How do toilets on airliners work? They have incredible suction. Is the low external pressure outside the aircraft’s fuselage used to create this? And why do they operate a few seconds after you press the flush button?
• On the aircraft I fly (a Brazilian-built jet) the two toilets are flushed by suction. To generate the low pressure required, two methods exist, depending on the stage of flight. If someone flushes the toilet at high altitude, the external atmospheric pressure is used, and the difference between internal and external pressure forces the waste into a holding tank at the rear of the fuselage. If the toilet is used at low altitude or on the ground a suction pump mounted near the waste tank provides the pressure difference and that is why it may take a second or two to flush once the button has been pressed.
“If somebody flushes the toilet at high altitude the external pressure forces the waste into a tank”
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Rob Cheesman, Pilot and First Officer, Belfast, UK
• Aircraft lavatories cannot be opened to the environment outside the plane for at least two reasons. First, flushing the toilet at altitude would cause explosive decompression of the cabin and second, if waste was scattered from the sky it would turn to ice and become a danger to people and structures on the ground.
Conversely, waste water from the food and drink galleys and hand basins is dumped from drain masts. These are electrically heated but occasionally chunks of ice do fall from the sky following a malfunction.
“Waste water is electrically heated but occasionally chunks of ice fall from the sky after a malfunction”
Toilet waste, on the other hand, is moved by a vacuum through a waste line to a holding tank that is emptied on the ground at the airport after a flight. If there is not already sufficient vacuum, pressing the flush switch starts up a generator, which depressurises the waste line. This takes about a second to operate, during which time a rinse valve opens and then stays open for a further second. A small, measured amount of rinse water is used to clean the toilet bowl. Then, after this 2-second delay, a flush valve opens and stays open for a further 4 seconds to ensure the toilet bowl is empty. The change in pressure eventually moves the waste to the holding tank.
The process cannot be left to chance. The flush sequence is governed by software and if a malfunction occurs the toilet shuts down and a signal is sent to the cockpit. If the waste tank becomes inoperative or fills up, the crew will be forced to put down for repairs.
Terence Hollingworth, Blagnac, France