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What is the exhaust from the end of jet wing prior to landing ? My guess,excess fuel from powering down engines.

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The only places where any fluid leaves a flying jet aircraft are the engine tail pipes, outflow valves and air cycle machine exhausts, and the auxiliary power unit exhaust, but that is usually only used on the ground.

None of these exhausts usually leaves any visible trail at low altitudes. The engine (and apu, if running; it is a small engine) exhausts contain a lot of water, but are very hot, so it remains a vapour; it only condenses at high altitudes where there is very cold. And the air from outflow valve—excess air leaving cabin—and from air cycle machine—air used for cooling—do not have any more water in them than the ambient air.

What on the other hand might leave visible trail is condensation is vortices behind wing-tips and sometimes end of flaps. This is just fog forming in very humid air due to decrease in temperature, like it might in the coldest hours around dawn, except here the cooling is local, caused by decrease of pressure that accompanies generation of lift by wings (this is called adiabatic cooling).

Regarding fuel, when the engine power is reduced, less fuel is simply pumped to the engines, so more of it stays in the tank. There is absolutely no reason to waste it by venting it anywhere—except for very, very rare case where the aircraft needs to return due to some malfunction, and needs to get lighter to avoid more damage on landing; but this intentional fuel dumping is always done at high altitude so the fuel evaporates before it could get to the ground and contaminate it.

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