Re: Three questions.

From:         Tom Speer <>
Organization: 412th Test Wing / TSFF
Date:         27 Jun 96 12:42:09 
References:   1
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Peter Smooker wrote:
>... What could have been the problem?
> 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?
> ....

What you saw was water vapor in the air condensing in the core of the
wingtip vortex.  The increase right after takeoff would have been due to
the aircraft rotating and pulling up to begin the climb.  The differences
between right and left were probably due to small variations in the
atmospheric humidity or temperature.  If you watch landing aircraft, you
will often see the same thing happening in the vortices shed off the ends
of the flaps.  Watch for momentary increases any time the pilot pulls
up, and vice versa, while tracking the glidepath.  Under the right
conditions, water vapor will also be visible in broader low pressure
regions, such as the top of the wings.