Re: Smoke Trail

From: (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         03 Jul 96 01:23:50 
References:   1
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In article <airliners.1996.1078@ohare.Chicago.COM>,
(Lars A Ewell) wrote:

> To Whom it May Concern,
>         I recently rode on a United Airlines 737
> (-300 I think) and I noticed something peculiar during
> take-off.  Just prior (approx 3sec) to the front
> wheels leaving the ground, a thin (approx 10cm diameter)
> stream of smoke was emitted from the front of the
> engine (I could only see one engine).  This stream of
> smoke traveled over the top of the wing and then
> back toward the tail of the plane.  It lasted about
> ten seconds.  The sight reminded me of pictures I have
> seen of objects being tested in a wind tunnel where smoke
> (or something similar) is used to make visible the
> airflow direction.  The same scenario was repeated on
> my return flight.

I suspect what you saw was water vapor condensing in the very low pressure
air coming off the engine nacelle and travelling back over the wing.  This
is a very common phenomenon, and is usually seen streaming back from
wingtips, flap corners, and sometimes over much of the upper surface of
the wing.  It occurs because the rapid lowering of the air pressure due to
the aerodynamic action of the wing, nacelle, etc. rapidly lowers the air's
temperature.  Under humid conditions, there is enough water vapor in the
air to condense into "clouds" as the temperature drops suddenly.  The
forward motion of the plane makes the "clouds" stream back across the
wings or off the wingtips.  It's a common sight here in Seattle.

C. Marin Faure
   author, Flying a Floatplane