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WATER SYSTEMS ABOARD AIRLINERS

The galleys, wash rooms and toilets on board a passenger aircraft are sophisticated high-tech compartments, but these are only the "tip of the icebergî. The passenger does not see anything of the water supply system. He expects to be served fresh coffee or tea even after the aircraft has been airborne for many hours, that the tap he uses to wash his hands will not go on strike, and that the toilets will continue to work despite the onslaught they have to endure. Normally he will take all this for granted without even thinking about it.

But for engineers at Airbus Deutschland in Hamburg-Finkenwerder it is a quite different matter. Having designed the flow of water around Airbus planes throughout the world, they turned their brains some time ago to the problem of providing equivalent tanks and pipework on board the mega-liner of the future, the A380. Three separate systems of water supply and disposal have to be fitted into the giant bird, one each for fresh water, grey water and waste water, on two passenger decks and in the hold.

First of all there is a requirement for fresh water, which must be of drinking water quality. The tanks installed for this supply the galleys, wash rooms and toilets and, to a lesser extent, the humidification systems as well. Customer airlines can even ask to have showers installed on the A380.

The tank capacity is fixed and must be big enough to ensure that water supplies do not run out before the aircraft touches down. Here, the engineers are guided by information supplied by the airlines, based on many years of experience. Arab and south-east Asian passengers, for example, consume more water than Europeans. Why this is so has yet to be researched, but in any case the customer has the option of ordering extra tanks.

To provide really fresh water it is necessary to fill up the tanks on the ground with pure drinking water. Because air travel entails a lot of people being cooped up in cramped conditions for long periods, disinfection is naturally very important. A special module is used to circulate and disinfect the water supply to prevent bacteria from having a field day in it. Small quantities of chlorine are used for this purpose, as specified in the drinking water regulations.

However, the fresh water system can currently only retain existing quality, it cannot improve it. Meanwhile the specialists are working on a water treatment module for the A380 in which the water undergoes the physical process of anodic oxidation and the depot effect of the chlorine ions is used. This means that the water remains fresh for longer even without chemicals.

Then it is important that the pressure in the system is always sufficiently high and is not allowed to collapse even if, for example, all the toilets are flushed at the same time. This does cause an objective loss of pressure but passengers are not aware of it, though they do have to get used to the frugal regime, for when the hands are washed normally only 12 seconds of the precious liquid are provided. Anyone who wants more water than that has to press the tap again. Only in First Class does the water run for longer ñ how long being decided by the airline.

The constant water pressure is maintained by compressed air at around 40psi. The drinking water taps and toilets are themselves designed to let air in or out. When water is taken from a tank, it is necessary for the vacuum created by the outflowing water to be filled by inflowing air; and when water is poured in, air must be able to escape.

The ìgrey waterî that is generated during hand washing is relatively easy to dispose of by finely spraying it outside with the aid of a ìdrain mastî. The spray head is heated so that even at minus 50ºC at cruising altitude no ice deposits can form. The design must ensure that this does not happen even when one or more valves develop(s) a leak; hence all the systems must have in-built redundancy. Only after spraying do the tiny drops of water freeze into fine ice crystals which are lost in the clouds.

But a major challenge for the specialists is the design of the waste water system, i.e. the disposal of the water used for toilet flushing and faecal matter. It goes without saying that passengers ñ and of course the aircraft crew as well ñ expect the toilets to function reliably at all times. No passenger can be asked to spend up to 16 hours in the air without using a toilet.

The system must therefore be reliable, even if the requirements it has to satisfy are not as stringent as for other safety-relevant aircraft systems. What is critical here is to arrive at a reasonable OI (operational interruption) rate, to use the engineersí jargon. This should be as small as possible and is expressed amongst other ways in how quickly a failed unit can be replaced during maintenance work on the ground.

One still comes across older systems, referred to as ìrecircî, even today. These can be recognised above all from the deep blue liquid disinfectant that appears when the toilet is flushed. They may be viewed as earth closets with circulating pumps, and if the drain valves do not close completely tightly, it is possible for unpleasant lumps to loom up from the blue ice.

But in recent years the tendency has been in modern commercial aircraft to install ìvacuum toiletsî in which the waste water is transported by the pressure difference between the higher external pressure and the lower internal pressure of the system. This ensures a higher standard of hygiene but is also significantly more expensive.

At Airbus in Hamburg a special 80 metre long test stand has been built for testing possible layouts for the A380ís waste water system. On three different levels tanks, toilets and galleys have been installed along with the associated pipework, and thanks to a sophisticated computer program these are being exposed to the same loadings that would be experienced during a longhaul flight.

How does one simulate the behaviour of around 600 passengers at the same time? Above each of the installed toilet basins there is a separate computer and a canister containing highly viscous, blue gel, a certain quantity of which is released and flushed away in response to a computer command. The test stand pipework is deliberately made out of glass so that one can see the waste water moving around clearly and identify possible problems areas. Every toilet, every tap and every sensor is linked with a microprocessor, and these are linked up together via data buses.

In the final aircraft the pipes will be made of titanium and run in such a fashion that they are obscured by other items and hence difficult to access. It is therefore imperative that there are no blockages, since nothing can be changed once the aircraft is operational.

Optimisation of the design is a highly complex problem and follows the laws of fluid mechanics. At the moment that the waste water gushes into the tank, the suppressed air must be able to escape having been pre-filtered and freed of bacteria. Waste water and faecal matter are collected and held in appropriate tanks until the aircraft lands, so that no unpleasant smelling lumps of ice can fall from the heavens like bombs.

Another factor to be considered is the fact that at certain flight attitudes the fluids in all the tanks begin to slosh around, as a result of which enormous loadings can occur. Nevertheless all the filters, valves and sensors must work reliably and be reasonably accessible for maintenance purposes. This is another aspect being examined on the test stand so as to ensure that any possible weak points can be identified and rectified in good time.

At the end of the day the airlines need ground handling staff to be able to rapidly fill and drain the systems from a single point and, in an emergency, without electrical power either. A slight incline must therefore be designed into the layout of the piping, while an ingenious heating system has to be accommodated so that the water supplies do not freeze on the ground.

As if this were not complicated enough, all the systems are controlled by a water management system that works with reliable level indicators; these in turn are interconnected via computers. If a toilet really fails, this must not add to the captainís workload, so the sensors automatically radio the failure to the ground where a decision is made after landing on spare parts provision and repairs. Passengers can thus rest assured that on every flight there will be enough fresh water on board and that the waste water will be disposed of reliably.

From FLUG REVUE 09/2001, page 88