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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
the interval for lights following
mediums in the same circumstances,
but will issue advisories.
Otherwise, a three minute interval
will apply in the wake of a heavy (or
if the projected flight path crosses),
or a light aircraft departing in the
wake of a medium if it starts to
takeoff from an intersection or a
point further along than the previous
aircraft, or if the controller thinks it
needs more runway for takeoff (and
will therefore encounter the wake).
Weather 123
You can request a pilot waiver of
separation, but it will never be
offered. It will not be granted for
intersection takeoffs of lights or
mediums behind heavy takeoff or
missed approach.
Helicopters
Rotor downwash is wake turbulence
from helicopters, which is easy to
forget when hovering near a runway
threshold or parked aircraft with
little wind (although it's quite useful
when crop spraying). Otherwise, the
effects are similar to fixed wing, in
that you get vortices from each side
of the rotor disc, but the lower
operating speed means they are
more concentrated. Downwash also
creates dust storms and can lift even
heavy objects into the air, instantly
presenting Foreign Object Damage
(FOD) hazards to engines, main and
tail rotor blades (so don't bolt your
FOD, it gives you ingestion!—old
RAF joke, on which I hope there's
no copyright). Plastic bags or
packaging sheets are FOD, too.
Generally speaking, the larger the
helicopter, the greater the potential
danger (obvious, really). Bell 212,
Sikorsky S76 and smaller machines
are Light, in terms of the above table,
but size is not significant when
creating vortices; use the table for
comparison purposes when avoiding
other types.
The Altimeter
This is simply an aneroid barometer
calibrated in feet rather than
millibars or inches of mercury (its
inner workings are described fully in
　
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