Toilets are one of the most pivotal inventions in the history of humanity. We use them every day, and they are an essential part of our daily life. Toilets are everywhere we go, in every park, inside every building and in most modes of transport, including aircraft. There was and still is always this mystery around aircraft toilets; a question that begs for an answer: "What happens when you flush an aircraft toilet?".
Scroll down to watch my Youtube Video on the topic.
What happens when you flush an aircraft toilet?
Surfing through the plumbing side of the internet, you will get to know various types of toilets: Dual- Flush Toilets, Pressure Assisted Toilets and more including the famous Gravity-Flush Toilet which we are all familiar with in our homes.
Looking at the aircraft, we would assume at first that it has the same type of toilets as our houses but that's not true. The one at our houses might seem small but there is a lot of work behind it, from digging to using large amounts of water for each flush (The average toilet uses approximately 7 Litres per flush).
This type of toilet (Gravity-Flush Toilet) is not effective to use in an aircraft then. The aircraft is limited in terms of weight for better efficiency and we can't afford to carry a large water tank, and large pipes as part of the toilet system. This would drastically increase the weight and leave less room for more important components.
Airplanes then adopt a different approach; a more weight-friendly and cost-effective approach, aircraft use Vaccum Flush Toilets.
How do airplane toilets work?
Above 16.000 ft:
We know that aircraft use a different type of toilet named a vacuum flush toilet so what happens when you flush an aircraft toilet?
Pressing the flush switch sends a signal through the logic control module (LCM) to the flush control unit (FCU).
The FCU then sends a signal to the rinse valve to inject a slight amount of water into the bowl (8 Ounces - 0.22 Litres), the rinse valve stays open for 0.7 seconds.
2 Seconds after that, the flush valve opens and a vacuum is created. You see, the waste tank is less pressurized than the cabin and air tend to flow from areas of high pressure to areas of low pressure. This means that your matter "Get Pushed" by air pressure or gets sucked (if you like) and this explains why the aircraft toilet is so loud when flushing. The flush valve stays open for 4 seconds.
Due to the vacuum created, the waste starts travelling through pipes until it reaches the waste tank where it's stored.
At this point, you will need to wait another 9 seconds before you can flush the toilet again.
Below 16.000 ft:
We mentioned that the aircraft uses the difference in pressure to create the vacuum needed to flush the toilet. This is easy to picture at higher altitudes as we know that aircraft is pressurized and the more we go up, the less pressure there is. But on the ground and at low altitudes? Where the air pressure is equal or nearly equal on the outside and the inside? And this isn't much differential pressure.
Below 16.000ft, the aircraft uses a vaccum blower. It works by pulling air from the waste tank and blowing it out of the aircraft This causes a low-pressure zone in the waste tank, so when the flush valve opens The higher pressure cabin air pushes the toilet waste to the "low pressure" waste tank.
The whole flush cycle remains the same, the only difference is that differential pressure is created by the vaccum blower.
Do airplanes dump waste in the air?
Regardless of the few articles that we read here and there about certain houses receiving gifts (human waste) from the air which makes people believe that airplanes dump their sewage overboard. This is not the case.
The waste tank is limited to 60 gallons (227 Litres) and pilots don't have a quantity indication about this in the cockpit, only flight attendants do. When the tank is full or nearly full, the flight attendants will inform the pilots who will inform ground personnel over a special radio frequency.
Upon landing, a special truck will approach the aircraft, open the service panel, insert the hose and empty the waste tank.
What happens if the waste tank gets full mid-flight?
If the waste tank gets full mid-flight, then the flush capability of all toilets will be automatically deactivated.
Worth noting that the waste tank must be rinsed at every service interval to avoid unwanted waste collecting on the sensors. As this would cause a false "Tank Full Signal" which would lead once again to the deactivation of all toilets.
Now we know what happens when you flush an aircraft toilet? What about the sink water, then?
Now we know what happens when you flush an aircraft toilet? What about the water that we use to wash our hands with? Where does the waste from the sink go?
This waste doesn't get stored but instead flows overboard. There are two drain masts, one in the front and the other in the back, where water flows through to dispense overboard.
The drain masts are heated to prevent water from freezing.
What happens when you flush an aircraft toilet? - Last words
I hope this post was beneficial and gave you an insight into the toilet system of an aircraft.
I'd like to mention that the toilet system is a bit more complex, and this post and my blog and youtube channel, in general, are directed toward the general public. That's why I tend to skip a few details.
Also, the post was specifically about the 737 toilet system, but the general principle remains the same throughout most different types of aeroplanes.