Terrible Turbulence, and the No Good, Very Bad Landing
Turbulence in the Landing Area
You’ve already seen turbulence before, but perhaps you never gave much thought to it. In water, eddies can show us some beautiful forms of turbulent flow. And, if you’ve watched smoke rise up from a campfire, then you may have noticed that sometimes it swirls around a bit. If you have more than 10 jumps, then you have probably already experienced turbulence in one form or another: that bumpy descent or maybe that landing where it felt like you dropped a few feet at the end.
Water eddies show turbulent flow.
Smoke rising from a fire can show turbulent flow.
Turbulence can be dangerous to skydivers and, unfortunately, turbulent air looks no different than smooth air. So, we must anticipate where it could be found. This post will take you through some common scenarios that skydivers should be aware of.
Definition of Turbulence
Turbulence is the violent or unsteady movement of a fluid.
From the world of physics, a fluid is any substance that flows. We usually only think of liquids as fluids, but gases are also fluids. Air flows and takes the shape of its container… therefore it is a fluid.
Downwind of An Obstacle
Most skydivers immediately think of this situation when asked where turbulence might exist in the landing area. It can be found downwind of an obstacle for 10-20 times the height of the obstacle. For example, if you have a row of trees that are about 40 feet tall, then you could expect to find turbulence 400-800 feet away from them on the downwind side.
From the above picture, we can also see that you would find some turbulent flow close to the obstacle on the upwind side and right on top of the obstacle as well. The less aerodynamically shaped the obstacle is, the more turbulent air.
This is a shortened version of a YouTube video that shows the effects of turbulence from wind flowing over obstacles on a tandem skydive landing. Full video is at https://www.youtube.com/watch?v=_yMJXcZocE8
Wind Shear
Turbulence can also occur as wind speed or wind direction changes over a vertical distance. On days where the wind speed drops off dramatically from one altitude to the next, then you can expect to feel some bumpiness.
It can also exist where there are temperature inversions, and can be found horizontally with incoming weather fronts.
Where Two Different Colors or Textures Meet
All over most airports, you will find asphalt that is next to grass. As the day progresses, the asphalt heats up faster than any surrounding areas of grass, trees, and water. The uneven heating of the air causes small areas of convective currents, giving you a bumpy flight as you cross over from one surface to the other. You should be careful flying over different surfaces as you are coming in to land, as turbulence from this phenomenon can lead to a hard landing.
Wake Turbulence
This type of turbulence is created by the propellers of passing planes, or from passing jets (we get the occasional visitor at our small airport). If you are landing near a runway or a taxiway then propeller wash may be a factor that you have to contend with….just another reason to try to stay away from both.
However, wake turbulence can also be present behind another parachute. Turbulent air can typically be felt more than 50 feet behind the flight path of the parachute that generated it. It’s incredibly important to keep ample distance between yourself and other parachutes while flying your landing pattern.
When You Encounter Turbulence
It used to be common for skydivers to believe that flying in some brakes (1/4 to 1/2 brakes) to “pressurize the canopy” was beneficial when encountering turbulence. Indeed, it did seem to help some of the older ram-air designs. Today, however, parachute manufacturers advise to keep your parachute in full flight.
If turbulence causes your parachute to suddenly dive, turn, or drop, then you should do the following:
stop or reduce any sudden loss of altitude
give needed input to keep your wing level
continue to steer for a clear, open landing area
be prepared for a hard landing (PLF)