Re: Caution: (757) Wake Turbulence ?

From: (Keith Barr)
Organization: Nyx, Public Access Unix at U. of Denver Math/CS dept.
Date:         09 Jan 94 18:13:34 PST
References:   1
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In article <>,
Barney Lum <> wrote:
>They're saying separation should be increased to about five miles and 
>only mentioned 757s, no other types or generalizations.  Is there anything 
>unique about that aircraft that it creates a particularly strong (or 
>longer lasting) wake?  Or was the statement made only to focus on the 
>situation at hand?

I went to a speech last February given by Robert Machol, who is one of the 
chief scientists involved in wake turbulence research at the FAA.  I have a
transcript of his speech, and here are a few pertinent statements.  These are
reprinted without his permission...I hope it is o.k.

>From Robert E. Machol's speech to NCAR on February 8, 1993

"We know a great deal about the initial strength of the vortices, say ten to
forty wingspans behind the aircraft (note that forty wingspans can exceed a
mile).  The vortex consists of a "core" in which the speeds might be quite
high (we have measured speeds over three hundred feet per second off a 757,
so the nickname "horizontal tornadoes" for wake vortices is quite reasonable).
The core may be very narrow, less than a foot in radius for as large an
airplane as a 757, but with considerable momentum also at radii many times as

"We understand that widely separated vortices tend to last longer than
closely spaced ones, which is one reason why jumbo vortices are so dangerous.
We know that 757s create vortices which dissipate more slowly (and therefore
are stronger at distances of three or four miles) than from other aircraft of
equal weight and similar configuration such as the A-300, but we don't 
entirely understand why."

"Assuming two aircraft of approximately the same size, we know both
theoretically and experimentally exactly what kinds of planes get rolled most
easily,  namely those with little roll inertia because the engines are
mounted on the fuselage and therefore very close to the axial center of
gravity -- the DC-9 (MD-80) and F-28.  When the engines are out on the wings
the airplane is much harder to roll, and if they're way out on the wings, as
in a 747, the plane is almost invulnerable to being rolled by a vortex.  Fuel
in the wing tanks also increases roll stability.  The U.S. doesn't have any
decent experimental data on this, but the CAA has a marvelous data set, and
we know that the planes that get rolled most often, all out of proportion to
their frequency, are the DC-9s (the ones causing the trouble all out of
proportion to their frequency are the 757 and 767)."
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