How aircraft toilets work, and why they often clog together

Hindustan Times

14-05-2025

Clogged lavatories have disrupted two of Air India's international flights in the span of two months, drawing attention to how aircraft toilets operate. All but one of the toilets on a Boeing 777 (B777) from Chicago to Delhi became unusable in March, forcing the aircraft to return, and at least five toilets clogged on a B777 from Toronto to Delhi this month, forcing a diversion to Frankfurt.
A B777 typically has 12 toilets, but why should so many clog simultaneously? The key factor is the location of the objects causing the blockage — often polythene bags or pieces of cloth — which can affect all toilets whose waste lines pass through that area. But, regardless of whether one or multiple toilets are affected, such blockages stem from a disruption in the physics that aircraft lavatory systems rely on.
Vacuum toilets
Aircraft must, by necessity, maintain cabin pressure at a level higher than the atmospheric pressure outside — and this pressure differential also underpins the scientific principle on which aircraft toilets operate. The primary reason for maintaining relatively high cabin pressure is passenger safety and comfort. At an altitude of, say, 35,000 feet, the air pressure is around 3.45 pounds per square inch or psi (for context sea-level pressure is roughly four times higher). To ensure passengers have enough oxygen to breathe, cabin pressure is typically maintained at 11 to 12 psi.
The aircraft toilet system takes advantage of the pressure differential between the cabin and the surrounding atmosphere by creating a vacuum so that air at higher pressure rushes toward the area of lower pressure, carrying waste along with it. This does not mean the waste is expelled into the atmosphere — as might happen to characters in an action movie when an aircraft door blows open. Instead, the waste is directed into a holding tank located in a lower-pressure zone in an area where the pressure is lower, similar to the conditions outside the aircraft.
Also Read: Polythene bags, clothes clogged toilets: Air India after flight returns to US
'The waste tank is located below the floor of the aircraft cabin, in an unpressurised area. This tank, when full, can be serviced (emptied of waste material) whenever the aircraft lands at an airport,' said Ashwani Sharma, a veteran aircraft maintenance engineer who served with Air India for many years and is currently a professor of practice at Chandigarh University's aerospace engineering department and chairman of the Aeronautical Society of India's Mumbai branch.
When one flushes a toilet at home, it releases a stream of water that carries the waste into a sewer line. In contrast, when one flushes an aircraft toilet, it opens a valve at the bottom of the toilet bowl, exposing the contents to the pressure differential. Air from the cabin immediately rushes into the bowl and, often noisily, passes through the valve on its way to the waste tank.
The science behind clogging
What is flushed down the toilet is often the reason behind clogging. Another cause can be mineral buildup — something that frequently affects household toilets — but clogs in aircraft toilets are more often associated with items flushed by fliers and where these end up in the network of drain lines.
Different aircraft models have different numbers of toilets and water tanks. Narrow-body, single-aisle aircraft such as the Boeing B737 MAX and Airbus A320 NEO have one tank each, while wide-body, twin-aisle aircraft have two or more. 'A B777 has two tanks, while an Airbus A350 has four. If there are two tanks, then half the toilets would be connected to each,' Sharma said.
Also Read: Air India flight returns to Chicago over clogged loos
HT has reported earlier that the B777 diverted to Frankfurt had three sewer lines. No details were immediately available on how these three lines were divided between two tanks (if there were indeed two), but it is easy to understand why some clogs affect a larger number of toilets than others.
A clog in a vacuum-based toilet means that one or more objects are acting as a barrier between the areas of higher and lower pressure, so the air in the cabin can no longer flow out with the waste, making the principle of differential pressure unworkable, Sharma said. 'If the clog is in a portion of drain line that is near the waste tank, then all toilets connected to that particular tank will be clogged as the clog will prevent differential pressure being created. If, however, the clog is in a toilet bowl or the drain line immediately below the bowl, then only that toilet would be clogged,' he added.
Size matters because some items may be small enough to pass through the toilet bowl hole but too large to go through the drain line, whose diameter is smaller. 'This can block the drain line. Once there's a block in the line, differential pressure cannot be created and all toilets connected to that waste tank would become unserviceable,' Sharma said.
Looking for solutions
Sharma stressed that the problem of clogging is not limited to Air India or any single airline, calling it a universal issue. As such, newer ways to prevent or reduce clogging continue to be discussed.
For example, a mesh could be placed at the end of the toilet bowl. 'The mesh will not allow a plastic bottle or a diaper to go through and block the line, but waste matter will go,' Sharma said.
'But it can create other issues,' he said. The suction pressure is so high that if a weak material is used, it will not last. Then again, if the material is too strong, the toilet bowl itself can get blocked. 'The difference is that if the toilet bowl gets blocked, only one toilet will become unserviceable. That particular toilet can be labelled as such, but the others will continue working.' Sharma said.
It remains an evolving area with scope for innovation.