I remember how they tested fuel dumping way back in the 50's or earlier...

they lit a fire on the ground, flew a plane over it and dumped fuel...turned out 4000' was the minimum magic altitude that the fuel didn't increase the fire on the ground.

of course, as some have said, if you need to dump earlier just to climb, dump!

Thanks for the replies. Do most of you plan on a higher altitude to dump if it is cold out. Lets say below 0°C?

According to company manuals it is 5,000 AGL, in the reference of ATC also.

Airbus FCOM

From experience of dumping obscene amounts of fuel on military ops, if you fly into your mates dumped fuel as he passes overhead a couple of thousand feet above it doesn't half stink up the cabin. We couldn't see any noticeable change in TGT and the engines didn't fluctuate noticeably but the windows got a little greasy.

Decided to avoid the trail after the first couple of days just in case.

I can also add that dumping whilst on the ground makes a mess too! Sorry Malmö circa 1992 :(

Found this. Seems reasonable and inline with what has been discussed.

http://leonardo-in-flight.nl/PDF/B76...%20DUMPING.pdf

Once we had to turn back and dump 80T of fuel after
one of the first class pax had a heart attack. LAX ATC
vectored us over the Pacific and insisted on 6,000 min
altitude (which corresponded to our QRH and SOP
requirements).

6,000 was also for the 727 in Oz so that altitude has
always stuck in my head. Haven't flown a dumpable
aircraft since so my thinking might be outdated a tad.

I did just find something by Boeing that says "Fuel Jettison above 4000 feet agl ensures complete fuel evaporation"