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THE LAST WORD

Loo clue 1While on a cruise ship in Spitsbergen, Norway, we had to use toilets which would only flush if the lid was closed to create a seal. Sometimes you had to hold the lid down to ensure the seal was intact. How did they work?

n I have seen this type of toilet on yachts. When the lid is closed to form a seal, the waste is automatically pumped out. This causes a reduction in the air pressure inside the bowl. The reduced pressure draws in water through a separate pipe, which is used to flush the bowl clean.

If the seat is damaged the seal may not form correctly and the toilet won’t work. Therefore it is essential for all yachts to carry a spare seat.Doug GriggCannonvale, Queensland, Australia

Loo clue 2How 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?

n On the aircraft I fly (a Brazilian-built Embraer 195 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.Rob CheesmanPilot and First OfficerBelfast, UK

n 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.

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 HollingworthBlagnac, France

This week’s questionswhaT a gasWhen I turn off a gas ring on my cooker the flame reduces as you might expect, but then it sometimes extinguishes with a small explosive popping sound. Why does it do that?David PrichardBluff Point, Western Australia

Brow noT BeaTenI have a friend with alopecia. He has lost all his hair except for that which forms his eyebrows. Why is this?Brian WalkerBelfast, UK

geT crackLingIf you want pork skin to be crackly when you roast it, recipes suggest rubbing salt into the skin first. What does the salt do?Peder OlafssonBruges, Belgium

Buzz offWhy do flies that enter your house in summer do so in such an ostentatious way, with loud and frenetic buzzing? Surely it would be better for their survival if they didn’t? Is there a purpose to this display?Greta BowmanBrighton, East Sussex, UK

geTTing LippyAs soon as the temperature falls below about 5 °C I have to avoid smiling because if I do, my lips crack and bleed.

Why does this happen and am I the only person to suffer in this way?Pascal RinaldiPerpignan, France

growTh spurTWhat factors determine the point on the surface of an apparently homogenous potato at which a new sprout bursts forth?Peter FoleyWorkington, Cumbria, UK

“waste water is electrically heated but occasionally chunks of ice fall from the sky after a malfunction”

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“if somebody flushes the toilet at high altitude the external pressure forces the waste into a tank”

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