Dumping Fuel

[cpbetts]

Why do commercial aircraft sometimes dump fuel before they land?

[sddsddean]

With fuel being as expensive as it is, this is usually only done in an emergency. An aircrafts design take off and landing weights vary a lot. A Boeing 747 on take off, fully loaded weighs over 300 tons, but if it was to land at that weight the undercarriage would collapse, as the designers have assumed that the aircraft will have 'gone' somewhere and be 100+ tons lighter due to the fuel that has been burnt. So if the aircraft has an emergency not long after take off, it must get rid of some weight, so that when it hits the ground the wheels have got half a chance of staying intact. If you are off on your holidays this year, just remember; every landing is a controlled crash!

[Gef]

I agree with sddsddean and presume another reason is to reduce the effect of a fire on landing (crashing). By the way, is the fuel dumped en masse (I wouldn't like to be smoking a cigar in a kerosene rainstorm) or is it sprayed out as fine droplets so that it has more chance of evaporating before reaching the ground?

[KARL]

Drains are opened on the wings and the fuel exits in a fairly modest stream, often not greatly dissimilar to the consumption rate - it takes quite a while to drain off the total capacity. Given the speed, this trail of fuel atomises and evaporates - you might be able to smell it if you were in the immediate vicinity but there is no risk of combustion at ground level.