vapour trails

[gazza3232]

can anyone tell me why most aeroplanes leave a "vapour" trail(is it vapour ?) but not all & it seems to be the odd very hot day when they don't leave one . Someone told me it was because they are dumping fuel but they leave a trail when taking off & surely they can't be dumping fuel then ? Also, are they allowed to dump their toilet waste over land & does it come out in an ice-cube ?

[JudgeJ]

Vapour trails are actually �condensation trails� (or contrails). They are formed when the water vapour which forms part of the aircraft�s exhaust freezes into ice crystals. (Look carefully and you will see one trail for each engine on the aircraft). Why they are evident in some circumstances and not others is dependent upon prevailing atmospheric conditions, in particular the local Relative Humidity. They are usually most noticeable on clear days from aircraft flying at high altitudes.

[heathfield]

JudgeJ's right, but there are also times when vapour trails are produced by an aircraft on landing and take-off. These tend to be produced by the flying surfaces rather than the engines. As the air flows over the wings, for example, a low pressure area is created. Because of the low pressure, this air can't hold as much moisture as the surrounding air, and the moisture condenses out in the form of a visible vapour.

[Catso]

No, you probably wouldn't see fuel being dumped, as it has to be a great deal colder to solidify it.

I believe the waste has to be kept in tanks, and is pumped out on the ground. If it (the liquid, anyway) were dumped, the slipsteam would disperse it into droplets which would then freeze. On the way to earth they would thaw out. As there isn't much of it, it might even all then evaporate before it hit the ground, but if it didn't it would fall to earth as 'rain'. Though you probably wouldn't notice it, as a pint or two would probably be spread over several square miles!

[Clanad]

About 95% of your question has been answered very well. Toilet waste is held in holding tanks on board the aircraft. These tanks are serviced at destination stations (as are those on buses and trains). On rare occasions, the liquid in the tanks forms a chunk of ice at the tank overboard vent(s). This can be quite large. I've seen pictures of some weighing perhaps 25 pounds or more. This is caused by a malfunction of the vent. In some cases the ice falls off the aircraft and causes some damage if it strikes a structure on the ground. The waste liquid is blue in color and hence the name for this kind of ice being "blue ice".

[JudgeJ]

Yes, heathfield is quite right. These trails of vapour are seen most commonly on fast aircraft flying at low level and usually manifest themselves as vortices of vapour appearing from the tips of the flying surfaces. They can also be seen on larger civil aircraft, particularly if humidity is high.

Most curiously they can also be seen on occasions coming from the aerodynamic �wings� of racing cars. These work on the same principle as aircraft wings (though they are designed to provide downforce to keep the car on the ground, rather than lift to help it take off!).

It should be remembered that the vapour produced in this way comes from water vapour already dissolved in the air, and not from the aircraft�s exhaust gases.

[Toureman]

The condensation trails are usually formed above 27,000ft.
In Vietnam the US airforce started ordering their bombers and fighters to stay below this hieght so the Viet cong could not see them and work out where they were heading to.

[heathfield]

Just out of interest, here's a photo showing a fuel dump, with a small vapour trail caused by an air vortex at the plane's wingtip.