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range. Fly parallel to the range in an
updraught, avoiding peaks.
Significant Temperature Inversions
Performance is affected by variations
in temperature, and inversions will
do so adversely. Large ones
encountered shortly after take-off
can seriously degrade climb
performance, particularly when
heavy. Even a small inversion in the
upper levels can prevent you
reaching a preferred cruising altitude.
At lower levels, expect deteriorating
visibility, as an inversion can prevent
fog clearance for prolonged periods.
Another good reason for avoiding
the top of an inversion is that all the
industrial pollutants collect there,
especially in the stubble burning
season which may include
incinerated pesticides.
Wake Turbulence
A by-product of lift behind every
aircraft (including helicopters) in
forward flight, particularly severe
from heavy ones. Wake vortices are
horizontally concentrated whirlwinds
streaming from the wingtips, from
the separation point between high
pressure below and low pressure
above the wing. The heavier and
slower the aircraft, the more severe
they will be, and flaps, etc. will only
have a small effect in breaking them
up. 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.
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, 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 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
116 Operational Flying
expand to occupy an oval area about
1 wingspan high and 2 wide. 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 and before the point
of its take-off and after the point of
landing. Although ATC will
normally suggest an interval, use
these tables as a guide:
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.
Same or parallel runways less than
760m apart (inc grass)
Leading Follow Departing From Min space
Heavy Med/Sm/Lt Same takeoff
posn
2 mins
Medium/Small Light Same takeoff
posn
2 mins
Heavy Med/Sm/ 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 m 2/3/3/4 m
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
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