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minutes, or more, though the
maximum intensity will start on
touching the ground and only last
for a couple of minutes.
They are problematical because they
involve a performance-increasing
shear to start with, followed by a
performance decreasing one. Being
so close to the ground, you are likely
to be taking off or landing, and
therefore more vulnerable. The angle
of attack reduces inside a downburst,
because it changes with the relative
airflow, so the nose should be placed
into a high pitch attitude on entry,
and reduced (quickly) on exit.
Microbursts are rarely isolated – if
you meet one, watch out for
another.
Wake Turbulence
A by-product of lift behind every
aircraft, (including helicopters) in
forward flight, arising from induced
drag, particularly severe from heavy
machines, and worst at slow speeds,
as on takeoff or landing. Wake vortices
are horizontally concentrated
whirlwinds streaming from the
wingtips, from the separation point
between high pressure below and
low pressure above the wing. Air
flowing over the top of the wing
tends to flow inward due to the
reduced pressure sucking it in, while
that under the wing tends to flow
outwards because it is of higher
pressure and pushes outwards.
Where the lower air curls over the
wingtip, it combines with the upper
air to form a counter clockwise flow.
The distance between the vortices
will be about ¾ of the wingspan or
rotor disc.
The heavier and slower the aircraft,
the more severe they will be, and
flaps, etc. will only have a small
Weather 85
effect in breaking them up, so even
clean aircraft are dangerous. The
effects become undetectable after a
time, varying from a few seconds to
a few minutes after the departure or
arrival, although they have been
detected at 20 minutes. Vortices are
most hazardous to other aircraft
during take-off, initial climb, final
approach and landing, but you
should be careful any time you are
within 1,000 feet below and behind a
heavy aircraft.
Although there is a danger of
shockloading, the biggest problem is
loss of control near the ground. You
are safest if you keep above the
approach and take-off path of the
other aircraft, or land beyond its
touchdown point (or lift off before
its takeoff point) but, for general
purposes, allow at least 3 minutes
behind any greater than the Light
category for the effects to disappear
(but see the table below).
Wake generation begins when the
nosewheel lifts off on take-off and
continues until it touches down
again after landing. Vortices (one
from each wing) will drift
downwind, at about 400-500 fpm for
larger aircraft, levelling out at about
900 feet below the altitude at which
they were generated. Eventually they
expand to occupy an oval area about
1 wingspan high and 2 wide, one on
each side of the aircraft.
Those from large aircraft tend to
move away from one another so, on
a calm day, the runway itself will
remain free, depending on how near
the runway edge the offending wings
were. They will also drift with wind,
so your landings and take-offs
should occur upwind of moving
heavy aircraft, before the point of
take-off and after the point of
landing. Inside a vortex core, you
could get roll rates as much as 80°
per second and experience
downdraughts of over 1500 feet per
minute, so avoid them.
Aircraft are grouped for wake
turbulence purposes into three
groups:
· Group 1 (Heavy). Over 300,000
lbs
· Group 2 (Medium). Between
12,500-300,000 lbs.
· Group 3 (Light). Up to 12,500
lbs.
Although ATC will normally suggest
an interval, the table below can be
used as a guide, although there is
never a guarantee you will not
encounter wake turbulence,
whatever separations are given:
Leading
Aircraft
Following
Aircraft
Min dist
(miles)
Heavy Heavy 4
Medium 5
Light 6
Medium Medium* 3
Light 4
* If the leading medium is a B757,
increase to 4 miles, as they are difficult to
slow down and lose height with, and often
fly steeper approaches.
Non-radar departures qualify for two
minute separation for aircraft taking
off in the wake of a heavy, if starting
from the threshold of the same
runway or a parallel one less than
2,500 feet away. ATC do not apply
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