Unsettling reason aeroplane toilets flush in the way they do with disgusting 'blue ice' on some aircraft

There are many things in life that we simply just accept without ever really questioning the science behind it.

One of those things, for me at least, is wondering where our waste goes when we go to the toilet on an aeroplane.

We think about many things when we arrive at the airport, whether it be making sure our suitcase is the right size or colour, ensuring we've brought an up-to-date passport or simply counting the correct number of kids.

But while most of us have the forethought to use the bathroom before takeoff on shorter flights, we can't always avoid using the airport bathroom on longer haul flights.

What happens in the air?

Of course, most toilets are connected to pipes that will take our waste down into sewers, but for reasons which I really hope I don't have to explain, that isn't possible on planes.

As funny as it might be to imagine that the plane simply dumps the waste onto unsuspecting folks below, this pretty much never happens, although one bloke in Windsor did get covered in the stuff when it landed in his garden.

There's actually some very interesting science behind it (Getty Stock)

You may well be familiar with the harsh 'whoosh' sound that you hear when flushing on board and that stems from the difference in pressure inside and outside the cabin.

Aerospace engineer Bill Crossley, the head of the School of Aeronautics and Astronautics at Purdue University in Indiana, said: “When you go up to high altitude and you’re flying fast, the pressure outside the cabin is a lot lower than it is inside.

“When you flush the toilet, you’re basically opening a valve to the outside, and the pressure differential sucks away whatever’s in the bowl."

Makes sense, right?

What happens on the ground?

In the unlikely event that you use an aeroplane toilet while it's on the ground, when the fasten seatbelt sign will usually be on, the pressure remains the same so the toilet works using a vacuum pump.

This pump extracts the particles from the toilet and remains in place until the atmospheric pressure builds to a point where it can work without the pump, and it is switched off.

What is blue ice?

Although they are pretty rare these days, some flights still use 'blue ice' on board, and while this might sound like part of a lovely cocktail to enjoy, it's pretty much the opposite.

Blue ice is defined as 'the frozen sewage material that has leaked mid-flight from commercial aircraft lavatory waste systems.'

Sadly this isn't the blue ice we're talking about (Getty Stock)

On rare occasions, blue ice that has been accumulated throughout the flight can be leaked from the aircraft during landing, which is just another reason why it's not the best idea to live close to the airport.

Heathrow Airport suggest that this happens on average 25 times a year in the UK, saying:

"Waste collection happens when the aircraft lands at an airport and is disposed of responsibly. However, there have been reported incidents where the hose valve that is used to empty the aircraft has been faulty and leaked fluid which has frozen at altitude.

"This rare occurrence usually results in discoloured ice and is commonly referred to as 'blue ice.'”

The waste is unlikely to be frozen by the time it hits the ground and in the unfortunate event you're stood close to where it lands, you'll definitely be the first to know.

When your peers tell you to keep your head out of the clouds, this is exactly what they mean.