Wake turbulence is a a rotating air disturbance that is created by all airplanes
and helicopters in flight. In airplanes it consists of wingtip vortices,
swirls of air escaping around the wingtips from the high pressure area under
the wing to the lower pressure area above. In helicopters the same effect
occurs at the rotor tips. Vortices dissapate very slowly over about
two minutes, spreading outward and dropping downward as they do so. In very windy or turbulent
conditions, the vortices break up, but light winds carry the vortices, and
a light crosswind may hold one vortex over the runway instead of letting it
spread off to the side.
General Information for Wake Turbulence
Wake turbulence from large aircraft can turn you upside down, or tear your
airplane apart. It is up to you to be aware of the possibility, and to remain
clear of the path of large aircraft. Even small aircraft can make you
feel a bump, if you take off right behind one. It's quite a distinctive
feeling, very abrupt. Here's a joke relevant to wake
Question-by-Question Explanation of Wake
7.01 It's always
the responsibility of the pilot (does this remind you of the previous section?)
7.02 Count on it
being around for two minutes at least.
7.03 All three are
7.04 Any of those
three is a possibility.
(1) The vortices won't be completely gone after two minutes.
(2) & (3) They dissappear slowly.
(4) The vortices will sink below the path of the aircraft.
7.06 The only
way to avoid the wake is to land past the touchdown point of the larger airplane.
If you land behind it, at it or beside it, you are subject to wake
can't see the vortices, so the only way to be safe is to take off before the
place that you know they start.
Note that the big and small airplanes are not on the runway at the same time.
You need to watch where the large airplane touched down or rotated
and remember that point for when you arrive.
All rotary and fixed wing aircraft produce wake turbulence, even gliders,
but you'd have to be very close behind in another glider to feel the wake
from a glider.
Rotation is the point when the airplane's nose lifts off the ground. Most
of the weight of the airplane is now on the wings, so wingtip vortices start
then. Small airplanes lift off at almost the same moment as rotation,
but a heavy airplane may travel a fair way down the runway between rotation
7.10 The wake turbulence
starts at rotation and falls below the flight path.
7.11 Wing tip vortices
have nothing to do with the engine.
clean (flaps up, gear up) and slow are the factors
contributing to the most wake turbulence. Low aspect ratio wings also
generate more wake turbulence because a greater proportion of the air under
the wings has a chance to escape around the wingtip.
7.13 Similar to
wingtip vortices, rotor downwash trails behind and below the helicopter.
7.14 I believe this
is because the speed of the rotor tips is about the same regardless of the
type of helicopter. I have experienced this when boarding helicopters
in flight, in the coastguard. The big ones are louder, but not harder
vortices spread out and drift off the runway as they dissipate. However,
a light wind can hold one of them over the runway. Note that the tendency
of the vortices to spread can also mean that an aircraft taking off or landing
can affect your operations on another runway or taxiway.
Back to the Questions for this Section
| PSTAR Index | On to the
Questions for the Next Section
Back to Commentary on Previous PSTAR Section
| On to Next PSTAR Commentary Section
This page written 8 October 2002 by Robyn Stewart.
Last revised 3 July 2003.
What Canadian student pilots
need to know
Links and how to use the A.I.P. the CARs and the CFS
Canada Exam Guides
Study guides and flight test standards
Search all of wabyn.net
Send me e-mail