What happens when you flush a plane toilet?

If you're unlucky enough to be hit by something falling from a plane, it's bad news -
but to be hit by a falling mass of human waste is adding insult to injury.

When in 2016 a woman from Aamkhoh in India was hit by a ball of frozen waste, dropped from a commercial plane, it was thought to be the first recorded case. According to The Times of India, the faecal matter had frozen at altitude into something resembling a cannon ball when it broke through the poor woman's roof.

So how come these incidents are so rare, why don't we hear more about the dangers of falling waste matter?

And what exactly happens when you flush an airplane toilet?

While most airplane restrooms ask you to place the seat down before flushing, it's still a dramatic-sounding operation.

That "whoosh" you hear? It's the noise of the bowl being "evacuated" using the pressure of the airplane cabin.

The modern airplane toilet built by James Kemper took Thomas Crapper's invention to another level, literally.

By using a valve to keep the evacuation system constantly at a lower pressure than the cabin, the waste matter is sucked out into tanks at the rear of the plane. There it remains, hopefully, until being emptied at the receiving airport.

A 10kg chunck of "blue ice" fell on the Indian viallage of Gurgaon. Photo / Supplied, Gurgaon Police

Stored out of sight and out of mind at the back of the plane, today's airplane toilet is a great improvement on previous models.

In the 1930s chemical drop toilets warned passengers to "avoid using if possible" because of the tendency for smells to leak into the cabin. This was especially true after turbulence, when a lot worse than odours would start leaking into the plane.

Then there was the "long drop" model of the Vickers Stranraer. In pre-pressurised cabins the toilet was open to the air, earning it the affectionate nickname of the "Whistling Shithouse".

In an almost perfect metaphor for the decade's general approach to air travel and waste management - using the Stranraer was a question of practicality over environmental impact.

"Don't look down, and try not to think too much about who or what you are flying over."

The Handley Page HP was one of the first civilian aircraft to contain a toilet. Photo / G. Eric and Edith Matson Photograph Collection

It was simpler times. In a decade when the annual number of passengers barely reached 1 million – compared to today's 4 billion – it was a risk people could take.

The current Kemper toilet really is a marvellous invention, although it is not fool proof.
On the rare occasion that the pressure valve does leak to the plane exterior it can result in falling frozen faecal matter.

The residue of waste matter and coloured Anotec cleaner mixes together to create something called "blue ice". In a leaky system chunks of this can clad the underside of an aircraft. As the plane descends for landing, chunks of frozen matter can be shaken free in great icicles. Although blue ice incidents are rare, they are not unheard of.

Every other year there are reports of chunks of falling dung breaking windscreens or damaging buildings.

However, the biggest danger from Kemper's toilet might not be directly beneath its flightpath.

Even more worrying than the 1930s sign warning passengers to "Refrain from using the toilet" is the far more sinister "Do not flush while seated on toilet."

The suction caused by an airplane toilet is incredibly powerful.

This is perhaps best demonstrated by the citizen science experiment mostly conducted by mischievous YouTubers.

By dropping the end of toilet tissue in the bowl and hitting flush, the suction is enough to empty an entire a roll. It's been recorded and viewed countless times in viral web videos.

Southwest airlines staff used to entertain bored passengers by holding "loo roll races" down the aisle of the plane.

Just hope you aren't next in line to use the toilet.

Push the button: Where does airplane waste go? Photo / Getty Images

However, if you happen to be sat on the crapper at the same time as pressing the flush, the result could be far less humorous.

Effects are perhaps best documented in Garrison Keillor's 2008 column for The New York Times, "How an airplane toilet can ruin your life". It's not pretty.

An urban myth that a woman was once "sucked out of a plane by pressing the flush" was successfully debunked by Adam Savage on the TV show Mythbusters, with severe discomfort.

However, it is not something you wish to experience.