What Happens When You Flush An Aircraft’s Toilet?

Have you ever wondered what happens when you flush an aircraft toilet? What causes the loud noise, and where is the waste stored on the aircraft? It is actually a great pressure-based system, developed in the 1970s, that is used on all commercial aircraft, as this article explores.

Aircraft toilet
The aircraft toilet – working differently to ground toilets Photo: Getty Images.

The modern aircraft toilet

The toilet that we all know and use on aircraft today is a vacuum-based system, with a non-stick bowl. It was invented (and patented) in 1975 by James Kemper (you can view the US patent details here). The system is mostly water-free, clearing the bowl instead by suction.

This offers several advantages over a water-filled system, as is usual in toilet systems on the ground. It is much cleaner for a start (imagine the spillages during turbulence or sudden movements). But it is also lighter as there is no need for a large tank of water for flushing. If you think how many times a toilet can be used on a full 747 or A380, you can imagine the weight saving.

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

There hasn’t been much change to aircraft toilets since these were introduced, at least not the technology. Airlines frequently change the toilet designs and layouts of their bathrooms. Sometimes these changes are towards the large and luxurious, such as the spacious toilets and showers on some A380 aircraft, and sometimes towards the small and simple, such as space-saving versions installed on the Boeing 737 MAX (measuring just 24 inches from wall-to-wall).

What Happens When You Flush An Aircraft’s Toilet?
The 737 MAX has some of the smallest toilets in the air. Photo: Getty Images

Vacuum suction

So how does the vacuum toilet work? When the flush button is pressed, a valve at the base of the toilet bowl opens, a small amount of blue disinfectant liquid (known as Skychem) flushes through, then the contents are sucked out of the bowl by suction. This is the ‘whoosh’ sound you hear when flushing.

This suction works by pressure difference. The waste tank is kept at a lower pressure than the cabin (it is not pressurized like the cabin is). There is also a vacuum pump in the system which is used when there is not sufficient differential pressure (this explains how the toilets still work on the ground). Once the valve in the toilet bowl is opened, the pressure difference causes air from the toilet bowl to be sucked rapidly out.

The waste and Skychem solution then flows through pipes to the rear of the aircraft. It is stored in large sealed tanks and emptied once on the ground, either by tanker or pumped into an underground storage system.

Aircraft lavatory service
The storage tanks are tempted via suction once the aircraft is one the ground. Photo: Mnts via Wikimedia

Old toilet systems – doing without the vacuum

Before the development of the vacuum system, aircraft toilets were chemical-based. These are flushed using a combination of water and disinfectant, and stored in tanks directly below the toilets. Such a system would be much heavier, though, and also potentially less pleasant, with the tank usually closer to the toilet rather than at the rear of the aircraft.

Chemical toilets date back to the 1920s, with British company Elsan developing some of the earliest ones. These were used extensively on transport and bombing aircraft during the Second World War. In the UK, there were known simply as ‘Elsans’ and, while practical, they would frequently spill their contents.

Wellington RAF toilets
An Elsan chemical toilet on a UK Royal Air Force Wellington aircraft in the early 1940s. Photo: Devon S A (RAF) via Wikimedia

Earlier than this, some aircraft would use chemical toilets, water-based toilets, or often none at all if flight times were shorter. Many flying boat aircraft, for example, featuring full porcelain, water flushing toilets.

Can the waste fall to earth?

There are tales of waste from aircraft toilets falling out of the sky. While this may have happened in the early days of aviation, it certainly does not with modern pressurized toilet systems. What does very occasionally happen is that chemical solutions can leak from the aircraft, freeze, and then fall from the aircraft fuselage – so-called ‘blue ice.’

There are even cases where such leaking toilet systems have caused damage to aircraft. In the 1970s and 1980s, there were a few cases of damage to the rear engine of Boeing 727 aircraft, such as this case reported in the Los Angeles Times, where ‘blue ice’ damaged the engine and caused an emergency landing.

Do you know of any different systems, or interesting examples, of aircraft toilets? Fell free to share in the comments.