What Happens When An Aircraft Toilet Is Flushed?

Aircraft toilets don’t work the same as those on the ground. Storing so much water would need large and heavy water tanks, and imagine the problems with water-filled toilets and turbulence! Instead, they use a vacuum system, as this article explores.

The modern toilet on a Qatar Airways Boeing 787. Photo: Arran Rice – Simple Flying

Early aircraft toilets

In the early days of aviation, toilets (if there were any) were quite different. Some of the luxurious flying boats had full size, water-based flushing toilets. Chemical toilets soon appeared, though. There date back to the 1920s, with British company Elsan developing some of the earliest ones.

Chemical toilets flush using a disinfecting chemical mixed with a minimal supply of water. This requires large tanks, usually located near the bathroom.

An early Elsan chemical toilet, seen here on a military Vickers aircraft. Photo: Devon S A (Mr) via Wikimedia

Developing the vacuum toilet

Chemical toilets were the most common option until the 1970s. A vacuum-based system was developed and patented in 1975 by James Kemper and remains the standard used on modern commercial aircraft today.

Toilets have a nonstick coating, which is emptied by suction. There is no water flush, just this suction. There is some liquid flushed through, usually blue disinfectant liquid (known as Skychem), but no continuous water passed through. This means only relatively small tanks are needed to store the disinfectant and the waste. And there is no water-filled system to slosh around.

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How does the flush work?

The toilet suction works by pressure difference. The toilet bowl is connected to the waste tank, which is kept at a lower pressure. In the air, the pressure difference is created using the lower external pressure compared to the pressurized cabin. When on the ground, there is a pump used to create a vacuum.

When the toilet flush button is pressed, the Skychem solution is first passed into the bowl. A valve then opens at the base of the toilet, and the contents are sucked out to the storage tank by the vacuum created. This is a rapid and strong suction. And it is what causes the loud noise whenever the toilet is flushed.

What can go wrong?

The toilet system may seem like a simple design, but it works well. There have been few changes made over the years since the 1970s. Toilets have changed, either larger and more luxurious in first class, or ever smaller as more passengers are packed on narrowbody aircraft (take a look at these on the 737 MAX, for example, measuring just 24 inches across).

Toilets and bathrooms may have improved on many aircraft (this is a first class bathroom with Emirates), but the simple toilet technology remains the same. Photo: Emirates

Contrary to some scare stories, it is not possible to be sucked into the toilet by the suction created. Nor do frozen pieces of toilet waste end up falling to the ground. There have been some cases of liquids leaking from the system, and either falling to the ground as ice or damaging parts of the aircraft, but this is rare.

And as a final thought, the aircraft toilet may not be a dirty as you would think. Some interesting research during the coronavirus pandemic highlighted the most contaminated parts of the aircraft that passengers come into contact with. Topping the list is the seatback table, up to ten times worse than the toilet areas.

Would you like to share any stories of your favorite aircraft toilets? Let us know of any interesting or different ones in the comments!

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