Why can’t we fly through volcanic ash?

Lots of people were getting very frustrated about not being able to fly through the volcanic ash cloud last year.
I read many angry comments online from people trying to go on holiday saying that airlines should just fly through the cloud and that it’s not going to cause any problems.
Let me just say that with the billions of dollars that it is costing the airline industry, the no-fly rule is not something that had been decided lightly. This is to protect lives, and for the safety of passengers and crew alike, I’m sure it’s frustrating if you are stuck somewhere, or missing your holiday, trust me I have a flight to England scheduled for next week, but if the airlines say I can’t go – I’m OK with that.
A lot of the comments have been from people who don’t really understand the consequences of volcanic ash in a jet turbine, so I thought I would try to explain it in a simple a form as possible. Some academics may rip this apart, but I’m not writing this for them, I’m writing it for you in the least technical format I can.
Firstly let’s start with how a jet turbine engine works:
When you look at an airplane, you will notice under the wings there are big cylindrical devices with fans inside – this is what gives the plane the power it needs to fly and are called turbine engines.
If we were to cut one in half and look through the cross section it would look like this:

The air is sucked into the engine by the rotating fan blades and then squeezed by the compressor; it is then injected into a combustion chamber and sprayed with fuel then ignited by a flame. This combustion is blown out of the exhaust via a turbine which accelerates the air out pushing the engine, and thus the airplane forward. The exhaustion gases also turn the turbine which keeps the compression cycle going.

So now we know how it works, what are the dangers of volcanic ash?
Let’s start at the front – those nice shiny metal fan blades.
Volcanic ash is very abrasive; it can scratch and wear away the fan blades causing damage.
Think of sand on the beach, sand is also abrasive; you wouldn’t scratch sand across anything valuable because you know that sand can damage surfaces – the same goes for the particles in the ash. Volcanic ash is like sand, but much, much finer, like flour, so it can go higher in the atmosphere and get into all the nooks and crannies of a plane.
Next let’s move into the combustion part of the engine – I have labelled this region hot, but what I really mean is 1200ºC to over 2000ºC! (put this into perspective, aluminium melts at 660ºC and titanium at 1670ºC). The little ash particles are made up of rock and glass and they melt into a liquid inside the combustion chamber where they pass through and hit the cooler surfaces of the turbine. This causes the liquid glass to solidify and create a glassy film on the turbine, which can prevent them from rotating.
There are vents on engines designed to flow air towards the turbines for cooling and these can become blocked. This results in the engines burning too hot causing them to stall and scarily shooting flames from the back!
In addition to the engine problems, another dangerous consequence of the ash is if it blocks the Pitot tube. This is a little tube mounted on the wing which has two holes and some sensors. The side hole measures the still air, and the front hole measures the airstream pressure, by measuring the difference between these two you can calculate airspeed.
Unfortunately if this tube gets blocked (for example by being clogged with volcanic ash) you will get an incorrect airspeed reading. Not knowing your airspeed makes piloting a plane very difficult (imagine trying to park or drive your car without a speedometer) and can affect the autopilot computer models.

So would I fly through a no-fly airspace?
Nope.

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