First of all, all that pee is on the plane when it lifts off, in one form or another. The only thing that changes is distribution of mass (depending on where the tank is and where they keep bottles with drinks). A tonne of pee gathered from all over the plane is most likely less effect than 3-4 people moving from one end to another end of the plane. So a stewardess with help and a cart.
Second, pilots can trim the plane while in flight.
Third, no safety margin is needed here because this is not a safety problem.
The amount of pee that can be deposited into the tanks is limited by the amount of pee they can physically hold at the start of the flight plus the amount of drinks they can consume during flight.
But more realistically, on a long flight, it is just the amount of drinks they can consume assuming they drinks are mostly used to replenish whatever they have peed. Most people try to empty their bladder before flight and are willing to have full bladder when they leave the plane (to relieve themselves afterwards) so I think even this is already a little pessimistic.
On 747-400 you have max 660 passengers. 1135L divided by 660 passengers gives ~1.7L per passenger on average. Which is a lot. Consider this is an average -- some people may drink way more than that but most people drink less than that in a day.
So it seems it might be possible to run out of the tank -- if you have a very long economy flight in a large plane converted to densest possible seating arrangement and if you are willing to believe people will be buying over 1.7L of fluids per person on an economy flight.
Of course, all that calculation goes out the window if you plan to fly an economy flight full of Australian Oktoberfest fans directly from Germany to Sydney.
> Third, no safety margin is needed here because this is not a safety problem.
Sewage overflow is certainly a safety problem. Just health safety though, not flight safety.
1.7L per passenger is a large volume of excretions, but flight operations may require multiple flights before emptying the tank, especially if there's a problem with the emptying equipment at some airport. Would a failure to empty the tank prevent a plane from taking off? Probably only if the remaining capacity is likely to be insufficient.
> The only thing that changes is distribution of mass (depending on where the tank is and where they keep bottles with drinks). A tonne of pee gathered from all over the plane is most likely less effect than 3-4 people moving from one end to another end of the plane. So a stewardess with help and a cart.
My informal search [1, 2] seems to indicate that every galley seems to have WC close by, though not vice-versa. Would never have thought to attribute that to an engineering decision before this discussion!
I imagine they have less than 1135 litres of beverages on board, which helps. Also 40k litres of fuel, much of which gets used up over the course of the flight...
> how much safety margin does the tank have build in
From the article: 1 liter (747) to 2 liters (A380) per passenger. More than enough, even if a dozen people manage to catch the runs and eject more liquid.
> how would an almost full tank affect flight behavior
Not that much, pilots can (and have to, as part of weight redistribution caused by the fuel tanks emptying) "trim" the airplane - basically, applying an offset to the neutral position of control surfaces (see https://en.wikipedia.org/wiki/Trim_tab). And all the content of the toilet waste tank was already in the plane before (either in the bodies of the passengers, or in the freshwater tank), so it's only a minor distribution change.
Second, pilots can trim the plane while in flight.
Third, no safety margin is needed here because this is not a safety problem.
The amount of pee that can be deposited into the tanks is limited by the amount of pee they can physically hold at the start of the flight plus the amount of drinks they can consume during flight.
But more realistically, on a long flight, it is just the amount of drinks they can consume assuming they drinks are mostly used to replenish whatever they have peed. Most people try to empty their bladder before flight and are willing to have full bladder when they leave the plane (to relieve themselves afterwards) so I think even this is already a little pessimistic.
On 747-400 you have max 660 passengers. 1135L divided by 660 passengers gives ~1.7L per passenger on average. Which is a lot. Consider this is an average -- some people may drink way more than that but most people drink less than that in a day.
So it seems it might be possible to run out of the tank -- if you have a very long economy flight in a large plane converted to densest possible seating arrangement and if you are willing to believe people will be buying over 1.7L of fluids per person on an economy flight.
Of course, all that calculation goes out the window if you plan to fly an economy flight full of Australian Oktoberfest fans directly from Germany to Sydney.