Re: Caution: (757) Wake Turbulence ?

From:         geoff@peck.com (Geoff Peck)
Organization: Geoffrey G. Peck, Consultant, San Jose CA
Date:         09 Jan 94 18:13:35 PST
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In article <airliners.1994.864@ohare.Chicago.COM>  barney@skat.usc.edu
(Barney Lum) writes:
> After concluding that the crash of a private twin jet (Westwind?)
> on approach to John Wayne/Orange County/Santa Ana/SNA on around
> the 15th of December was caused by jet wake turbulence, the FAA
> issued a warning pilots not to fly closely behind 757s (reported
> in The Advertiser, Honolulu).

Indeed, I got a copy of said warning in the mail a few days ago.  It
appears to be a mass mailing to all pilots with active medical
certificates.  (Read:  THIS IS SERIOUS.)

> The smaller jet's pilot was warned sevarl times by ATC that he was
> getting too close (two miles) behind the UA jet, with the pilot 
> acknowledging.  It was on a short, 20 mile hop from Brackett in VMC.  
> Two crew and two pax (In-N-Out execs) were killed.

Aargh.  I think (hope?) that most pilots of piston-powered light aircraft
are _very_ aware of wake turbulence, and of avoidance techniques.  It may
be that the crew of the Westwind was thinking "gee, he's a jet, we're a
jet, no problem."  Wrong.  There have been cases of 727s and 737s being
upset by 747 wake turbulence...

I just happened to fly in to SNA the day after the crash.  It was, of
course, all over the newspapers down there, and folks at the airport were
busy speculating what had happened.  (Like, duh, given the mode-C readout
traces which were published in the Orange County register, how could you
come to any other conclusion?)

> 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?

Yes, the 757 appears to generate wake turbulence which is much more
severe than would be expected for an aircraft of its weight.  (Remember
that the "heavy" designation applies to aircraft which may have a
takeoff weight of 300,000 pounds or m ore whether or not they are
operating at this weight during a particular phase of flight.  The 757
doesn't qualify as a heavy, the 767 does.)  I've seen theories that the
exceptional wake turbulence of the 757 is a function of the
exceptionally clean wing design of the aircraft ...  perhaps someone
from Boeing could comment on this?

> Naturally, smaller aircraft should give a wide berth to larger ones 
> regardless of powerplant.

    Twr: "Tomahawk xyz, you're cleared to land, number two behind a
	  Saratoga.  Caution wake turbulence."

							Geoff