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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 into three groups:
Category ICAO and
Flight Plan
UK
Heavy
(H)
136 000 or
greater
136 000 or
greater
Medium
(M)
<136 000
and >7000
< 136 000 and >
40 000
Small (S) – 40 000 or less
and >17 000
Light (L) 7000 or less 17 000 or less
Successive aircraft on finals
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
Small 6
Light 8
Medium Medium* 3
Small 4
Light 6
Small Med or Small 3
Light 4
Note: 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. BV234, Puma, Super Puma, EH
101 and S61N helicopters are Small. Bell 212,
Sikorsky S76 and smaller machines are Light.
Departing aircraft
Applies to IFR and VFR flights.
86 JAR Private Pilot Studies
Same or parallel runways less than
760m apart (inc grass)
Leading Follow Departing
From
Min
space
Heavy Med/S
m/Lt
Same takeoff
posn
2 mins
Medium/
Small
Light Same takeoff
posn
2 mins
Heavy Med/S
m/ Lt
Intermediate
posn
3 mins
Medium/
Small
Light Intermediate
posn
3 mins
Runways with displaced landing
thresholds where flight paths cross
Leadiing Following Min space
Heavy Arrival Med/Small/Light Dep
2 mins
Heavy Departure Med/Small/Light Arr
2 mins
Medium Arrival Light/Small Dep 2
mins
Medium Departure Light/Small Arr 2
mins
Crossing and diverging or parallel
runways over 760m apart
Lead Crossing
Behind
Min Dist Time
Equiv
Hvy Hvy/Med/Sm
/Lt
4/5/6/8
miles
2/3/3/4
mins
Med Med/Sm/Lt 3/4/6
miles
2/2/3
mins
Small Med or
Sm/Lt
¾ miles 2/2
mins
Opposite direction runways
There should be at least 2 minutes
between a light, small or medium
and a heavy, and between light and a
small or medium within 760 m (a
grass strip is a runway).
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.
　
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