Re: Are commuter aircrafts really that disliked ?

From: (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         03 Nov 95 02:51:38 
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In article <airliners.1995.1701@ohare.Chicago.COM> (Timothy Kono) writes:

   Robert Ashcroft ( wrote:
   : In article <45n0oe$>,
   : Michael Coch <> wrote:

   : >There are pros and cons to everything.  According to my pilot friends, you
   : >don't want to be in a light jet during landing if there's wind shear --
   : >takes a little too long for the engines to spool up to counter the shear.

   : Is this true?

      I'm just wondering but, are lighter jets more likely to be unstable
   during such things as wind sheer?  Also, would faster and more powerful
   engines counter the sheer much faster and better?  I just want to know.

In a windshear encounter, there are a couple of things going on.  (I'm
actually going to talk about microbursts, since they are the most
"dangerous" form of windshear.)

First, there (usually) is a pocket or stream of cold air descending
vertically from a thunderstorm cloud.  When this gets near the
ground, it turns out in the radial direction and accelerates to high
velocities.  An ASCIIgram of the cross section (use fixed width font
for best results):

      ||| <-Descending air
   \--/|\--/  <- flattening and "rollup" near the ground ( < 2000 ft AGL)
================= <- the ground

In the typical "hazard" case, the aircraft is flying an approach only
a couple of thousand feet above ground.  What the aircraft experiences
is first a headwind, then a sharp change (shear) into a downwind
followed immediately by a tailwind.

Since the aircraft cannot change ground speed fast enough to maintain a
constant airspeed, it "appears" that the airspeed drops quickly (in a
few seconds) by large amounts (30+ knots).  Obviously, this is a bad
situation for any aircraft, including propellor and jet aircraft.

However, with jet transports, there is a second, more dangerous effect
than just airspeed loss.  An airframe in flight, like any dynamic
system, has "modes" of motion.  One mode which every pilot becomes
familiar with when first learning to fly is the phugoid mode.  It is a
pendulum-like motion caused by an out-of-phase oscillation of altitude
and airspeed.

It just so happens that the typical approach speed of a jet transport
and the size of the microburst end up exciting this mode of
oscillation, which makes recovery extremely difficult.  Furthermore,
since jet engines have a spool-up time latency (even just a couple of
seconds), this can be the difference between hitting the ground and
not hitting the ground.

It turns out the recommended "escape manuever" for jets in a microburst
is to command maximum thrust and pitch to "stick shaker", or maximum
angle-of-attack.  This is because the aircraft needs to combat
altitude loss as quickly as possible.

For a slow piston-engine aircraft, the "escape manuever" is not as
clear:  it turns out that for some aircraft, it's better to INCREASE
airspeed by pitching down in order to escape the downdraft portion of
the microburst as quickly as possible.  (Slower aircraft by their
nature are less susceptible to the airspeed loss because the slower
speed translates to a weaker shear, but instead spend more time in the
downdraft part.)  However, research continues in this area.

Bottom lines:  1) smaller, slower aircraft are not as greatly affected
by the dynamics of a microburst encounter compared to jets.  2) If
you're on a jet in a microburst encounter, you want to have as much
excess thrust as possible to combat altitude loss.

Sorry for the long reply, hope this helps,


--------   Ed Hahn | | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.