Re: Three questions.

From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         27 Jun 96 12:42:10 
References:   1
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In article <airliners.1996.1051@ohare.Chicago.COM>, Peter Smooker
<peter.smooker@med.monash.edu.au> wrote:


> 3. An Evergreen cargo 747 (200?) was about to take off. There was
> substantial vapour coming from the left wingtip. It took off and the
> vapour increased. After takeoff vapour was coming from the right hand
> wingtip as well, obscuring the plane. Was this fuel, and if so, is it not
> dangerous? Why was it being released?

Can't answer your first two questions, but the vapor you saw was
condesation generated in the low-pressure wingtip vortices.  The
aerodynamic action of the wing causes the air pressure to drop very
rapidly above the wing and in the vortices coming off the wingtips, flap
edges, etc.  As the air pressure drops rapidly, its temperature does, too,
and if there is enough moisture in the air (humidity) it will condense in
the low-pressure, low-temperature air and form instant "clouds."  Its a
very common phenomenon, and some planes do it more often than others.  In
Hawaii, it was normal for 737s to trail long vapor streams off each
wingtip on final approach.  The Air Force's F-4 Phantoms did it, too.
Here in Seattle, our planes often almost totally disappear in the sudden
cloud of vapor that comes off the entire upper wing surface at rotation on
a drizzly day.

C. Marin Faure
   author, Flying a Floatplane