the atmosphere? We heard about global cooling and what are some of the problems with the gases that are released as well? Because we heard about asphyxiation, but can they poison crops and things like that?

Jess - They can, yeah. Sulphur dioxide, one of the main gases that’s released from a lot of volcanoes, when it mixes with water it causes sulphuric acid, and that can cause acid rain that can erode metal and can also poison crops and livestock. People who live downwind of Kallawaya, for example, will be very familiar with what they call “vog,” it’s volcanic fog made up of sulphur dioxide.


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