What is a Wake?
You've probably heard some variation of the term "left in a wake," used
to refer to someone or something which was sorely beaten in a race.
However, a plane "left in a wake" is in a serious situation, which can
be potentially dangerous to a small airplane.
A vortex is a circulating cylinder of air; basically, a group of air
spinning in a circle. Every wing attached to a plane which is flying
produces vortices; it's impossible to have one without the other. Lift
accompanies vortices, just as vortices accompany lift. A vortex is
formed as air is pulled down over the wing of a plane. Since the wind
is moving faster over the top of the wing, as it meets the air coming
under the bottom of the wing, it spins into a vortex.
As the vortices are formed coming off the wings, they form a wake
vortex, which may extend for miles, of two long, horizontal cylinders
of spinning air. Eventually, the two spinning vortices meet, and the
vortex cancels itself out.
Why is this a Problem?
At first, it may appear that vortices are not a problem. They produce
no effect on the plane which creates them, besides lift. What vortices
can do is harm planes which enter their paths. A large, heavy plane
creates a very strong vortex, one which potentially could flip a small
plane 180°. The vortices create a spinning motion, one which can
easily act on any object in its trail. Imagine feeling a 30 mph (48
km/hour) wind against your back. A wind of such magnitude, while it
would likely not flip you over, would certainly push you around. The
wake vortex off a very large plane can be many times as strong as this,
and certainly strong enough to make a plane toss around.
Whenever you feel a slight movement in the plane, or hear the pilot
announce turbulence ahead, what he's really saying is that another
plane has left a wake. In planes you fly in commercially, turbulence is
hardly ever more than a slight annoyance, except in isolated incidents.
However, for small planes, they are a potentially deadly phenomena
which must be avoided.