Wake Turbulence Commentary

General Information for Wake Turbulence

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. wingtip vortices behind an airplane

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.

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 turbulence.

Question-by-Question Explanation of Wake Turbulence

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 correct.

7.04 Any of those three is a possibility.

7.05
(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 turbulence.
staying above the path of a landing aircraft

7.07 You can't see the vortices, so the only way to be safe is to take off before the place that you know they start. small aircraft rotates before the rotation point of the large one

small aircraft rotates before the rotation point of the large one

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.

7.08
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.

7.09
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 and liftoff.

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.

7.12 Heavy, 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 The current AIM AIR 2.9 states that "the heavier the helicopter, the more intense the wake turbulence." At some point between 2006 and 2008 the text was changed. The previous version read "the size of the aircraft is not a factor on the intensity of the vortexes." It is very likely that some old PSTAR answer keys will reflext the old information, so be ready with AIR AIM 2.9 wif you get that question marked wrong.

7.15 Ordinarily 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.
wind holds the vortex over the runway